fix: prevent duplicate Authorization header in SSEClientTransport#2087
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Christian-Sidak wants to merge 1 commit into
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fix: prevent duplicate Authorization header in SSEClientTransport#2087Christian-Sidak wants to merge 1 commit into
Christian-Sidak wants to merge 1 commit into
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When both `requestInit.headers` and `eventSourceInit.fetch` provided the same `Authorization` header, the SDK's internal SSE wrapper passed a `Headers` instance (which lowercases all keys) to the user's fetch. If that fetch spread the headers into a plain object and then overlaid its own closure headers (with original casing), the result had both `authorization` and `Authorization` as distinct keys. `new Headers()` joins them as "Bearer X, Bearer X", causing strict servers to reject the request with 401. Fix: use `opts.fetch ?? globalThis.fetch` (not `eventSourceInit.fetch`) as the underlying transport inside the SDK's internal SSE fetch wrapper. Users who relied on `eventSourceInit.fetch` for header injection should migrate to the top-level `fetch` option. Fixes modelcontextprotocol#1872 Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Summary
requestInit.headersandeventSourceInit.fetchwith the sameAuthorizationheader caused the server to receiveBearer X, Bearer X(comma-joined duplicate) instead of a singleBearer Xvalue._startOrAuth()usedeventSourceInit.fetchas the underlying HTTP transport inside the SDK's internal SSE wrapper. The wrapper passed aHeadersinstance (which normalizes keys to lowercase) to the user's fetch. If the user's fetch spread those headers into a plain object and overlaid its own closure headers (with original casing), the result had bothauthorizationandAuthorizationas separate keys;new Headers()joins them as"X, X"per the HTTP multi-value spec.fetchImplfromthis._eventSourceInit?.fetch ?? this._fetch ?? fetchtothis._fetch ?? fetch. The SDK's wrapper fully replaceseventSourceInit.fetchfor the EventSource library, sofetchImplshould be the raw transport (opts.fetchorglobalThis.fetch), not the user's header-injecting wrapper.Test plan
does not duplicate Authorization header when requestInit.headers and eventSourceInit.fetch both set itthat reproduces the exact user pattern from the bug report (spreadHeadersthen overlay plain-object closure headers).uses custom fetch implementation from EventSourceInit to add auth headersto use the top-levelfetchoption (the correct post-fix migration path).pnpm --filter @modelcontextprotocol/client test).Fixes #1872