diff --git a/src/runtime/transport.ts b/src/runtime/transport.ts
index 983c3be..e49ed6f 100644
--- a/src/runtime/transport.ts
+++ b/src/runtime/transport.ts
@@ -163,6 +163,13 @@ export async function createClientContext(
           await oauthSession?.close().catch(() => {});
           throw primaryError;
         }
+        // When OAuth is active the server already accepted a POST (returning 401),
+        // so it speaks Streamable HTTP.  Falling back to SSE (GET) would fail with
+        // 405 Method Not Allowed or a fresh 401 that can never be resolved.
+        if (activeDefinition.auth === 'oauth') {
+          await oauthSession?.close().catch(() => {});
+          throw primaryError;
+        }
         if (primaryError instanceof Error) {
           logger.info(`Falling back to SSE transport for '${activeDefinition.name}': ${primaryError.message}`);
         }
diff --git a/tests/runtime-transport.test.ts b/tests/runtime-transport.test.ts
index d198a6f..c883848 100644
--- a/tests/runtime-transport.test.ts
+++ b/tests/runtime-transport.test.ts
@@ -46,6 +46,24 @@ describe('createClientContext (HTTP)', () => {
     expect(clientConnect).toHaveBeenCalledTimes(2);
   });
 
+  it('does not fall back to SSE when OAuth is active', async () => {
+    const definition: ServerDefinition = {
+      ...stubHttpDefinition('https://example.com/mcp'),
+      auth: 'oauth',
+    };
+    const { Client } = await import('@modelcontextprotocol/sdk/client/index.js');
+    const { StreamableHTTPClientTransport } = await import('@modelcontextprotocol/sdk/client/streamableHttp.js');
+
+    vi.spyOn(Client.prototype, 'connect').mockImplementation(async (transport) => {
+      expect(transport).toBeInstanceOf(StreamableHTTPClientTransport);
+      throw new Error('connection failed');
+    });
+
+    await expect(
+      createClientContext(definition, logger, clientInfo, { maxOAuthAttempts: 0 })
+    ).rejects.toThrow('connection failed');
+  });
+
   it.skip('promotes ad-hoc HTTP servers to OAuth after unauthorized, then retries', async () => {
     const fetchSpy = vi.spyOn(globalThis, 'fetch').mockImplementation(async () => {
       return new Response(null, { status: 401, statusText: 'Unauthorized' });