Test mcpc's MCP extension declarations on every request - #420
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The client matrix (modelcontextprotocol.io/extensions/client-matrix) has
clients declare extension support in the `extensions` field of the client
capabilities sent in every request's `_meta`. Unit tests covered what
buildClientCapabilities() returns, but nothing checked the wire.
Both e2e test servers now record the client capabilities in effect for the
latest tools/call (GET /control/get-client-capabilities), and
basic/extensions asserts, in both protocol eras, that mcpc declares exactly
the two auth extensions with `{}` settings, and not MCP Apps, skills
(server-declared only) or tasks.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhPWh9m3jHeNxPJKoXCP4y
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Checked mcpc against the extension client matrix: it already declares the right extensions (the two auth extensions as
{}; not skills, which only servers declare, and not MCP Apps or tasks). Until now only unit tests coveredbuildClientCapabilities(), so this adds an e2e check that the declarations actually reach a server.tools/call(GET /control/get-client-capabilities)basic/extensionsasserts the exact declared set in both protocol eras; on 2026-07-28 this also shows the_metaenvelope rides regular requests, not justserver/discoverTest-only, no user-facing change.
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https://claude.ai/code/session_01EhPWh9m3jHeNxPJKoXCP4y