Anthropic error-envelope parity matrix

This matrix compares nexus /anthropic/* error responses against live Anthropic Messages API behavior for the common failure set exercised by certification. Status codes and error.type values are the SDK-facing contract.

Condition	HTTP status	error.type	Nexus behavior	Anthropic reference	Notes
Missing/invalid nexus key	401	authentication_error	Matches	Matches	Accepts x-api-key or Authorization: Bearer
Missing max_tokens	400	invalid_request_error	Matches	Matches	Not defaulted on the native edge
Unknown model / route resolution failure	400	invalid_request_error	Matches	Matches	Registry-owned reject
Malformed JSON body	400	invalid_request_error	Matches	Matches
SDK echo fields on replayed blocks (citations, parsed_output, caller)	—	—	Stripped before upstream	N/A	Prevents upstream 400 on streamed tool follow-ups
Policy deny	403	permission_error	Matches	Matches	Includes x-nexus-request-id when scoped
Unknown /anthropic/* route	404	not_found_error	Matches	Matches	Fallback handler on the Anthropic router
Gateway rate limit	429	rate_limit_error	Matches	Matches	Includes Retry-After (delta-seconds, same as OpenAI edge)
Upstream 429	429	rate_limit_error	Matches	Matches	Preserved via UpstreamStatus
Upstream 529	529	overloaded_error	Matches	Matches	Preserved via UpstreamStatus
Upstream 400	400	invalid_request_error	Matches	Matches	Body message forwarded when JSON
Wallet / budget cap	400	invalid_request_error	Matches	Matches	Anthropic uses 400 for credit balance, not 402
Internal / all attempts failed	500	api_error	Matches	Matches	Includes x-nexus-request-id when scoped
Mid-stream upstream failure	200 SSE → event: error	api_error	Matches	Matches	In-band error event, not OpenAI [DONE]

Headers

Header	Success responses	Error responses	Notes
x-nexus-request-id	Present	Present when a request id is assigned (request-scoped errors)	Stable correlation id for analytics
anthropic-version (inbound)	Accepted, ignored at edge	Accepted	Upstream Anthropic calls set their own version
Retry-After	—	Present on gateway 429	Value is the retry window in whole seconds (string), rounded up
Upstream anthropic-ratelimit-*	Not forwarded	Not forwarded	Nexus emits its own rate-limit surface

Divergences

None unexplained for the matrix above. Upstream provider rate-limit headers are not forwarded because nexus terminates auth and applies its own limits before upstream calls complete.

Certification date: 2026-06-14. Pinned SDK versions: spec/anthropic-sdk-provenance.json.