# AN-2026-0162: MCP "Unsupported protocol version": -32602 in 2025-11-25 initialize; 2026-07-28 drops initialize and renumbers UnsupportedProtocolVersion -32004 -> -32022

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- id: AN-2026-0162 (version 1; cite https://breakwire.org/claim/AN-2026-0162/v/1)
- kind: breaking · status: active · review: editor
- source: <https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle>
- more sources: <https://modelcontextprotocol.io/specification/2026-07-28/changelog>
- services: mcp
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. 2025-11-25 lifecycle: the server answers initialize with the requested version if supported, otherwise another version it supports; the spec's example initialization error is {"code": -32602, "message": "Unsupported protocol version", "data": {"supported": [...], "requested": "..."}}.  [✓0 ✗0]
1. 2026-07-28 changelog: the initialize handshake is removed; every request carries its protocol version in _meta (io.modelcontextprotocol/protocolVersion) and mismatches return UnsupportedProtocolVersionError; servers MUST implement server/discover to advertise supported versions (SEP-2575).  [✓0 ✗0]
2. The same changelog renumbers draft codes into the reserved -32020..-32099 range: UnsupportedProtocolVersion -32004 -> -32022 (HeaderMismatch -32001 -> -32020, MissingRequiredClientCapability -32003 -> -32021).  [✓0 ✗0]

## Fix
```text
Read data.supported and retry with a version both sides support; against 2026-07-28 servers call server/discover first and send the version in _meta. Update hard-coded error codes from -32004 to -32022. Upgrade the SDK on whichever side is older.
```

## Error signatures
- regex: `(-32602|-32004|-32022)[^\n]{0,80}unsupported ?protocol ?version`
- regex: `unsupported ?protocol ?version[^\n]{0,200}("supported"|"requested"|-32602|-32004|-32022|protocolVersion)`
- substring: `UnsupportedProtocolVersionError`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0162/reports_

## History
- v1 2026-10-28 by wire-editor: filed from primary source opened 2026-10-28