diff --git a/src/scenarios/server/lifecycle.test.ts b/src/scenarios/server/lifecycle.test.ts
new file mode 100644
index 0000000..15ee6d8
--- /dev/null
+++ b/src/scenarios/server/lifecycle.test.ts
@@ -0,0 +1,94 @@
+import { ServerInitializeScenario } from './lifecycle';
+import { connectToServer } from './client-helper';
+
+vi.mock('./client-helper', () => ({
+  connectToServer: vi.fn()
+}));
+
+describe('ServerInitializeScenario', () => {
+  const serverUrl = 'http://localhost:3000/mcp';
+  const closeMock = vi.fn().mockResolvedValue(undefined);
+  const fetchMock = vi.fn();
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    vi.stubGlobal('fetch', fetchMock);
+    vi.mocked(connectToServer).mockResolvedValue({
+      client: {} as any,
+      close: closeMock
+    });
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('returns INFO when the server does not provide an MCP-Session-Id header', async () => {
+    fetchMock.mockResolvedValue(new Response(null));
+
+    const checks = await new ServerInitializeScenario().run(serverUrl);
+
+    expect(connectToServer).toHaveBeenCalledWith(serverUrl);
+    expect(closeMock).toHaveBeenCalled();
+    expect(fetchMock).toHaveBeenCalledWith(
+      serverUrl,
+      expect.objectContaining({
+        method: 'POST'
+      })
+    );
+
+    expect(checks).toHaveLength(2);
+    expect(checks[0]?.id).toBe('server-initialize');
+    expect(checks[0]?.status).toBe('SUCCESS');
+    expect(checks[1]).toMatchObject({
+      id: 'server-session-id-visible-ascii',
+      status: 'INFO',
+      details: {
+        message:
+          'Server did not provide an MCP-Session-Id header (session ID is optional)'
+      }
+    });
+  });
+
+  it('returns SUCCESS when the server provides a visible ASCII session ID', async () => {
+    fetchMock.mockResolvedValue(
+      new Response(null, {
+        headers: {
+          'mcp-session-id': 'session-123_ABC'
+        }
+      })
+    );
+
+    const checks = await new ServerInitializeScenario().run(serverUrl);
+
+    expect(checks[1]).toMatchObject({
+      id: 'server-session-id-visible-ascii',
+      status: 'SUCCESS',
+      details: {
+        sessionId: 'session-123_ABC'
+      }
+    });
+  });
+
+  it('returns FAILURE when the server provides a non-ASCII session ID', async () => {
+    fetchMock.mockResolvedValue(
+      new Response(null, {
+        headers: {
+          'mcp-session-id': 'session-123-é'
+        }
+      })
+    );
+
+    const checks = await new ServerInitializeScenario().run(serverUrl);
+
+    expect(checks[1]).toMatchObject({
+      id: 'server-session-id-visible-ascii',
+      status: 'FAILURE',
+      errorMessage:
+        'Session ID contains characters outside visible ASCII range (0x21-0x7E)',
+      details: {
+        sessionId: 'session-123-é'
+      }
+    });
+  });
+});
diff --git a/src/scenarios/server/lifecycle.ts b/src/scenarios/server/lifecycle.ts
index 392a932..3758615 100644
--- a/src/scenarios/server/lifecycle.ts
+++ b/src/scenarios/server/lifecycle.ts
@@ -5,6 +5,15 @@
 import { ClientScenario, ConformanceCheck, SpecVersion } from '../../types';
 import { connectToServer } from './client-helper';
 
+const VISIBLE_ASCII_REGEX = /^[\x21-\x7E]+$/;
+
+const SESSION_SPEC_REFERENCES = [
+  {
+    id: 'MCP-Session-Management',
+    url: 'https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#session-management'
+  }
+];
+
 export class ServerInitializeScenario implements ClientScenario {
   name = 'server-initialize';
   specVersions: SpecVersion[] = ['2025-06-18', '2025-11-25'];
@@ -18,8 +27,10 @@ export class ServerInitializeScenario implements ClientScenario {
 - Accept \`initialize\` request with client info and capabilities
 - Return valid initialize response with server info, protocol version, and capabilities
 - Accept \`initialized\` notification from client after handshake
+- If a session ID is assigned, it MUST only contain visible ASCII characters (0x21 to 0x7E)
 
-This test verifies the server can complete the two-phase initialization handshake successfully.`;
+This test verifies the server can complete the two-phase initialization handshake successfully,
+and validates session ID format if one is assigned.`;
 
   async run(serverUrl: string): Promise<ConformanceCheck[]> {
     const checks: ConformanceCheck[] = [];
@@ -65,6 +76,91 @@ This test verifies the server can complete the two-phase initialization handshak
           }
         ]
       });
+      return checks;
+    }
+
+    // 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.
+    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'
+        },
+        body: JSON.stringify({
+          jsonrpc: '2.0',
+          id: 1,
+          method: 'initialize',
+          params: {
+            protocolVersion: '2025-11-25',
+            capabilities: {},
+            clientInfo: {
+              name: 'conformance-session-id-test',
+              version: '1.0.0'
+            }
+          }
+        })
+      });
+
+      const sessionId = response.headers.get('mcp-session-id');
+
+      if (!sessionId) {
+        checks.push({
+          id: 'server-session-id-visible-ascii',
+          name: 'ServerSessionIdVisibleAscii',
+          description:
+            'Server-provided session ID uses only visible ASCII characters',
+          status: 'INFO',
+          timestamp: new Date().toISOString(),
+          specReferences: SESSION_SPEC_REFERENCES,
+          details: {
+            message:
+              'Server did not provide an MCP-Session-Id header (session ID is optional)'
+          }
+        });
+      } else if (VISIBLE_ASCII_REGEX.test(sessionId)) {
+        checks.push({
+          id: 'server-session-id-visible-ascii',
+          name: 'ServerSessionIdVisibleAscii',
+          description:
+            'Server-provided session ID uses only visible ASCII characters',
+          status: 'SUCCESS',
+          timestamp: new Date().toISOString(),
+          specReferences: SESSION_SPEC_REFERENCES,
+          details: {
+            sessionId
+          }
+        });
+      } else {
+        checks.push({
+          id: 'server-session-id-visible-ascii',
+          name: 'ServerSessionIdVisibleAscii',
+          description:
+            'Server-provided session ID uses only visible ASCII characters',
+          status: 'FAILURE',
+          timestamp: new Date().toISOString(),
+          errorMessage:
+            'Session ID contains characters outside visible ASCII range (0x21-0x7E)',
+          specReferences: SESSION_SPEC_REFERENCES,
+          details: {
+            sessionId
+          }
+        });
+      }
+    } catch (error) {
+      checks.push({
+        id: 'server-session-id-visible-ascii',
+        name: 'ServerSessionIdVisibleAscii',
+        description:
+          'Server-provided session ID uses only visible ASCII characters',
+        status: 'FAILURE',
+        timestamp: new Date().toISOString(),
+        errorMessage: `Failed to send initialize request for session ID check: ${error instanceof Error ? error.message : String(error)}`,
+        specReferences: SESSION_SPEC_REFERENCES
+      });
     }
 
     return checks;