fix: inject Accept header in MCP middleware to prevent 406 errors

Some MCP clients (e.g., Claude Code) don't send an Accept header, causing the MCP SDK to reject requests with 406 Not Acceptable. The middleware now ensures Accept includes application/json and text/event-stream when missing. Co-Authored-By: Claude Opus 4.6 <[email protected]>
2026-03-13 12:17:27 -07:00
1 changed files with 9 additions and 0 deletions
@@ -381,6 +381,15 @@ class MCPMiddleware:
            # Clear root_path since we're passing directly to the app
            new_scope["root_path"] = ""

+            # Ensure Accept header includes required MIME types for MCP SDK.
+            # Some clients (e.g., Claude Code) don't send Accept, causing
+            # the SDK to reject with 406 Not Acceptable.
+            accept_header = self._get_header(new_scope, "accept")
+            if not accept_header or "text/event-stream" not in accept_header:
+                headers = [(k, v) for k, v in new_scope.get("headers", []) if k.lower() != b"accept"]
+                headers.append((b"accept", b"application/json, text/event-stream"))
+                new_scope["headers"] = headers
+
            # Wrap send to rewrite the SSE endpoint URL to include bank_id if using path-based routing.
            # Only rewrite SSE (text/event-stream) responses to avoid corrupting tool results
            # that might contain the literal string "data: /messages".