Crate llmposter 

llmposter

A fixture-driven mock server for LLM APIs. Speaks OpenAI Chat Completions, Anthropic Messages, Gemini generateContent, and OpenAI Responses API — both streaming (SSE) and non-streaming.

Quick Start

use llmposter::{ServerBuilder, Fixture};

#[tokio::test]
async fn test_llm_client() {
    let server = ServerBuilder::new()
        .fixture(
            Fixture::new()
                .match_user_message("hello")
                .respond_with_content("Hi from the mock!"),
        )
        .build()
        .await
        .unwrap();

    // Point your LLM client at server.url() instead of the real API
    let base_url = server.url();
    // ... your client code here ...
}

Features

• 4 LLM API formats: OpenAI, Anthropic, Gemini, OpenAI Responses
• Streaming & non-streaming: SSE and JSON-array responses
• Fixture-driven: YAML files or programmatic builder API
• Failure simulation: latency, truncation, disconnect, corruption, error codes
• Stateful scenarios: multi-turn matching via named state machines
• Request capture: verify what your client sent with server.get_requests()
• Auth simulation: bearer tokens, OAuth2 mock server (default oauth feature)
• Record & replay (VCR): proxy requests to the real provider API and save responses as replayable fixtures. The default record feature pulls in reqwest’s rustls TLS stack — disable via default-features = false if you don’t need recording and want the smaller dependency footprint.
• Deterministic: fixed IDs, sequential counters, no randomness

Modules

• fixture — fixture types, matching, YAML loading
• server — ServerBuilder, MockServer, CapturedRequest
• auth — bearer token and OAuth2 middleware
• record — VCR record/replay: VcrMode, recording proxy (record feature)
• cli — CLI binary entry point

Re-exports

pub use auth::AuthState;

pub use auth::TokenStatus;

pub use fixture::FailureConfig;

pub use fixture::Fixture;

pub use fixture::Refusal;

pub use fixture::ScenarioConfig;

pub use fixture::StreamingConfig;

pub use fixture::ToolCall;

pub use record::VcrMode;

pub use server::OAuthConfig;

pub use server::CapturedRequest;

pub use server::MockServer;

pub use server::RequestOutcome;

pub use server::ServerBuilder;

Modules

auth

Bearer token authentication and OAuth2 middleware.

cli

CLI binary entry point and argument parsing.

fixture

Fixture types, matching logic, and YAML loading.

record

VCR record/replay: proxy unmatched requests to a real provider API and save the response as a replayable fixture in a cassette file (a YAML fixture file the recorder appends to). Enabled by the record feature.

server

Server builder, mock server, and request capture.

Enums

Provider

Provider identifier — which endpoint was hit.