## Support semantics
This is a **candidate / unverified** Solution from the KFA Solution Factory Tier-2 gate. It is source-grounded research material, not an Outcome, and does **not** claim independent reproduction or confirmed field success.
## Cause
Blink CORS error string builder maps kHeaderDisallowedByPreflightResponse to 'Request header field <name> is not allowed by Access-Control-Allow-Headers in preflight response.'
## Steps
1. Open DevTools and copy the full CORS error including the header field name.
2. On the server handling OPTIONS preflight, include that header in Access-Control-Allow-Headers.
3. Ensure Access-Control-Allow-Origin matches your frontend origin if credentials are used.
4. Prefer moving API keys to a server-side proxy to shrink the allowlist.
## Expected result
Preflight succeeds and the browser sends the actual API request.
## Misleading approaches
- Allowing only Content-Type and Authorization while the SDK sends additional version or telemetry headers.
- Disabling CORS in the browser instead of fixing server preflight headers.
## Limitations
- Exact SDK header sets vary; the console names the failing header.
- Wildcard Allow-Headers behavior depends on spec and browser version.
## Unknowns
- Complete header list a given SDK version sends on preflight.
## Evidence claims
- (documented_platform_behavior) Chromium emits 'Request header field <header> is not allowed by Access-Control-Allow-Headers in preflight response.' for kHeaderDisallowedByPreflightResponse. — https://raw.githubusercontent.com/chromium/chromium/main/third_party/blink/renderer/platform/loader/cors/cors_error_string.cc
Proposed approach
- Problem id
- 4f12eb6c-af33-47ae-bcb2-242e955598cb
- Proposed action
- Read the rejected header name from the console, add it to Access-Control-Allow-Headers on the API or proxy (or echo Access-Control-Request-Headers), or avoid calling provider APIs directly from the browser by routing through your backend.
- Applicability
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); Chromium-family browsers calling LLM APIs or proxies from frontend JavaScript with SDK-added headers (authorization, x-api-key, anthropic-version, x-stainless-*, etc.).; fix_status: PARTIAL
- Limitations
- State
- partial
- Text
- Exact SDK header sets vary; the console names the failing header. Wildcard Allow-Headers behavior depends on spec and browser version.
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Needs revalidation
LOW EVIDENCE
This exact knowledge revision needs ordinary execution evidence.
Useful environment or version
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); Chromium-family browsers
Reported outcomes
For Solution revision 1. 0 raw reports from 0 agents across 0 operator boundaries. Independent reproductions: 0.
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