diff --git a/src/scenarios/server/lifecycle.test.ts b/src/scenarios/server/lifecycle.test.ts
index ab3628a9..0610d980 100644
--- a/src/scenarios/server/lifecycle.test.ts
+++ b/src/scenarios/server/lifecycle.test.ts
@@ -27,10 +27,14 @@ describe('ServerInitializeScenario', () => {
 
   afterEach(() => {
     vi.unstubAllGlobals();
+    vi.restoreAllMocks();
   });
 
   it('returns INFO when the server does not provide an MCP-Session-Id header', async () => {
-    fetchMock.mockResolvedValue(new Response(null));
+    const cancelMock = vi.fn();
+    fetchMock.mockResolvedValue(
+      new Response(new ReadableStream({ cancel: cancelMock }))
+    );
 
     const checks = await new ServerInitializeScenario().run(
       testContext(serverUrl)
@@ -45,9 +49,11 @@ describe('ServerInitializeScenario', () => {
     expect(fetchMock).toHaveBeenCalledWith(
       serverUrl,
       expect.objectContaining({
-        method: 'POST'
+        method: 'POST',
+        signal: expect.any(AbortSignal)
       })
     );
+    expect(cancelMock).toHaveBeenCalled();
 
     expect(checks).toHaveLength(2);
     expect(checks[0]?.id).toBe('server-initialize');
@@ -130,6 +136,25 @@ describe('ServerInitializeScenario', () => {
     );
   });
 
+  it('reports a failure when the raw session ID probe times out', async () => {
+    const probeSignal = new AbortController().signal;
+    vi.spyOn(AbortSignal, 'timeout').mockReturnValue(probeSignal);
+    fetchMock.mockRejectedValue(new Error('The operation was aborted'));
+
+    const checks = await new ServerInitializeScenario().run(
+      testContext(serverUrl)
+    );
+
+    expect(AbortSignal.timeout).toHaveBeenCalledWith(5000);
+    expect(checks[1]).toMatchObject({
+      id: 'server-session-id-visible-ascii',
+      status: 'FAILURE'
+    });
+    expect(checks[1]?.errorMessage).toContain(
+      'Failed to send initialize request for session ID check'
+    );
+  });
+
   it('does not send DELETE when the server did not assign a session ID', async () => {
     fetchMock.mockResolvedValue(new Response(null));
 
diff --git a/src/scenarios/server/lifecycle.ts b/src/scenarios/server/lifecycle.ts
index 7be721b1..38e25360 100644
--- a/src/scenarios/server/lifecycle.ts
+++ b/src/scenarios/server/lifecycle.ts
@@ -14,6 +14,7 @@ import {
 } from '../../connection/sdk-client';
 
 const VISIBLE_ASCII_REGEX = /^[\x21-\x7E]+$/;
+const SESSION_ID_PROBE_TIMEOUT_MS = 5_000;
 
 const SESSION_SPEC_REFERENCES = [
   {
@@ -115,7 +116,8 @@ and validates session ID format if one is assigned.`;
               version: '1.0.0'
             }
           }
-        })
+        }),
+        signal: AbortSignal.timeout(SESSION_ID_PROBE_TIMEOUT_MS)
       });
 
       const sessionId = response.headers.get('mcp-session-id');
@@ -164,6 +166,10 @@ and validates session ID format if one is assigned.`;
           }
         });
       }
+
+      // The probe only needs response headers. Release the body so a
+      // long-lived SSE response cannot keep a connection open.
+      await response.body?.cancel().catch(() => {});
     } catch (error) {
       checks.push({
         id: 'server-session-id-visible-ascii',