diff --git a/.changeset/fix-capability-error.md b/.changeset/fix-capability-error.md
new file mode 100644
index 000000000..48db14a66
--- /dev/null
+++ b/.changeset/fix-capability-error.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/sdk": patch
+---
+
+fix: throw SdkError for unsupported capabilities in assertCapability()
diff --git a/packages/client/src/client/client.ts b/packages/client/src/client/client.ts
index edb08ee58..df9e703b1 100644
--- a/packages/client/src/client/client.ts
+++ b/packages/client/src/client/client.ts
@@ -434,7 +434,7 @@ export class Client extends Protocol<ClientContext> {
 
     protected assertCapability(capability: keyof ServerCapabilities, method: string): void {
         if (!this._serverCapabilities?.[capability]) {
-            throw new Error(`Server does not support ${capability} (required for ${method})`);
+            throw new SdkError(SdkErrorCode.CapabilityNotSupported, `Server does not support ${capability} (required for ${method})`);
         }
     }
 
diff --git a/test/integration/test/client/client.test.ts b/test/integration/test/client/client.test.ts
index 948d16e17..cfc4d180b 100644
--- a/test/integration/test/client/client.test.ts
+++ b/test/integration/test/client/client.test.ts
@@ -418,6 +418,12 @@ test('should respect server capabilities', async () => {
             argument: { name: 'test', value: 'test' }
         })
     ).rejects.toThrow('Server does not support completions');
+
+    // Verify that unsupported capability errors are SdkError with CapabilityNotSupported code
+    await expect(client.listPrompts()).rejects.toThrow(SdkError);
+    await expect(client.listPrompts()).rejects.toMatchObject({
+        code: SdkErrorCode.CapabilityNotSupported
+    });
 });
 
 /***