switchyard-protocol 0.3.0

// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
// SPDX-License-Identifier: Apache-2.0

//! The routed-call server trait and its shared error types.
//!
//! [`RoutedLlmClient`] is the one piece of I/O the protocol does not own: a host
//! implements it to actually perform a model call. It lives here — rather than in
//! libsy's orchestration crate — so a client crate that depends only on the protocol
//! can serve routed calls without pulling in the orchestrator.

use async_trait::async_trait;
use thiserror::Error;

use crate::{ModelId, Request, Response};

/// A boxed client-specific error preserved as the source of a routed call failure.
pub type BoxError = Box<dyn std::error::Error + Send + Sync + 'static>;

/// Failures a routed LLM client can surface to its caller.
///
/// The variants classify failures that routing hosts commonly need to handle,
/// while boxed sources preserve implementation-specific detail. `General` is the
/// escape hatch for failures that do not fit a shared category.
#[non_exhaustive]
#[derive(Debug, Error)]
pub enum LlmClientError {
    /// The request cannot be served as supplied.
    #[error("invalid request: {message}")]
    InvalidRequest {
        /// Human-readable request validation failure.
        message: String,
    },

    /// Decoding the inbound request failed in the translation engine.
    #[error("request translation failed: {0}")]
    RequestTranslation(String),

    /// Encoding the request for the upstream failed in the translation engine.
    #[error("outbound request encoding failed: {0}")]
    RequestEncoding(String),

    /// Decoding or encoding the response failed in the translation engine.
    #[error("response translation failed: {0}")]
    ResponseTranslation(String),

    /// The client is not configured to serve the selected target.
    #[error("client configuration error: {message}")]
    Configuration {
        /// Human-readable configuration failure.
        message: String,
    },

    /// The route cannot record another provider-owned response or conversation ID.
    #[error(
        "Responses state tracking reached its limit of {limit} IDs; no existing records were removed"
    )]
    ResponseStateLimitExceeded {
        /// Maximum number of IDs retained by this route.
        limit: usize,
    },

    /// Two configured models reported the same saved response or conversation ID.
    #[error("Responses state ID is already recorded for another model; its owner was not changed")]
    ResponseStateConflict,

    /// The upstream could not be reached or the request could not be sent.
    #[error("upstream transport error: {source}")]
    Transport {
        /// Client-specific transport failure.
        #[source]
        source: BoxError,
    },

    /// The upstream request exceeded its timeout.
    #[error("upstream request timed out: {source}")]
    Timeout {
        /// Client-specific timeout failure.
        #[source]
        source: BoxError,
    },

    /// The upstream rejected the request because it exceeds the model's context window.
    #[error("context window exceeded for model {model}: {message}")]
    ContextWindowExceeded {
        /// Model whose context window was exceeded.
        model: ModelId,
        /// Upstream error message.
        message: String,
    },

    /// The upstream returned a non-success HTTP response.
    #[error("upstream returned HTTP {status}: {body}")]
    UpstreamHttp {
        /// Upstream HTTP status code.
        status: http::StatusCode,
        /// Raw upstream error body.
        body: String,
    },

    /// The upstream returned a response the client could not decode.
    #[error("invalid upstream response: {source}")]
    InvalidResponse {
        /// Client-specific decoding or validation failure.
        #[source]
        source: BoxError,
    },

    /// A call across a foreign-function boundary (e.g. a Python-implemented client)
    /// failed. The boxed source is the foreign error itself.
    #[error("foreign function interface error: {source}")]
    Ffi {
        /// Foreign-language failure, preserved verbatim.
        #[source]
        source: BoxError,
    },

    /// A string message. Useful in testing, but prefer adding variants over using this.
    #[error("{0}")]
    General(String),
}

/// Why routing replaced a selected target with another eligible target.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum RoutingFallbackReason {
    /// The selected target rejected the request because its context window was too small.
    ContextWindow,
    /// The selected target was unavailable after its client retries finished.
    Unavailable,
}

impl RoutingFallbackReason {
    /// Stable value used when logging a routing fallback.
    pub const fn as_str(self) -> &'static str {
        match self {
            Self::ContextWindow => "context_window",
            Self::Unavailable => "unavailable",
        }
    }
}

/// Performs the actual model call for a target. This is the one piece of I/O the
/// library does not own — a host implements it over its own transport (HTTP SDK,
/// in-process model, mock). It serves a call the stream consumer chose not to
/// override, reached as a routed request's `default_client`.
///
/// # Concurrency
///
/// A client may be shared by many targets and concurrent algorithm runs. Calls may
/// overlap, so implementations must synchronize mutable state internally and should
/// not serialize requests unless their transport requires it.
#[async_trait]
pub trait RoutedLlmClient: Send + Sync {
    /// Make a request
    async fn call(&self, request: Request) -> Result<Response, LlmClientError>;
}