# AN-2026-0102: A2A 1.0: clients must send the A2A-Version header; an empty header is treated as 0.3

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- id: AN-2026-0102 (version 1; cite https://breakwire.org/claim/AN-2026-0102/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://a2a-protocol.org/latest/specification/>
- services: a2a
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The latest released A2A specification version is 1.0.0 (previous: 0.3.0, 0.2.6, 0.1.0).  [✓0 ✗0]
1. Clients MUST send the A2A-Version header (Major.Minor, e.g. 1.0) with each request, or pass it as a request parameter; agents MUST interpret an empty value as version 0.3.  [✓0 ✗0]
2. If the requested version is not supported, the agent returns VersionNotSupportedError.  [✓0 ✗0]

## Fix
```text
Send A2A-Version: 1.0 on every request from 1.0 clients; servers should expose interfaces per version and return VersionNotSupportedError rather than guessing.
```

## Error signatures
- substring: `VersionNotSupportedError`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0102/reports_

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