Summary

internal/brokercore/brokercore.go's PassthroughHeaders allowlist (Content-Type, Content-Encoding, Accept, Accept-Encoding, Accept-Language, User-Agent, Idempotency-Key, X-Request-Id) drops any other client-supplied request headers for credentialed (non-passthrough) services. This breaks provider APIs that require non-standard request headers — most notably Anthropic's anthropic-version header which is mandatory on every /v1/messages request.

Repro

agent-vault server -d
# attach api.anthropic.com via API-key auth, x-api-key header, ANTHROPIC_API_KEY credential
agent-vault run --vault default -- curl -s -X POST https://api.anthropic.com/v1/messages \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{"model":"claude-haiku-4-5","max_tokens":3,"messages":[{"role":"user","content":"hi"}]}'

Returns:

{"type":"error","error":{"type":"invalid_request_error","message":"anthropic-version: header is required"}}

The cred injection works (no 401), but anthropic-version was dropped during MITM rewriting.

Confirmed via debug log + source review — PassthroughHeaders doesn't include anthropic-version, so it's stripped in the agent → upstream hop.

Suggested fix

Add a per-service config field (e.g., extra_passthrough_headers: ["anthropic-version", "openai-organization", ...]) so service operators can extend the allowlist for the provider they're brokering for. The catalog templates could pre-fill known-required headers per provider.

Workarounds tried

• Sending Anthropic-Version (capitalized) — also dropped (same allowlist).
• Sending as X-Anthropic-Version — dropped.
• Sending as a query param — Anthropic API doesn't accept it that way.

Currently the only workaround is fork agent-vault and edit PassthroughHeaders, which violates the no-fork stance for adopters.

Context

I'm adopting agent-vault for a credential brokering layer alongside an egress-policy proxy at SourcePad. The cred-broker is working perfectly otherwise (v0.10.0 commit 2b8e020) — this header-stripping is the one rough edge that makes Anthropic-via-vault fail.

Happy to PR if you want a reviewer's hand on it.