Fix Gemini 3 on OpenRouter (#43416)

Release Notes:

- Gemini 3 now works on OpenRouter in the Agent Panel

crates/agent/src/thread.rs

@@ -113,6 +113,7 @@ impl Message {
                 role: Role::User,
                 content: vec!["Continue where you left off".into()],
                 cache: false,
+                reasoning_details: None,
             }],
         }
     }
@@ -177,6 +178,7 @@ impl UserMessage {
             role: Role::User,
             content: Vec::with_capacity(self.content.len()),
             cache: false,
+            reasoning_details: None,
         };
 
         const OPEN_CONTEXT: &str = "<context>\n\
@@ -444,6 +446,7 @@ impl AgentMessage {
             role: Role::Assistant,
             content: Vec::with_capacity(self.content.len()),
             cache: false,
+            reasoning_details: self.reasoning_details.clone(),
         };
         for chunk in &self.content {
             match chunk {
@@ -479,6 +482,7 @@ impl AgentMessage {
             role: Role::User,
             content: Vec::new(),
             cache: false,
+            reasoning_details: None,
         };
 
         for tool_result in self.tool_results.values() {
@@ -508,6 +512,7 @@ impl AgentMessage {
 pub struct AgentMessage {
     pub content: Vec<AgentMessageContent>,
     pub tool_results: IndexMap<LanguageModelToolUseId, LanguageModelToolResult>,
+    pub reasoning_details: Option<serde_json::Value>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
@@ -1398,6 +1403,18 @@ impl Thread {
                 self.handle_thinking_event(text, signature, event_stream, cx)
             }
             RedactedThinking { data } => self.handle_redacted_thinking_event(data, cx),
+            ReasoningDetails(details) => {
+                let last_message = self.pending_message();
+                // Store the last non-empty reasoning_details (overwrites earlier ones)
+                // This ensures we keep the encrypted reasoning with signatures, not the early text reasoning
+                if let serde_json::Value::Array(ref arr) = details {
+                    if !arr.is_empty() {
+                        last_message.reasoning_details = Some(details);
+                    }
+                } else {
+                    last_message.reasoning_details = Some(details);
+                }
+            }
             ToolUse(tool_use) => {
                 return Ok(self.handle_tool_use_event(tool_use, event_stream, cx));
             }
@@ -1673,6 +1690,7 @@ impl Thread {
             role: Role::User,
             content: vec![SUMMARIZE_THREAD_DETAILED_PROMPT.into()],
             cache: false,
+            reasoning_details: None,
         });
 
         let task = cx
@@ -1737,6 +1755,7 @@ impl Thread {
             role: Role::User,
             content: vec![SUMMARIZE_THREAD_PROMPT.into()],
             cache: false,
+            reasoning_details: None,
         });
         self.pending_title_generation = Some(cx.spawn(async move |this, cx| {
             let mut title = String::new();
@@ -1984,6 +2003,7 @@ impl Thread {
             role: Role::System,
             content: vec![system_prompt.into()],
             cache: false,
+            reasoning_details: None,
         }];
         for message in &self.messages {
             messages.extend(message.to_request());