extensions

Writing your own extension (SEP-2133): one identifier bundles a settings entry under ServerCapabilities.extensions, a contributed tool, and a vendor request method gated on the client declaring the extension back.

Run it

# stdio (default — the client spawns the server as a subprocess)
uv run python -m stories.extensions.client

# HTTP — the client self-hosts the server on a free port, runs, then tears it down
uv run python -m stories.extensions.client --http

What to look at

• server.py class Catalog(Extension) — the whole extension: settings() becomes the advertised capability entry, tools() contributes a regular tool, methods() registers a vendor verb. The extension never holds the server; it declares contributions and MCPServer(extensions=[...]) consumes them.
• server.py require_client_extension(ctx, EXTENSION_ID) — the vendor method rejects clients that did not declare the extension with -32021 (missing required client capability) and a machine-readable requiredCapabilities payload.
• client.py Client(target, extensions=[advertise(EXTENSION_ID)]) — the client-side half of the negotiation; on 2026-07-28 it travels in the per-request _meta envelope.
• client.py client.session.send_request(...) — vendor methods have no Client-level helper; the session escape hatch sends them.

Spec

SEP-2133 — extensions capability · Capabilities — _meta key grammar

See also

apps/ (the built-in MCP Apps extension) · custom_methods/ (the same verb registered on the lowlevel Server by hand, without an extension).