fix(server): add structured content fallback without content

Author: mattzcarey

packages/server/src/server/mcp.ts

@@ -29,7 +29,6 @@ import type {
 import {
     assertCompleteRequestPrompt,
     assertCompleteRequestResourceTemplate,
-    isCallToolResult,
     normalizeRawShapeSchema,
     promptArgumentsFromStandardSchema,
     ProtocolError,
@@ -177,7 +176,7 @@ export class McpServer {
                 // Per SEP-2106, a server returning array or primitive structuredContent MUST also emit a
                 // TextContent block with the serialized JSON, so pre-SEP clients that only understand
                 // object-typed structuredContent can fall back to the text content.
-                return isCallToolResult(result) ? withStructuredContentTextFallback(result) : result;
+                return withStructuredContentTextFallback(result);
             } catch (error) {
                 if (error instanceof ProtocolError && error.code === ProtocolErrorCode.UrlElicitationRequired) {
                     throw error; // Return the error to the caller without wrapping in CallToolResult
@@ -1191,10 +1190,11 @@ function withStructuredContentTextFallback(result: CallToolResult): CallToolResu
     if (isPlainObject) {
         return result;
     }
-    if (result.content.some(block => block.type === 'text')) {
+    const content = Array.isArray(result.content) ? result.content : [];
+    if (content.some(block => block.type === 'text')) {
         return result;
     }
-    return { ...result, content: [...result.content, { type: 'text', text: JSON.stringify(structuredContent) }] };
+    return { ...result, content: [...content, { type: 'text', text: JSON.stringify(structuredContent) }] };
 }
 
 /**

packages/server/test/server/mcp.compat.test.ts

@@ -1,4 +1,4 @@
-import type { JSONRPCMessage } from '@modelcontextprotocol/core';
+import type { CallToolResultWithStructuredContent, JSONRPCMessage } from '@modelcontextprotocol/core';
 import { InMemoryTransport, isStandardSchema, LATEST_PROTOCOL_VERSION } from '@modelcontextprotocol/core';
 import { describe, expect, expectTypeOf, it, vi } from 'vitest';
 import * as z from 'zod/v4';
@@ -119,6 +119,50 @@ describe('registerTool/registerPrompt accept raw Zod shape (auto-wrapped)', () =
 
         await server.close();
     });
+
+    it('adds text fallback for structuredContent when an untyped handler omits content', async () => {
+        const server = new McpServer({ name: 't', version: '1.0.0' });
+        const untypedHandler = (async () => ({ structuredContent: [1, 2, 3] })) as unknown as () => Promise<
+            CallToolResultWithStructuredContent<number[]>
+        >;
+
+        server.registerTool('forecast', { outputSchema: z.array(z.number()) }, untypedHandler);
+
+        const [client, srv] = InMemoryTransport.createLinkedPair();
+        await server.connect(srv);
+        await client.start();
+
+        const responses: JSONRPCMessage[] = [];
+        client.onmessage = m => responses.push(m);
+
+        await client.send({
+            jsonrpc: '2.0',
+            id: 1,
+            method: 'initialize',
+            params: {
+                protocolVersion: LATEST_PROTOCOL_VERSION,
+                capabilities: {},
+                clientInfo: { name: 'c', version: '1.0.0' }
+            }
+        } as JSONRPCMessage);
+        await client.send({ jsonrpc: '2.0', method: 'notifications/initialized' } as JSONRPCMessage);
+        await client.send({
+            jsonrpc: '2.0',
+            id: 2,
+            method: 'tools/call',
+            params: { name: 'forecast', arguments: {} }
+        } as JSONRPCMessage);
+
+        await vi.waitFor(() => expect(responses.some(r => 'id' in r && r.id === 2)).toBe(true));
+
+        const response = responses.find(r => 'id' in r && r.id === 2) as {
+            result?: { content?: Array<{ type: string; text?: string }>; structuredContent?: unknown };
+        };
+        expect(response.result?.structuredContent).toEqual([1, 2, 3]);
+        expect(response.result?.content).toEqual([{ type: 'text', text: '[1,2,3]' }]);
+
+        await server.close();
+    });
 });
 
 describe('InferRawShape', () => {