diff --git a/.changeset/pink-goats-push.md b/.changeset/pink-goats-push.md
new file mode 100644
index 000000000..4e2b3c6b7
--- /dev/null
+++ b/.changeset/pink-goats-push.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/examples-server": patch
+---
+
+fix(examples): return 404 for invalid session IDs
diff --git a/examples/server/src/elicitationFormExample.ts b/examples/server/src/elicitationFormExample.ts
index 9c13b739e..e7feb78d3 100644
--- a/examples/server/src/elicitationFormExample.ts
+++ b/examples/server/src/elicitationFormExample.ts
@@ -338,6 +338,19 @@ async function main() {
             if (sessionId && transports[sessionId]) {
                 // Reuse existing transport for this session
                 transport = transports[sessionId];
+            } else if (sessionId && !transports[sessionId]) {
+                // Session ID provided but not found - return 404 to signal client should start new session
+                // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
+                // an Mcp-Session-Id, it MUST start a new session"
+                res.status(404).json({
+                    jsonrpc: '2.0',
+                    error: {
+                        code: -32_000,
+                        message: 'Session not found'
+                    },
+                    id: null
+                });
+                return;
             } else if (!sessionId && isInitializeRequest(req.body)) {
                 // New initialization request - create new transport
                 transport = new NodeStreamableHTTPServerTransport({
@@ -365,12 +378,14 @@ async function main() {
                 await transport.handleRequest(req, res, req.body);
                 return;
             } else {
-                // Invalid request - no session ID or not initialization request
+                // No session ID and not an initialization request - return 400
+                // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
+                // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
                 res.status(400).json({
                     jsonrpc: '2.0',
                     error: {
                         code: -32_000,
-                        message: 'Bad Request: No valid session ID provided'
+                        message: 'Bad Request: Session ID required'
                     },
                     id: null
                 });
@@ -399,8 +414,14 @@ async function main() {
     // Handle GET requests for SSE streams
     const mcpGetHandler = async (req: Request, res: Response) => {
         const sessionId = req.headers['mcp-session-id'] as string | undefined;
-        if (!sessionId || !transports[sessionId]) {
-            res.status(400).send('Invalid or missing session ID');
+        if (!sessionId) {
+            // No session ID provided
+            res.status(400).send('Session ID required');
+            return;
+        }
+        if (!transports[sessionId]) {
+            // Session ID provided but not found - return 404 per MCP spec
+            res.status(404).send('Session not found');
             return;
         }
 
@@ -414,8 +435,14 @@ async function main() {
     // Handle DELETE requests for session termination
     const mcpDeleteHandler = async (req: Request, res: Response) => {
         const sessionId = req.headers['mcp-session-id'] as string | undefined;
-        if (!sessionId || !transports[sessionId]) {
-            res.status(400).send('Invalid or missing session ID');
+        if (!sessionId) {
+            // No session ID provided
+            res.status(400).send('Session ID required');
+            return;
+        }
+        if (!transports[sessionId]) {
+            // Session ID provided but not found - return 404 per MCP spec
+            res.status(404).send('Session not found');
             return;
         }
 
diff --git a/examples/server/src/elicitationUrlExample.ts b/examples/server/src/elicitationUrlExample.ts
index c38dd75e8..272f42b0a 100644
--- a/examples/server/src/elicitationUrlExample.ts
+++ b/examples/server/src/elicitationUrlExample.ts
@@ -571,6 +571,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
         if (sessionId && transports[sessionId]) {
             // Reuse existing transport
             transport = transports[sessionId];
+        } else if (sessionId && !transports[sessionId]) {
+            // Session ID provided but not found - return 404 to signal client should start new session
+            // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
+            // an Mcp-Session-Id, it MUST start a new session"
+            res.status(404).json({
+                jsonrpc: '2.0',
+                error: {
+                    code: -32_000,
+                    message: 'Session not found'
+                },
+                id: null
+            });
+            return;
         } else if (!sessionId && isInitializeRequest(req.body)) {
             const server = getServer();
             // New initialization request
@@ -607,12 +620,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
             await transport.handleRequest(req, res, req.body);
             return; // Already handled
         } else {
-            // Invalid request - no session ID or not initialization request
+            // No session ID and not an initialization request - return 400
+            // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
+            // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
             res.status(400).json({
                 jsonrpc: '2.0',
                 error: {
                     code: -32_000,
-                    message: 'Bad Request: No valid session ID provided'
+                    message: 'Bad Request: Session ID required'
                 },
                 id: null
             });
@@ -643,8 +658,14 @@ app.post('/mcp', authMiddleware, mcpPostHandler);
 // Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
 const mcpGetHandler = async (req: Request, res: Response) => {
     const sessionId = req.headers['mcp-session-id'] as string | undefined;
-    if (!sessionId || !transports[sessionId]) {
-        res.status(400).send('Invalid or missing session ID');
+    if (!sessionId) {
+        // No session ID provided
+        res.status(400).send('Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        // Session ID provided but not found - return 404 per MCP spec
+        res.status(404).send('Session not found');
         return;
     }
 
@@ -682,8 +703,14 @@ app.get('/mcp', authMiddleware, mcpGetHandler);
 // Handle DELETE requests for session termination (according to MCP spec)
 const mcpDeleteHandler = async (req: Request, res: Response) => {
     const sessionId = req.headers['mcp-session-id'] as string | undefined;
-    if (!sessionId || !transports[sessionId]) {
-        res.status(400).send('Invalid or missing session ID');
+    if (!sessionId) {
+        // No session ID provided
+        res.status(400).send('Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        // Session ID provided but not found - return 404 per MCP spec
+        res.status(404).send('Session not found');
         return;
     }
 
diff --git a/examples/server/src/jsonResponseStreamableHttp.ts b/examples/server/src/jsonResponseStreamableHttp.ts
index 7a3aad67a..3e40495d1 100644
--- a/examples/server/src/jsonResponseStreamableHttp.ts
+++ b/examples/server/src/jsonResponseStreamableHttp.ts
@@ -92,6 +92,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
         if (sessionId && transports[sessionId]) {
             // Reuse existing transport
             transport = transports[sessionId];
+        } else if (sessionId && !transports[sessionId]) {
+            // Session ID provided but not found - return 404 to signal client should start new session
+            // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
+            // an Mcp-Session-Id, it MUST start a new session"
+            res.status(404).json({
+                jsonrpc: '2.0',
+                error: {
+                    code: -32_000,
+                    message: 'Session not found'
+                },
+                id: null
+            });
+            return;
         } else if (!sessionId && isInitializeRequest(req.body)) {
             // New initialization request - use JSON response mode
             transport = new NodeStreamableHTTPServerTransport({
@@ -111,12 +124,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
             await transport.handleRequest(req, res, req.body);
             return; // Already handled
         } else {
-            // Invalid request - no session ID or not initialization request
+            // No session ID and not an initialization request - return 400
+            // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
+            // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
             res.status(400).json({
                 jsonrpc: '2.0',
                 error: {
                     code: -32_000,
-                    message: 'Bad Request: No valid session ID provided'
+                    message: 'Bad Request: Session ID required'
                 },
                 id: null
             });
diff --git a/examples/server/src/simpleStreamableHttp.ts b/examples/server/src/simpleStreamableHttp.ts
index be025c04c..1833a0760 100644
--- a/examples/server/src/simpleStreamableHttp.ts
+++ b/examples/server/src/simpleStreamableHttp.ts
@@ -654,6 +654,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
         if (sessionId && transports[sessionId]) {
             // Reuse existing transport
             transport = transports[sessionId];
+        } else if (sessionId && !transports[sessionId]) {
+            // Session ID provided but not found - return 404 to signal client should start new session
+            // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
+            // an Mcp-Session-Id, it MUST start a new session"
+            res.status(404).json({
+                jsonrpc: '2.0',
+                error: {
+                    code: -32_000,
+                    message: 'Session not found'
+                },
+                id: null
+            });
+            return;
         } else if (!sessionId && isInitializeRequest(req.body)) {
             // New initialization request
             const eventStore = new InMemoryEventStore();
@@ -685,12 +698,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
             await transport.handleRequest(req, res, req.body);
             return; // Already handled
         } else {
-            // Invalid request - no session ID or not initialization request
+            // No session ID and not an initialization request - return 400
+            // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
+            // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
             res.status(400).json({
                 jsonrpc: '2.0',
                 error: {
                     code: -32_000,
-                    message: 'Bad Request: No valid session ID provided'
+                    message: 'Bad Request: Session ID required'
                 },
                 id: null
             });
@@ -725,8 +740,14 @@ if (useOAuth && authMiddleware) {
 // Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
 const mcpGetHandler = async (req: Request, res: Response) => {
     const sessionId = req.headers['mcp-session-id'] as string | undefined;
-    if (!sessionId || !transports[sessionId]) {
-        res.status(400).send('Invalid or missing session ID');
+    if (!sessionId) {
+        // No session ID provided
+        res.status(400).send('Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        // Session ID provided but not found - return 404 per MCP spec
+        res.status(404).send('Session not found');
         return;
     }
 
@@ -756,8 +777,14 @@ if (useOAuth && authMiddleware) {
 // Handle DELETE requests for session termination (according to MCP spec)
 const mcpDeleteHandler = async (req: Request, res: Response) => {
     const sessionId = req.headers['mcp-session-id'] as string | undefined;
-    if (!sessionId || !transports[sessionId]) {
-        res.status(400).send('Invalid or missing session ID');
+    if (!sessionId) {
+        // No session ID provided
+        res.status(400).send('Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        // Session ID provided but not found - return 404 per MCP spec
+        res.status(404).send('Session not found');
         return;
     }
 
diff --git a/examples/server/src/simpleTaskInteractive.ts b/examples/server/src/simpleTaskInteractive.ts
index 9092926d9..d0c8bf718 100644
--- a/examples/server/src/simpleTaskInteractive.ts
+++ b/examples/server/src/simpleTaskInteractive.ts
@@ -649,6 +649,16 @@ app.post('/mcp', async (req: Request, res: Response) => {
 
         if (sessionId && transports[sessionId]) {
             transport = transports[sessionId];
+        } else if (sessionId && !transports[sessionId]) {
+            // Session ID provided but not found - return 404 to signal client should start new session
+            // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
+            // an Mcp-Session-Id, it MUST start a new session"
+            res.status(404).json({
+                jsonrpc: '2.0',
+                error: { code: -32_000, message: 'Session not found' },
+                id: null
+            });
+            return;
         } else if (!sessionId && isInitializeRequest(req.body)) {
             transport = new NodeStreamableHTTPServerTransport({
                 sessionIdGenerator: () => randomUUID(),
@@ -671,9 +681,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
             await transport.handleRequest(req, res, req.body);
             return;
         } else {
+            // No session ID and not an initialization request - return 400
+            // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
+            // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
             res.status(400).json({
                 jsonrpc: '2.0',
-                error: { code: -32_000, message: 'Bad Request: No valid session ID' },
+                error: { code: -32_000, message: 'Bad Request: Session ID required' },
                 id: null
             });
             return;
@@ -695,8 +708,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 // Handle GET requests for SSE streams
 app.get('/mcp', async (req: Request, res: Response) => {
     const sessionId = req.headers['mcp-session-id'] as string | undefined;
-    if (!sessionId || !transports[sessionId]) {
-        res.status(400).send('Invalid or missing session ID');
+    if (!sessionId) {
+        // No session ID provided
+        res.status(400).send('Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        // Session ID provided but not found - return 404 per MCP spec
+        res.status(404).send('Session not found');
         return;
     }
 
@@ -707,8 +726,14 @@ app.get('/mcp', async (req: Request, res: Response) => {
 // Handle DELETE requests for session termination
 app.delete('/mcp', async (req: Request, res: Response) => {
     const sessionId = req.headers['mcp-session-id'] as string | undefined;
-    if (!sessionId || !transports[sessionId]) {
-        res.status(400).send('Invalid or missing session ID');
+    if (!sessionId) {
+        // No session ID provided
+        res.status(400).send('Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        // Session ID provided but not found - return 404 per MCP spec
+        res.status(404).send('Session not found');
         return;
     }
 
diff --git a/examples/server/src/standaloneSseWithGetStreamableHttp.ts b/examples/server/src/standaloneSseWithGetStreamableHttp.ts
index b1b2ccf51..c500922b9 100644
--- a/examples/server/src/standaloneSseWithGetStreamableHttp.ts
+++ b/examples/server/src/standaloneSseWithGetStreamableHttp.ts
@@ -57,6 +57,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
         if (sessionId && transports[sessionId]) {
             // Reuse existing transport
             transport = transports[sessionId];
+        } else if (sessionId && !transports[sessionId]) {
+            // Session ID provided but not found - return 404 to signal client should start new session
+            // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
+            // an Mcp-Session-Id, it MUST start a new session"
+            res.status(404).json({
+                jsonrpc: '2.0',
+                error: {
+                    code: -32_000,
+                    message: 'Session not found'
+                },
+                id: null
+            });
+            return;
         } else if (!sessionId && isInitializeRequest(req.body)) {
             // New initialization request - create a fresh server for this client
             const server = getServer();
@@ -87,12 +100,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
             await transport.handleRequest(req, res, req.body);
             return; // Already handled
         } else {
-            // Invalid request - no session ID or not initialization request
+            // No session ID and not an initialization request - return 400
+            // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
+            // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
             res.status(400).json({
                 jsonrpc: '2.0',
                 error: {
                     code: -32_000,
-                    message: 'Bad Request: No valid session ID provided'
+                    message: 'Bad Request: Session ID required'
                 },
                 id: null
             });
@@ -119,8 +134,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 // Handle GET requests for SSE streams (now using built-in support from StreamableHTTP)
 app.get('/mcp', async (req: Request, res: Response) => {
     const sessionId = req.headers['mcp-session-id'] as string | undefined;
-    if (!sessionId || !transports[sessionId]) {
-        res.status(400).send('Invalid or missing session ID');
+    if (!sessionId) {
+        // No session ID provided
+        res.status(400).send('Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        // Session ID provided but not found - return 404 per MCP spec
+        res.status(404).send('Session not found');
         return;
     }