fix: add google json mode support + additionalProperties (#5464)

* fix: add json mode support + additionalProperties

* copilot PR comments applied

Author: H2Shami

packages/__tests__/llm-mapper/openai-to-google-response-format.test.ts

@@ -0,0 +1,187 @@
+import { describe, expect, it } from "@jest/globals";
+import { toGoogle } from "../../llm-mapper/transform/providers/openai/request/toGoogle";
+
+describe("toGoogle response_format transformation", () => {
+  it("should convert response_format with json_schema to Google's responseMimeType and responseSchema", () => {
+    const openAIRequest = {
+      model: "gemini-2.5-flash",
+      messages: [
+        {
+          role: "user" as const,
+          content: "Generate a JSON object with title and bullets fields",
+        },
+      ],
+      response_format: {
+        type: "json_schema" as const,
+        json_schema: {
+          name: "article",
+          schema: {
+            type: "object",
+            properties: {
+              title: { type: "string" },
+              bullets: {
+                type: "array",
+                items: { type: "string" },
+              },
+            },
+            required: ["title", "bullets"],
+          },
+        },
+      },
+    };
+
+    const result = toGoogle(openAIRequest);
+
+    // Should have generationConfig with responseMimeType and responseSchema
+    expect(result.generationConfig).toBeDefined();
+    expect(result.generationConfig?.responseMimeType).toBe("application/json");
+    expect(result.generationConfig?.responseSchema).toBeDefined();
+    expect(result.generationConfig?.responseSchema).toEqual({
+      type: "object",
+      properties: {
+        title: { type: "string" },
+        bullets: {
+          type: "array",
+          items: { type: "string" },
+        },
+      },
+      required: ["title", "bullets"],
+    });
+  });
+
+  it("should convert response_format with type json_object to responseMimeType application/json", () => {
+    const openAIRequest = {
+      model: "gemini-2.5-flash",
+      messages: [
+        {
+          role: "user" as const,
+          content: "Return a JSON object",
+        },
+      ],
+      response_format: {
+        type: "json_object" as const,
+      },
+    };
+
+    const result = toGoogle(openAIRequest);
+
+    expect(result.generationConfig).toBeDefined();
+    expect(result.generationConfig?.responseMimeType).toBe("application/json");
+    // No responseSchema since json_object doesn't provide one
+    expect(result.generationConfig?.responseSchema).toBeUndefined();
+  });
+
+  it("should handle response_format with type text (default behavior)", () => {
+    const openAIRequest = {
+      model: "gemini-2.5-flash",
+      messages: [
+        {
+          role: "user" as const,
+          content: "Just return plain text",
+        },
+      ],
+      response_format: {
+        type: "text" as const,
+      },
+    };
+
+    const result = toGoogle(openAIRequest);
+
+    // Should NOT set responseMimeType for text (default behavior)
+    expect(result.generationConfig?.responseMimeType).toBeUndefined();
+  });
+
+  it("should strip additionalProperties from json_schema when converting", () => {
+    const openAIRequest = {
+      model: "gemini-2.5-flash",
+      messages: [
+        {
+          role: "user" as const,
+          content: "Generate structured output",
+        },
+      ],
+      response_format: {
+        type: "json_schema" as const,
+        json_schema: {
+          name: "result",
+          strict: true,
+          schema: {
+            type: "object",
+            properties: {
+              name: { type: "string" },
+              nested: {
+                type: "object",
+                properties: {
+                  value: { type: "number" },
+                },
+                additionalProperties: false,
+              },
+            },
+            required: ["name"],
+            additionalProperties: false,
+          },
+        },
+      },
+    };
+
+    const result = toGoogle(openAIRequest);
+
+    expect(result.generationConfig?.responseSchema).toBeDefined();
+    // additionalProperties should be stripped
+    expect(result.generationConfig?.responseSchema?.additionalProperties).toBeUndefined();
+    expect(result.generationConfig?.responseSchema?.properties?.nested?.additionalProperties).toBeUndefined();
+  });
+
+  it("should handle Vercel AI SDK generateObject style request", () => {
+    // This simulates what Vercel AI SDK sends for generateObject
+    const openAIRequest = {
+      model: "gemini-2.5-flash",
+      messages: [
+        {
+          role: "system" as const,
+          content: "You are a helpful assistant that generates structured data.",
+        },
+        {
+          role: "user" as const,
+          content: "Generate an article about AI",
+        },
+      ],
+      response_format: {
+        type: "json_schema" as const,
+        json_schema: {
+          name: "article",
+          schema: {
+            type: "object",
+            properties: {
+              title: { type: "string", description: "The article title" },
+              content: { type: "string", description: "The article content" },
+              tags: {
+                type: "array",
+                items: { type: "string" },
+                description: "Tags for the article",
+              },
+            },
+            required: ["title", "content", "tags"],
+            additionalProperties: false,
+          },
+          strict: true,
+        },
+      },
+    };
+
+    const result = toGoogle(openAIRequest);
+
+    // Should properly convert to Google format
+    expect(result.generationConfig).toBeDefined();
+    expect(result.generationConfig?.responseMimeType).toBe("application/json");
+    expect(result.generationConfig?.responseSchema).toBeDefined();
+
+    // Schema should be present without additionalProperties
+    expect(result.generationConfig?.responseSchema?.type).toBe("object");
+    expect(result.generationConfig?.responseSchema?.properties?.title).toEqual({
+      type: "string",
+      description: "The article title",
+    });
+    expect(result.generationConfig?.responseSchema?.additionalProperties).toBeUndefined();
+  });
+});