docs: add FixSpec wrapper for CreateMessageResult schema divergence

Document the intentional divergence between SDK's CreateMessageResult
(single content for v1.x backwards compat) and the spec's version
(array content per SEP-1577).

The full array-capable type is available as CreateMessageResultWithTools.
This will be aligned with schema in v2.0.

Author: felixweinberger

src/examples/server/toolWithSampleServer.ts

@@ -33,7 +33,7 @@ mcpServer.registerTool(
             maxTokens: 500
         });
 
-        // Since we're not using tools, response.content is guaranteed to be single content
+        // Since we're not passing tools param to createMessage, response.content is single content
         return {
             content: [
                 {

src/spec.types.test.ts

@@ -80,6 +80,21 @@ type FixSpecInitializeRequest<T> = T extends { params: infer P } ? Omit<T, 'para
 
 type FixSpecClientRequest<T> = T extends { params: infer P } ? Omit<T, 'params'> & { params: FixSpecInitializeRequestParams<P> } : T;
 
+// Targeted fix: CreateMessageResult in SDK uses single content for v1.x backwards compat.
+// The full array-capable type is CreateMessageResultWithTools.
+// This will be aligned with schema in v2.0.
+// Narrows content from SamplingMessageContentBlock (includes tool types) to basic content types only.
+type NarrowToBasicContent<C> = C extends { type: 'text' | 'image' | 'audio' } ? C : never;
+type FixSpecCreateMessageResult<T> = T extends { content: infer C; role: infer R; model: infer M }
+    ? {
+          _meta?: { [key: string]: unknown };
+          model: M;
+          role: R;
+          stopReason?: string;
+          content: C extends (infer U)[] ? NarrowToBasicContent<U> : NarrowToBasicContent<C>;
+      }
+    : T;
+
 const sdkTypeChecks = {
     RequestParams: (sdk: RemovePassthrough<SDKTypes.RequestParams>, spec: SpecTypes.RequestParams) => {
         sdk = spec;
@@ -369,7 +384,10 @@ const sdkTypeChecks = {
         sdk = spec;
         spec = sdk;
     },
-    CreateMessageResult: (sdk: RemovePassthrough<SDKTypes.CreateMessageResult>, spec: SpecTypes.CreateMessageResult) => {
+    CreateMessageResult: (
+        sdk: RemovePassthrough<SDKTypes.CreateMessageResult>,
+        spec: FixSpecCreateMessageResult<SpecTypes.CreateMessageResult>
+    ) => {
         sdk = spec;
         spec = sdk;
     },