fix(google-common): pass structured objects directly in FunctionResponse

Parse JSON string content and pass the resulting object directly as
FunctionResponse.response instead of wrapping in { content: ... }.
Plain strings and arrays are wrapped in { result: ... }.

Author: jackjin1997

.changeset/fix-google-tool-message-structured.md

@@ -0,0 +1,5 @@
+---
+"@langchain/google-common": patch
+---
+
+Pass structured JSON objects directly in `FunctionResponse.response` instead of wrapping them in `{ content: ... }`, matching the Gemini API's expected format.

libs/providers/langchain-google-common/src/tests/utils.test.ts

@@ -1092,3 +1092,81 @@ describe("gemini empty text content handling", () => {
     expect(textPart.text).toBe("I am doing well");
   });
 });
+
+describe("gemini tool message formatting", () => {
+  test("passes JSON object content directly in FunctionResponse", async () => {
+    const api = getGeminiAPI();
+    const messages = [
+      new HumanMessage("What is the weather?"),
+      new AIMessage({
+        content: "",
+        tool_calls: [
+          { id: "call_1", name: "get_weather", args: { city: "SF" } },
+        ],
+      }),
+      new ToolMessage({
+        tool_call_id: "call_1",
+        content: JSON.stringify({ temperature: 72, unit: "F" }),
+      }),
+    ];
+
+    const formatted = (await api.formatData(messages, {})) as GeminiRequest;
+    const functionContent = formatted.contents?.find(
+      (c) => c.role === "function"
+    );
+    const part = functionContent?.parts[0] as any;
+    expect(part.functionResponse.response.temperature).toBe(72);
+    expect(part.functionResponse.response.unit).toBe("F");
+  });
+
+  test("wraps plain string content in result field", async () => {
+    const api = getGeminiAPI();
+    const messages = [
+      new HumanMessage("What is the weather?"),
+      new AIMessage({
+        content: "",
+        tool_calls: [
+          { id: "call_1", name: "get_weather", args: { city: "SF" } },
+        ],
+      }),
+      new ToolMessage({
+        tool_call_id: "call_1",
+        content: "sunny and 72°F",
+      }),
+    ];
+
+    const formatted = (await api.formatData(messages, {})) as GeminiRequest;
+    const functionContent = formatted.contents?.find(
+      (c) => c.role === "function"
+    );
+    const part = functionContent?.parts[0] as any;
+    expect(part.functionResponse.response).toEqual({ result: "sunny and 72°F" });
+  });
+
+  test("wraps JSON array content in result field", async () => {
+    const api = getGeminiAPI();
+    const messages = [
+      new HumanMessage("Search"),
+      new AIMessage({
+        content: "",
+        tool_calls: [
+          { id: "call_1", name: "search", args: { q: "restaurants" } },
+        ],
+      }),
+      new ToolMessage({
+        tool_call_id: "call_1",
+        content: JSON.stringify([{ name: "A" }, { name: "B" }]),
+      }),
+    ];
+
+    const formatted = (await api.formatData(messages, {})) as GeminiRequest;
+    const functionContent = formatted.contents?.find(
+      (c) => c.role === "function"
+    );
+    const part = functionContent?.parts[0] as any;
+    expect(part.functionResponse.response.result).toEqual([
+      { name: "A" },
+      { name: "B" },
+    ]);
+  });
+});

libs/providers/langchain-google-common/src/utils/gemini.ts

@@ -893,41 +893,38 @@ export function getGeminiAPI(config?: GeminiAPIConfig): GoogleAIAPI {
             },
             ""
           );
-    // Hacky :(
     const responseName =
       (isAIMessage(prevMessage) && !!prevMessage.tool_calls?.length
         ? prevMessage.tool_calls[0].name
         : prevMessage.name) ?? message.tool_call_id;
+
+    // Gemini accepts any JSON object as FunctionResponse.response.
+    // Parse JSON strings to pass structured data directly instead of
+    // wrapping in { content: stringified }.
+    let response: Record<string, unknown>;
     try {
-      const content = JSON.parse(contentStr);
-      return [
-        {
-          role: "function",
-          parts: [
-            {
-              functionResponse: {
-                name: responseName,
-                response: { content },
-              },
-            },
-          ],
-        },
-      ];
+      const parsed = JSON.parse(contentStr);
+      response =
+        typeof parsed === "object" && parsed !== null && !Array.isArray(parsed)
+          ? parsed
+          : { result: parsed };
     } catch (_) {
-      return [
-        {
-          role: "function",
-          parts: [
-            {
-              functionResponse: {
-                name: responseName,
-                response: { content: contentStr },
-              },
-            },
-          ],
-        },
-      ];
+      response = { result: contentStr };
     }
+
+    return [
+      {
+        role: "function",
+        parts: [
+          {
+            functionResponse: {
+              name: responseName,
+              response,
+            },
+          },
+        ],
+      },
+    ];
   }
 
   async function baseMessageToContent(