fix(server): validate Content-Type by parsed media type instead of substring match (#2441)

packages/server/src/server/createMcpHandler.ts

@@ -41,6 +41,8 @@ import {
     CLIENT_CAPABILITIES_META_KEY,
     CLIENT_INFO_META_KEY,
     httpStatusForErrorCode,
+    isJsonContentType,
+    mediaTypeEssence,
     missingClientCapabilities,
     MissingRequiredClientCapabilityError,
     modernOnlyStrictRejection,
@@ -330,7 +332,7 @@ export function legacyStatelessFallback(factory: McpServerFactory, onerror?: (er
                 ...(options?.authInfo !== undefined && { authInfo: options.authInfo }),
                 ...(options?.parsedBody !== undefined && { parsedBody: options.parsedBody })
             });
-            if (response.body === null || !(response.headers.get('content-type') ?? '').includes('text/event-stream')) {
+            if (response.body === null || mediaTypeEssence(response.headers.get('content-type')) !== 'text/event-stream') {
                 // Non-streaming exchange (a buffered JSON body or a body-less
                 // ack): the response is complete, release the pair now.
                 teardown();
@@ -483,7 +485,11 @@ async function classifyEntryRequest(request: Request, providedParsedBody?: unkno
  * This is the entry's own classification step exported as a predicate — it
  * runs exactly the code `createMcpHandler` runs to make the routing decision,
  * not a re-implementation — so a hand-wired composition that branches on it
- * can never disagree with the entry. Use it to keep an existing legacy
+ * can never disagree with the entry. It is classification only: hand-wired
+ * compositions must validate Content-Type themselves (415 for POSTs whose
+ * media type is not `application/json`, via {@linkcode isJsonContentType})
+ * before dispatching either leg — routing the legacy leg into the SDK
+ * transports gets their built-in check, but a custom modern leg has none. Use it to keep an existing legacy
  * deployment (for example a sessionful streamable HTTP wiring) serving 2025
  * traffic next to a strict modern endpoint, now that the entry has no
  * handler-valued `legacy` option:
@@ -574,7 +580,10 @@ export async function isLegacyRequest(request: Request, parsedBody?: unknown): P
  * pattern. Power users composing transport-neutral routing can also use the
  * exported building blocks directly: {@linkcode classifyInboundRequest} for
  * the era decision and `PerRequestHTTPServerTransport` for single-exchange
- * serving.
+ * serving — such compositions must reject POSTs whose Content-Type media type
+ * is not `application/json` (415) before parsing the body, using
+ * {@linkcode isJsonContentType}; neither building block performs this
+ * validation itself.
  *
  * The entry performs no token verification: `authInfo` given to `fetch` is
  * passed through to handlers and the factory as-is and is never derived from
@@ -817,6 +826,18 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
 
     async function handle(request: Request, requestOptions?: McpHandlerRequestOptions): Promise<Response> {
         const authInfo = requestOptions?.authInfo;
+
+        // Content-Type check, answered before the body is read (parsed media
+        // type — see isJsonContentType). Load-bearing for the modern leg,
+        // whose ladder does not inspect Content-Type; the legacy transport
+        // keeps its own check for hand-wired use, so via this entry a
+        // doubly-invalid request answers 415 here before the transport's 406
+        // Accept check would.
+        if (request.method.toUpperCase() === 'POST' && !isJsonContentType(request.headers.get('content-type'))) {
+            reportError(new Error('Unsupported Media Type: Content-Type must be application/json'));
+            return jsonRpcErrorResponse(415, -32_000, 'Unsupported Media Type: Content-Type must be application/json');
+        }
+
         const classified = await classifyEntryRequest(request, requestOptions?.parsedBody);
 
         if (classified.step === 'unreadable-body') {

packages/server/src/server/perRequestTransport.ts

@@ -32,6 +32,10 @@
  * `createMcpHandler`) inherit the spec's per-error HTTP mandate with it: a
  * terminal `MissingRequiredClientCapabilityError` (-32021) MUST answer 400,
  * which this transport applies whenever the response is still uncommitted.
+ * A custom entry must also reject POSTs whose Content-Type media type is not
+ * `application/json` (415) before parsing the body — use `isJsonContentType`
+ * from the package root; this transport never sees the HTTP headers, so it
+ * cannot perform that validation itself.
  */
 import type {
     AuthInfo,

packages/server/src/server/streamableHttp.ts

@@ -11,6 +11,7 @@ import type { AuthInfo, JSONRPCMessage, MessageExtraInfo, RequestId, Transport }
 import {
     DEFAULT_NEGOTIATED_PROTOCOL_VERSION,
     isInitializeRequest,
+    isJsonContentType,
     isJSONRPCErrorResponse,
     isJSONRPCRequest,
     isJSONRPCResultResponse,
@@ -680,6 +681,8 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             // Validate the Accept header
             const acceptHeader = req.headers.get('accept');
             // The client MUST include an Accept header, listing both application/json and text/event-stream as supported content types.
+            // Accept is a comma-separated list, so a substring check is the intended semantics here (unlike Content-Type below).
+            // eslint-disable-next-line no-restricted-syntax
             if (!acceptHeader?.includes('application/json') || !acceptHeader.includes('text/event-stream')) {
                 this.onerror?.(new Error('Not Acceptable: Client must accept both application/json and text/event-stream'));
                 return this.createJsonErrorResponse(
@@ -689,8 +692,12 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
                 );
             }
 
+            // Parsed media type, never a substring match — see
+            // isJsonContentType. This check stays here for hand-wired
+            // transports; via createMcpHandler the entry's own check answers
+            // first.
             const ct = req.headers.get('content-type');
-            if (!ct || !ct.includes('application/json')) {
+            if (!isJsonContentType(ct)) {
                 this.onerror?.(new Error('Unsupported Media Type: Content-Type must be application/json'));
                 return this.createJsonErrorResponse(415, -32_000, 'Unsupported Media Type: Content-Type must be application/json');
             }