feat: Add CORS configuration for browser-based MCP clients #713
+54 −14
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- Add cors middleware to example servers with Mcp-Session-Id exposed - Add CORS documentation section to README - Configure minimal CORS settings (only expose required headers) This enables browser-based clients to connect to MCP servers by properly exposing the Mcp-Session-Id header required for session management. Reported-by: Jerome
jerome3o-anthropic force-pushed the jerome/cors-browser-support branch from 6c23ed6 to bda811a Compare 
felixweinberger requested review from felixweinberger and removed request for felixweinberger 
ihrpr requested changes Jul 7, 2025
| @@ -584,6 +584,26 @@ app.listen(3000); | |||
| > ); | |||
| > ``` | |||
| | |||
| | |||
| #### CORS Configuration for Browser-Based Clients | |||
| | |||
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Ah up - that's just above this, I've merged them
| @@ -96,6 +97,12 @@ const getServer = () => { | |||
| const app = express(); | |||
| app.use(express.json()); | |||
| | |||
| // Configure CORS to expose Mcp-Session-Id header for browser-based clients | |||
| app.use(cors({ | |||
| origin: '*', // Allow all origins - adjust as needed for production | |||
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I'd probably add a very big "WARNING" here not to include "*"
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If your MCP server will be accessible by browsers, then you might actually want this
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Summary
Mcp-Session-IdheaderProblem
Browser-based MCP clients cannot access the
Mcp-Session-Idheader from initialization responses due to CORS restrictions. Without this header, they cannot establish sessions with MCP servers.Solution
This PR adds the
corsnpm package to example servers and configures it to expose theMcp-Session-Idheader viaAccess-Control-Expose-Headers. The configuration is minimal, only exposing what's necessary for MCP protocol operation.Changes
corsimport and middleware to:sseAndStreamableHttpCompatibleServer.tssimpleStatelessStreamableHttp.tsjsonResponseStreamableHttp.tsTest plan
Mcp-Session-Idheader from responsesReported-by: Jerome