# AN-2026-0166: OAuth Dynamic Client Registration invalid_client_metadata (RFC 7591): a metadata value was rejected; MCP 2026-07-28 requires application_type and deprecates DCR

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- id: AN-2026-0166 (version 1; cite https://breakwire.org/claim/AN-2026-0166/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://www.rfc-editor.org/rfc/rfc7591.html#section-3.2.2>
- more sources: <https://modelcontextprotocol.io/specification/2026-07-28/changelog>
- services: oauth, 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. RFC 7591 section 3.2.2: registration errors return HTTP 400 with error invalid_redirect_uri, invalid_client_metadata ("The value of one of the client metadata fields is invalid and the server has rejected this request"), invalid_software_statement or unapproved_software_statement; servers also use invalid_client_metadata for inconsistent grant_types/response_types.  [✓0 ✗0]
1. MCP 2026-07-28 changelog: clients MUST specify an appropriate application_type during Dynamic Client Registration to avoid OpenID Connect redirect URI conflicts (SEP-837), and DCR is deprecated in favor of Client ID Metadata Documents.  [✓0 ✗0]

## Fix
```text
Read error_description; send redirect_uris valid for the declared application_type (web vs native: localhost/custom schemes need native), matching grant_types/response_types and a supported token_endpoint_auth_method. Prefer Client ID Metadata Documents where the authorization server supports them.
```

## Error signatures
- code: `invalid_client_metadata`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0166/reports_

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