fix: strip $schema and convert array types in Gemini schema conversion

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

@@ -184,4 +184,80 @@ describe("toGoogle response_format transformation", () => {
     });
     expect(result.generationConfig?.responseSchema?.additionalProperties).toBeUndefined();
   });
+
+  it("should strip $schema 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",
+          schema: {
+            $schema: "http://json-schema.org/draft-07/schema#",
+            type: "object",
+            properties: {
+              name: { type: "string" },
+            },
+            additionalProperties: false,
+          },
+        },
+      },
+    };
+
+    const result = toGoogle(openAIRequest);
+
+    expect(result.generationConfig?.responseSchema).toBeDefined();
+    // $schema should be stripped
+    expect(result.generationConfig?.responseSchema?.$schema).toBeUndefined();
+    expect(result.generationConfig?.responseSchema?.type).toBe("object");
+  });
+
+  it("should convert array type with null to nullable for Gemini", () => {
+    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",
+          schema: {
+            type: "object",
+            properties: {
+              name: { type: "string" },
+              nickname: { type: ["string", "null"] },
+              age: { type: ["number", "null"] },
+            },
+          },
+        },
+      },
+    };
+
+    const result = toGoogle(openAIRequest);
+
+    expect(result.generationConfig?.responseSchema).toBeDefined();
+    // Array types should be converted to single type with nullable
+    expect(result.generationConfig?.responseSchema?.properties?.nickname).toEqual({
+      type: "string",
+      nullable: true,
+    });
+    expect(result.generationConfig?.responseSchema?.properties?.age).toEqual({
+      type: "number",
+      nullable: true,
+    });
+    // Non-nullable field should remain unchanged
+    expect(result.generationConfig?.responseSchema?.properties?.name).toEqual({
+      type: "string",
+    });
+  });
 });

packages/llm-mapper/transform/providers/openai/request/toGoogle.ts

@@ -352,6 +352,10 @@ function buildImageConfig(body: HeliconeChatCreateParams): GeminiImageConfig | u
  * OpenAI's strict mode requires additionalProperties: false on all object schemas,
  * but Gemini's API rejects this field with:
  * "Unknown name 'additionalProperties' at 'tools[0].function_declarations[0].parameters'"
+ *
+ * Also handles:
+ * - $schema: JSON Schema version identifier (not supported by Gemini)
+ * - type as array: OpenAI uses ["string", "null"] for nullable, Gemini uses nullable: true
  */
 function stripOpenAISchemaFields(schema: Record<string, any> | undefined): Record<string, any> | undefined {
   if (!schema || typeof schema !== 'object' || Array.isArray(schema)) {
@@ -363,6 +367,32 @@ function stripOpenAISchemaFields(schema: Record<string, any> | undefined): Recor
 
   // Remove OpenAI-specific fields
   delete cleaned.additionalProperties;
+  delete cleaned.$schema;
+
+  // Handle type as array (e.g., ["string", "null"] for nullable types)
+  // Gemini expects type as a single string and uses nullable: true separately
+  if (Array.isArray(cleaned.type)) {
+    const types = cleaned.type as string[];
+    const hasNull = types.includes("null");
+    const nonNullTypes = types.filter((t) => t !== "null");
+
+    if (nonNullTypes.length === 1) {
+      cleaned.type = nonNullTypes[0];
+      if (hasNull) {
+        cleaned.nullable = true;
+      }
+    } else if (nonNullTypes.length > 1) {
+      // Multiple non-null types - just take the first one as Gemini doesn't support union types
+      cleaned.type = nonNullTypes[0];
+      if (hasNull) {
+        cleaned.nullable = true;
+      }
+    } else if (hasNull && nonNullTypes.length === 0) {
+      // Only null type - shouldn't happen but handle gracefully
+      cleaned.type = "string";
+      cleaned.nullable = true;
+    }
+  }
 
   // Recurse into properties
   if (cleaned.properties && typeof cleaned.properties === 'object') {