use Gemini systemInstruction instead of fake user/model exchange

Author: nvms

threads/src/providers/google.ts

@@ -20,18 +20,6 @@ export const callGoogle = async (
   const apiKey = getApiKey(configApiKey);
 
   const contents = [];
-
-  if (instructions) {
-    contents.push({
-      role: "user",
-      parts: [{ text: instructions }],
-    });
-    contents.push({
-      role: "model",
-      parts: [{ text: "I understand." }],
-    });
-  }
-
   const toolCallMap = new Map<string, string>();
 
   for (let i = 0; i < ctx.history.length; i++) {
@@ -101,9 +89,13 @@ export const callGoogle = async (
     }
   }
 
-  const body: any = {
-    contents,
-  };
+  const body: any = { contents };
+
+  if (instructions) {
+    body.systemInstruction = {
+      parts: [{ text: instructions }],
+    };
+  }
 
   if (ctx.tools && ctx.tools.length > 0) {
     body.tools = [

threads/tests/google-provider.test.ts

@@ -219,7 +219,7 @@ describe("Google provider tool call formatting", () => {
     expect(capturedBody.contents[0].parts[0].text).toBe("Do something");
   });
 
-  it("includes instructions as initial user/model exchange", async () => {
+  it("passes instructions as systemInstruction", async () => {
     const ctx: ConversationContext = {
       history: [{ role: "user", content: "Hello" }],
       tools: [],
@@ -230,10 +230,23 @@ describe("Google provider tool call formatting", () => {
       ctx
     );
 
-    expect(capturedBody.contents).toHaveLength(3);
-    expect(capturedBody.contents[0].parts[0].text).toBe("You are helpful");
-    expect(capturedBody.contents[1].parts[0].text).toBe("I understand.");
-    expect(capturedBody.contents[2].parts[0].text).toBe("Hello");
+    expect(capturedBody.contents).toHaveLength(1);
+    expect(capturedBody.contents[0].parts[0].text).toBe("Hello");
+    expect(capturedBody.systemInstruction).toEqual({
+      parts: [{ text: "You are helpful" }],
+    });
+  });
+
+  it("omits systemInstruction when no instructions provided", async () => {
+    const ctx: ConversationContext = {
+      history: [{ role: "user", content: "Hello" }],
+      tools: [],
+    };
+
+    await callGoogle({ model: "gemini-2.0-flash" }, ctx);
+
+    expect(capturedBody.systemInstruction).toBeUndefined();
+    expect(capturedBody.contents).toHaveLength(1);
   });
 
   it("streaming: formats tool results correctly", async () => {