fix: use text injection for Gemini models to avoid thought signature errors

Gemini 3+ models have strict validation requiring thoughtSignature on functionCall parts. When the plugin injects synthetic assistant messages with tool parts, providers without robust signature handling will fail with 400 errors.

This change detects Gemini models and injects as text parts instead of tool parts, avoiding the thought signature requirement entirely.

Author: Tarquinen

lib/messages/utils.ts

@@ -7,32 +7,58 @@ const SYNTHETIC_MESSAGE_ID = "msg_01234567890123456789012345"
 const SYNTHETIC_PART_ID = "prt_01234567890123456789012345"
 const SYNTHETIC_CALL_ID = "call_01234567890123456789012345"
 
+const isGeminiModel = (modelID: string): boolean => {
+    const lowerModelID = modelID.toLowerCase()
+    return lowerModelID.includes("gemini")
+}
+
 export const createSyntheticAssistantMessageWithToolPart = (
     baseMessage: WithParts,
     content: string,
     variant?: string,
 ): WithParts => {
     const userInfo = baseMessage.info as UserMessage
     const now = Date.now()
-    return {
-        info: {
-            id: SYNTHETIC_MESSAGE_ID,
-            sessionID: userInfo.sessionID,
-            role: "assistant",
-            agent: userInfo.agent || "code",
-            parentID: userInfo.id,
-            modelID: userInfo.model.modelID,
-            providerID: userInfo.model.providerID,
-            mode: "default",
-            path: {
-                cwd: "/",
-                root: "/",
-            },
-            time: { created: now, completed: now },
-            cost: 0,
-            tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
-            ...(variant !== undefined && { variant }),
+
+    const baseInfo = {
+        id: SYNTHETIC_MESSAGE_ID,
+        sessionID: userInfo.sessionID,
+        role: "assistant" as const,
+        agent: userInfo.agent || "code",
+        parentID: userInfo.id,
+        modelID: userInfo.model.modelID,
+        providerID: userInfo.model.providerID,
+        mode: "default",
+        path: {
+            cwd: "/",
+            root: "/",
         },
+        time: { created: now, completed: now },
+        cost: 0,
+        tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+        ...(variant !== undefined && { variant }),
+    }
+
+    // For Gemini models, inject as text to avoid thought signature requirements
+    // Gemini 3+ has strict validation requiring thoughtSignature on functionCall parts
+    if (isGeminiModel(userInfo.model.modelID)) {
+        return {
+            info: baseInfo,
+            parts: [
+                {
+                    id: SYNTHETIC_PART_ID,
+                    sessionID: userInfo.sessionID,
+                    messageID: SYNTHETIC_MESSAGE_ID,
+                    type: "text",
+                    text: content,
+                },
+            ],
+        }
+    }
+
+    // For other models, use tool part for cleaner context
+    return {
+        info: baseInfo,
         parts: [
             {
                 id: SYNTHETIC_PART_ID,