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Implement MCP 2026-07-28 (#2286)

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Co-authored-by: Matt <77928207+mattzcarey@users.noreply.github.com>
Co-authored-by: Konstantin Konstantinov <KKonstantinov@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
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Felix Weinberger 2026-06-26 14:01:10 +01:00 • committed by GitHub
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.changeset/envelope-auto-emission.md Normal file
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---
'@modelcontextprotocol/client': minor
'@modelcontextprotocol/core-internal': minor
---
Per-request `_meta` envelope auto-emission on modern-era connections: once a client negotiates a 2026-07-28+ protocol revision (via `versionNegotiation: { mode: 'auto' }` or `{ pin }`), it automatically attaches the reserved protocol-version / client-info / client-capabilities
`_meta` keys to every outgoing request and notification — you no longer set the envelope by hand. User-supplied `_meta` keys take precedence over the auto-attached ones; the auto-attached client-capabilities reflect what the client actually registered. Legacy-era connections
(the default, and the `'auto'`-mode fallback) never gain these keys, so 2025-era outbound traffic is byte-identical to before.
Adds `Client.getProtocolEra()` (`'legacy' | 'modern' | undefined`), the `ProtocolEra` type, `Client.setVersionNegotiation()` for configuring negotiation pre-connect on an already-constructed instance, and the `probe.maxRetries` knob (default `0`) which governs probe-timeout
re-sends only — the spec-mandated `-32022` corrective continuation is never counted against it. The `versionNegotiation` default remains `'legacy'`: absent (or `mode: 'legacy'`), `connect()` runs the plain 2025 sequence, byte-identical to a v1.x client.