fix(engine): offer localhost_preview in the request that describes it

rustcode/engine/src/network.rs

@@ -198,6 +198,25 @@ pub(crate) const CHATGPT_TURN_STATE_HEADER: &str = "x-codex-turn-state";
 /// synchronous MCP/filesystem work. Cache metadata is snapshotted before the
 /// computation and sticky names are committed only if that snapshot is still
 /// current when the computation finishes.
+/// The root agent's tool policy for a request. The system prompt and the
+/// provider request both build it here, so a tool the prompt describes
+/// (`localhost_preview`) is always in the request's `tools` too.
+pub(crate) fn root_schema_policy(
+    s: &AppState,
+    compact_tool_prompt: bool,
+) -> crate::tools::ToolSchemaPolicy {
+    let policy = crate::tools::ToolSchemaPolicy::root_for_mode_with_compact_prompt(
+        s.delegation_active,
+        s.agent_mode,
+        compact_tool_prompt,
+    );
+    if s.preview_client.is_some() && !compact_tool_prompt {
+        policy.with_preview_tool()
+    } else {
+        policy
+    }
+}
+
 pub(crate) async fn prepare_native_tool_schemas(
     state: &Arc<Mutex<AppState>>,
     policy: crate::tools::ToolSchemaPolicy,
@@ -1558,17 +1577,10 @@ pub(crate) async fn prepare_turn_request_with_checkpoint_and_prefix_cache(
             && !matches!(protocol, crate::config::ToolProtocol::ApiNative);
         let session_title_tool_available =
             s.session_title_tool_available && agent_mode != crate::config::AgentMode::Plan;
-        let mut schema_policy = crate::tools::ToolSchemaPolicy::root_for_mode_with_compact_prompt(
-            delegation_active,
-            agent_mode,
-            compact_tool_prompt,
-        );
+        let mut schema_policy = root_schema_policy(&s, compact_tool_prompt);
         if session_title_tool_available {
             schema_policy = schema_policy.with_session_title_tool();
         }
-        if s.preview_client.is_some() && !compact_tool_prompt {
-            schema_policy = schema_policy.with_preview_tool();
-        }
         // The session-title tool only changes the native `tools` field; its
         // guidance goes in the context tail so the system prompt (and the
         // cached prefix) stays identical between the first and later turns.

rustcode/engine/src/network/turn/request.rs

@@ -586,11 +586,7 @@ pub(super) async fn collect_round(
         (
             s.api_base_url.clone(),
             s.model_name.clone(),
-            crate::tools::ToolSchemaPolicy::root_for_mode_with_compact_prompt(
-                s.delegation_active,
-                s.agent_mode,
-                compact_tool_prompt,
-            ),
+            crate::network::root_schema_policy(&s, compact_tool_prompt),
             s.active_session_id.clone(),
         )
     };

rustcode/engine/src/preview.rs

@@ -754,6 +754,28 @@ mod tests {
         })
     }
 
+    /// The prompt's hint and the provider request's `tools` come from one
+    /// policy: a session with a preview offers the tool it is told about.
+    #[test]
+    fn request_policy_offers_preview_with_a_client() {
+        let mut state = crate::app::AppState::new();
+        assert!(!crate::network::root_schema_policy(&state, false).include_preview_tool);
+        state.preview_client = Some(answering(Value::Null) as Arc<dyn PreviewClient>);
+        let policy = crate::network::root_schema_policy(&state, false);
+        assert!(policy.include_preview_tool);
+        let mut prompt = String::new();
+        crate::tools::schema::append_preview_hint(&mut prompt, policy);
+        assert!(prompt.contains(PREVIEW_TOOL_NAME));
+        let tools = crate::tools::native_tools_schema_for_context(policy, &[]).0;
+        assert!(
+            tools
+                .iter()
+                .any(|tool| tool.to_string().contains(PREVIEW_TOOL_NAME))
+        );
+        // A compact prompt describes no preview, so offers none.
+        assert!(!crate::network::root_schema_policy(&state, true).include_preview_tool);
+    }
+
     fn no_save(_: &[u8], _: &str) -> std::io::Result<PathBuf> {
         panic!("nothing should be saved")
     }