diff --git a/examples/server/src/simpleStreamableHttp.ts b/examples/server/src/simpleStreamableHttp.ts
index be025c04c..d3560c91e 100644
--- a/examples/server/src/simpleStreamableHttp.ts
+++ b/examples/server/src/simpleStreamableHttp.ts
@@ -726,7 +726,7 @@ if (useOAuth && authMiddleware) {
 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');
+        res.status(404).send('Invalid or missing session ID');
         return;
     }
 
@@ -757,7 +757,7 @@ if (useOAuth && authMiddleware) {
 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');
+        res.status(404).send('Invalid or missing session ID');
         return;
     }
 
diff --git a/examples/server/src/standaloneSseWithGetStreamableHttp.ts b/examples/server/src/standaloneSseWithGetStreamableHttp.ts
index b1b2ccf51..1a4018b3a 100644
--- a/examples/server/src/standaloneSseWithGetStreamableHttp.ts
+++ b/examples/server/src/standaloneSseWithGetStreamableHttp.ts
@@ -120,7 +120,7 @@ app.post('/mcp', async (req: Request, res: Response) => {
 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');
+        res.status(404).send('Invalid or missing session ID');
         return;
     }
 
diff --git a/test/conformance/src/everythingServer.ts b/test/conformance/src/everythingServer.ts
index 25a6feb0f..7b79012c4 100644
--- a/test/conformance/src/everythingServer.ts
+++ b/test/conformance/src/everythingServer.ts
@@ -962,7 +962,7 @@ 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');
+        res.status(404).send('Invalid or missing session ID');
         return;
     }
 
@@ -989,7 +989,7 @@ 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');
+        res.status(404).send('Invalid or missing session ID');
         return;
     }