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 (optional oauth feature)
• Deterministic: fixed IDs, sequential counters, no randomness

Modules

• fixture — fixture types, matching, YAML loading
• server — ServerBuilder, MockServer, CapturedRequest
• auth — bearer token and OAuth2 middleware
• 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 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.

server

Server builder, mock server, and request capture.

Enums

Provider

Provider identifier — which endpoint was hit.