Crate emperor_mcp 

Opinionated framework for building MCP servers: stateless Streamable HTTP, content-aware output framing, and per-request credential forwarding.

Hatched from the kowalski workspace; servers built there (kowalski-mcp-rookery, kowalski-mcp-datafusion) are the reference consumers.

Two server styles share the same conventions (stateless HTTP, output framing, no auth shortcuts):

1. transport::McpHandler — hand-rolled JSON-RPC dispatch; use transport::run_stdio (local/dev) or transport::serve_http.
2. serve::serve — rmcp ServerHandler + #[tool_router] bootstrap at /mcp + /health, with credential-forwarding middleware for multi-tenant deployments.

Every server binary also ships a doctor subcommand built from doctor checks.

Also in the box: credentials (an upstream 401/403 becomes the MCP response’s status), limits (caps on what a tool returns), schema (lean tool definitions: strips JSON Schema boilerplate from tools/list, opt in with Self::tool_router().compact_schemas()), and two optional features: cli (shared --bind/--stdio/--allowed-host/--allowed-origin flags, cli::ServeArgs) and testkit (tool-list conformance checks for a server’s own tests, testkit::assert_tools).

See MCP_REQUIREMENTS.md and MANIFEST_SPEC.md in the repository root.

Re-exports

pub use credentials::Rejection;

pub use credentials::credentials_rejected;

pub use framing::FrameKind;

pub use framing::frame;

pub use framing::structured_framed;

pub use headers::ForwardConfig;

pub use headers::ForwardedHeaders;

pub use headers::forward_headers_middleware;

pub use limits::Limits;

pub use limits::truncate_chars;

pub use serve::ServeOptions;

pub use serve::serve;

pub use transport::ACCEPT_STREAMABLE;

pub use transport::McpHandler;

pub use transport::http_router;

pub use transport::run_stdio;

pub use transport::serve_http;

pub use transport::wants_sse;

Modules

clicli

Shared command-line flags for server binaries (feature cli).

credentials

Upstream credential rejection: tell the MCP client to sign in again.

doctor

doctor: an environment check every server binary ships as a subcommand.

framing

Content-aware output framing for first-party MCP tool results.

headers

Credential forwarding.

limits

Output and input caps every tool should apply: characters returned, results per call, and bytes downloaded or read from a file.

schema

Lean tool definitions: strip JSON Schema boilerplate a model pays for on every request.

serve

Streamable-HTTP server bootstrap.

testkittestkit

Conformance checks a server runs in its own tests (feature testkit).

transport

Lightweight JSON-RPC transport for hand-rolled MCP servers (McpHandler).

Functions

init_tracing

Initialise tracing/logging to stderr (stdout stays clean for the stdio transport) with an RUST_LOG-configurable filter, defaulting to info.