fix: strip markdown JSON code blocks in OpenAI adapter responseFormat

packages/ai/__tests__/strip-json-code-blocks.test.ts

@@ -0,0 +1,47 @@
+/**
+ * Tests for stripJsonCodeBlock — strips markdown JSON code fences.
+ */
+
+import { describe, it, expect } from "vitest";
+import { stripJsonCodeBlock } from "../src/adapters/openai-adapter.js";
+
+describe("stripJsonCodeBlock", () => {
+  it("strips ```json code fences", () => {
+    const input = '```json\n{"name": "Alice", "age": 30}\n```';
+    expect(stripJsonCodeBlock(input)).toBe('{"name": "Alice", "age": 30}');
+  });
+
+  it("strips ``` code fences without language tag", () => {
+    const input = '```\n{"name": "Bob"}\n```';
+    expect(stripJsonCodeBlock(input)).toBe('{"name": "Bob"}');
+  });
+
+  it("strips fences with extra whitespace", () => {
+    const input = '```json  \n  {"key": "value"}  \n  ```  ';
+    expect(stripJsonCodeBlock(input)).toBe('{"key": "value"}');
+  });
+
+  it("handles multiline JSON in code block", () => {
+    const input = '```json\n{\n  "name": "Alice",\n  "age": 30\n}\n```';
+    expect(stripJsonCodeBlock(input)).toBe('{\n  "name": "Alice",\n  "age": 30\n}');
+  });
+
+  it("returns plain JSON unchanged", () => {
+    const input = '{"name": "Alice"}';
+    expect(stripJsonCodeBlock(input)).toBe('{"name": "Alice"}');
+  });
+
+  it("returns non-JSON text unchanged", () => {
+    const input = "Hello, world!";
+    expect(stripJsonCodeBlock(input)).toBe("Hello, world!");
+  });
+
+  it("handles empty input", () => {
+    expect(stripJsonCodeBlock("")).toBe("");
+  });
+
+  it("does not strip if code block is not the whole input", () => {
+    const input = 'Here is the JSON:\n```json\n{"key": "value"}\n```\nDone.';
+    expect(stripJsonCodeBlock(input)).toBe(input);
+  });
+});

packages/ai/src/adapters/openai-adapter.ts

@@ -25,6 +25,32 @@ import {
   TimeoutError,
 } from "../errors.js";
 
+// ---------------------------------------------------------------------------
+// JSON code-block stripping
+// ---------------------------------------------------------------------------
+
+/**
+ * Strip markdown JSON code blocks from model output.
+ *
+ * Many LLMs wrap JSON responses in ```json ... ``` fences even when
+ * asked for plain JSON. This function detects and removes them.
+ *
+ * Handles:
+ * - ```json ... ```
+ * - ``` ... ```
+ * - Leading/trailing whitespace around the code block
+ */
+export function stripJsonCodeBlock(input: string): string {
+  const text = input.trim();
+  // Match ```json ... ``` or ``` ... ```
+  const codeBlockPattern = /^```(?:json)?\s*\n?([\s\S]*?)\n?```\s*$/;
+  const match = text.match(codeBlockPattern);
+  if (match) {
+    return match[1].trim();
+  }
+  return text;
+}
+
 // ---------------------------------------------------------------------------
 // Think-tag stripping
 // ---------------------------------------------------------------------------
@@ -473,6 +499,11 @@ export abstract class OpenAIAdapter implements ModelProvider {
       reasoning = result.reasoning;
     }
 
+    // Strip markdown code blocks when JSON response format is requested
+    if (text && options.responseFormat?.type === "json") {
+      text = stripJsonCodeBlock(text);
+    }
+
     return {
       text,
       toolCalls,

packages/ai/src/index.ts

@@ -122,6 +122,7 @@ export {
   parseFinishReason,
   parseUsage,
   stripThinkTags,
+  stripJsonCodeBlock,
   type StripThinkTagsResult,
   type OpenAIMessage,
   type OpenAIToolCall,