fix: send HTTP DELETE to terminate server sessions after each scenario

src/connection/sdk-client.test.ts

@@ -0,0 +1,77 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import express from 'express';
+import type { Server as HttpServer } from 'http';
+import { connectToServer } from './sdk-client';
+
+// Regression guard for the session-termination behavior of close(): a bad
+// merge of connectToServer must not silently drop the DELETE (issue #79).
+describe('connectToServer close()', () => {
+  let httpServer: HttpServer | undefined;
+
+  afterEach(
+    () =>
+      new Promise<void>((resolve) => {
+        if (!httpServer) return resolve();
+        httpServer.closeAllConnections?.();
+        httpServer.close(() => resolve());
+        httpServer = undefined;
+      })
+  );
+
+  function startServer(sessionId?: string): Promise<{
+    url: string;
+    deletes: Array<string | undefined>;
+  }> {
+    const deletes: Array<string | undefined> = [];
+    const app = express();
+    app.use(express.json());
+    app.post('/mcp', (req, res) => {
+      if (req.body?.method === 'initialize') {
+        if (sessionId) res.set('mcp-session-id', sessionId);
+        res.json({
+          jsonrpc: '2.0',
+          id: req.body.id,
+          result: {
+            protocolVersion: '2025-11-25',
+            capabilities: {},
+            serverInfo: { name: 'delete-probe', version: '1.0.0' }
+          }
+        });
+      } else {
+        res.status(202).end();
+      }
+    });
+    app.delete('/mcp', (req, res) => {
+      deletes.push(req.headers['mcp-session-id'] as string | undefined);
+      res.status(200).end();
+    });
+    return new Promise((resolve, reject) => {
+      const server = app.listen(0);
+      httpServer = server;
+      server.on('error', reject);
+      server.on('listening', () => {
+        const addr = server.address();
+        const port = typeof addr === 'object' && addr ? addr.port : 0;
+        resolve({ url: `http://localhost:${port}/mcp`, deletes });
+      });
+    });
+  }
+
+  it('sends an HTTP DELETE with the session ID to terminate the session', async () => {
+    const { url, deletes } = await startServer('session-abc-123');
+
+    const connection = await connectToServer(url, {}, '2025-11-25');
+    await connection.close();
+
+    expect(deletes).toEqual(['session-abc-123']);
+  });
+
+  it('does not send DELETE when the server assigned no session ID', async () => {
+    const { url, deletes } = await startServer();
+
+    const connection = await connectToServer(url, {}, '2025-11-25');
+    await connection.close();
+
+    expect(deletes).toEqual([]);
+  });
+});

src/connection/sdk-client.ts

@@ -121,6 +121,11 @@ function instrumentTransport(
  * conformant `initialize`. The transport is instrumented so every wire
  * message in both directions is validated against `specVersion`'s spec
  * JSON schema; scenarios that call this directly pass `ctx.specVersion`.
+ *
+ * The returned `close()` sends an HTTP DELETE to terminate the server-side
+ * session (per the Streamable HTTP transport spec) before closing the client,
+ * so each scenario leaves the server hermetic. A server that responds 405
+ * (DELETE not supported) is tolerated, since the spec allows that.
  */
 export async function connectToServer(
   serverUrl: string,
@@ -139,11 +144,60 @@ export async function connectToServer(
   return {
     client,
     close: async () => {
+      await terminateSessionBestEffort(transport, serverUrl);
       await client.close();
     }
   };
 }
 
+// Shared teardown bound so a wedged server-under-test cannot stall the suite.
+const SESSION_TERMINATE_TIMEOUT_MS = 5000;
+
+/**
+ * Best-effort session termination for an SDK transport. Delegates to
+ * terminateSessionRaw (with the transport's negotiated protocol version) so
+ * the DELETE is truly bounded — the socket is aborted at the timeout rather
+ * than left pending against a wedged server. No-op when the server never
+ * issued a session.
+ */
+export async function terminateSessionBestEffort(
+  transport: StreamableHTTPClientTransport,
+  serverUrl: string
+): Promise<void> {
+  if (!transport.sessionId || !transport.protocolVersion) return;
+  await terminateSessionRaw(
+    serverUrl,
+    transport.sessionId,
+    transport.protocolVersion
+  );
+}
+
+/**
+ * Best-effort HTTP DELETE to terminate a raw (non-SDK) session.
+ *
+ * Used by scenarios that open sessions via raw `fetch` instead of going through
+ * the SDK's StreamableHTTPClientTransport. Failures are swallowed because the
+ * spec allows servers to respond 405, and cleanup must not derail the scenario.
+ */
+export async function terminateSessionRaw(
+  serverUrl: string,
+  sessionId: string,
+  protocolVersion: string
+): Promise<void> {
+  try {
+    await fetch(serverUrl, {
+      method: 'DELETE',
+      headers: {
+        'mcp-session-id': sessionId,
+        'MCP-Protocol-Version': protocolVersion
+      },
+      signal: AbortSignal.timeout(SESSION_TERMINATE_TIMEOUT_MS)
+    });
+  } catch {
+    // best-effort cleanup
+  }
+}
+
 /**
  * Helper to collect notifications (logging and progress)
  */

src/scenarios/server/lifecycle.test.ts

@@ -2,9 +2,14 @@ import { testContext } from '../../connection/testing';
 import { ServerInitializeScenario } from './lifecycle';
 import { connectToServer } from '../../connection/sdk-client';
 
-vi.mock('../../connection/sdk-client', () => ({
-  connectToServer: vi.fn()
-}));
+vi.mock('../../connection/sdk-client', async (importOriginal) => {
+  const actual =
+    await importOriginal<typeof import('../../connection/sdk-client')>();
+  return {
+    ...actual,
+    connectToServer: vi.fn()
+  };
+});
 
 describe('ServerInitializeScenario', () => {
   const serverUrl = 'http://localhost:3000/mcp';
@@ -102,4 +107,37 @@ describe('ServerInitializeScenario', () => {
       }
     });
   });
+
+  it('sends an HTTP DELETE with the issued session ID to terminate the raw session', async () => {
+    fetchMock.mockResolvedValue(
+      new Response(null, {
+        headers: {
+          'mcp-session-id': 'session-123_ABC'
+        }
+      })
+    );
+
+    await new ServerInitializeScenario().run(testContext(serverUrl));
+
+    expect(fetchMock).toHaveBeenCalledWith(
+      serverUrl,
+      expect.objectContaining({
+        method: 'DELETE',
+        headers: expect.objectContaining({
+          'mcp-session-id': 'session-123_ABC'
+        })
+      })
+    );
+  });
+
+  it('does not send DELETE when the server did not assign a session ID', async () => {
+    fetchMock.mockResolvedValue(new Response(null));
+
+    await new ServerInitializeScenario().run(testContext(serverUrl));
+
+    const deleteCalls = fetchMock.mock.calls.filter(
+      ([, init]) => (init as RequestInit | undefined)?.method === 'DELETE'
+    );
+    expect(deleteCalls).toHaveLength(0);
+  });
 });

src/scenarios/server/lifecycle.ts

@@ -8,7 +8,10 @@ import {
   DRAFT_PROTOCOL_VERSION
 } from '../../types';
 import type { RunContext } from '../../connection';
-import { connectToServer } from '../../connection/sdk-client';
+import {
+  connectToServer,
+  terminateSessionRaw
+} from '../../connection/sdk-client';
 
 const VISIBLE_ASCII_REGEX = /^[\x21-\x7E]+$/;
 
@@ -91,20 +94,21 @@ and validates session ID format if one is assigned.`;
     // Check: Session ID visible ASCII validation
     // Use a raw fetch to inspect the MCP-Session-Id response header,
     // since the SDK client transport does not expose it.
+    let rawSessionId: string | null = null;
     try {
       const response = await fetch(serverUrl, {
         method: 'POST',
         headers: {
           'Content-Type': 'application/json',
           Accept: 'application/json, text/event-stream',
-          'mcp-protocol-version': '2025-11-25'
+          'mcp-protocol-version': ctx.specVersion
         },
         body: JSON.stringify({
           jsonrpc: '2.0',
           id: 1,
           method: 'initialize',
           params: {
-            protocolVersion: '2025-11-25',
+            protocolVersion: ctx.specVersion,
             capabilities: {},
             clientInfo: {
               name: 'conformance-session-id-test',
@@ -115,6 +119,7 @@ and validates session ID format if one is assigned.`;
       });
 
       const sessionId = response.headers.get('mcp-session-id');
+      rawSessionId = sessionId;
 
       if (!sessionId) {
         checks.push({
@@ -170,6 +175,10 @@ and validates session ID format if one is assigned.`;
         errorMessage: `Failed to send initialize request for session ID check: ${error instanceof Error ? error.message : String(error)}`,
         specReferences: SESSION_SPEC_REFERENCES
       });
+    } finally {
+      if (rawSessionId) {
+        await terminateSessionRaw(serverUrl, rawSessionId, ctx.specVersion);
+      }
     }
 
     return checks;

src/scenarios/server/sse-multiple-streams.ts

@@ -18,6 +18,7 @@ import { buildStandardHeaders, type RunContext } from '../../connection';
 import { EventSourceParserStream } from 'eventsource-parser/stream';
 import { Client } from '@modelcontextprotocol/sdk/client/index.js';
 import { StreamableHTTPClientTransport } from '@modelcontextprotocol/sdk/client/streamableHttp.js';
+import { terminateSessionBestEffort } from '../../connection/sdk-client';
 
 export class ServerSSEMultipleStreamsScenario implements ClientScenario {
   name = 'server-sse-multiple-streams';
@@ -293,7 +294,11 @@ export class ServerSSEMultipleStreamsScenario implements ClientScenario {
         ]
       });
     } finally {
-      // Clean up
+      // Clean up: terminate the session before closing the client so the
+      // server is left hermetic.
+      if (transport) {
+        await terminateSessionBestEffort(transport, serverUrl);
+      }
       if (client) {
         try {
           await client.close();

src/scenarios/server/sse-polling.ts

@@ -17,6 +17,7 @@ import type { RunContext } from '../../connection';
 import { EventSourceParserStream } from 'eventsource-parser/stream';
 import { Client } from '@modelcontextprotocol/sdk/client/index.js';
 import { StreamableHTTPClientTransport } from '@modelcontextprotocol/sdk/client/streamableHttp.js';
+import { terminateSessionBestEffort } from '../../connection/sdk-client';
 
 function createLoggingFetch(checks: ConformanceCheck[]) {
   return async (url: string, options: RequestInit): Promise<Response> => {
@@ -598,7 +599,11 @@ export class ServerSSEPollingScenario implements ClientScenario {
         ]
       });
     } finally {
-      // Clean up
+      // Clean up: terminate the session before closing the client so the
+      // server is left hermetic.
+      if (transport) {
+        await terminateSessionBestEffort(transport, serverUrl);
+      }
       if (client) {
         try {
           await client.close();