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42 lines
1.7 KiB
Markdown
42 lines
1.7 KiB
Markdown
# extensions
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Writing your own extension (SEP-2133): one identifier bundles a settings entry
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under `ServerCapabilities.extensions`, a contributed tool, and a vendor request
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method gated on the client declaring the extension back.
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## Run it
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```bash
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# stdio (default — the client spawns the server as a subprocess)
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uv run python -m stories.extensions.client
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# HTTP — the client self-hosts the server on a free port, runs, then tears it down
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uv run python -m stories.extensions.client --http
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```
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## What to look at
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- `server.py` `class Catalog(Extension)` — the whole extension: `settings()`
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becomes the advertised capability entry, `tools()` contributes a regular tool,
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`methods()` registers a vendor verb. The extension never holds the server; it
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declares contributions and `MCPServer(extensions=[...])` consumes them.
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- `server.py` `require_client_extension(ctx, EXTENSION_ID)` — the vendor method
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rejects clients that did not declare the extension with `-32021` (missing
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required client capability) and a machine-readable `requiredCapabilities`
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payload.
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- `client.py` `Client(target, extensions=[advertise(EXTENSION_ID)])` — the client-side
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half of the negotiation; on 2026-07-28 it travels in the per-request `_meta`
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envelope.
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- `client.py` `client.session.send_request(...)` — vendor methods have no
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`Client`-level helper; the session escape hatch sends them.
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## Spec
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[SEP-2133 — extensions capability](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/2133)
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· [Capabilities — `_meta` key grammar](https://modelcontextprotocol.io/specification/draft/basic/index)
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## See also
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`apps/` (the built-in MCP Apps extension) · `custom_methods/` (the same verb
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registered on the lowlevel `Server` by hand, without an extension).
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