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

.changeset/content-type-media-type-validation.md

@@ -0,0 +1,23 @@
+---
+'@modelcontextprotocol/server': patch
+'@modelcontextprotocol/client': patch
+'@modelcontextprotocol/hono': patch
+'@modelcontextprotocol/node': patch
+---
+
+POSTs whose `Content-Type` media type is not `application/json` are now
+rejected with `415 Unsupported Media Type`; the header is parsed instead of
+substring-matched. Previously any value merely containing the substring
+passed the check (for example `text/plain; a=application/json`), case
+variants were wrongly rejected, and the 2026-07-28 entry did not inspect
+`Content-Type` at all — requests with a missing or non-JSON header that used
+to be served on that path now also answer 415. Values with parameters
+(`application/json; charset=utf-8`, including malformed parameter sections
+like `application/json;`) continue to work. SDK clients always send the
+correct header and are unaffected.
+
+The new `isJsonContentType(header)` helper is exported for transport and
+framework-adapter authors — custom entries composing the exported building
+blocks (`classifyInboundRequest`, `PerRequestHTTPServerTransport`) must apply
+it themselves. The hono adapter's JSON body pre-parse and the client's
+response dispatch now use the same parsed-media-type comparison.

common/eslint-config/eslint.config.mjs

@@ -47,6 +47,18 @@ export default defineConfig(
             'unicorn/prevent-abbreviations': 'off',
             'unicorn/no-null': 'off',
             'unicorn/prefer-add-event-listener': 'off',
+            'no-restricted-syntax': [
+                'error',
+                {
+                    selector:
+                        ":matches(CallExpression[callee.property.name='includes'], CallExpression[callee.property.name='indexOf'], " +
+                        "CallExpression[callee.property.name='startsWith'])[arguments.0.value='application/json']",
+                    message:
+                        "Substring-matching 'application/json' misclassifies Content-Type values whose media type is different " +
+                        "(e.g. 'text/plain; a=application/json') and mishandles parameters and case. " +
+                        'Parse the media type instead: isJsonContentType() from core-internal.'
+                }
+            ],
             'unicorn/no-useless-undefined': ['error', { checkArguments: false }],
             '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_' }],
             'n/prefer-node-protocol': 'error',

docs/migration/upgrade-to-v2.md

@@ -817,6 +817,18 @@ same as every released v1.x — only the import path changes. Framework-agnostic
 (`validateHostHeader`, `localhostAllowedHostnames`, `hostHeaderValidationResponse`) are
 in `@modelcontextprotocol/server`.
 
+Server entries validate the request `Content-Type` by its **parsed media type**, not a
+substring: every POST whose media type is not `application/json` answers
+`415 Unsupported Media Type`. Previously any value merely containing the substring
+passed (for example `text/plain; a=application/json`), case variants were wrongly
+rejected, and the 2026-07-28 entry did not inspect `Content-Type` at all — so
+hand-rolled clients that omit the header (or send a non-JSON type) must now set
+`Content-Type: application/json`. Parameters (`; charset=utf-8`) and unambiguous
+values with malformed parameter sections (`application/json;`) keep working; SDK
+clients always sent the correct header and are unaffected. Custom entries that
+compose `classifyInboundRequest` / `PerRequestHTTPServerTransport` directly must
+apply the same validation themselves — use the exported `isJsonContentType(header)`.
+
 ### Errors
 
 The SDK now distinguishes three error kinds:

examples/json-response/client.ts

@@ -8,7 +8,7 @@
  * the stateless legacy fallback unaffected.
  */
 import { check, parseExampleArgs } from '@mcp-examples/shared';
-import { Client, StreamableHTTPClientTransport } from '@modelcontextprotocol/client';
+import { Client, isJsonContentType, StreamableHTTPClientTransport } from '@modelcontextprotocol/client';
 
 const { url } = parseExampleArgs();
 
@@ -39,7 +39,7 @@ const probe = await fetch(url, {
         }
     })
 });
-check.match(probe.headers.get('content-type') ?? '', /application\/json/);
+check.equal(isJsonContentType(probe.headers.get('content-type')), true);
 check.equal(probe.status, 200);
 
 // High-level: the regular Client works unchanged.

packages/client/src/client/streamableHttp.ts

@@ -10,6 +10,7 @@ import {
     isJSONRPCResultResponse,
     isModernProtocolVersion,
     JSONRPCMessageSchema,
+    mediaTypeEssence,
     normalizeHeaders,
     PROTOCOL_VERSION_META_KEY,
     SdkError,
@@ -1080,11 +1081,12 @@ export class StreamableHTTPClientTransport implements Transport {
 
             const hasRequests = messages.some(msg => 'method' in msg && 'id' in msg && msg.id !== undefined);
 
-            // Check the response type
+            // Check the response type (parsed media type — see mediaTypeEssence)
             const contentType = response.headers.get('content-type');
+            const responseMediaType = mediaTypeEssence(contentType);
 
             if (hasRequests) {
-                if (contentType?.includes('text/event-stream')) {
+                if (responseMediaType === 'text/event-stream') {
                     // Handle SSE stream responses for requests
                     // We use the same handler as standalone streams, which now supports
                     // reconnection with the last event ID
@@ -1097,7 +1099,7 @@ export class StreamableHTTPClientTransport implements Transport {
                         },
                         false
                     );
-                } else if (contentType?.includes('application/json')) {
+                } else if (responseMediaType === 'application/json') {
                     // For non-streaming servers, we might get direct JSON responses
                     const data = await response.json();
                     const responseMessages = Array.isArray(data)

packages/client/test/client/middleware.test.ts

@@ -1,4 +1,5 @@
 import type { FetchLike } from '@modelcontextprotocol/core-internal';
+import { isJsonContentType } from '@modelcontextprotocol/core-internal';
 import type { Mocked, MockedFunction, MockInstance } from 'vitest';
 
 import type { OAuthClientProvider } from '../../src/client/auth';
@@ -1016,7 +1017,7 @@ describe('createMiddleware', () => {
         const transformMiddleware = createMiddleware(async (next, input, init) => {
             const response = await next(input, init);
 
-            if (response.headers.get('content-type')?.includes('application/json')) {
+            if (isJsonContentType(response.headers.get('content-type'))) {
                 const data = (await response.json()) as Record<string, unknown>;
                 const transformed = { ...data, timestamp: 123_456_789 };
 

packages/core-internal/package.json

@@ -51,6 +51,7 @@
         "test:watch": "vitest"
     },
     "dependencies": {
+        "content-type": "catalog:runtimeShared",
         "json-schema-typed": "catalog:runtimeShared",
         "zod": "catalog:runtimeShared"
     },

packages/core-internal/src/exports/public/index.ts

@@ -39,6 +39,9 @@ export type {
 // Auth utilities
 export { checkResourceAllowed, resourceUrlFromServerUrl } from '../../shared/authUtils';
 
+// Media-type utilities (for transport and framework-adapter authors)
+export { isJsonContentType } from '../../shared/mediaType';
+
 // Metadata utilities
 export { getDisplayName } from '../../shared/metadataUtils';
 

packages/core-internal/src/index.ts

@@ -10,6 +10,7 @@ export * from './shared/inputRequired';
 export * from './shared/inputRequiredDriver';
 export * from './shared/inputRequiredEngine';
 export * from './shared/mcpParamHeaders';
+export * from './shared/mediaType';
 export * from './shared/metadataUtils';
 export * from './shared/protocol';
 export * from './shared/protocolEras';

packages/core-internal/src/shared/mediaType.ts

@@ -0,0 +1,57 @@
+import contentType from 'content-type';
+
+/**
+ * Extracts the media type (the lowercased `type/subtype` pair, without
+ * parameters) from a raw `Content-Type` header value, or `undefined` when the
+ * header is missing or empty.
+ *
+ * Content-Type comparisons must use the parsed media type, never a substring
+ * search of the raw header: a value like `text/plain; a=application/json`
+ * contains the substring `application/json` but its media type is
+ * `text/plain`, and case variants or parameters make naive string comparison
+ * wrong in both directions.
+ *
+ * "Essence" is the WHATWG MIME Sniffing standard's term for the bare
+ * `type/subtype` pair (https://mimesniff.spec.whatwg.org/#mime-type-essence);
+ * the Fetch standard's request classification is defined against it
+ * (https://fetch.spec.whatwg.org/#cors-safelisted-request-header).
+ *
+ * Parsing is RFC 9110 (`content-type` package) first. When the parameter
+ * section is malformed (`application/json;`, `application/json; charset=`),
+ * browsers and most HTTP stacks still derive the media type from the segment
+ * before the first `;` — the fallback matches that widely-implemented
+ * behavior, so a header whose media type is unambiguous is not rejected for
+ * a sloppy parameter section.
+ */
+export function mediaTypeEssence(header: string | null | undefined): string | undefined {
+    if (!header) {
+        return undefined;
+    }
+    try {
+        return contentType.parse(header).type;
+    } catch {
+        const essence = (header.split(';', 1)[0] ?? '').trim().toLowerCase();
+        // A comma in the parameter tail of an unparseable value indicates
+        // joined duplicate headers — ambiguous, so no essence at all (keeps
+        // duplicate-header handling uniform whether or not the first copy
+        // carries parameters).
+        if (essence === '' || header.slice(essence.length).includes(',')) {
+            return undefined;
+        }
+        return essence;
+    }
+}
+
+/**
+ * Whether a raw `Content-Type` header value denotes `application/json`.
+ * Parameters (for example `charset=utf-8`) are allowed and ignored; malformed
+ * parameter sections do not reject a header whose media type is unambiguously
+ * `application/json` (see `mediaTypeEssence` for the exact grammar).
+ */
+export function isJsonContentType(header: string | null | undefined): boolean {
+    // Fast path: the exact literal is what SDK clients send on every POST.
+    if (header === 'application/json') {
+        return true;
+    }
+    return mediaTypeEssence(header) === 'application/json';
+}

packages/core-internal/test/shared/mediaType.test.ts

@@ -0,0 +1,68 @@
+import { describe, expect, it } from 'vitest';
+
+import { isJsonContentType, mediaTypeEssence } from '../../src/shared/mediaType';
+
+describe('mediaTypeEssence', () => {
+    it('parses well-formed headers', () => {
+        expect(mediaTypeEssence('application/json')).toBe('application/json');
+        expect(mediaTypeEssence('text/event-stream; charset=utf-8')).toBe('text/event-stream');
+        expect(mediaTypeEssence('Application/JSON; charset=utf-8')).toBe('application/json');
+    });
+
+    it('falls back to the pre-parameter segment for malformed parameter sections', () => {
+        expect(mediaTypeEssence('application/json;')).toBe('application/json');
+        expect(mediaTypeEssence('application/json; charset=')).toBe('application/json');
+        expect(mediaTypeEssence('text/plain;')).toBe('text/plain');
+    });
+
+    it('returns undefined for missing or empty headers', () => {
+        expect(mediaTypeEssence(null)).toBeUndefined();
+        expect(mediaTypeEssence(undefined)).toBeUndefined();
+        expect(mediaTypeEssence('')).toBeUndefined();
+        expect(mediaTypeEssence('   ')).toBeUndefined();
+        expect(mediaTypeEssence(';charset=utf-8')).toBeUndefined();
+    });
+
+    it('yields no essence for joined duplicate headers, with or without parameters', () => {
+        // Headers.get() joins repeated headers with ', '. Without parameters
+        // the comma lands in the first segment; with parameters it hides in
+        // the tail — both must behave the same.
+        expect(mediaTypeEssence('application/json, application/json')).toBe('application/json, application/json');
+        expect(mediaTypeEssence('application/json; charset=utf-8, text/plain')).toBeUndefined();
+        expect(mediaTypeEssence('application/json; charset=utf-8, application/json')).toBeUndefined();
+    });
+});
+
+describe('isJsonContentType', () => {
+    it('accepts application/json with or without parameters', () => {
+        expect(isJsonContentType('application/json')).toBe(true);
+        expect(isJsonContentType('application/json; charset=utf-8')).toBe(true);
+        expect(isJsonContentType('Application/JSON')).toBe(true);
+        expect(isJsonContentType('  application/json ; charset=utf-8')).toBe(true);
+    });
+
+    it('accepts unambiguous media types with malformed parameter sections', () => {
+        expect(isJsonContentType('application/json;')).toBe(true);
+        expect(isJsonContentType('application/json; charset=')).toBe(true);
+        expect(isJsonContentType('application/json; charset=utf-8; charset=x')).toBe(true);
+    });
+
+    it('never matches on substrings: parameters and sibling types are not application/json', () => {
+        expect(isJsonContentType('text/plain; a=application/json')).toBe(false);
+        expect(isJsonContentType('text/plain;')).toBe(false);
+        expect(isJsonContentType('text/plain, application/json')).toBe(false);
+        expect(isJsonContentType('application/json, application/json')).toBe(false);
+        expect(isJsonContentType('application/json; charset=utf-8, text/plain')).toBe(false);
+        expect(isJsonContentType('text/plain; charset=utf-8, application/json')).toBe(false);
+        expect(isJsonContentType('application/json-patch+json')).toBe(false);
+        expect(isJsonContentType('application/jsonp')).toBe(false);
+    });
+
+    it('rejects missing, empty, and non-JSON types', () => {
+        expect(isJsonContentType(null)).toBe(false);
+        expect(isJsonContentType(undefined)).toBe(false);
+        expect(isJsonContentType('')).toBe(false);
+        expect(isJsonContentType('text/plain')).toBe(false);
+        expect(isJsonContentType('multipart/form-data')).toBe(false);
+    });
+});

packages/middleware/hono/src/hono.ts

@@ -1,3 +1,4 @@
+import { isJsonContentType } from '@modelcontextprotocol/server';
 import type { Context } from 'hono';
 import { Hono } from 'hono';
 
@@ -45,8 +46,8 @@ export interface CreateMcpHonoAppOptions {
  * DNS rebinding attacks on localhost servers.
  *
  * This also installs a small JSON body parsing middleware (similar to `express.json()`)
- * that stashes the parsed body into `c.set('parsedBody', ...)` when `Content-Type` includes
- * `application/json`.
+ * that stashes the parsed body into `c.set('parsedBody', ...)` when the `Content-Type`
+ * media type is `application/json`.
  *
  * @param options - Configuration options
  * @returns A configured Hono application
@@ -63,8 +64,10 @@ export function createMcpHonoApp(options: CreateMcpHonoAppOptions = {}): Hono {
             return await next();
         }
 
-        const ct = c.req.header('content-type') ?? '';
-        if (!ct.includes('application/json')) {
+        // Parsed media type, never a substring match — see isJsonContentType.
+        // A body left unparsed here is answered 415 by the transport's own
+        // Content-Type check downstream.
+        if (!isJsonContentType(c.req.header('content-type'))) {
             return await next();
         }
 

packages/middleware/hono/test/hono.test.ts

@@ -90,6 +90,34 @@ describe('@modelcontextprotocol/hono', () => {
         expect(await res.text()).toBe('Invalid JSON');
     });
 
+    test('createMcpHonoApp does not parse a non-JSON media type whose parameters contain application/json', async () => {
+        const app = createMcpHonoApp();
+        app.post('/echo', (c: Context) => c.json({ parsed: c.get('parsedBody') ?? null }));
+
+        // `text/plain; a=application/json` contains the substring but its media
+        // type is text/plain — it must never be treated as a JSON body.
+        const res = await app.request('http://localhost/echo', {
+            method: 'POST',
+            headers: { Host: 'localhost:3000', 'content-type': 'text/plain; a=application/json' },
+            body: JSON.stringify({ a: 1 })
+        });
+        expect(res.status).toBe(200);
+        expect(await res.json()).toEqual({ parsed: null });
+    });
+
+    test('createMcpHonoApp parses application/json with parameters', async () => {
+        const app = createMcpHonoApp();
+        app.post('/echo', (c: Context) => c.json(c.get('parsedBody')));
+
+        const res = await app.request('http://localhost/echo', {
+            method: 'POST',
+            headers: { Host: 'localhost:3000', 'content-type': 'application/json; charset=utf-8' },
+            body: JSON.stringify({ a: 1 })
+        });
+        expect(res.status).toBe(200);
+        expect(await res.json()).toEqual({ a: 1 });
+    });
+
     test('createMcpHonoApp does not override parsedBody if upstream middleware set it', async () => {
         const app = createMcpHonoApp();
         app.use('/echo', async (c: Context, next) => {

packages/server-legacy/package.json

@@ -85,7 +85,7 @@
     "dependencies": {
         "zod": "catalog:runtimeShared",
         "raw-body": "catalog:runtimeServerOnly",
-        "content-type": "catalog:runtimeServerOnly",
+        "content-type": "catalog:runtimeShared",
         "cors": "catalog:runtimeServerOnly",
         "express-rate-limit": "^8.2.1",
         "pkce-challenge": "catalog:runtimeShared"