diff --git a/examples/server/src/elicitationFormExample.ts b/examples/server/src/elicitationFormExample.ts
index 70ff8ecb5..3bb14cf4d 100644
--- a/examples/server/src/elicitationFormExample.ts
+++ b/examples/server/src/elicitationFormExample.ts
@@ -362,13 +362,25 @@ async function main() {
 
                 await transport.handleRequest(req, res, req.body);
                 return;
+            } else if (sessionId) {
+                // Session ID provided but not found - per spec, return 404 to signal
+                // client should start a new session
+                res.status(404).json({
+                    jsonrpc: '2.0',
+                    error: {
+                        code: -32_000,
+                        message: 'Session not found'
+                    },
+                    id: null
+                });
+                return;
             } else {
-                // Invalid request - no session ID or not initialization request
+                // No session ID and not an initialization request - return 400
                 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
                 });
@@ -397,8 +409,12 @@ 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) {
+            res.status(400).send('Bad Request: Session ID required');
+            return;
+        }
+        if (!transports[sessionId]) {
+            res.status(404).send('Session not found');
             return;
         }
 
@@ -412,8 +428,12 @@ 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) {
+            res.status(400).send('Bad Request: Session ID required');
+            return;
+        }
+        if (!transports[sessionId]) {
+            res.status(404).send('Session not found');
             return;
         }
 
diff --git a/examples/server/src/elicitationUrlExample.ts b/examples/server/src/elicitationUrlExample.ts
index e72845f1a..677969fa0 100644
--- a/examples/server/src/elicitationUrlExample.ts
+++ b/examples/server/src/elicitationUrlExample.ts
@@ -605,13 +605,25 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 
             await transport.handleRequest(req, res, req.body);
             return; // Already handled
+        } else if (sessionId) {
+            // Session ID provided but not found - per spec, return 404 to signal
+            // client should start a new session
+            res.status(404).json({
+                jsonrpc: '2.0',
+                error: {
+                    code: -32_000,
+                    message: 'Session not found'
+                },
+                id: null
+            });
+            return;
         } else {
-            // Invalid request - no session ID or not initialization request
+            // No session ID and not an initialization request - return 400
             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
             });
@@ -642,8 +654,12 @@ 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) {
+        res.status(400).send('Bad Request: Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        res.status(404).send('Session not found');
         return;
     }
 
@@ -681,8 +697,12 @@ 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) {
+        res.status(400).send('Bad Request: Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        res.status(404).send('Session not found');
         return;
     }
 
diff --git a/examples/server/src/jsonResponseStreamableHttp.ts b/examples/server/src/jsonResponseStreamableHttp.ts
index fa0e3a300..a82e073e6 100644
--- a/examples/server/src/jsonResponseStreamableHttp.ts
+++ b/examples/server/src/jsonResponseStreamableHttp.ts
@@ -128,13 +128,25 @@ app.post('/mcp', async (req: Request, res: Response) => {
             await server.connect(transport);
             await transport.handleRequest(req, res, req.body);
             return; // Already handled
+        } else if (sessionId) {
+            // Session ID provided but not found - per spec, return 404 to signal
+            // client should start a new session
+            res.status(404).json({
+                jsonrpc: '2.0',
+                error: {
+                    code: -32_000,
+                    message: 'Session not found'
+                },
+                id: null
+            });
+            return;
         } else {
-            // Invalid request - no session ID or not initialization request
+            // No session ID and not an initialization request - return 400
             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 75308beff..0bdcdfba0 100644
--- a/examples/server/src/simpleStreamableHttp.ts
+++ b/examples/server/src/simpleStreamableHttp.ts
@@ -595,13 +595,25 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 
             await transport.handleRequest(req, res, req.body);
             return; // Already handled
+        } else if (sessionId) {
+            // Session ID provided but not found - per spec, return 404 to signal
+            // client should start a new session
+            res.status(404).json({
+                jsonrpc: '2.0',
+                error: {
+                    code: -32_000,
+                    message: 'Session not found'
+                },
+                id: null
+            });
+            return;
         } else {
-            // Invalid request - no session ID or not initialization request
+            // No session ID and not an initialization request - return 400
             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
             });
@@ -636,8 +648,12 @@ 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) {
+        res.status(400).send('Bad Request: Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        res.status(404).send('Session not found');
         return;
     }
 
@@ -667,8 +683,12 @@ 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) {
+        res.status(400).send('Bad Request: Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        res.status(404).send('Session not found');
         return;
     }
 
diff --git a/examples/server/src/simpleTaskInteractive.ts b/examples/server/src/simpleTaskInteractive.ts
index 1b8532525..97fe19942 100644
--- a/examples/server/src/simpleTaskInteractive.ts
+++ b/examples/server/src/simpleTaskInteractive.ts
@@ -679,10 +679,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
             await server.connect(transport);
             await transport.handleRequest(req, res, req.body);
             return;
+        } else if (sessionId) {
+            // Session ID provided but not found - per spec, return 404
+            res.status(404).json({
+                jsonrpc: '2.0',
+                error: { code: -32_000, message: 'Session not found' },
+                id: null
+            });
+            return;
         } else {
+            // No session ID and not an initialization request - return 400
             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;
@@ -704,8 +713,12 @@ 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) {
+        res.status(400).send('Bad Request: Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        res.status(404).send('Session not found');
         return;
     }
 
@@ -716,8 +729,12 @@ 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) {
+        res.status(400).send('Bad Request: Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        res.status(404).send('Session not found');
         return;
     }
 
diff --git a/examples/server/src/standaloneSseWithGetStreamableHttp.ts b/examples/server/src/standaloneSseWithGetStreamableHttp.ts
index 1fd351ef1..f20b4df6a 100644
--- a/examples/server/src/standaloneSseWithGetStreamableHttp.ts
+++ b/examples/server/src/standaloneSseWithGetStreamableHttp.ts
@@ -66,13 +66,25 @@ app.post('/mcp', async (req: Request, res: Response) => {
             // Handle the request - the onsessioninitialized callback will store the transport
             await transport.handleRequest(req, res, req.body);
             return; // Already handled
+        } else if (sessionId) {
+            // Session ID provided but not found - per spec, return 404 to signal
+            // client should start a new session
+            res.status(404).json({
+                jsonrpc: '2.0',
+                error: {
+                    code: -32_000,
+                    message: 'Session not found'
+                },
+                id: null
+            });
+            return;
         } else {
-            // Invalid request - no session ID or not initialization request
+            // No session ID and not an initialization request - return 400
             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
             });
@@ -99,8 +111,12 @@ 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) {
+        res.status(400).send('Bad Request: Session ID required');
+        return;
+    }
+    if (!transports[sessionId]) {
+        res.status(404).send('Session not found');
         return;
     }