fix(server): mirror structured MCP results into text

.changeset/calm-tools-show.md

@@ -0,0 +1,5 @@
+---
+'@adcp/sdk': patch
+---
+
+Mirror final MCP `structuredContent` into a serialized text block so text-only clients retain complete AdCP results.

src/lib/server/create-adcp-server.ts

@@ -5201,6 +5201,18 @@ export function createAdcpServer<TAccount = unknown>(config: AdcpServerConfig<TA
 
   const applyResponseEnhancer = (response: McpToolResponse): McpToolResponse => {
     responseEnhancer?.(response);
+    const structuredContent = response.structuredContent;
+    if (structuredContent !== undefined) {
+      const serialized = JSON.stringify(structuredContent);
+      const hasSerializedFallback = response.content.some(block => block.type === 'text' && block.text === serialized);
+      if (!hasSerializedFallback) {
+        // MCP recommends mirroring structuredContent into a TextContent block
+        // for clients that do not expose the structured channel to the model.
+        // Preserve any adopter-authored summary as the first block and append
+        // the exact final wire object after every framework/enhancer rewrite.
+        response.content.push({ type: 'text', text: serialized });
+      }
+    }
     return response;
   };
 

test/server-create-adcp-server.test.js

@@ -397,6 +397,40 @@ describe('createAdcpServer', () => {
     assert.deepStrictEqual(sawParams, { brief: 'premium' });
     assert.ok(result.content, 'CallToolResult should carry content');
     assert.ok(result.structuredContent, 'CallToolResult should carry structuredContent');
+    assert.ok(
+      result.content.some(block => block.type === 'text' && block.text === JSON.stringify(result.structuredContent)),
+      'CallToolResult should serialize structuredContent for text-only MCP clients'
+    );
+  });
+
+  it('preserves the human summary and appends one final structured-content fallback', async () => {
+    const server = createAdcpServer({
+      name: 'Test',
+      version: '1.0.0',
+      responseEnhancer: response => {
+        response.structuredContent.enhanced = true;
+      },
+      mediaBuy: {
+        getProducts: async () => ({
+          products: [
+            {
+              product_id: 'p1',
+              name: 'CTV package',
+              pricing_options: [{ pricing_model: 'cpm', rate: 12, currency: 'USD' }],
+            },
+          ],
+          cache_scope: 'public',
+        }),
+      },
+    });
+
+    const result = await callToolRaw(server, 'get_products', {});
+    const serialized = JSON.stringify(result.structuredContent);
+
+    assert.strictEqual(result.content[0].text, 'Found 1 products');
+    assert.strictEqual(result.content.filter(block => block.text === serialized).length, 1);
+    assert.strictEqual(JSON.parse(result.content.at(-1).text).products[0].name, 'CTV package');
+    assert.strictEqual(JSON.parse(result.content.at(-1).text).enhanced, true);
   });
 
   it('dispatchTestRequest throws for unknown tools and methods', async () => {