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

packages/server/test/server/contentTypeValidation.test.ts

@@ -0,0 +1,186 @@
+/**
+ * Content-Type validation at the HTTP entries — the parsed media type decides,
+ * never a substring search of the raw header.
+ *
+ * The shape pinned here: `Content-Type: text/plain; a=application/json`
+ * contains the substring `application/json`, but its media type is
+ * `text/plain` — every entry must answer it (and any other non-JSON media
+ * type) with 415 before the body is dispatched, while values whose media type
+ * is `application/json` keep working regardless of parameters or case.
+ */
+import { CLIENT_CAPABILITIES_META_KEY, CLIENT_INFO_META_KEY, PROTOCOL_VERSION_META_KEY } from '@modelcontextprotocol/core-internal';
+import { beforeEach, describe, expect, it } from 'vitest';
+import * as z from 'zod/v4';
+
+import { createMcpHandler } from '../../src/server/createMcpHandler';
+import { McpServer } from '../../src/server/mcp';
+import { WebStandardStreamableHTTPServerTransport } from '../../src/server/streamableHttp';
+
+const MODERN_REVISION = '2026-07-28';
+
+const executed: string[] = [];
+
+function factory(): McpServer {
+    const mcpServer = new McpServer({ name: 'ct-fixture', version: '1.0.0' });
+    mcpServer.registerTool('run', { description: 'records each dispatch', inputSchema: { cmd: z.string() } }, async ({ cmd }) => {
+        executed.push(cmd);
+        return { content: [{ type: 'text', text: `ran: ${cmd}` }] };
+    });
+    return mcpServer;
+}
+
+const LEGACY_CALL = {
+    jsonrpc: '2.0',
+    id: 1,
+    method: 'tools/call',
+    params: { name: 'run', arguments: { cmd: 'hello' } }
+};
+
+const MODERN_CALL = {
+    jsonrpc: '2.0',
+    id: 1,
+    method: 'tools/call',
+    params: {
+        name: 'run',
+        arguments: { cmd: 'hello' },
+        _meta: {
+            [PROTOCOL_VERSION_META_KEY]: MODERN_REVISION,
+            [CLIENT_INFO_META_KEY]: { name: 'ct-client', version: '1.0.0' },
+            [CLIENT_CAPABILITIES_META_KEY]: {}
+        }
+    }
+};
+
+function postRequest(body: unknown, headers: Record<string, string>): Request {
+    return new Request('http://127.0.0.1/mcp', {
+        method: 'POST',
+        headers: {
+            Accept: 'application/json, text/event-stream',
+            ...headers
+        },
+        body: JSON.stringify(body)
+    });
+}
+
+beforeEach(() => {
+    executed.length = 0;
+});
+
+describe('createMcpHandler Content-Type validation (default options, legacy stateless fallback active)', () => {
+    it('serves an application/json POST (control)', async () => {
+        const handler = createMcpHandler(factory);
+        const response = await handler.fetch(postRequest(LEGACY_CALL, { 'Content-Type': 'application/json' }));
+        expect(response.status).toBe(200);
+        expect(executed).toEqual(['hello']);
+    });
+
+    it('accepts application/json with parameters', async () => {
+        const handler = createMcpHandler(factory);
+        const response = await handler.fetch(postRequest(LEGACY_CALL, { 'Content-Type': 'application/json; charset=utf-8' }));
+        expect(response.status).toBe(200);
+        expect(executed).toEqual(['hello']);
+    });
+
+    it('rejects text/plain with 415', async () => {
+        const handler = createMcpHandler(factory);
+        const response = await handler.fetch(postRequest(LEGACY_CALL, { 'Content-Type': 'text/plain' }));
+        expect(response.status).toBe(415);
+        expect(executed).toEqual([]);
+    });
+
+    it('rejects a non-JSON media type whose parameters contain `application/json` and does not dispatch', async () => {
+        const handler = createMcpHandler(factory);
+        const response = await handler.fetch(postRequest(LEGACY_CALL, { 'Content-Type': 'text/plain; a=application/json' }));
+        expect(response.status).toBe(415);
+        const body = (await response.json()) as { error: { code: number; message: string } };
+        expect(body.error.code).toBe(-32_000);
+        expect(body.error.message).toContain('Content-Type must be application/json');
+        expect(executed).toEqual([]);
+    });
+
+    it('rejects a POST with no Content-Type header with 415', async () => {
+        const handler = createMcpHandler(factory);
+        // A string body makes Request auto-attach `text/plain;charset=UTF-8`;
+        // delete it so this actually exercises the absent-header branch.
+        const request = postRequest(LEGACY_CALL, {});
+        request.headers.delete('content-type');
+        expect(request.headers.get('content-type')).toBeNull();
+        const response = await handler.fetch(request);
+        expect(response.status).toBe(415);
+        expect(executed).toEqual([]);
+    });
+
+    it('accepts an unambiguous media type with a malformed parameter section (trailing semicolon)', async () => {
+        const handler = createMcpHandler(factory);
+        const response = await handler.fetch(postRequest(LEGACY_CALL, { 'Content-Type': 'application/json;' }));
+        expect(response.status).toBe(200);
+        expect(executed).toEqual(['hello']);
+    });
+
+    it('rejects joined duplicate Content-Type headers', async () => {
+        const handler = createMcpHandler(factory);
+        const response = await handler.fetch(postRequest(LEGACY_CALL, { 'Content-Type': 'application/json, application/json' }));
+        expect(response.status).toBe(415);
+        expect(executed).toEqual([]);
+    });
+
+    it('validates the modern leg too: enveloped request with a non-JSON media type is answered 415 before dispatch', async () => {
+        const handler = createMcpHandler(factory);
+        const response = await handler.fetch(
+            postRequest(MODERN_CALL, {
+                'Content-Type': 'text/plain; a=application/json',
+                'mcp-protocol-version': MODERN_REVISION,
+                'Mcp-Method': 'tools/call',
+                'Mcp-Name': 'run'
+            })
+        );
+        expect(response.status).toBe(415);
+        expect(executed).toEqual([]);
+    });
+
+    it('still serves the modern leg for application/json (control)', async () => {
+        const handler = createMcpHandler(factory);
+        const response = await handler.fetch(
+            postRequest(MODERN_CALL, {
+                'Content-Type': 'application/json',
+                'mcp-protocol-version': MODERN_REVISION,
+                'Mcp-Method': 'tools/call',
+                'Mcp-Name': 'run'
+            })
+        );
+        expect(response.status).toBe(200);
+        expect(executed).toEqual(['hello']);
+    });
+
+    it('does not apply the POST check to bodyless methods', async () => {
+        const handler = createMcpHandler(factory);
+        const response = await handler.fetch(new Request('http://127.0.0.1/mcp', { method: 'DELETE' }));
+        expect(response.status).not.toBe(415);
+    });
+});
+
+describe('WebStandardStreamableHTTPServerTransport Content-Type validation (direct wiring)', () => {
+    async function postToTransport(headers: Record<string, string>): Promise<Response> {
+        const server = factory();
+        const transport = new WebStandardStreamableHTTPServerTransport({
+            sessionIdGenerator: undefined,
+            enableJsonResponse: true
+        });
+        await server.connect(transport);
+        const response = await transport.handleRequest(postRequest(LEGACY_CALL, headers));
+        await server.close();
+        return response;
+    }
+
+    it('rejects a non-JSON media type whose parameters contain `application/json` and does not dispatch', async () => {
+        const response = await postToTransport({ 'Content-Type': 'text/plain; a=application/json' });
+        expect(response.status).toBe(415);
+        expect(executed).toEqual([]);
+    });
+
+    it('serves application/json (control)', async () => {
+        const response = await postToTransport({ 'Content-Type': 'application/json' });
+        expect(response.status).toBe(200);
+        expect(executed).toEqual(['hello']);
+    });
+});