From 4729c877083edf7bc6b73ab32961dc5bbdecc6d5 Mon Sep 17 00:00:00 2001
From: Adam Boudjemaa <boudjemaa.adam@gmail.com>
Date: Fri, 6 Mar 2026 03:56:43 +0100
Subject: [PATCH 1/2] fix(transport): handle 404 and 406 for GET SSE streams

When a server returns 404 or 406 for SSE GET requests, the client
should fall back gracefully instead of only handling 405.

Fixes #1635
---
 packages/client/src/client/streamableHttp.ts  |  5 +-
 .../client/test/client/streamableHttp.test.ts | 64 +++++++++++++++++++
 2 files changed, 67 insertions(+), 2 deletions(-)

diff --git a/packages/client/src/client/streamableHttp.ts b/packages/client/src/client/streamableHttp.ts
index 79a20adfc..b4fbf6162 100644
--- a/packages/client/src/client/streamableHttp.ts
+++ b/packages/client/src/client/streamableHttp.ts
@@ -236,8 +236,9 @@ export class StreamableHTTPClientTransport implements Transport {
                 }
 
                 // 405 indicates that the server does not offer an SSE stream at GET endpoint
-                // This is an expected case that should not trigger an error
-                if (response.status === 405) {
+                // 404 indicates the endpoint does not exist, 406 indicates the content type is not accepted
+                // All three are expected cases that should not trigger an error
+                if (response.status === 405 || response.status === 404 || response.status === 406) {
                     return;
                 }
 
diff --git a/packages/client/test/client/streamableHttp.test.ts b/packages/client/test/client/streamableHttp.test.ts
index 8a550feae..1dd123691 100644
--- a/packages/client/test/client/streamableHttp.test.ts
+++ b/packages/client/test/client/streamableHttp.test.ts
@@ -282,6 +282,70 @@ describe('StreamableHTTPClientTransport', () => {
         expect(globalThis.fetch).toHaveBeenCalledTimes(2);
     });
 
+    it('should attempt initial GET connection and handle 404 gracefully', async () => {
+        // Mock the server returning 404 for the SSE GET endpoint (endpoint does not exist)
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: false,
+            status: 404,
+            statusText: 'Not Found'
+        });
+
+        // We expect the 404 error to be caught and handled gracefully
+        // This should not throw an error that breaks the transport
+        await transport.start();
+        await expect(transport['_startOrAuthSse']({})).resolves.not.toThrow('Failed to open SSE stream: Not Found');
+        // Check that GET was attempted
+        expect(globalThis.fetch).toHaveBeenCalledWith(
+            expect.anything(),
+            expect.objectContaining({
+                method: 'GET',
+                headers: expect.any(Headers)
+            })
+        );
+
+        // Verify transport still works after 404
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: true,
+            status: 202,
+            headers: new Headers()
+        });
+
+        await transport.send({ jsonrpc: '2.0', method: 'test', params: {} } as JSONRPCMessage);
+        expect(globalThis.fetch).toHaveBeenCalledTimes(2);
+    });
+
+    it('should attempt initial GET connection and handle 406 gracefully', async () => {
+        // Mock the server returning 406 for the SSE GET endpoint (content type not acceptable)
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: false,
+            status: 406,
+            statusText: 'Not Acceptable'
+        });
+
+        // We expect the 406 error to be caught and handled gracefully
+        // This should not throw an error that breaks the transport
+        await transport.start();
+        await expect(transport['_startOrAuthSse']({})).resolves.not.toThrow('Failed to open SSE stream: Not Acceptable');
+        // Check that GET was attempted
+        expect(globalThis.fetch).toHaveBeenCalledWith(
+            expect.anything(),
+            expect.objectContaining({
+                method: 'GET',
+                headers: expect.any(Headers)
+            })
+        );
+
+        // Verify transport still works after 406
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: true,
+            status: 202,
+            headers: new Headers()
+        });
+
+        await transport.send({ jsonrpc: '2.0', method: 'test', params: {} } as JSONRPCMessage);
+        expect(globalThis.fetch).toHaveBeenCalledTimes(2);
+    });
+
     it('should handle successful initial GET connection for SSE', async () => {
         // Set up readable stream for SSE events
         const encoder = new TextEncoder();

From c8e861a650d3f1b933636a8d7cb2917e1537e29b Mon Sep 17 00:00:00 2001
From: Adam Boudjemaa <boudjemaa.adam@gmail.com>
Date: Fri, 6 Mar 2026 04:35:08 +0100
Subject: [PATCH 2/2] chore: add changeset for 404/406 SSE handling

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .changeset/stale-mayflies-beam.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/stale-mayflies-beam.md

diff --git a/.changeset/stale-mayflies-beam.md b/.changeset/stale-mayflies-beam.md
new file mode 100644
index 000000000..7b2234ba9
--- /dev/null
+++ b/.changeset/stale-mayflies-beam.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Handle 404 and 406 status codes gracefully for GET SSE streams, matching existing 405 behavior