[BUG] Vertex AI: 400 "diagnostics: Extra inputs are not permitted" — new top-level diagnostics request field rejected (same class as #37746 output_config)

Environment

• Claude Code v2.1.220 (autoUpdatesChannel: latest), macOS (Darwin 25.5)
• CLAUDE_CODE_USE_VERTEX=1, CLOUD_ML_REGION=global
• Model: claude-fable-5 (session also had earlier claude-opus-5 turns)

What happens

Mid-session, every /v1/messages request starts failing with:

API Error: 400 diagnostics: Extra inputs are not permitted

The Pydantic error path is the bare top-level field (diagnostics, not messages.N.diagnostics), i.e. Vertex is rejecting a top-level diagnostics field in the request body. Once it starts the session is unusable: the main loop 400s on every turn, and Workflow-tool subagents die with the same error mid-run (2 of 6 agents in one workflow). Retrying / "continue" does not recover.

Evidence it is the client-side diagnostics field, feature-gated

• Affected sessions' transcripts stamp "diagnostics": null onto every stored assistant message (message.diagnostics, hundreds of occurrences). An unaffected session on the same machine, same v2.1.220, same model, same Vertex project has zero occurrences and works fine — so the field is gate-controlled per process, not version-tied.
• The field is already known to exist and carry previous_message_id from reports against the direct API, e.g. #76008 and #73776 (400 diagnostics.previous_message_id: must be the id from a prior /v1/messages response). The direct API recognizes the field and validates its contents; Vertex rejects it outright as an unknown key.

Precedent — same bug class, previously fixed for a different field

• #37746: Vertex rejecting output_config with the identical "Extra inputs are not permitted" error
• CHANGELOG v2.1.122: "Fixed Vertex AI / Bedrock returning invalid_request_error: output_config: Extra inputs are not permitted on session-title generation and other structured-output queries"
• CHANGELOG v2.1.113: similar fix for CLAUDE_CODE_EXTRA_BODY output_config.effort on Vertex

Whatever strips first-party-only request fields for Vertex/Bedrock does not cover diagnostics.

Expected

Omit diagnostics on Vertex/Bedrock requests (as was done for output_config), or disable the gated feature when CLAUDE_CODE_USE_VERTEX/CLAUDE_CODE_USE_BEDROCK is set.