diff --git a/src/client/index.test.ts b/src/client/index.test.ts
index bbfa80faf..d555aa4d9 100644
--- a/src/client/index.test.ts
+++ b/src/client/index.test.ts
@@ -843,6 +843,7 @@ describe('outputSchema validation', () => {
       if (request.params.name === 'test-tool') {
         return {
           structuredContent: { result: 'success', count: 42 },
+          content: [],  // compatibility: remove in 1.12+
         };
       }
       throw new Error('Unknown tool');
@@ -918,6 +919,7 @@ describe('outputSchema validation', () => {
         // Return invalid structured content (count is string instead of number)
         return {
           structuredContent: { result: 'success', count: 'not a number' },
+          content: [],  // compatibility: remove in 1.12+
         };
       }
       throw new Error('Unknown tool');
@@ -1155,6 +1157,7 @@ describe('outputSchema validation', () => {
               created: '2023-01-01T00:00:00Z',
             },
           },
+          content: [],  // compatibility: remove in 1.12+
         };
       }
       throw new Error('Unknown tool');
@@ -1235,6 +1238,7 @@ describe('outputSchema validation', () => {
             name: 'John',
             extraField: 'not allowed',
           },
+          content: [],  // compatibility: remove in 1.12+
         };
       }
       throw new Error('Unknown tool');
diff --git a/src/server/mcp.ts b/src/server/mcp.ts
index a5f2d0f1a..4f506c003 100644
--- a/src/server/mcp.ts
+++ b/src/server/mcp.ts
@@ -40,12 +40,36 @@ import {
   ServerRequest,
   ServerNotification,
   ToolAnnotations,
+  ResultSchema,
+  ContentListSchema,
+  CallToolUnstructuredResultSchema,
+  Infer,
 } from "../types.js";
 import { Completable, CompletableDef } from "./completable.js";
 import { UriTemplate, Variables } from "../shared/uriTemplate.js";
 import { RequestHandlerExtra } from "../shared/protocol.js";
 import { Transport } from "../shared/transport.js";
 
+/**
+ * In SDK versions 1.12 and forward, the `content` array will be optional for structured
+ * tool call results. For convenience we use this type for defining tool callbacks on the 
+ * server side, but provide a backward-compatible schema for the client in 1.11.
+ */
+const CallToolStructuredResultSchema_forward = ResultSchema.extend({
+  structuredContent: z.object({}).passthrough(),
+  content: z.optional(ContentListSchema),
+  isError: z.optional(z.boolean()),
+});
+
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const CallToolResultSchema_forward = z.union([
+  CallToolUnstructuredResultSchema,
+  CallToolStructuredResultSchema_forward,
+]);
+
+type CallToolResult_forward = Infer<typeof CallToolResultSchema_forward>;
+
+
 /**
  * High-level MCP server that provides a simpler API for working with resources, tools, and prompts.
  * For advanced usage (like sending notifications or setting custom request handlers), use the underlying
@@ -153,7 +177,7 @@ export class McpServer {
           );
         }
 
-        let result: CallToolResult;
+        let result: CallToolResult_forward;
 
         if (tool.inputSchema) {
           const parseResult = await tool.inputSchema.safeParseAsync(
@@ -236,7 +260,7 @@ export class McpServer {
           }
         }
 
-        return result;
+        return result as CallToolResult;
       },
     );
 
@@ -1009,8 +1033,8 @@ export type ToolCallback<Args extends undefined | ZodRawShape = undefined> =
   ? (
     args: z.objectOutputType<Args, ZodTypeAny>,
     extra: RequestHandlerExtra<ServerRequest, ServerNotification>,
-  ) => CallToolResult | Promise<CallToolResult>
-  : (extra: RequestHandlerExtra<ServerRequest, ServerNotification>) => CallToolResult | Promise<CallToolResult>;
+  ) => CallToolResult_forward | Promise<CallToolResult_forward>
+  : (extra: RequestHandlerExtra<ServerRequest, ServerNotification>) => CallToolResult_forward | Promise<CallToolResult_forward>;
 
 export type RegisteredTool = {
   description?: string;
diff --git a/src/types.ts b/src/types.ts
index bd299c8f7..f118e1ed4 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -924,14 +924,14 @@ export const CallToolStructuredResultSchema = ResultSchema.extend({
   /**
    * A list of content objects that represent the result of the tool call.
    *
-   * If the Tool defines an outputSchema, this field MAY be present in the result.
+   * Per the spec, if the Tool defines an outputSchema, this field MAY be present in the result.
    * 
-   * Tools may use this field to provide compatibility with older clients that 
-   * do not support structured content.
+   * In this SDK we automatically generate backwards-compatible `content` for older clients, 
+   * so we define this field as non-optional for backwards compatibility in SDK versions 1.11.*.
    * 
    * Clients that support structured content should ignore this field.
    */
-  content: z.optional(ContentListSchema),
+  content: ContentListSchema,
 
   /**
    * Whether the tool call ended in an error.
@@ -1319,7 +1319,7 @@ type Flatten<T> = T extends Primitive
   ? { [K in keyof T]: Flatten<T[K]> }
   : T;
 
-type Infer<Schema extends ZodTypeAny> = Flatten<z.infer<Schema>>;
+export type Infer<Schema extends ZodTypeAny> = Flatten<z.infer<Schema>>;
 
 /* JSON-RPC types */
 export type ProgressToken = Infer<typeof ProgressTokenSchema>;