fix(devserver): add x-bt-use-gateway to CORS allowed headers#119
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Curtis Galione (cjgalione) merged 1 commit intomainfrom Mar 23, 2026
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The Braintrust Playground sends x-bt-use-gateway when gateway routing is enabled. The api-ts CORS config already allows it, but the Python devserver's ALLOWED_HEADERS list was missing it, causing browsers to block preflight requests to remote eval servers with a CORS error.
Curtis Galione (cjgalione)
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Summary
x-bt-use-gatewaytoALLOWED_HEADERSin the Python devserver CORS config, matching whatapi-ts/src/cors.tsalready allowsContext
The Braintrust Playground sends
x-bt-use-gateway: truewhen gateway routing is enabled. The main data plane (api-ts) added this header to its CORS allowlist in Feb (commitee5f4f65e0), but the Python SDK devserver was never updated to match.This causes browsers to block preflight
OPTIONSrequests to any remote eval server built onbraintrust.devserver, with an error like:Multiple demo projects (
pydantic-supervisor,langgraph-supervisor,google-adk-supervisor) independently worked around this by monkey-patchingbt_cors.ALLOWED_HEADERSat startup. This fix addresses it at the source.Test plan
test_cors_preflight_allows_gateway_headersends an OPTIONS request withx-bt-use-gatewayand asserts it appears in the response'saccess-control-allow-headers