feat: Added support for thought-signatures for Gemini 3+ models

src/client/vertexai.rs

@@ -219,7 +219,14 @@ pub async fn gemini_chat_completions_streaming(
                         part["functionCall"]["name"].as_str(),
                         part["functionCall"]["args"].as_object(),
                     ) {
-                        handler.tool_call(ToolCall::new(name.to_string(), json!(args), None))?;
+                        let thought_signature = part["thoughtSignature"]
+                          .as_str()
+                          .or_else(|| part["thought_signature"].as_str())
+                          .map(|s| s.to_string());
+                        handler.tool_call(
+                            ToolCall::new(name.to_string(), json!(args), None)
+                              .with_thought_signature(thought_signature),
+                        )?;
                     }
                 }
             } else if let Some("SAFETY") = data["promptFeedback"]["blockReason"]
@@ -280,7 +287,14 @@ fn gemini_extract_chat_completions_text(data: &Value) -> Result<ChatCompletionsO
                 part["functionCall"]["name"].as_str(),
                 part["functionCall"]["args"].as_object(),
             ) {
-                tool_calls.push(ToolCall::new(name.to_string(), json!(args), None));
+                let thought_signature = part["thoughtSignature"]
+                  .as_str()
+                  .or_else(|| part["thought_signature"].as_str())
+                  .map(|s| s.to_string());
+                tool_calls.push(
+                    ToolCall::new(name.to_string(), json!(args), None)
+                      .with_thought_signature(thought_signature),
+                );
             }
         }
     }
@@ -347,12 +361,16 @@ pub fn gemini_build_chat_completions_body(
                     },
                     MessageContent::ToolCalls(MessageContentToolCalls { tool_results, .. }) => {
                         let model_parts: Vec<Value> = tool_results.iter().map(|tool_result| {
-                            json!({
+                            let mut part = json!({
                                 "functionCall": {
                                     "name": tool_result.call.name,
                                     "args": tool_result.call.arguments,
                                 }
-                            })
+                            });
+                            if let Some(sig) = &tool_result.call.thought_signature {
+                                part["thoughtSignature"] = json!(sig);
+                            }
+                            part
                         }).collect();
                         let function_parts: Vec<Value> = tool_results.into_iter().map(|tool_result| {
                             json!({

src/function/mod.rs

@@ -756,6 +756,10 @@ pub struct ToolCall {
     pub name: String,
     pub arguments: Value,
     pub id: Option<String>,
+    /// Gemini 3's thought signature for stateful reasoning in function calling.
+    /// Must be preserved and sent back when submitting function responses.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub thought_signature: Option<String>,
 }
 
 type CallConfig = (String, String, Vec<String>, HashMap<String, String>);
@@ -785,9 +789,15 @@ impl ToolCall {
             name,
             arguments,
             id,
+            thought_signature: None,
         }
     }
 
+    pub fn with_thought_signature(mut self, thought_signature: Option<String>) -> Self {
+        self.thought_signature = thought_signature;
+        self
+    }
+
     pub async fn eval(&self, config: &GlobalConfig) -> Result<Value> {
         let (call_name, cmd_name, mut cmd_args, envs) = match &config.read().agent {
             Some(agent) => self.extract_call_config_from_agent(config, agent)?,