molo_core/provider/

mod.rs

//! Provider: the interface for communicating with an LLM.
//!
//! This module defines the `Provider` trait and its companion data types:
//! requests ([`ChatRequest`]), responses ([`ChatResponse`] / [`StreamEvent`]),
//! errors ([`ProviderError`]), capabilities ([`ProviderCapabilities`]),
//! request context ([`ProviderRequestContext`]), and usage ([`Usage`]). The
//! trait itself is vendor-agnostic; the lightweight implementations
//! ([`FakeProvider`] and [`RetryProvider`]) live here. Concrete network
//! providers live in optional adapter crates such as `molo-openai`.
//!
//! The provider contract is intentionally explicit: successful `chat`
//! returns exactly one assistant message, successful streams terminate with
//! one `Done`, usage is reported only when the provider supplies it, and
//! local decode/protocol/size-limit failures are distinct from vendor API
//! errors. Context-aware methods are the runtime boundary: runtimes pass run
//! ids, model request ids, deadlines, cancellation, and sanitized metadata to
//! providers explicitly. The plain `chat` and `stream_chat` methods remain
//! convenience entry points for direct provider use.

mod fake;
mod retry;

pub use fake::{FakeProvider, FakeReply};
pub use retry::{Backoff, RetryPolicy, RetryProvider, Retryable};

use crate::message::Message;
use crate::run::{RunContext, RunMetadata};
use crate::tool::ToolSchema;
use futures::stream::BoxStream;
use serde::{Deserialize, Serialize};
use std::collections::BTreeMap;
use std::fmt;
use std::time::{Duration, Instant};

/// Provider capability metadata used by hosts and conformance tests.
///
/// Capabilities are descriptive, not a security boundary. When a provider
/// declares support for an optional capability, it should pass the matching
/// provider conformance cases. Unsupported direct calls should return
/// [`ProviderError::Unsupported`] rather than panic.
#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
pub struct ProviderCapabilities {
    /// Supports [`Provider::stream_chat`] / [`Provider::stream_chat_with_context`].
    pub streaming: bool,
    /// Maps provider reasoning/thinking output to [`Message`] or
    /// [`StreamEvent::Reasoning`].
    pub reasoning: bool,
    /// Supports model tool-call requests.
    pub tool_calls: bool,
    /// Preserves multiple tool calls in one assistant turn.
    pub parallel_tool_calls: bool,
    /// Accepts [`ModelOptions::structured`] as a provider-side best-effort
    /// constraint.
    pub structured_output: bool,
    /// Reports provider token usage when the backend supplies it.
    pub usage: bool,
    /// Cooperatively observes cancellation from [`ProviderRequestContext`].
    pub context_cancellation: bool,
    /// Cooperatively observes deadlines from [`ProviderRequestContext`].
    pub context_deadline: bool,
}

impl ProviderCapabilities {
    /// Baseline provider capabilities: non-streaming text only.
    pub fn baseline() -> Self {
        Self::default()
    }
}

/// Request-scoped provider context.
///
/// This is the provider-boundary projection of [`RunContext`]: it carries
/// correlation ids, cancellation/deadline controls, an optional timeout hint,
/// and sanitized host metadata. Raw prompts, source code, auth headers, API
/// keys, and environment values should not be placed in metadata by default.
#[derive(Clone)]
pub struct ProviderRequestContext {
    /// Run id shared with run summaries, event records, and tracing spans.
    pub run_id: String,
    /// Model request id unique within the run.
    pub model_request_id: String,
    /// Cooperative cancellation source.
    pub cancellation: tokio_util::sync::CancellationToken,
    /// Optional absolute deadline.
    pub deadline: Option<Instant>,
    /// Optional per-provider-call timeout hint.
    pub timeout: Option<Duration>,
    /// Host/framework metadata for observability and routing.
    pub metadata: RunMetadata,
}

impl fmt::Debug for ProviderRequestContext {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("ProviderRequestContext")
            .field("run_id", &self.run_id)
            .field("model_request_id", &self.model_request_id)
            .field("cancellation", &"CancellationToken")
            .field("deadline", &self.deadline)
            .field("timeout", &self.timeout)
            .field("metadata", &self.metadata)
            .finish()
    }
}

impl ProviderRequestContext {
    /// Builds provider context from a run context and model request id.
    pub fn from_run_context(model_request_id: impl Into<String>, context: &RunContext) -> Self {
        Self {
            run_id: context.run_id.clone(),
            model_request_id: model_request_id.into(),
            cancellation: context.cancellation.clone(),
            deadline: context.deadline,
            timeout: context.remaining(),
            metadata: context.metadata.clone(),
        }
    }

    /// Sets a provider-call timeout hint.
    pub fn with_timeout(mut self, timeout: Duration) -> Self {
        self.timeout = Some(timeout);
        self
    }

    /// Whether cancellation has already been requested.
    pub fn is_cancelled(&self) -> bool {
        self.cancellation.is_cancelled()
    }

    /// Whether the deadline has elapsed.
    pub fn is_expired(&self) -> bool {
        self.deadline
            .is_some_and(|deadline| Instant::now() >= deadline)
    }

    /// Remaining time before the deadline.
    pub fn remaining(&self) -> Option<Duration> {
        self.deadline
            .map(|deadline| deadline.saturating_duration_since(Instant::now()))
    }
}

/// The interface for chatting with an LLM.
///
/// Implementations are responsible for communicating with a specific LLM
/// service and mapping vendor responses back to this framework's [`Message`];
/// [`chat`](Provider::chat) returns the full reply at once, while
/// [`stream_chat`](Provider::stream_chat) returns the same reply incrementally
/// as a stream of events. Both share the same semantics and differ only in
/// delivery.
///
/// `Send + Sync` guarantees that `Box<dyn Provider>` can be held across
/// awaits in Agent implementations (for the same reason as
/// [`Tool`](crate::tool::Tool)).
///
/// # Examples
///
/// The calling convention is identical for every implementation; the example
/// below uses [`FakeProvider`]:
///
/// ```rust
/// # extern crate molo_core as molo;
/// # #[tokio::main]
/// # async fn main() -> Result<(), molo::ProviderError> {
/// use molo::provider::{ChatRequest, FakeProvider, FakeReply, Provider};
///
/// let fake = FakeProvider::new([FakeReply::Text("hi".into())]);
/// let response = fake.chat(ChatRequest::default()).await?;
/// assert_eq!(response.message, molo::message::Message::assistant("hi"));
/// # Ok(())
/// # }
/// ```
#[async_trait::async_trait]
pub trait Provider: Send + Sync {
    /// Model identifier exposed by this provider, when known.
    ///
    /// Agents copy this value into run summaries for observability. Providers
    /// that are not bound to one model can keep the default `None`.
    fn model(&self) -> Option<&str> {
        None
    }

    /// Capability metadata for this provider instance.
    fn capabilities(&self) -> ProviderCapabilities {
        ProviderCapabilities::baseline()
    }

    /// Sends one turn with request-scoped provider context.
    ///
    /// # Errors
    ///
    /// Network failures / timeouts / rate limits / vendor business errors are
    /// all returned as [`ProviderError`]; see that type's docs for error
    /// classification and retry guidance.
    async fn chat_with_context(
        &self,
        request: ChatRequest,
        context: &ProviderRequestContext,
    ) -> Result<ChatResponse, ProviderError>;

    /// Sends one turn of conversation and returns the model's reply (text, or
    /// a request to call tools).
    ///
    /// This direct-use convenience wrapper creates a generated run context.
    /// Runtimes that already have a [`RunContext`] should call
    /// [`chat_with_context`](Provider::chat_with_context).
    async fn chat(&self, request: ChatRequest) -> Result<ChatResponse, ProviderError> {
        let run = RunContext::generated();
        let context = ProviderRequestContext::from_run_context("direct-chat", &run);
        self.chat_with_context(request, &context).await
    }

    /// Streams one turn of conversation for direct provider use.
    ///
    /// Semantically identical to [`chat`](Provider::chat), except that the
    /// reply is returned as a stream of events: several [`StreamEvent::Delta`]
    /// items concatenated in order form the full reply, and the stream ends
    /// with [`StreamEvent::Done`].
    ///
    /// # Errors
    ///
    /// Failures during request setup (connection / timeout / vendor rejection)
    /// are returned as `Err`; event errors after the stream is established are
    /// produced as `Err` items in the stream, and no success events are
    /// produced after an error item (see [`StreamEvent`] for termination
    /// semantics).
    async fn stream_chat(
        &self,
        request: ChatRequest,
    ) -> Result<BoxStream<'static, Result<StreamEvent, ProviderError>>, ProviderError> {
        let run = RunContext::generated();
        let context = ProviderRequestContext::from_run_context("direct-stream", &run);
        self.stream_chat_with_context(request, &context).await
    }

    /// Streams one turn with request-scoped provider context.
    ///
    /// Runtimes should call this method so providers can observe cancellation,
    /// deadlines, request ids, and sanitized metadata.
    async fn stream_chat_with_context(
        &self,
        request: ChatRequest,
        context: &ProviderRequestContext,
    ) -> Result<BoxStream<'static, Result<StreamEvent, ProviderError>>, ProviderError>;
}

/// `Box<dyn Provider>` is itself a Provider: re-exposes the trait object as a
/// value, for assembly patterns that need to "hold an instance and create a
/// new loop per call" (e.g., a sub-agent factory that captures a provider and
/// constructs a fresh loop for each invocation).
#[async_trait::async_trait]
impl Provider for Box<dyn Provider> {
    fn model(&self) -> Option<&str> {
        self.as_ref().model()
    }

    fn capabilities(&self) -> ProviderCapabilities {
        self.as_ref().capabilities()
    }

    async fn chat(&self, request: ChatRequest) -> Result<ChatResponse, ProviderError> {
        self.as_ref().chat(request).await
    }

    async fn chat_with_context(
        &self,
        request: ChatRequest,
        context: &ProviderRequestContext,
    ) -> Result<ChatResponse, ProviderError> {
        self.as_ref().chat_with_context(request, context).await
    }

    async fn stream_chat(
        &self,
        request: ChatRequest,
    ) -> Result<BoxStream<'static, Result<StreamEvent, ProviderError>>, ProviderError> {
        self.as_ref().stream_chat(request).await
    }

    async fn stream_chat_with_context(
        &self,
        request: ChatRequest,
        context: &ProviderRequestContext,
    ) -> Result<BoxStream<'static, Result<StreamEvent, ProviderError>>, ProviderError> {
        self.as_ref()
            .stream_chat_with_context(request, context)
            .await
    }
}

/// A single conversation request.
///
/// # Examples
///
/// ```rust
/// # extern crate molo_core as molo;
/// use molo::message::Message;
/// use molo::provider::ChatRequest;
///
/// let request = ChatRequest {
///     messages: vec![Message::user("hi")],
///     ..Default::default()
/// };
/// # let _ = request;
/// ```
#[derive(Debug, Clone, Default, PartialEq, Serialize, Deserialize)]
pub struct ChatRequest {
    /// Conversation history (in order of occurrence, assembled by the caller).
    pub messages: Vec<Message>,
    /// Tool definitions offered to the model; when empty, the model sees no
    /// tools.
    pub tools: Vec<ToolSchema>,
    /// Model options; `Default` means all vendor defaults.
    pub options: ModelOptions,
}

/// Model options for one conversation.
///
/// Common parameters are provided as typed fields (temperature / max tokens,
/// where `None` means vendor default); **vendor-specific or framework-unknown
/// parameters go into [`extra`](ModelOptions::extra)** and are passed through
/// to the vendor verbatim under their wire field names — so users can use new
/// parameters without waiting for a framework update:
///
/// ```rust
/// # extern crate molo_core as molo;
/// use molo::ModelOptions;
///
/// let mut options = ModelOptions::default();
/// options.extra.insert("top_p".into(), serde_json::json!(0.9));
/// ```
///
/// Extra keys that collide with framework-managed fields are ignored in favor
/// of the typed fields.
#[derive(Debug, Clone, Default, PartialEq, Serialize, Deserialize)]
pub struct ModelOptions {
    /// Sampling temperature; `None` means vendor default.
    pub temperature: Option<f32>,
    /// Maximum number of tokens for the reply; `None` means vendor default.
    pub max_tokens: Option<u32>,
    /// Vendor extension parameters: keys are wire request field names,
    /// serialized into the request body verbatim.
    pub extra: BTreeMap<String, serde_json::Value>,
    /// Structured output: the final answer must be JSON conforming to this
    /// **JSON Schema document** (the serialized `RootSchema` produced by
    /// schemars, or a hand-written schema).
    ///
    /// Two layers of semantics:
    /// - Provider side: compatible endpoints receive it via `response_format`
    ///   to best-effort constrain the model (the OpenAI-compatible
    ///   `json_schema` shape; unsupported endpoints ignore it or error);
    /// - Agent side: the final answer is **validated framework-side**, and on
    ///   mismatch the validation error is fed back to the model for a retry
    ///   (counted against the turn budget by the agent runtime's structured
    ///   output support).
    ///
    /// `None` = free-form text reply.
    pub structured: Option<serde_json::Value>,
}

/// Token usage for one conversation.
///
/// Field names match the OpenAI wire format; `total_tokens` follows the
/// vendor's convention (not necessarily the sum of the other two). `Default`
/// = all zeros, with nothing reported.
///
/// Presence is carried by the enclosing type: [`ChatResponse::usage`] /
/// [`StreamEvent::Done::usage`] are `Option<Usage>` — `None` means the
/// endpoint did not report usage for this turn, `Some` means the reported
/// values. The distinction matters for observability: "not reported" is not
/// the same as "reportedly zero" (the Agent layer also tracks it in
/// [`RunSummary::usage_omitted`](crate::run::RunSummary)).
///
/// [`cached_tokens`](Self::cached_tokens) carries the same distinction inside
/// a reported usage: the prompt-cache breakdown is optional per turn even
/// when the totals are present.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Serialize, Deserialize)]
pub struct Usage {
    /// Input tokens for this turn.
    pub prompt_tokens: u32,
    /// Output tokens for this turn.
    pub completion_tokens: u32,
    /// Total for this turn (vendor convention).
    pub total_tokens: u32,
    /// Prompt tokens served from the provider's prompt cache.
    ///
    /// A subset of [`prompt_tokens`](Self::prompt_tokens), not an addition to
    /// it (vendors bill cached input at a lower rate).
    ///
    /// `None` = the endpoint did not report a cache breakdown for this turn;
    /// `Some(0)` = it reported that nothing hit the cache. Endpoints differ
    /// here — prompt-caching ones report the breakdown (OpenAI / DeepSeek as
    /// `prompt_tokens_details.cached_tokens`), others omit it — so a reported
    /// zero must not be inferred from an omission. When turns are summed,
    /// only the reported parts contribute and
    /// [`RunSummary::cache_omitted`](crate::run::RunSummary) marks whether
    /// the sum is a lower bound.
    pub cached_tokens: Option<u32>,
}

impl Usage {
    /// Constructs from input / output counts; the total is summed
    /// automatically and no cache breakdown is reported
    /// ([`with_cached_tokens`](Self::with_cached_tokens) sets one).
    pub fn new(prompt_tokens: u32, completion_tokens: u32) -> Self {
        Self {
            prompt_tokens,
            completion_tokens,
            total_tokens: prompt_tokens + completion_tokens,
            cached_tokens: None,
        }
    }

    /// Sets the prompt tokens served from the provider's cache, marking the
    /// breakdown as reported (`Some` in [`cached_tokens`](Self::cached_tokens)).
    pub fn with_cached_tokens(mut self, cached_tokens: u32) -> Self {
        self.cached_tokens = Some(cached_tokens);
        self
    }
}

/// Usage accumulates per turn (the Agent layer sums tokens across turns).
///
/// `cached_tokens` sums the parts that were reported: a turn that omitted the
/// breakdown contributes nothing rather than zero.
impl std::ops::AddAssign for Usage {
    fn add_assign(&mut self, rhs: Self) {
        self.prompt_tokens += rhs.prompt_tokens;
        self.completion_tokens += rhs.completion_tokens;
        self.total_tokens += rhs.total_tokens;
        self.cached_tokens = match (self.cached_tokens, rhs.cached_tokens) {
            (Some(lhs), Some(rhs)) => Some(lhs + rhs),
            (lhs, rhs) => lhs.or(rhs),
        };
    }
}

/// The reply to one conversation.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct ChatResponse {
    /// This turn's reply: a single [`Message::Assistant`] message
    /// (text + reasoning + any tool requests from the same turn stay together,
    /// one-to-one with the vendor wire structure);
    /// when the model produces nothing it is still an empty Assistant.
    ///
    /// The Agent loop executes tool requests when it sees them and appends the
    /// results as [`Message::ToolResult`] before continuing the conversation.
    pub message: Message,
    /// Why the model ended its reply; vendor-specific reasons are surfaced via
    /// [`FinishReason::Other`].
    pub finish_reason: FinishReason,
    /// Token usage for this turn; `None` when the endpoint did not return it
    /// (compatible endpoints may omit usage; see [`Usage`] for the presence
    /// semantics shared with [`StreamEvent::Done::usage`]).
    pub usage: Option<Usage>,
}

/// Why the model ended its reply.
///
/// Common reasons are typed (Stop / Length); vendor-specific or
/// framework-unknown reasons are surfaced via [`Other`](FinishReason::Other)
/// carrying the vendor's raw string — users can recognize new reasons without
/// waiting for a framework update. `#[non_exhaustive]` guarantees that adding
/// new common categories in the future is not a breaking change.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[non_exhaustive]
pub enum FinishReason {
    /// The model ended naturally (including ending the turn by requesting
    /// tool calls).
    Stop,
    /// Truncated by hitting the max_tokens limit.
    Length,
    /// A vendor-specific or framework-unknown reason, carrying the vendor's
    /// raw string.
    Other(String),
}

/// An event in a streamed conversation reply.
///
/// One streamed reply = several [`StreamEvent::Delta`] /
/// [`StreamEvent::ToolCall`] increments + one closing [`StreamEvent::Done`];
/// the caller concatenates the Deltas in order to get the full reply.
///
/// Stream termination semantics: on normal termination `Done` is always the
/// last success event on the stream; errors terminate the stream with an
/// `Err` item, and no events are produced after the error item.
///
/// The enum is `#[non_exhaustive]` (reserved for extension): matches must
/// include a wildcard arm.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[non_exhaustive]
pub enum StreamEvent {
    /// An incremental fragment of the reply content.
    Delta(String),
    /// The model requests a tool call; argument fragments have already been
    /// aggregated by the Provider into full JSON text, with fields matching
    /// [`crate::ToolCall`] so the Agent can use them directly.
    ToolCall {
        /// Unique id of this call; the execution result is paired back to it
        /// via this id.
        id: String,
        /// Tool name, corresponding to the name in
        /// [`Tool::schema`](crate::tool::Tool::schema).
        name: String,
        /// Arguments generated by the model (JSON text).
        arguments: String,
    },
    /// An incremental fragment of the model's reasoning (thinking); the
    /// vendor delivers it in fragments during the stream, and the Provider
    /// forwards each fragment as it arrives (like [`StreamEvent::Delta`]) —
    /// consumers concatenate them in order to get the full text.
    ///
    /// Corresponds to the reasoning field of [`Message`]; the Agent must store
    /// it in this turn's message and carry it back verbatim in the history
    /// (otherwise thinking models like DeepSeek / Qwen3 reject the request).
    Reasoning(String),
    /// The model finished its reply; the stream produces no more events after
    /// this.
    Done {
        /// Why the model ended its reply.
        reason: FinishReason,
        /// Token usage for this turn; `None` when the endpoint did not return
        /// it (`include_usage` off or unsupported by the endpoint).
        usage: Option<Usage>,
    },
}

/// Why a Provider call failed.
///
/// The enum categories cover the cases that need distinguishing, with details
/// carried by fields; vendor-specific errors are mapped into this type at the
/// implementation boundary. `#[non_exhaustive]` guarantees that adding new
/// categories in the future is not a breaking change.
///
/// Error classification is the basis for retry decisions (see the `Default`
/// judgment of [`Retryable`]): Network / Timeout / RateLimited are worth
/// retrying, while `Api` is judged by status (5xx retried, 4xx not — retrying
/// would not change the outcome). Local decode/protocol errors are distinct
/// from vendor API errors and are not retried by default.
#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
#[non_exhaustive]
pub enum ProviderError {
    /// A business/API error returned by the vendor (auth failure / invalid
    /// arguments, quota exhaustion, server error), carrying the HTTP status
    /// and optional vendor error code.
    // No "provider " prefix: the type name already expresses the domain,
    // avoiding a double prefix when wrapped by AgentError::Provider
    // ("provider error: provider api error …").
    #[error("api error (status {status}): {message}")]
    Api {
        /// HTTP status code returned by the vendor.
        status: u16,
        /// Optional vendor error code.
        code: Option<String>,
        /// Error description text.
        message: String,
    },
    /// Provider response was syntactically decoded but violated either the
    /// vendor contract or molo's provider contract.
    #[error("provider protocol error: {message}")]
    Protocol {
        /// Error description text.
        message: String,
    },
    /// Provider response body, SSE frame, or encoded payload could not be
    /// decoded.
    #[error("response decode error: {message}")]
    Decode {
        /// Error description text.
        message: String,
    },
    /// Rate limited (HTTP 429): retrying is meaningful, and the default retry
    /// policy waits before retrying.
    ///
    /// `retry_after`: the wait duration parsed from the vendor's
    /// `Retry-After` response header (numeric seconds); `None` when absent
    /// (HTTP date formats are not parsed).
    #[error("rate limited")]
    RateLimited {
        /// The wait duration indicated by the vendor (numeric seconds);
        /// `None` when missing / not numeric.
        retry_after: Option<Duration>,
    },
    /// A network-layer failure (connection failure and other transport
    /// errors).
    ///
    /// Implementations map concrete transport errors (e.g. reqwest's) into
    /// carried text, so the error type does not depend on a concrete
    /// implementation library's types.
    #[error("network error: {0}")]
    Network(String),
    /// Request timeout, carrying the stage at which it occurred (see
    /// [`TimeoutStage`]): distinguishes "cannot connect", "total duration
    /// elapsed" and "event interval stalled" for easier diagnosis.
    #[error("request timed out during {0:?}")]
    Timeout(TimeoutStage),
    /// Provider request was cancelled through provider context.
    #[error("provider request cancelled")]
    Cancelled,
    /// Provider response exceeded a configured local size limit.
    #[error("response exceeded configured limit ({limit_bytes} bytes)")]
    ResponseTooLarge {
        /// Configured limit that was exceeded.
        limit_bytes: usize,
    },
    /// Capability was requested from a provider that does not support it.
    #[error("unsupported provider capability: {capability}")]
    Unsupported {
        /// Unsupported capability name.
        capability: &'static str,
    },
}

/// The stage at which a timeout occurred: one-to-one with
/// `OpenAiProvider`'s four timeouts (connect / non-streaming total /
/// streaming event interval / streaming total), plus the error-response-body
/// read timeout and a generic transport timeout.
///
/// The enum is `#[non_exhaustive]` (reserved for extension): matches must
/// include a wildcard arm.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[non_exhaustive]
pub enum TimeoutStage {
    /// Connect timeout (Client-level connect timeout).
    Connect,
    /// Request timeout: for non-streaming, covers until the response body is
    /// fully read; for streaming, covers the connect and response-header wait.
    Request,
    /// Streaming event interval exceeded: no data between two events (idle
    /// timeout).
    Idle,
    /// Streaming total duration exceeded: the wall-clock deadline has elapsed
    /// and an active but never-ending stream is terminated (stream timeout).
    StreamTotal,
    /// Total timeout for reading an error response body.
    ResponseBody,
    /// Generic transport timeout (reqwest cannot distinguish the stage, e.g.
    /// during connect or read).
    Transport,
}

#[cfg(test)]
mod tests {
    use super::Usage;

    #[test]
    fn usage_add_assign_sums_reported_cache_counts() {
        let mut total = Usage::new(10, 2).with_cached_tokens(4);
        total += Usage::new(20, 5).with_cached_tokens(6);
        assert_eq!(total, Usage::new(30, 7).with_cached_tokens(10));
    }

    /// A turn that did not report the breakdown contributes nothing (not
    /// zero); a turn that reported one still contributes, so the sum stays
    /// usable as the lower bound it is (the Agent layer marks it via
    /// `RunSummary::cache_omitted`).
    #[test]
    fn usage_add_assign_ignores_turns_without_a_cache_breakdown() {
        let mut total = Usage::new(10, 2).with_cached_tokens(4);
        total += Usage::new(20, 5);
        assert_eq!(total, Usage::new(30, 7).with_cached_tokens(4));

        let mut none_reported = Usage::new(10, 2);
        none_reported += Usage::new(20, 5);
        assert_eq!(none_reported.cached_tokens, None);

        // Reported zero is not the same as not reported.
        let mut reported_zero = Usage::default();
        reported_zero += Usage::new(10, 2).with_cached_tokens(0);
        assert_eq!(reported_zero.cached_tokens, Some(0));
    }
}