google_cloud_auth/credentials/

external_account.rs

// Copyright 2025 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! [Workload Identity Federation] (or External Account) credentials.
//!
//! Workload identity federation allows applications running outside of Google Cloud
//! to access Google Cloud resources without using a [Service Account Key]. Instead of
//! a long-lived credential (like a service account key), you can exchange a credential
//! from your workload's identity provider for a short-lived Google Cloud access token.
//! As per [best practices], Workload Identity Federation is the recommended method of
//! authentication if your workload is using an external identity provider.
//!
//! Refer to [Obtain short-lived tokens for Workforce Identity Federation] for
//! creating configurations that can be used with this library for loading credentials
//! using various external toke provider sources such as file, URL, or an executable.
//!
//! ## Example: Creating credentials from a JSON object
//!
//! ```
//! # use google_cloud_auth::credentials::external_account;
//! # use http::Extensions;
//! #
//! # async fn sample() -> anyhow::Result<()> {
//! let project_id = "your-gcp-project-id";
//! let pool_id = "your-workload-identity-pool-id";
//! let provider_id = "your-provider-id";
//!
//! let audience = format!(
//!     "//iam.googleapis.com/projects/{}/locations/global/workloadIdentityPools/{}/providers/{}",
//!     project_id, pool_id, provider_id
//! );
//!
//! // This is an example of a configuration for a file-sourced credential.
//! // The actual configuration will depend on your identity provider.
//! let external_account_config = serde_json::json!({
//!     "type": "external_account",
//!     "audience": audience,
//!     "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
//!     "token_url": "https://sts.googleapis.com/v1/token",
//!     "credential_source": {
//!         "file": "/path/to/your/oidc/token.jwt"
//!     }
//! });
//!
//! let credentials = external_account::Builder::new(external_account_config)
//!     .build()
//!     .unwrap();
//! let headers = credentials.headers(Extensions::new()).await?;
//! println!("Headers: {headers:?}");
//! # Ok(()) }
//! ```
//!
//! ## Example: Creating credentials with custom retry behavior
//!
//! ```
//! # use google_cloud_auth::credentials::external_account;
//! # use http::Extensions;
//! # use std::time::Duration;
//! # async fn sample() -> anyhow::Result<()> {
//! use google_cloud_gax::retry_policy::{AlwaysRetry, RetryPolicyExt};
//! use google_cloud_gax::exponential_backoff::ExponentialBackoff;
//! # let project_id = "your-gcp-project-id";
//! # let pool_id = "your-workload-identity-pool-id";
//! # let provider_id = "your-provider-id";
//! #
//! # let audience = format!(
//! #     "//iam.googleapis.com/projects/{}/locations/global/workloadIdentityPools/{}/providers/{}",
//! #     project_id, pool_id, provider_id
//! # );
//! #
//! # let external_account_config = serde_json::json!({
//! #     "type": "external_account",
//! #     "audience": audience,
//! #     "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
//! #     "token_url": "https://sts.googleapis.com/v1/token",
//! #     "credential_source": {
//! #         "file": "/path/to/your/oidc/token.jwt"
//! #     }
//! # });
//! let backoff = ExponentialBackoff::default();
//! let credentials = external_account::Builder::new(external_account_config)
//!     .with_retry_policy(AlwaysRetry.with_attempt_limit(3))
//!     .with_backoff_policy(backoff)
//!     .build()
//!     .unwrap();
//! let headers = credentials.headers(Extensions::new()).await?;
//! println!("Headers: {headers:?}");
//! # Ok(()) }
//! ```
//!
//! [Workload Identity Federation]: https://cloud.google.com/iam/docs/workload-identity-federation
//! [AIP-4117]: https://google.aip.dev/auth/4117
//! [Service Account Key]: https://cloud.google.com/iam/docs/service-account-creds#key-types
//! [best practices]: https://cloud.google.com/docs/authentication#auth-decision-tree
//! [Obtain short-lived tokens for Workforce Identity Federation]: https://cloud.google.com/iam/docs/workforce-obtaining-short-lived-credentials#use_configuration_files_for_sign-in

use super::dynamic::CredentialsProvider;
use super::external_account_sources::aws_sourced::AwsSourcedCredentials;
use super::external_account_sources::executable_sourced::ExecutableSourcedCredentials;
use super::external_account_sources::file_sourced::FileSourcedCredentials;
use super::external_account_sources::url_sourced::UrlSourcedCredentials;
use super::impersonated;
use super::internal::sts_exchange::{ClientAuthentication, ExchangeTokenRequest, STSHandler};
use super::{CacheableResource, Credentials};
use crate::access_boundary::{CredentialsWithAccessBoundary, external_account_lookup_url};
use crate::build_errors::Error as BuilderError;
use crate::constants::{DEFAULT_SCOPE, DEFAULT_UNIVERSE_DOMAIN, STS_TOKEN_URL};
use crate::credentials::dynamic::AccessTokenCredentialsProvider;
use crate::credentials::external_account_sources::programmatic_sourced::ProgrammaticSourcedCredentials;
use crate::credentials::subject_token::dynamic;
use crate::credentials::{AccessToken, AccessTokenCredentials};
use crate::errors::non_retryable;
use crate::headers_util::AuthHeadersBuilder;
use crate::retry::Builder as RetryTokenProviderBuilder;
use crate::token::{CachedTokenProvider, Token, TokenProvider};
use crate::token_cache::TokenCache;
use crate::{BuildResult, Result};
use google_cloud_gax::backoff_policy::BackoffPolicyArg;
use google_cloud_gax::retry_policy::RetryPolicyArg;
use google_cloud_gax::retry_throttler::RetryThrottlerArg;
use http::{Extensions, HeaderMap};
use serde::{Deserialize, Serialize};
use serde_json::Value;
use std::collections::HashMap;
use std::sync::Arc;
use tokio::time::{Duration, Instant};

const IAM_SCOPE: &str = "https://www.googleapis.com/auth/iam";

#[derive(Serialize, Deserialize, Debug, Clone, PartialEq)]
pub(crate) struct CredentialSourceFormat {
    #[serde(rename = "type")]
    pub format_type: String,
    #[serde(skip_serializing_if = "Option::is_none")]
    pub subject_token_field_name: Option<String>,
}

#[derive(Serialize, Deserialize, Debug, Clone, Default)]
pub(crate) struct ExecutableConfig {
    pub command: String,
    pub timeout_millis: Option<u32>,
    pub output_file: Option<String>,
}

#[derive(Serialize, Deserialize, Debug, Clone)]
#[serde(untagged)]
enum CredentialSourceFile {
    // Most specific variants first for untagged enum
    Aws {
        environment_id: String,
        region_url: Option<String>,
        url: Option<String>,
        regional_cred_verification_url: Option<String>,
        imdsv2_session_token_url: Option<String>,
    },
    Executable {
        executable: ExecutableConfig,
    },
    Url {
        url: String,
        headers: Option<HashMap<String, String>>,
        format: Option<CredentialSourceFormat>,
    },
    File {
        file: String,
        format: Option<CredentialSourceFormat>,
    },
}

/// Check if the audience parameter matches the pattern for a valid workforce pool.
/// Example: https://iam.googleapis.com/locations/global/workforcePools/pool/providers/provider
fn is_valid_workforce_pool_audience(audience: &str) -> bool {
    let path = audience
        .strip_prefix("//iam.googleapis.com/")
        .unwrap_or(audience);

    let mut s = path.split('/');
    matches!((s.next(), s.next(), s.next(), s.next(), s.next(), s.next(), s.next()), (
            Some("locations"),
            Some(_location),
            Some("workforcePools"),
            Some(pool),
            Some("providers"),
            Some(provider),
            None,
        ) if !pool.is_empty() && !provider.is_empty())
}

/// A representation of a [external account config file].
///
/// [external account config file]: https://google.aip.dev/auth/4117#configuration-file-generation-and-usage
#[derive(Serialize, Deserialize, Debug, Clone)]
struct ExternalAccountFile {
    audience: String,
    subject_token_type: String,
    service_account_impersonation_url: Option<String>,
    token_url: String,
    client_id: Option<String>,
    client_secret: Option<String>,
    scopes: Option<Vec<String>>,
    credential_source: CredentialSourceFile,
    workforce_pool_user_project: Option<String>,
    universe_domain: Option<String>,
}

impl From<ExternalAccountFile> for ExternalAccountConfig {
    fn from(config: ExternalAccountFile) -> Self {
        let mut scope = config.scopes.unwrap_or_default();
        if scope.is_empty() {
            scope.push(DEFAULT_SCOPE.to_string());
        }
        Self {
            audience: config.audience.clone(),
            client_id: config.client_id,
            client_secret: config.client_secret,
            subject_token_type: config.subject_token_type,
            token_url: config.token_url,
            service_account_impersonation_url: config.service_account_impersonation_url,
            credential_source: CredentialSource::from_file(
                config.credential_source,
                &config.audience,
            ),
            scopes: scope,
            workforce_pool_user_project: config.workforce_pool_user_project,
            universe_domain: config.universe_domain,
        }
    }
}

impl CredentialSource {
    fn from_file(source: CredentialSourceFile, audience: &str) -> Self {
        match source {
            CredentialSourceFile::Url {
                url,
                headers,
                format,
            } => Self::Url(UrlSourcedCredentials::new(url, headers, format)),
            CredentialSourceFile::Executable { executable } => {
                Self::Executable(ExecutableSourcedCredentials::new(executable))
            }
            CredentialSourceFile::File { file, format } => {
                Self::File(FileSourcedCredentials::new(file, format))
            }
            CredentialSourceFile::Aws {
                region_url,
                url,
                regional_cred_verification_url,
                imdsv2_session_token_url,
                ..
            } => Self::Aws(AwsSourcedCredentials::new(
                region_url,
                url,
                regional_cred_verification_url,
                imdsv2_session_token_url,
                audience.to_string(),
            )),
        }
    }
}

#[derive(Debug, Clone)]
struct ExternalAccountConfig {
    audience: String,
    subject_token_type: String,
    token_url: String,
    service_account_impersonation_url: Option<String>,
    client_id: Option<String>,
    client_secret: Option<String>,
    scopes: Vec<String>,
    credential_source: CredentialSource,
    workforce_pool_user_project: Option<String>,
    universe_domain: Option<String>,
}

#[derive(Debug, Default)]
struct ExternalAccountConfigBuilder {
    audience: Option<String>,
    subject_token_type: Option<String>,
    token_url: Option<String>,
    service_account_impersonation_url: Option<String>,
    client_id: Option<String>,
    client_secret: Option<String>,
    scopes: Option<Vec<String>>,
    credential_source: Option<CredentialSource>,
    workforce_pool_user_project: Option<String>,
    universe_domain: Option<String>,
}

impl ExternalAccountConfigBuilder {
    fn with_audience<S: Into<String>>(mut self, audience: S) -> Self {
        self.audience = Some(audience.into());
        self
    }

    fn with_subject_token_type<S: Into<String>>(mut self, subject_token_type: S) -> Self {
        self.subject_token_type = Some(subject_token_type.into());
        self
    }

    fn with_token_url<S: Into<String>>(mut self, token_url: S) -> Self {
        self.token_url = Some(token_url.into());
        self
    }

    fn with_service_account_impersonation_url<S: Into<String>>(mut self, url: S) -> Self {
        self.service_account_impersonation_url = Some(url.into());
        self
    }

    fn with_client_id<S: Into<String>>(mut self, client_id: S) -> Self {
        self.client_id = Some(client_id.into());
        self
    }

    fn with_client_secret<S: Into<String>>(mut self, client_secret: S) -> Self {
        self.client_secret = Some(client_secret.into());
        self
    }

    fn with_scopes(mut self, scopes: Vec<String>) -> Self {
        self.scopes = Some(scopes);
        self
    }

    fn with_credential_source(mut self, source: CredentialSource) -> Self {
        self.credential_source = Some(source);
        self
    }

    fn with_workforce_pool_user_project<S: Into<String>>(mut self, project: S) -> Self {
        self.workforce_pool_user_project = Some(project.into());
        self
    }

    fn with_universe_domain<S: Into<String>>(mut self, universe_domain: S) -> Self {
        self.universe_domain = Some(universe_domain.into());
        self
    }

    fn build(self) -> BuildResult<ExternalAccountConfig> {
        let audience = self
            .audience
            .clone()
            .ok_or(BuilderError::missing_field("audience"))?;

        if self.workforce_pool_user_project.is_some()
            && !is_valid_workforce_pool_audience(&audience)
        {
            return Err(BuilderError::parsing(
                "workforce_pool_user_project should not be set for non-workforce pool credentials",
            ));
        }

        Ok(ExternalAccountConfig {
            audience,
            subject_token_type: self
                .subject_token_type
                .ok_or(BuilderError::missing_field("subject_token_type"))?,
            token_url: self
                .token_url
                .ok_or(BuilderError::missing_field("token_url"))?,
            scopes: self.scopes.ok_or(BuilderError::missing_field("scopes"))?,
            credential_source: self
                .credential_source
                .ok_or(BuilderError::missing_field("credential_source"))?,
            service_account_impersonation_url: self.service_account_impersonation_url,
            client_id: self.client_id,
            client_secret: self.client_secret,
            workforce_pool_user_project: self.workforce_pool_user_project,
            universe_domain: self.universe_domain,
        })
    }
}

#[derive(Debug, Clone)]
enum CredentialSource {
    Url(UrlSourcedCredentials),
    Executable(ExecutableSourcedCredentials),
    File(FileSourcedCredentials),
    Aws(AwsSourcedCredentials),
    Programmatic(ProgrammaticSourcedCredentials),
}

impl ExternalAccountConfig {
    fn make_credentials(
        self,
        quota_project_id: Option<String>,
        retry_builder: RetryTokenProviderBuilder,
    ) -> ExternalAccountCredentials<TokenCache> {
        let config = self.clone();
        match self.credential_source {
            CredentialSource::Url(source) => {
                Self::make_credentials_from_source(source, config, quota_project_id, retry_builder)
            }
            CredentialSource::Executable(source) => {
                Self::make_credentials_from_source(source, config, quota_project_id, retry_builder)
            }
            CredentialSource::Programmatic(source) => {
                Self::make_credentials_from_source(source, config, quota_project_id, retry_builder)
            }
            CredentialSource::File(source) => {
                Self::make_credentials_from_source(source, config, quota_project_id, retry_builder)
            }
            CredentialSource::Aws(source) => {
                Self::make_credentials_from_source(source, config, quota_project_id, retry_builder)
            }
        }
    }

    fn make_credentials_from_source<T>(
        subject_token_provider: T,
        config: ExternalAccountConfig,
        quota_project_id: Option<String>,
        retry_builder: RetryTokenProviderBuilder,
    ) -> ExternalAccountCredentials<TokenCache>
    where
        T: dynamic::SubjectTokenProvider + 'static,
    {
        let universe_domain = config.universe_domain.clone();
        let token_provider = ExternalAccountTokenProvider {
            subject_token_provider,
            config,
        };
        let token_provider_with_retry = retry_builder.build(token_provider);
        let cache = TokenCache::new(token_provider_with_retry);
        ExternalAccountCredentials {
            token_provider: cache,
            quota_project_id,
            universe_domain,
        }
    }
}

#[derive(Debug)]
struct ExternalAccountTokenProvider<T>
where
    T: dynamic::SubjectTokenProvider,
{
    subject_token_provider: T,
    config: ExternalAccountConfig,
}

#[async_trait::async_trait]
impl<T> TokenProvider for ExternalAccountTokenProvider<T>
where
    T: dynamic::SubjectTokenProvider,
{
    async fn token(&self) -> Result<Token> {
        let subject_token = self.subject_token_provider.subject_token().await?;

        let audience = self.config.audience.clone();
        let subject_token_type = self.config.subject_token_type.clone();
        let user_scopes = self.config.scopes.clone();
        let url = self.config.token_url.clone();

        let extra_options =
            if self.config.client_id.is_none() && self.config.client_secret.is_none() {
                let workforce_pool_user_project = self.config.workforce_pool_user_project.clone();
                workforce_pool_user_project.map(|project| {
                    let mut options = HashMap::new();
                    options.insert("userProject".to_string(), project);
                    options
                })
            } else {
                None
            };

        // User provides the scopes to be set on the final token they receive. The scopes they
        // provide does not necessarily have to include the IAM scope or cloud-platform scope
        // which are needed to make the call to `iamcredentials` endpoint. So when minting the
        // STS token, we use the user provided or IAM scope depending on whether the STS provided
        // token is to be used with impersonation or directly.
        let sts_scope = if self.config.service_account_impersonation_url.is_some() {
            vec![IAM_SCOPE.to_string()]
        } else {
            user_scopes.clone()
        };

        let req = ExchangeTokenRequest {
            url,
            audience: Some(audience),
            subject_token: subject_token.token,
            subject_token_type,
            scope: sts_scope,
            authentication: ClientAuthentication {
                client_id: self.config.client_id.clone(),
                client_secret: self.config.client_secret.clone(),
            },
            extra_options,
            ..ExchangeTokenRequest::default()
        };

        let token_res = STSHandler::default().exchange_token(req).await?;

        if let Some(impersonation_url) = &self.config.service_account_impersonation_url {
            let mut headers = HeaderMap::new();
            headers.insert(
                http::header::AUTHORIZATION,
                http::HeaderValue::from_str(&format!("Bearer {}", token_res.access_token))
                    .map_err(non_retryable)?,
            );

            return impersonated::generate_access_token(
                headers,
                None,
                user_scopes,
                impersonated::DEFAULT_LIFETIME,
                impersonation_url,
            )
            .await;
        }

        let token = Token {
            token: token_res.access_token,
            token_type: token_res.token_type,
            expires_at: Some(Instant::now() + Duration::from_secs(token_res.expires_in)),
            metadata: None,
        };
        Ok(token)
    }
}

#[derive(Debug)]
pub(crate) struct ExternalAccountCredentials<T>
where
    T: CachedTokenProvider,
{
    token_provider: T,
    quota_project_id: Option<String>,
    universe_domain: Option<String>,
}

/// A builder for external account [Credentials] instances.
///
/// # Example
/// ```
/// # use google_cloud_auth::credentials::external_account::Builder;
/// # async fn sample() -> anyhow::Result<()> {
/// let project_id = project_id();
/// let workload_identity_pool_id = workload_identity_pool();
/// let provider_id = workload_identity_provider();
/// let provider_name = format!(
///     "//iam.googleapis.com/projects/{project_id}/locations/global/workloadIdentityPools/{workload_identity_pool_id}/providers/{provider_id}"
/// );
/// let config = serde_json::json!({
///     "type": "external_account",
///     "audience": provider_name,
///     "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
///     "token_url": "https://sts.googleapis.com/v1beta/token",
///     "credential_source": {
///         "url": format!("http://169.254.169.254/metadata/identity/oauth2/token?api-version=2018-02-01&resource={provider_name}"),
///         "headers": {
///           "Metadata": "True"
///         },
///         "format": {
///           "type": "json",
///           "subject_token_field_name": "access_token"
///         }
///     }
/// });
/// let credentials = Builder::new(config)
///     .with_quota_project_id("quota_project")
///     .build()?;
/// # Ok(()) }
///
/// # fn project_id() -> String {
/// #     "test-only".to_string()
/// # }
/// # fn workload_identity_pool() -> String {
/// #     "test-only".to_string()
/// # }
/// # fn workload_identity_provider() -> String {
/// #     "test-only".to_string()
/// # }
/// ```
pub struct Builder {
    external_account_config: Value,
    quota_project_id: Option<String>,
    scopes: Option<Vec<String>>,
    universe_domain: Option<String>,
    retry_builder: RetryTokenProviderBuilder,
    iam_endpoint_override: Option<String>,
}

impl Builder {
    /// Creates a new builder using [external_account_credentials] JSON value.
    ///
    /// [external_account_credentials]: https://google.aip.dev/auth/4117#configuration-file-generation-and-usage
    pub fn new(external_account_config: Value) -> Self {
        Self {
            external_account_config,
            quota_project_id: None,
            scopes: None,
            universe_domain: None,
            retry_builder: RetryTokenProviderBuilder::default(),
            iam_endpoint_override: None,
        }
    }

    /// Sets the [quota project] for this credentials.
    ///
    /// In some services, you can use a service account in
    /// one project for authentication and authorization, and charge
    /// the usage to a different project. This requires that the
    /// service account has `serviceusage.services.use` permissions on the quota project.
    ///
    /// [quota project]: https://cloud.google.com/docs/quotas/quota-project
    pub fn with_quota_project_id<S: Into<String>>(mut self, quota_project_id: S) -> Self {
        self.quota_project_id = Some(quota_project_id.into());
        self
    }

    /// Sets the Google Cloud universe domain for these credentials.
    ///
    /// Any value provided here overrides a `universe_domain` value from the input service account JSON.
    ///
    /// # Example
    /// ```
    /// # use google_cloud_auth::credentials::external_account::Builder;
    /// # use serde_json::json;
    /// # async fn sample() -> anyhow::Result<()> {
    /// # let config = json!({
    /// #     "type": "external_account",
    /// #     "audience": "audience",
    /// #     "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
    /// #     "token_url": "https://sts.googleapis.com/v1beta/token",
    /// #     "credential_source": { "url": "url" }
    /// # });
    /// let credentials = Builder::new(config)
    ///     .with_universe_domain("googleapis.com")
    ///     .build()?;
    /// # Ok(()) }
    /// ```
    ///
    /// [universe domain]: https://cloud.google.com/docs/authentication/universe-domain
    pub fn with_universe_domain<S: Into<String>>(mut self, universe_domain: S) -> Self {
        self.universe_domain = Some(universe_domain.into());
        self
    }

    /// Overrides the [scopes] for this credentials.
    ///
    /// [scopes]: https://developers.google.com/identity/protocols/oauth2/scopes
    pub fn with_scopes<I, S>(mut self, scopes: I) -> Self
    where
        I: IntoIterator<Item = S>,
        S: Into<String>,
    {
        self.scopes = Some(scopes.into_iter().map(|s| s.into()).collect());
        self
    }

    /// Configure the retry policy for fetching tokens.
    ///
    /// The retry policy controls how to handle retries, and sets limits on
    /// the number of attempts or the total time spent retrying.
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::Builder;
    /// # async fn sample() -> anyhow::Result<()> {
    /// use google_cloud_gax::retry_policy::{AlwaysRetry, RetryPolicyExt};
    /// let config = serde_json::json!({
    ///     "type": "external_account",
    ///     "audience": "audience",
    ///     "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
    ///     "token_url": "https://sts.googleapis.com/v1beta/token",
    ///     "credential_source": { "file": "/path/to/your/oidc/token.jwt" }
    /// });
    /// let credentials = Builder::new(config)
    ///     .with_retry_policy(AlwaysRetry.with_attempt_limit(3))
    ///     .build()?;
    /// # Ok(()) }
    /// ```
    pub fn with_retry_policy<V: Into<RetryPolicyArg>>(mut self, v: V) -> Self {
        self.retry_builder = self.retry_builder.with_retry_policy(v.into());
        self
    }

    /// Configure the retry backoff policy.
    ///
    /// The backoff policy controls how long to wait in between retry attempts.
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::Builder;
    /// # use std::time::Duration;
    /// # async fn sample() -> anyhow::Result<()> {
    /// use google_cloud_gax::exponential_backoff::ExponentialBackoff;
    /// let config = serde_json::json!({
    ///     "type": "external_account",
    ///     "audience": "audience",
    ///     "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
    ///     "token_url": "https://sts.googleapis.com/v1beta/token",
    ///     "credential_source": { "file": "/path/to/your/oidc/token.jwt" }
    /// });
    /// let policy = ExponentialBackoff::default();
    /// let credentials = Builder::new(config)
    ///     .with_backoff_policy(policy)
    ///     .build()?;
    /// # Ok(()) }
    /// ```
    pub fn with_backoff_policy<V: Into<BackoffPolicyArg>>(mut self, v: V) -> Self {
        self.retry_builder = self.retry_builder.with_backoff_policy(v.into());
        self
    }

    /// Configure the retry throttler.
    ///
    /// Advanced applications may want to configure a retry throttler to
    /// [Address Cascading Failures] and when [Handling Overload] conditions.
    /// The authentication library throttles its retry loop, using a policy to
    /// control the throttling algorithm. Use this method to fine tune or
    /// customize the default retry throttler.
    ///
    /// [Handling Overload]: https://sre.google/sre-book/handling-overload/
    /// [Address Cascading Failures]: https://sre.google/sre-book/addressing-cascading-failures/
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::Builder;
    /// # async fn sample() -> anyhow::Result<()> {
    /// use google_cloud_gax::retry_throttler::AdaptiveThrottler;
    /// let config = serde_json::json!({
    ///     "type": "external_account",
    ///     "audience": "audience",
    ///     "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
    ///     "token_url": "https://sts.googleapis.com/v1beta/token",
    ///     "credential_source": { "file": "/path/to/your/oidc/token.jwt" }
    /// });
    /// let credentials = Builder::new(config)
    ///     .with_retry_throttler(AdaptiveThrottler::default())
    ///     .build()?;
    /// # Ok(()) }
    /// ```
    pub fn with_retry_throttler<V: Into<RetryThrottlerArg>>(mut self, v: V) -> Self {
        self.retry_builder = self.retry_builder.with_retry_throttler(v.into());
        self
    }

    #[cfg(all(test, google_cloud_unstable_trusted_boundaries))]
    fn maybe_iam_endpoint_override(mut self, iam_endpoint_override: Option<String>) -> Self {
        self.iam_endpoint_override = iam_endpoint_override;
        self
    }

    /// Returns a [Credentials] instance with the configured settings.
    ///
    /// # Errors
    ///
    /// Returns a [BuilderError] if the `external_account_config`
    /// provided to [`Builder::new`] cannot be successfully deserialized into the
    /// expected format for an external account configuration. This typically happens if the
    /// JSON value is malformed or missing required fields.
    ///
    /// For more information, on the expected format, consult the relevant
    /// section in the [external_account_credentials] guide.
    ///
    /// [external_account_credentials]: https://google.aip.dev/auth/4117#configuration-file-generation-and-usage
    pub fn build(self) -> BuildResult<Credentials> {
        Ok(self.build_credentials()?.into())
    }

    /// Returns an [AccessTokenCredentials] instance with the configured settings.
    ///
    /// # Errors
    ///
    /// Returns a [BuilderError] if the `external_account_config`
    /// provided to [`Builder::new`] cannot be successfully deserialized into the
    /// expected format for an external account configuration. This typically happens if the
    /// JSON value is malformed or missing required fields.
    ///
    /// For more information, on the expected format, consult the relevant
    /// section in the [external_account_credentials] guide.
    ///
    /// [external_account_credentials]: https://google.aip.dev/auth/4117#configuration-file-generation-and-usage
    pub fn build_access_token_credentials(self) -> BuildResult<AccessTokenCredentials> {
        Ok(self.build_credentials()?.into())
    }

    fn build_credentials(
        self,
    ) -> BuildResult<CredentialsWithAccessBoundary<ExternalAccountCredentials<TokenCache>>> {
        let mut file: ExternalAccountFile =
            serde_json::from_value(self.external_account_config).map_err(BuilderError::parsing)?;

        if let Some(scopes) = self.scopes {
            file.scopes = Some(scopes);
        }

        if let Some(ref ud) = self.universe_domain {
            file.universe_domain = Some(ud.clone());
        }

        if file.workforce_pool_user_project.is_some()
            && !is_valid_workforce_pool_audience(&file.audience)
        {
            return Err(BuilderError::parsing(
                "workforce_pool_user_project should not be set for non-workforce pool credentials",
            ));
        }

        let config: ExternalAccountConfig = file.into();

        let access_boundary_url =
            external_account_lookup_url(&config.audience, self.iam_endpoint_override.as_deref());

        let creds = config.make_credentials(self.quota_project_id, self.retry_builder);

        Ok(CredentialsWithAccessBoundary::new(
            creds,
            access_boundary_url,
        ))
    }
}

/// A builder for external account [Credentials] that uses a user provided subject
/// token provider.
///
/// This builder is designed for advanced use cases where the subject token is
/// provided directly by the application through a custom implementation of the
/// [SubjectTokenProvider] trait.
///
/// # Example
///
/// ```
/// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
/// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
/// # use google_cloud_auth::errors::SubjectTokenProviderError;
/// # use std::error::Error;
/// # use std::fmt;
/// # use std::sync::Arc;
/// #
/// # #[derive(Debug)]
/// # struct MyTokenProvider;
/// #
/// # #[derive(Debug)]
/// # struct MyProviderError;
/// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
/// # impl Error for MyProviderError {}
/// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
/// #
/// # impl SubjectTokenProvider for MyTokenProvider {
/// #     type Error = MyProviderError;
/// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
/// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
/// #     }
/// # }
/// #
/// # async fn sample() -> anyhow::Result<()> {
/// let provider = Arc::new(MyTokenProvider);
///
/// let credentials = ProgrammaticBuilder::new(provider)
///     .with_audience("//iam.googleapis.com/projects/123/locations/global/workloadIdentityPools/my-pool/providers/my-provider".to_string())
///     .with_subject_token_type("urn:ietf:params:oauth:token-type:jwt".to_string())
///     .with_token_url("https://sts.googleapis.com/v1beta/token".to_string())
///     .with_quota_project_id("my-quota-project")
///     .with_scopes(vec!["https://www.googleapis.com/auth/devstorage.read_only".to_string()])
///     .build()
///     .unwrap();
/// # Ok(()) }
/// ```
/// [SubjectTokenProvider]: crate::credentials::subject_token::SubjectTokenProvider
pub struct ProgrammaticBuilder {
    quota_project_id: Option<String>,
    config: ExternalAccountConfigBuilder,
    retry_builder: RetryTokenProviderBuilder,
}

impl ProgrammaticBuilder {
    /// Creates a new builder that uses the provided [SubjectTokenProvider] to
    /// fetch the third-party subject token.
    ///
    /// # Example
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// let provider = Arc::new(MyTokenProvider);
    /// let builder = ProgrammaticBuilder::new(provider);
    /// ```
    /// [SubjectTokenProvider]: crate::credentials::subject_token::SubjectTokenProvider
    pub fn new(subject_token_provider: Arc<dyn dynamic::SubjectTokenProvider>) -> Self {
        let config = ExternalAccountConfigBuilder::default().with_credential_source(
            CredentialSource::Programmatic(ProgrammaticSourcedCredentials::new(
                subject_token_provider,
            )),
        );
        Self {
            quota_project_id: None,
            config,
            retry_builder: RetryTokenProviderBuilder::default(),
        }
    }

    /// Sets the optional [quota project] for this credentials.
    ///
    /// In some services, you can use a service account in
    /// one project for authentication and authorization, and charge
    /// the usage to a different project. This requires that the
    /// service account has `serviceusage.services.use` permissions on the quota project.
    ///
    /// # Example
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// # let provider = Arc::new(MyTokenProvider);
    /// let builder = ProgrammaticBuilder::new(provider)
    ///     .with_quota_project_id("my-quota-project");
    /// ```
    ///
    /// [quota project]: https://cloud.google.com/docs/quotas/quota-project
    pub fn with_quota_project_id<S: Into<String>>(mut self, quota_project_id: S) -> Self {
        self.quota_project_id = Some(quota_project_id.into());
        self
    }

    /// Overrides the optional [scopes] for this credentials. If this method is not
    /// called, a default scope will be used.
    ///
    /// # Example
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// # let provider = Arc::new(MyTokenProvider);
    /// let builder = ProgrammaticBuilder::new(provider)
    ///     .with_scopes(vec!["scope1", "scope2"]);
    /// ```
    ///
    /// [scopes]: https://developers.google.com/identity/protocols/oauth2/scopes
    pub fn with_scopes<I, S>(mut self, scopes: I) -> Self
    where
        I: IntoIterator<Item = S>,
        S: Into<String>,
    {
        self.config = self.config.with_scopes(
            scopes
                .into_iter()
                .map(|s| s.into())
                .collect::<Vec<String>>(),
        );
        self
    }

    /// Sets the Google Cloud universe domain for these credentials.
    ///
    /// Any value provided here overrides a `universe_domain` value from the input service account JSON.
    ///
    /// # Example
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::sync::Arc;
    /// # #[derive(Debug)] struct MyProvider;
    /// # #[derive(Debug)] struct MyError;
    /// # impl std::fmt::Display for MyError { fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { write!(f, "err") } }
    /// # impl std::error::Error for MyError {}
    /// # impl SubjectTokenProviderError for MyError { fn is_transient(&self) -> bool { false } }
    /// # impl SubjectTokenProvider for MyProvider { type Error = MyError; async fn subject_token(&self) -> Result<SubjectToken, Self::Error> { Ok(SubjectTokenBuilder::new("token".to_string()).build()) } }
    /// # async fn sample() -> anyhow::Result<()> {
    /// # let provider = Arc::new(MyProvider);
    /// let credentials = ProgrammaticBuilder::new(provider)
    ///     .with_universe_domain("googleapis.com");
    /// # Ok(()) }
    /// ```
    ///
    /// [universe domain]: https://cloud.google.com/docs/authentication/universe-domain
    pub fn with_universe_domain<S: Into<String>>(mut self, universe_domain: S) -> Self {
        self.config = self.config.with_universe_domain(universe_domain);
        self
    }

    /// Sets the required audience for the token exchange.
    ///
    /// This is the resource name for the workload identity pool and the provider
    /// identifier in that pool.
    ///
    /// # Example
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// # let provider = Arc::new(MyTokenProvider);
    /// let builder = ProgrammaticBuilder::new(provider)
    ///     .with_audience("my-audience");
    /// ```
    pub fn with_audience<S: Into<String>>(mut self, audience: S) -> Self {
        self.config = self.config.with_audience(audience);
        self
    }

    /// Sets the required subject token type.
    ///
    /// This is the STS subject token type based on the OAuth 2.0 token exchange spec.
    ///
    /// # Example
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// # let provider = Arc::new(MyTokenProvider);
    /// let builder = ProgrammaticBuilder::new(provider)
    ///     .with_subject_token_type("my-token-type");
    /// ```
    pub fn with_subject_token_type<S: Into<String>>(mut self, subject_token_type: S) -> Self {
        self.config = self.config.with_subject_token_type(subject_token_type);
        self
    }

    /// Sets the optional token URL for the STS token exchange. If not provided,
    /// `https://sts.googleapis.com/v1/token` is used.
    ///
    /// # Example
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// # let provider = Arc::new(MyTokenProvider);
    /// let builder = ProgrammaticBuilder::new(provider)
    ///     .with_token_url("http://my-token-url.com");
    /// ```
    pub fn with_token_url<S: Into<String>>(mut self, token_url: S) -> Self {
        self.config = self.config.with_token_url(token_url);
        self
    }

    /// Sets the optional client ID for client authentication.
    ///
    /// # Example
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// # let provider = Arc::new(MyTokenProvider);
    /// let builder = ProgrammaticBuilder::new(provider)
    ///     .with_client_id("my-client-id");
    /// ```
    pub fn with_client_id<S: Into<String>>(mut self, client_id: S) -> Self {
        self.config = self.config.with_client_id(client_id.into());
        self
    }

    /// Sets the optional client secret for client authentication.
    ///
    /// # Example
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// # let provider = Arc::new(MyTokenProvider);
    /// let builder = ProgrammaticBuilder::new(provider)
    ///     .with_client_secret("my-client-secret");
    /// ```
    pub fn with_client_secret<S: Into<String>>(mut self, client_secret: S) -> Self {
        self.config = self.config.with_client_secret(client_secret.into());
        self
    }

    /// Sets the optional target principal.
    ///
    /// Target principal is the email of the service account to impersonate.
    ///
    /// # Example
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// # let provider = Arc::new(MyTokenProvider);
    /// let builder = ProgrammaticBuilder::new(provider)
    ///     .with_target_principal("test-principal");
    /// ```
    pub fn with_target_principal<S: Into<String>>(mut self, target_principal: S) -> Self {
        let url = format!(
            "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/{}:generateAccessToken",
            target_principal.into()
        );
        self.config = self.config.with_service_account_impersonation_url(url);
        self
    }

    /// Sets the optional workforce pool user project.
    ///
    /// This is the project ID used for quota and billing in the workforce pool context.
    ///
    /// # Example
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// # let provider = Arc::new(MyTokenProvider);
    /// let builder = ProgrammaticBuilder::new(provider)
    ///     .with_workforce_pool_user_project("my-project-id");
    /// ```
    pub fn with_workforce_pool_user_project<S: Into<String>>(mut self, project: S) -> Self {
        self.config = self.config.with_workforce_pool_user_project(project);
        self
    }

    /// Configure the retry policy for fetching tokens.
    ///
    /// The retry policy controls how to handle retries, and sets limits on
    /// the number of attempts or the total time spent retrying.
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// #
    /// # async fn sample() -> anyhow::Result<()> {
    /// use google_cloud_gax::retry_policy::{AlwaysRetry, RetryPolicyExt};
    /// let provider = Arc::new(MyTokenProvider);
    /// let credentials = ProgrammaticBuilder::new(provider)
    ///     .with_audience("test-audience")
    ///     .with_subject_token_type("test-token-type")
    ///     .with_retry_policy(AlwaysRetry.with_attempt_limit(3))
    ///     .build()?;
    /// # Ok(()) }
    /// ```
    pub fn with_retry_policy<V: Into<RetryPolicyArg>>(mut self, v: V) -> Self {
        self.retry_builder = self.retry_builder.with_retry_policy(v.into());
        self
    }

    /// Configure the retry backoff policy.
    ///
    /// The backoff policy controls how long to wait in between retry attempts.
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// #
    /// # async fn sample() -> anyhow::Result<()> {
    /// use google_cloud_gax::exponential_backoff::ExponentialBackoff;
    /// let provider = Arc::new(MyTokenProvider);
    /// let policy = ExponentialBackoff::default();
    /// let credentials = ProgrammaticBuilder::new(provider)
    ///     .with_audience("test-audience")
    ///     .with_subject_token_type("test-token-type")
    ///     .with_backoff_policy(policy)
    ///     .build()?;
    /// # Ok(()) }
    /// ```
    pub fn with_backoff_policy<V: Into<BackoffPolicyArg>>(mut self, v: V) -> Self {
        self.retry_builder = self.retry_builder.with_backoff_policy(v.into());
        self
    }

    /// Configure the retry throttler.
    ///
    /// Advanced applications may want to configure a retry throttler to
    /// [Address Cascading Failures] and when [Handling Overload] conditions.
    /// The authentication library throttles its retry loop, using a policy to
    /// control the throttling algorithm. Use this method to fine tune or
    /// customize the default retry throttler.
    ///
    /// [Handling Overload]: https://sre.google/sre-book/handling-overload/
    /// [Address Cascading Failures]: https://sre.google/sre-book/addressing-cascading-failures/
    ///
    /// ```
    /// # use google_cloud_auth::credentials::external_account::ProgrammaticBuilder;
    /// # use google_cloud_auth::credentials::subject_token::{SubjectTokenProvider, SubjectToken, Builder as SubjectTokenBuilder};
    /// # use google_cloud_auth::errors::SubjectTokenProviderError;
    /// # use std::error::Error;
    /// # use std::fmt;
    /// # use std::sync::Arc;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyTokenProvider;
    /// #
    /// # #[derive(Debug)]
    /// # struct MyProviderError;
    /// # impl fmt::Display for MyProviderError { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "MyProviderError") } }
    /// # impl Error for MyProviderError {}
    /// # impl SubjectTokenProviderError for MyProviderError { fn is_transient(&self) -> bool { false } }
    /// #
    /// # impl SubjectTokenProvider for MyTokenProvider {
    /// #     type Error = MyProviderError;
    /// #     async fn subject_token(&self) -> Result<SubjectToken, Self::Error> {
    /// #         Ok(SubjectTokenBuilder::new("my-programmatic-token".to_string()).build())
    /// #     }
    /// # }
    /// #
    /// # async fn sample() -> anyhow::Result<()> {
    /// use google_cloud_gax::retry_throttler::AdaptiveThrottler;
    /// let provider = Arc::new(MyTokenProvider);
    /// let credentials = ProgrammaticBuilder::new(provider)
    ///     .with_audience("test-audience")
    ///     .with_subject_token_type("test-token-type")
    ///     .with_retry_throttler(AdaptiveThrottler::default())
    ///     .build()?;
    /// # Ok(()) }
    /// ```
    pub fn with_retry_throttler<V: Into<RetryThrottlerArg>>(mut self, v: V) -> Self {
        self.retry_builder = self.retry_builder.with_retry_throttler(v.into());
        self
    }

    /// Returns a [Credentials] instance with the configured settings.
    ///
    /// # Errors
    ///
    /// Returns a [BuilderError] if any of the required fields (such as
    /// `audience` or `subject_token_type`) have not been set.
    pub fn build(self) -> BuildResult<Credentials> {
        let (config, quota_project_id, retry_builder) = self.build_components()?;
        let creds = config.make_credentials(quota_project_id, retry_builder);
        Ok(Credentials {
            inner: Arc::new(creds),
        })
    }

    /// Consumes the builder and returns its configured components.
    fn build_components(
        self,
    ) -> BuildResult<(
        ExternalAccountConfig,
        Option<String>,
        RetryTokenProviderBuilder,
    )> {
        let Self {
            quota_project_id,
            config,
            retry_builder,
        } = self;

        let mut config_builder = config;
        if config_builder.scopes.is_none() {
            config_builder = config_builder.with_scopes(vec![DEFAULT_SCOPE.to_string()]);
        }
        if config_builder.token_url.is_none() {
            let mut token_url = STS_TOKEN_URL.to_string();
            if let Some(ref ud) = config_builder.universe_domain {
                if ud != DEFAULT_UNIVERSE_DOMAIN {
                    token_url = token_url.replace(DEFAULT_UNIVERSE_DOMAIN, ud);
                }
            }
            config_builder = config_builder.with_token_url(token_url);
        }
        let final_config = config_builder.build()?;

        Ok((final_config, quota_project_id, retry_builder))
    }
}

#[async_trait::async_trait]
impl<T> CredentialsProvider for ExternalAccountCredentials<T>
where
    T: CachedTokenProvider,
{
    async fn headers(&self, extensions: Extensions) -> Result<CacheableResource<HeaderMap>> {
        let token = self.token_provider.token(extensions).await?;

        AuthHeadersBuilder::new(&token)
            .maybe_quota_project_id(self.quota_project_id.as_deref())
            .build()
    }

    async fn universe_domain(&self) -> Option<String> {
        self.universe_domain.clone()
    }
}

#[async_trait::async_trait]
impl<T> AccessTokenCredentialsProvider for ExternalAccountCredentials<T>
where
    T: CachedTokenProvider,
{
    async fn access_token(&self) -> Result<AccessToken> {
        let token = self.token_provider.token(Extensions::new()).await?;
        token.into()
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::constants::{
        ACCESS_TOKEN_TYPE, DEFAULT_SCOPE, JWT_TOKEN_TYPE, TOKEN_EXCHANGE_GRANT_TYPE,
    };
    use crate::credentials::subject_token::{
        Builder as SubjectTokenBuilder, SubjectToken, SubjectTokenProvider,
    };
    use crate::credentials::tests::{
        find_source_error, get_mock_auth_retry_policy, get_mock_backoff_policy,
        get_mock_retry_throttler, get_token_from_headers,
    };
    use crate::errors::{CredentialsError, SubjectTokenProviderError};
    use httptest::{
        Expectation, Server, cycle,
        matchers::{all_of, contains, request, url_decoded},
        responders::{json_encoded, status_code},
    };
    use serde_json::*;
    use std::collections::HashMap;
    use std::error::Error;
    use std::fmt;
    use test_case::test_case;
    use time::OffsetDateTime;

    #[derive(Debug)]
    struct TestProviderError;
    impl fmt::Display for TestProviderError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "TestProviderError")
        }
    }
    impl Error for TestProviderError {}
    impl SubjectTokenProviderError for TestProviderError {
        fn is_transient(&self) -> bool {
            false
        }
    }

    #[derive(Debug)]
    struct TestSubjectTokenProvider;
    impl SubjectTokenProvider for TestSubjectTokenProvider {
        type Error = TestProviderError;
        async fn subject_token(&self) -> std::result::Result<SubjectToken, Self::Error> {
            Ok(SubjectTokenBuilder::new("test-subject-token".to_string()).build())
        }
    }

    #[tokio::test]
    async fn create_external_account_builder() {
        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": "https://sts.googleapis.com/v1beta/token",
            "credential_source": {
                "url": "https://example.com/token",
                "format": {
                  "type": "json",
                  "subject_token_field_name": "access_token"
                }
            }
        });

        let creds = Builder::new(contents)
            .with_quota_project_id("test_project")
            .with_scopes(["a", "b"])
            .build()
            .unwrap();

        // Use the debug output to verify the right kind of credentials are created.
        let fmt = format!("{creds:?}");
        assert!(fmt.contains("ExternalAccountCredentials"));
    }

    #[tokio::test]
    async fn create_external_account_with_universe_domain() {
        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": "https://sts.my-custom-universe.com/v1beta/token",
            "credential_source": {
                "url": "https://example.com/token",
                "format": {
                  "type": "json",
                  "subject_token_field_name": "access_token"
                }
            },
            "universe_domain": "my-custom-universe.com"
        });

        let creds = Builder::new(contents).build().unwrap();

        assert_eq!(
            creds.universe_domain().await,
            Some("my-custom-universe.com".to_string())
        );
    }

    #[tokio::test]
    async fn create_external_account_with_universe_domain_override() {
        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": "https://sts.my-custom-universe.com/v1beta/token",
            "credential_source": {
                "url": "https://example.com/token",
                "format": {
                  "type": "json",
                  "subject_token_field_name": "access_token"
                }
            },
            "universe_domain": "my-custom-universe.com"
        });

        let creds = Builder::new(contents)
            .with_universe_domain("my-overridden-universe.com")
            .build()
            .unwrap();

        assert_eq!(
            creds.universe_domain().await,
            Some("my-overridden-universe.com".to_string())
        );
    }

    #[tokio::test]
    async fn test_programmatic_builder_with_universe_domain() {
        let provider = Arc::new(TestSubjectTokenProvider);
        let builder = ProgrammaticBuilder::new(provider)
            .with_audience("test-audience")
            .with_subject_token_type("test-token-type")
            .with_token_url(STS_TOKEN_URL)
            .with_universe_domain("my-custom-universe.com");

        let creds = builder.build().unwrap();

        assert_eq!(
            creds.universe_domain().await,
            Some("my-custom-universe.com".to_string())
        );
    }

    #[tokio::test]
    async fn test_programmatic_builder_sts_url_updates_with_universe_domain() {
        let provider = Arc::new(TestSubjectTokenProvider);
        let builder = ProgrammaticBuilder::new(provider)
            .with_audience("test-audience")
            .with_subject_token_type("test-token-type")
            .with_universe_domain("my-custom-universe.com");

        let (config, _, _) = builder.build_components().unwrap();

        assert_eq!(
            config.token_url,
            "https://sts.my-custom-universe.com/v1/token"
        );
    }

    #[tokio::test]
    async fn create_external_account_detect_url_sourced() {
        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": "https://sts.googleapis.com/v1beta/token",
            "credential_source": {
                "url": "https://example.com/token",
                "headers": {
                  "Metadata": "True"
                },
                "format": {
                  "type": "json",
                  "subject_token_field_name": "access_token"
                }
            }
        });

        let file: ExternalAccountFile =
            serde_json::from_value(contents).expect("failed to parse external account config");
        let config: ExternalAccountConfig = file.into();
        let source = config.credential_source;

        match source {
            CredentialSource::Url(source) => {
                assert_eq!(source.url, "https://example.com/token");
                assert_eq!(
                    source.headers,
                    HashMap::from([("Metadata".to_string(), "True".to_string()),]),
                );
                assert_eq!(source.format, "json");
                assert_eq!(source.subject_token_field_name, "access_token");
            }
            _ => {
                unreachable!("expected Url Sourced credential")
            }
        }
    }

    #[tokio::test]
    async fn create_external_account_detect_executable_sourced() {
        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": "https://sts.googleapis.com/v1beta/token",
            "credential_source": {
                "executable": {
                    "command": "cat /some/file",
                    "output_file": "/some/file",
                    "timeout_millis": 5000
                }
            }
        });

        let file: ExternalAccountFile =
            serde_json::from_value(contents).expect("failed to parse external account config");
        let config: ExternalAccountConfig = file.into();
        let source = config.credential_source;

        match source {
            CredentialSource::Executable(source) => {
                assert_eq!(source.command, "cat");
                assert_eq!(source.args, vec!["/some/file"]);
                assert_eq!(source.output_file.as_deref(), Some("/some/file"));
                assert_eq!(source.timeout, Duration::from_secs(5));
            }
            _ => {
                unreachable!("expected Executable Sourced credential")
            }
        }
    }

    #[tokio::test]
    async fn create_external_account_detect_file_sourced() {
        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": "https://sts.googleapis.com/v1beta/token",
            "credential_source": {
                "file": "/foo/bar",
                "format": {
                    "type": "json",
                    "subject_token_field_name": "token"
                }
            }
        });

        let file: ExternalAccountFile =
            serde_json::from_value(contents).expect("failed to parse external account config");
        let config: ExternalAccountConfig = file.into();
        let source = config.credential_source;

        match source {
            CredentialSource::File(source) => {
                assert_eq!(source.file, "/foo/bar");
                assert_eq!(source.format, "json");
                assert_eq!(source.subject_token_field_name, "token");
            }
            _ => {
                unreachable!("expected File Sourced credential")
            }
        }
    }

    #[tokio::test]
    async fn test_external_account_with_impersonation_success() {
        let subject_token_server = Server::run();
        let sts_server = Server::run();
        let impersonation_server = Server::run();

        let impersonation_path = "/projects/-/serviceAccounts/sa@test.com:generateAccessToken";
        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": sts_server.url("/token").to_string(),
            "service_account_impersonation_url": impersonation_server.url(impersonation_path).to_string(),
            "credential_source": {
                "url": subject_token_server.url("/subject_token").to_string(),
                "format": {
                  "type": "json",
                  "subject_token_field_name": "access_token"
                }
            }
        });

        subject_token_server.expect(
            Expectation::matching(request::method_path("GET", "/subject_token")).respond_with(
                json_encoded(json!({
                    "access_token": "subject_token",
                })),
            ),
        );

        sts_server.expect(
            Expectation::matching(all_of![
                request::method_path("POST", "/token"),
                request::body(url_decoded(contains((
                    "grant_type",
                    TOKEN_EXCHANGE_GRANT_TYPE
                )))),
                request::body(url_decoded(contains(("subject_token", "subject_token")))),
                request::body(url_decoded(contains((
                    "requested_token_type",
                    ACCESS_TOKEN_TYPE
                )))),
                request::body(url_decoded(contains((
                    "subject_token_type",
                    JWT_TOKEN_TYPE
                )))),
                request::body(url_decoded(contains(("audience", "audience")))),
                request::body(url_decoded(contains(("scope", IAM_SCOPE)))),
            ])
            .respond_with(json_encoded(json!({
                "access_token": "sts-token",
                "issued_token_type": "urn:ietf:params:oauth:token-type:access_token",
                "token_type": "Bearer",
                "expires_in": 3600,
            }))),
        );

        let expire_time = (OffsetDateTime::now_utc() + time::Duration::hours(1))
            .format(&time::format_description::well_known::Rfc3339)
            .unwrap();
        impersonation_server.expect(
            Expectation::matching(all_of![
                request::method_path("POST", impersonation_path),
                request::headers(contains(("authorization", "Bearer sts-token"))),
            ])
            .respond_with(json_encoded(json!({
                "accessToken": "final-impersonated-token",
                "expireTime": expire_time
            }))),
        );

        let creds = Builder::new(contents).build().unwrap();
        let headers = creds.headers(Extensions::new()).await.unwrap();
        match headers {
            CacheableResource::New { data, .. } => {
                let token = data.get("authorization").unwrap().to_str().unwrap();
                assert_eq!(token, "Bearer final-impersonated-token");
            }
            CacheableResource::NotModified => panic!("Expected new headers"),
        }
    }

    #[tokio::test]
    async fn test_external_account_without_impersonation_success() {
        let subject_token_server = Server::run();
        let sts_server = Server::run();

        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": sts_server.url("/token").to_string(),
            "credential_source": {
                "url": subject_token_server.url("/subject_token").to_string(),
                "format": {
                  "type": "json",
                  "subject_token_field_name": "access_token"
                }
            }
        });

        subject_token_server.expect(
            Expectation::matching(request::method_path("GET", "/subject_token")).respond_with(
                json_encoded(json!({
                    "access_token": "subject_token",
                })),
            ),
        );

        sts_server.expect(
            Expectation::matching(all_of![
                request::method_path("POST", "/token"),
                request::body(url_decoded(contains((
                    "grant_type",
                    TOKEN_EXCHANGE_GRANT_TYPE
                )))),
                request::body(url_decoded(contains(("subject_token", "subject_token")))),
                request::body(url_decoded(contains((
                    "requested_token_type",
                    ACCESS_TOKEN_TYPE
                )))),
                request::body(url_decoded(contains((
                    "subject_token_type",
                    JWT_TOKEN_TYPE
                )))),
                request::body(url_decoded(contains(("audience", "audience")))),
                request::body(url_decoded(contains(("scope", DEFAULT_SCOPE)))),
            ])
            .respond_with(json_encoded(json!({
                "access_token": "sts-only-token",
                "issued_token_type": "urn:ietf:params:oauth:token-type:access_token",
                "token_type": "Bearer",
                "expires_in": 3600,
            }))),
        );

        let creds = Builder::new(contents).build().unwrap();
        let headers = creds.headers(Extensions::new()).await.unwrap();
        match headers {
            CacheableResource::New { data, .. } => {
                let token = data.get("authorization").unwrap().to_str().unwrap();
                assert_eq!(token, "Bearer sts-only-token");
            }
            CacheableResource::NotModified => panic!("Expected new headers"),
        }
    }

    #[tokio::test]
    async fn test_external_account_access_token_credentials_success() {
        let server = Server::run();

        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": server.url("/token").to_string(),
            "credential_source": {
                "url": server.url("/subject_token").to_string(),
                "format": {
                  "type": "json",
                  "subject_token_field_name": "access_token"
                }
            }
        });

        server.expect(
            Expectation::matching(request::method_path("GET", "/subject_token")).respond_with(
                json_encoded(json!({
                    "access_token": "subject_token",
                })),
            ),
        );

        server.expect(
            Expectation::matching(all_of![
                request::method_path("POST", "/token"),
                request::body(url_decoded(contains((
                    "grant_type",
                    TOKEN_EXCHANGE_GRANT_TYPE
                )))),
                request::body(url_decoded(contains(("subject_token", "subject_token")))),
                request::body(url_decoded(contains((
                    "requested_token_type",
                    ACCESS_TOKEN_TYPE
                )))),
                request::body(url_decoded(contains((
                    "subject_token_type",
                    JWT_TOKEN_TYPE
                )))),
                request::body(url_decoded(contains(("audience", "audience")))),
                request::body(url_decoded(contains(("scope", DEFAULT_SCOPE)))),
            ])
            .respond_with(json_encoded(json!({
                "access_token": "sts-only-token",
                "issued_token_type": "urn:ietf:params:oauth:token-type:access_token",
                "token_type": "Bearer",
                "expires_in": 3600,
            }))),
        );

        let creds = Builder::new(contents)
            .build_access_token_credentials()
            .unwrap();
        let access_token = creds.access_token().await.unwrap();
        assert_eq!(access_token.token, "sts-only-token");
    }

    #[tokio::test]
    async fn test_impersonation_flow_sts_call_fails() {
        let subject_token_server = Server::run();
        let sts_server = Server::run();
        let impersonation_server = Server::run();

        let impersonation_path = "/projects/-/serviceAccounts/sa@test.com:generateAccessToken";
        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": sts_server.url("/token").to_string(),
            "service_account_impersonation_url": impersonation_server.url(impersonation_path).to_string(),
            "credential_source": {
                "url": subject_token_server.url("/subject_token").to_string(),
                "format": {
                  "type": "json",
                  "subject_token_field_name": "access_token"
                }
            }
        });

        subject_token_server.expect(
            Expectation::matching(request::method_path("GET", "/subject_token")).respond_with(
                json_encoded(json!({
                    "access_token": "subject_token",
                })),
            ),
        );

        sts_server.expect(
            Expectation::matching(request::method_path("POST", "/token"))
                .respond_with(status_code(500)),
        );

        let creds = Builder::new(contents).build().unwrap();
        let err = creds.headers(Extensions::new()).await.unwrap_err();
        let original_err = find_source_error::<CredentialsError>(&err).unwrap();
        assert!(
            original_err
                .to_string()
                .contains("failed to exchange token")
        );
        assert!(original_err.is_transient());
    }

    #[tokio::test]
    async fn test_impersonation_flow_iam_call_fails() {
        let subject_token_server = Server::run();
        let sts_server = Server::run();
        let impersonation_server = Server::run();

        let impersonation_path = "/projects/-/serviceAccounts/sa@test.com:generateAccessToken";
        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": sts_server.url("/token").to_string(),
            "service_account_impersonation_url": impersonation_server.url(impersonation_path).to_string(),
            "credential_source": {
                "url": subject_token_server.url("/subject_token").to_string(),
                "format": {
                  "type": "json",
                  "subject_token_field_name": "access_token"
                }
            }
        });

        subject_token_server.expect(
            Expectation::matching(request::method_path("GET", "/subject_token")).respond_with(
                json_encoded(json!({
                    "access_token": "subject_token",
                })),
            ),
        );

        sts_server.expect(
            Expectation::matching(request::method_path("POST", "/token")).respond_with(
                json_encoded(json!({
                    "access_token": "sts-token",
                    "issued_token_type": "urn:ietf:params:oauth:token-type:access_token",
                    "token_type": "Bearer",
                    "expires_in": 3600,
                })),
            ),
        );

        impersonation_server.expect(
            Expectation::matching(request::method_path("POST", impersonation_path))
                .respond_with(status_code(403)),
        );

        let creds = Builder::new(contents).build().unwrap();
        let err = creds.headers(Extensions::new()).await.unwrap_err();
        let original_err = find_source_error::<CredentialsError>(&err).unwrap();
        assert!(original_err.to_string().contains("failed to fetch token"));
        assert!(!original_err.is_transient());
    }

    #[test_case(Some(vec!["scope1", "scope2"]), Some("http://custom.com/token") ; "with custom scopes and token_url")]
    #[test_case(None, Some("http://custom.com/token") ; "with default scopes and custom token_url")]
    #[test_case(Some(vec!["scope1", "scope2"]), None ; "with custom scopes and default token_url")]
    #[test_case(None, None ; "with default scopes and default token_url")]
    #[tokio::test]
    async fn create_programmatic_builder(scopes: Option<Vec<&str>>, token_url: Option<&str>) {
        let provider = Arc::new(TestSubjectTokenProvider);
        let mut builder = ProgrammaticBuilder::new(provider)
            .with_audience("test-audience")
            .with_subject_token_type("test-token-type")
            .with_client_id("test-client-id")
            .with_client_secret("test-client-secret")
            .with_target_principal("test-principal");

        let expected_scopes = if let Some(scopes) = scopes.clone() {
            scopes.iter().map(|s| s.to_string()).collect()
        } else {
            vec![DEFAULT_SCOPE.to_string()]
        };

        let expected_token_url = token_url.unwrap_or(STS_TOKEN_URL).to_string();

        if let Some(scopes) = scopes {
            builder = builder.with_scopes(scopes);
        }
        if let Some(token_url) = token_url {
            builder = builder.with_token_url(token_url);
        }

        let (config, _, _) = builder.build_components().unwrap();

        assert_eq!(config.audience, "test-audience");
        assert_eq!(config.subject_token_type, "test-token-type");
        assert_eq!(config.client_id, Some("test-client-id".to_string()));
        assert_eq!(config.client_secret, Some("test-client-secret".to_string()));
        assert_eq!(config.scopes, expected_scopes);
        assert_eq!(config.token_url, expected_token_url);
        assert_eq!(
            config.service_account_impersonation_url,
            Some("https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/test-principal:generateAccessToken".to_string())
        );
    }

    #[tokio::test]
    async fn create_programmatic_builder_with_quota_project_id() {
        let provider = Arc::new(TestSubjectTokenProvider);
        let builder = ProgrammaticBuilder::new(provider)
            .with_audience("test-audience")
            .with_subject_token_type("test-token-type")
            .with_token_url(STS_TOKEN_URL)
            .with_quota_project_id("test-quota-project");

        let creds = builder.build().unwrap();

        let fmt = format!("{creds:?}");
        assert!(
            fmt.contains("ExternalAccountCredentials"),
            "Expected 'ExternalAccountCredentials', got: {fmt}"
        );
        assert!(
            fmt.contains("test-quota-project"),
            "Expected 'test-quota-project', got: {fmt}"
        );
    }

    #[tokio::test]
    async fn programmatic_builder_returns_correct_headers() {
        let provider = Arc::new(TestSubjectTokenProvider);
        let sts_server = Server::run();
        let builder = ProgrammaticBuilder::new(provider)
            .with_audience("test-audience")
            .with_subject_token_type("test-token-type")
            .with_token_url(sts_server.url("/token").to_string())
            .with_quota_project_id("test-quota-project");

        let creds = builder.build().unwrap();

        sts_server.expect(
            Expectation::matching(all_of![
                request::method_path("POST", "/token"),
                request::body(url_decoded(contains((
                    "grant_type",
                    TOKEN_EXCHANGE_GRANT_TYPE
                )))),
                request::body(url_decoded(contains((
                    "subject_token",
                    "test-subject-token"
                )))),
                request::body(url_decoded(contains((
                    "requested_token_type",
                    ACCESS_TOKEN_TYPE
                )))),
                request::body(url_decoded(contains((
                    "subject_token_type",
                    "test-token-type"
                )))),
                request::body(url_decoded(contains(("audience", "test-audience")))),
                request::body(url_decoded(contains(("scope", DEFAULT_SCOPE)))),
            ])
            .respond_with(json_encoded(json!({
                "access_token": "sts-only-token",
                "issued_token_type": "urn:ietf:params:oauth:token-type:access_token",
                "token_type": "Bearer",
                "expires_in": 3600,
            }))),
        );

        let headers = creds.headers(Extensions::new()).await.unwrap();
        match headers {
            CacheableResource::New { data, .. } => {
                let token = data.get("authorization").unwrap().to_str().unwrap();
                assert_eq!(token, "Bearer sts-only-token");
                let quota_project = data.get("x-goog-user-project").unwrap().to_str().unwrap();
                assert_eq!(quota_project, "test-quota-project");
            }
            CacheableResource::NotModified => panic!("Expected new headers"),
        }
    }

    #[tokio::test]
    async fn create_programmatic_builder_fails_on_missing_required_field() {
        let provider = Arc::new(TestSubjectTokenProvider);
        let result = ProgrammaticBuilder::new(provider)
            .with_subject_token_type("test-token-type")
            // Missing .with_audience(...)
            .with_token_url("http://test.com/token")
            .build();

        assert!(result.is_err(), "{result:?}");
        let error_string = result.unwrap_err().to_string();
        assert!(
            error_string.contains("missing required field: audience"),
            "Expected error about missing 'audience', got: {error_string}"
        );
    }

    #[tokio::test]
    async fn test_external_account_retries_on_transient_failures() {
        let mut subject_token_server = Server::run();
        let mut sts_server = Server::run();

        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": sts_server.url("/token").to_string(),
            "credential_source": {
                "url": subject_token_server.url("/subject_token").to_string(),
            }
        });

        subject_token_server.expect(
            Expectation::matching(request::method_path("GET", "/subject_token"))
                .times(3)
                .respond_with(json_encoded(json!({
                    "access_token": "subject_token",
                }))),
        );

        sts_server.expect(
            Expectation::matching(request::method_path("POST", "/token"))
                .times(3)
                .respond_with(status_code(503)),
        );

        let creds = Builder::new(contents)
            .with_retry_policy(get_mock_auth_retry_policy(3))
            .with_backoff_policy(get_mock_backoff_policy())
            .with_retry_throttler(get_mock_retry_throttler())
            .build()
            .unwrap();

        let err = creds.headers(Extensions::new()).await.unwrap_err();
        assert!(err.is_transient(), "{err:?}");
        sts_server.verify_and_clear();
        subject_token_server.verify_and_clear();
    }

    #[tokio::test]
    async fn test_external_account_does_not_retry_on_non_transient_failures() {
        let subject_token_server = Server::run();
        let mut sts_server = Server::run();

        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": sts_server.url("/token").to_string(),
            "credential_source": {
                "url": subject_token_server.url("/subject_token").to_string(),
            }
        });

        subject_token_server.expect(
            Expectation::matching(request::method_path("GET", "/subject_token")).respond_with(
                json_encoded(json!({
                    "access_token": "subject_token",
                })),
            ),
        );

        sts_server.expect(
            Expectation::matching(request::method_path("POST", "/token"))
                .times(1)
                .respond_with(status_code(401)),
        );

        let creds = Builder::new(contents)
            .with_retry_policy(get_mock_auth_retry_policy(1))
            .with_backoff_policy(get_mock_backoff_policy())
            .with_retry_throttler(get_mock_retry_throttler())
            .build()
            .unwrap();

        let err = creds.headers(Extensions::new()).await.unwrap_err();
        assert!(!err.is_transient());
        sts_server.verify_and_clear();
    }

    #[tokio::test]
    async fn test_external_account_retries_for_success() {
        let mut subject_token_server = Server::run();
        let mut sts_server = Server::run();

        let contents = json!({
            "type": "external_account",
            "audience": "audience",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": sts_server.url("/token").to_string(),
            "credential_source": {
                "url": subject_token_server.url("/subject_token").to_string(),
            }
        });

        subject_token_server.expect(
            Expectation::matching(request::method_path("GET", "/subject_token"))
                .times(3)
                .respond_with(json_encoded(json!({
                    "access_token": "subject_token",
                }))),
        );

        sts_server.expect(
            Expectation::matching(request::method_path("POST", "/token"))
                .times(3)
                .respond_with(cycle![
                    status_code(503).body("try-again"),
                    status_code(503).body("try-again"),
                    json_encoded(json!({
                        "access_token": "sts-only-token",
                        "issued_token_type": "urn:ietf:params:oauth:token-type:access_token",
                        "token_type": "Bearer",
                        "expires_in": 3600,
                    }))
                ]),
        );

        let creds = Builder::new(contents)
            .with_retry_policy(get_mock_auth_retry_policy(3))
            .with_backoff_policy(get_mock_backoff_policy())
            .with_retry_throttler(get_mock_retry_throttler())
            .build()
            .unwrap();

        let headers = creds.headers(Extensions::new()).await.unwrap();
        match headers {
            CacheableResource::New { data, .. } => {
                let token = data.get("authorization").unwrap().to_str().unwrap();
                assert_eq!(token, "Bearer sts-only-token");
            }
            CacheableResource::NotModified => panic!("Expected new headers"),
        }
        sts_server.verify_and_clear();
        subject_token_server.verify_and_clear();
    }

    #[tokio::test]
    async fn test_programmatic_builder_retries_on_transient_failures() {
        let provider = Arc::new(TestSubjectTokenProvider);
        let mut sts_server = Server::run();

        sts_server.expect(
            Expectation::matching(request::method_path("POST", "/token"))
                .times(3)
                .respond_with(status_code(503)),
        );

        let creds = ProgrammaticBuilder::new(provider)
            .with_audience("test-audience")
            .with_subject_token_type("test-token-type")
            .with_token_url(sts_server.url("/token").to_string())
            .with_retry_policy(get_mock_auth_retry_policy(3))
            .with_backoff_policy(get_mock_backoff_policy())
            .with_retry_throttler(get_mock_retry_throttler())
            .build()
            .unwrap();

        let err = creds.headers(Extensions::new()).await.unwrap_err();
        assert!(err.is_transient(), "{err:?}");
        sts_server.verify_and_clear();
    }

    #[tokio::test]
    async fn test_programmatic_builder_does_not_retry_on_non_transient_failures() {
        let provider = Arc::new(TestSubjectTokenProvider);
        let mut sts_server = Server::run();

        sts_server.expect(
            Expectation::matching(request::method_path("POST", "/token"))
                .times(1)
                .respond_with(status_code(401)),
        );

        let creds = ProgrammaticBuilder::new(provider)
            .with_audience("test-audience")
            .with_subject_token_type("test-token-type")
            .with_token_url(sts_server.url("/token").to_string())
            .with_retry_policy(get_mock_auth_retry_policy(1))
            .with_backoff_policy(get_mock_backoff_policy())
            .with_retry_throttler(get_mock_retry_throttler())
            .build()
            .unwrap();

        let err = creds.headers(Extensions::new()).await.unwrap_err();
        assert!(!err.is_transient());
        sts_server.verify_and_clear();
    }

    #[tokio::test]
    async fn test_programmatic_builder_retries_for_success() {
        let provider = Arc::new(TestSubjectTokenProvider);
        let mut sts_server = Server::run();

        sts_server.expect(
            Expectation::matching(request::method_path("POST", "/token"))
                .times(3)
                .respond_with(cycle![
                    status_code(503).body("try-again"),
                    status_code(503).body("try-again"),
                    json_encoded(json!({
                        "access_token": "sts-only-token",
                        "issued_token_type": "urn:ietf:params:oauth:token-type:access_token",
                        "token_type": "Bearer",
                        "expires_in": 3600,
                    }))
                ]),
        );

        let creds = ProgrammaticBuilder::new(provider)
            .with_audience("test-audience")
            .with_subject_token_type("test-token-type")
            .with_token_url(sts_server.url("/token").to_string())
            .with_retry_policy(get_mock_auth_retry_policy(3))
            .with_backoff_policy(get_mock_backoff_policy())
            .with_retry_throttler(get_mock_retry_throttler())
            .build()
            .unwrap();

        let headers = creds.headers(Extensions::new()).await.unwrap();
        match headers {
            CacheableResource::New { data, .. } => {
                let token = data.get("authorization").unwrap().to_str().unwrap();
                assert_eq!(token, "Bearer sts-only-token");
            }
            CacheableResource::NotModified => panic!("Expected new headers"),
        }
        sts_server.verify_and_clear();
    }

    #[test_case(
        "//iam.googleapis.com/projects/12345/locations/global/workloadIdentityPools/my-pool/providers/my-provider",
        "/v1/projects/12345/locations/global/workloadIdentityPools/my-pool/allowedLocations";
        "workload_identity_pool"
    )]
    #[test_case(
        "//iam.googleapis.com/locations/global/workforcePools/my-pool/providers/my-provider",
        "/v1/locations/global/workforcePools/my-pool/allowedLocations";
        "workforce_pool"
    )]
    #[tokio::test]
    #[cfg(google_cloud_unstable_trusted_boundaries)]
    async fn e2e_access_boundary(audience: &str, iam_path: &str) -> anyhow::Result<()> {
        use crate::credentials::tests::get_access_boundary_from_headers;

        let audience = audience.to_string();
        let iam_path = iam_path.to_string();

        let server = Server::run();

        server.expect(
            Expectation::matching(request::method_path("GET", "/subject_token")).respond_with(
                json_encoded(json!({
                    "access_token": "subject_token",
                })),
            ),
        );

        server.expect(
            Expectation::matching(all_of![
                request::method_path("POST", "/token"),
                request::body(url_decoded(contains(("subject_token", "subject_token")))),
                request::body(url_decoded(contains(("audience", audience.clone())))),
            ])
            .respond_with(json_encoded(json!({
                "access_token": "sts-only-token",
                "issued_token_type": "urn:ietf:params:oauth:token-type:access_token",
                "token_type": "Bearer",
                "expires_in": 3600,
            }))),
        );

        server.expect(
            Expectation::matching(all_of![request::method_path("GET", iam_path.clone()),])
                .respond_with(json_encoded(json!({
                    "locations": ["us-central1"],
                    "encodedLocations": "0x1234"
                }))),
        );

        let contents = json!({
            "type": "external_account",
            "audience": audience.to_string(),
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": server.url("/token").to_string(),
            "credential_source": {
                "url": server.url("/subject_token").to_string(),
                "format": {
                  "type": "json",
                  "subject_token_field_name": "access_token"
                }
            }
        });

        let iam_endpoint = server.url("").to_string().trim_end_matches('/').to_string();

        let creds = Builder::new(contents)
            .maybe_iam_endpoint_override(Some(iam_endpoint))
            .build_credentials()?;

        // wait for the access boundary background thread to update
        creds.wait_for_boundary().await;

        let headers = creds.headers(Extensions::new()).await?;
        let token = get_token_from_headers(headers.clone());
        let access_boundary = get_access_boundary_from_headers(headers);

        assert!(token.is_some(), "should have some token");
        assert_eq!(access_boundary.as_deref(), Some("0x1234"));

        Ok(())
    }

    #[tokio::test]
    async fn test_kubernetes_wif_direct_identity_parsing() {
        let contents = json!({
            "audience": "//iam.googleapis.com/projects/123456789/locations/global/workloadIdentityPools/pool-name/providers/k8s-cluster",
            "credential_source": {
                "file": "/var/run/service-account/token"
            },
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": "https://sts.googleapis.com/v1/token",
            "type": "external_account"
        });

        let file: ExternalAccountFile = serde_json::from_value(contents)
            .expect("failed to parse kubernetes WIF direct identity config");
        let config: ExternalAccountConfig = file.into();

        match config.credential_source {
            CredentialSource::File(source) => {
                assert_eq!(source.file, "/var/run/service-account/token");
                assert_eq!(source.format, "text"); // Default format
                assert_eq!(source.subject_token_field_name, ""); // Default empty
            }
            _ => {
                unreachable!("expected File sourced credential")
            }
        }
    }

    #[tokio::test]
    async fn test_kubernetes_wif_impersonation_parsing() {
        let contents = json!({
            "audience": "//iam.googleapis.com/projects/123456789/locations/global/workloadIdentityPools/pool-name/providers/k8s-cluster",
            "credential_source": {
                "file": "/var/run/service-account/token",
                "format": {
                    "type": "text"
                }
            },
            "service_account_impersonation_url": "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/test-sa@test-project.iam.gserviceaccount.com:generateAccessToken",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": "https://sts.googleapis.com/v1/token",
            "type": "external_account",
            "universe_domain": "googleapis.com"
        });

        let file: ExternalAccountFile = serde_json::from_value(contents)
            .expect("failed to parse kubernetes WIF impersonation config");
        let config: ExternalAccountConfig = file.into();

        match config.credential_source {
            CredentialSource::File(source) => {
                assert_eq!(source.file, "/var/run/service-account/token");
                assert_eq!(source.format, "text");
                assert_eq!(source.subject_token_field_name, ""); // Empty for text format
            }
            _ => {
                unreachable!("expected File sourced credential")
            }
        }

        assert_eq!(
            config.service_account_impersonation_url,
            Some("https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/test-sa@test-project.iam.gserviceaccount.com:generateAccessToken".to_string())
        );
    }

    #[tokio::test]
    async fn test_aws_parsing() {
        let contents = json!({
            "audience": "audience",
            "credential_source": {
                "environment_id": "aws1",
                "region_url": "http://169.254.169.254/latest/meta-data/placement/availability-zone",
                "url": "http://169.254.169.254/latest/meta-data/iam/security-credentials/",
                "regional_cred_verification_url": "https://sts.{region}.amazonaws.com?Action=GetCallerIdentity&Version=2011-06-15",
                "imdsv2_session_token_url": "http://169.254.169.254/latest/api/token"
            },
            "subject_token_type": "urn:ietf:params:aws:token-type:aws4_request",
            "token_url": "https://sts.googleapis.com/v1/token",
            "type": "external_account"
        });

        let file: ExternalAccountFile =
            serde_json::from_value(contents).expect("failed to parse AWS config");
        let config: ExternalAccountConfig = file.into();

        match config.credential_source {
            CredentialSource::Aws(source) => {
                assert_eq!(
                    source.region_url,
                    Some(
                        "http://169.254.169.254/latest/meta-data/placement/availability-zone"
                            .to_string()
                    )
                );
                assert_eq!(
                    source.regional_cred_verification_url,
                    Some(
                        "https://sts.{region}.amazonaws.com?Action=GetCallerIdentity&Version=2011-06-15"
                            .to_string()
                    )
                );
                assert_eq!(
                    source.imdsv2_session_token_url,
                    Some("http://169.254.169.254/latest/api/token".to_string())
                );
            }
            _ => {
                unreachable!("expected Aws sourced credential")
            }
        }
    }

    #[tokio::test]
    async fn builder_workforce_pool_user_project_fails_without_workforce_pool_audience() {
        let contents = json!({
            "type": "external_account",
            "audience": "not-a-workforce-pool",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": "http://test.com/token",
            "credential_source": {
                "url": "http://test.com/subject_token",
            },
            "workforce_pool_user_project": "test-project"
        });

        let result = Builder::new(contents).build();

        assert!(result.is_err(), "{result:?}");
        let err = result.unwrap_err();
        assert!(err.is_parsing(), "{err:?}");
    }

    #[tokio::test]
    async fn sts_handler_ignores_workforce_pool_user_project_with_client_auth()
    -> std::result::Result<(), Box<dyn std::error::Error>> {
        let subject_token_server = Server::run();
        let sts_server = Server::run();

        let contents = json!({
            "type": "external_account",
            "audience": "//iam.googleapis.com/locations/global/workforcePools/pool/providers/provider",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": sts_server.url("/token").to_string(),
            "client_id": "client-id",
            "credential_source": {
                "url": subject_token_server.url("/subject_token").to_string(),
            },
            "workforce_pool_user_project": "test-project"
        });

        subject_token_server.expect(
            Expectation::matching(request::method_path("GET", "/subject_token")).respond_with(
                json_encoded(json!({
                    "access_token": "subject_token",
                })),
            ),
        );

        sts_server.expect(
            Expectation::matching(all_of![request::method_path("POST", "/token"),]).respond_with(
                json_encoded(json!({
                    "access_token": "sts-only-token",
                    "issued_token_type": "urn:ietf:params:oauth:token-type:access_token",
                    "token_type": "Bearer",
                    "expires_in": 3600,
                })),
            ),
        );

        let creds = Builder::new(contents).build()?;
        let headers = creds.headers(Extensions::new()).await?;
        let token = get_token_from_headers(headers);
        assert_eq!(token.as_deref(), Some("sts-only-token"));

        Ok(())
    }

    #[tokio::test]
    async fn sts_handler_receives_workforce_pool_user_project()
    -> std::result::Result<(), Box<dyn std::error::Error>> {
        let subject_token_server = Server::run();
        let sts_server = Server::run();

        let contents = json!({
            "type": "external_account",
            "audience": "//iam.googleapis.com/locations/global/workforcePools/pool/providers/provider",
            "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
            "token_url": sts_server.url("/token").to_string(),
            "credential_source": {
                "url": subject_token_server.url("/subject_token").to_string(),
            },
            "workforce_pool_user_project": "test-project-123"
        });

        subject_token_server.expect(
            Expectation::matching(request::method_path("GET", "/subject_token")).respond_with(
                json_encoded(json!({
                    "access_token": "subject_token",
                })),
            ),
        );

        sts_server.expect(
            Expectation::matching(all_of![
                request::method_path("POST", "/token"),
                request::body(url_decoded(contains((
                    "options",
                    "{\"userProject\":\"test-project-123\"}"
                )))),
            ])
            .respond_with(json_encoded(json!({
                "access_token": "sts-only-token",
                "issued_token_type": "urn:ietf:params:oauth:token-type:access_token",
                "token_type": "Bearer",
                "expires_in": 3600,
            }))),
        );

        let creds = Builder::new(contents).build()?;
        let headers = creds.headers(Extensions::new()).await?;
        let token = get_token_from_headers(headers);
        assert_eq!(token.as_deref(), Some("sts-only-token"));

        Ok(())
    }
}