Route standalone image generation through host finalization md (#25176)

## Why

Standalone image-generation extensions emitted turn items through the
low-level event path, bypassing host-owned finalization such as image
persistence and contributor processing. At the same time, the
generated-image save-path hint must remain visible to the model through
the extension tool's `FunctionCallOutput`, rather than the legacy
built-in developer-message path.

## What changed

- Extended `ExtensionTurnItem` to support image-generation items while
keeping the extension-facing emitter API limited to `emit_started` and
`emit_completed`.
- Routed extension completion through core `finalize_turn_item`, so
standalone image-generation items receive host-owned processing and
persisted `saved_path` values before publication.
- Kept legacy built-in image generation on its existing
developer-message hint path, while standalone image generation returns
its deterministic saved-path hint in `FunctionCallOutput`.
- Shared the image artifact path and output-hint formatting used by core
and the image-generation extension.
- Passed thread identity through extension tool calls so standalone
image generation can construct the same intended artifact path as core.
- Added an app-server integration test covering real standalone image
generation, saved artifact publication, model-visible output hint
wiring, and absence of the legacy developer-message hint.

## Validation

- `just fmt`
- `just test -p codex-image-generation-extension`
- `just test -p codex-web-search-extension`
- `just test -p codex-goal-extension`
- `just test -p codex-memories-extension`
- Targeted `codex-core` tests for image save history, extension
completion finalization, and contributor execution
- `just test -p codex-app-server
standalone_image_generation_returns_saved_path_hint_to_model`
- `just fix -p codex-core`
- `just fix -p codex-image-generation-extension`
- `just bazel-lock-update`
- `just bazel-lock-check`

codex-rs/ext/extension-api/src/lib.rs

@@ -13,7 +13,6 @@ pub use capabilities::ResponseItemInjector;
 pub use codex_tools::ConversationHistory;
 pub use codex_tools::ExtensionTurnItem;
 pub use codex_tools::FunctionCallError;
-pub use codex_tools::ImageGenerationCompletionFuture;
 pub use codex_tools::JsonToolOutput;
 pub use codex_tools::NoopTurnItemEmitter;
 pub use codex_tools::ResponsesApiTool;

codex-rs/ext/image-generation/src/tests.rs

@@ -26,7 +26,6 @@ use crate::IMAGE_GEN_NAMESPACE;
 use crate::IMAGEGEN_TOOL_NAME;
 
 const RESULT: &str = "cG5n";
-const OUTPUT_HINT: &str = "Generated images are saved to /tmp as /tmp/call-1.png by default.";
 
 #[test]
 fn uses_reserved_image_gen_namespace() {
@@ -55,10 +54,9 @@ fn generate_uses_fixed_request_defaults() {
 }
 
 #[test]
-fn generated_output_returns_image_input_and_output_hint() {
+fn generated_output_returns_image_input() {
     let output = GeneratedImageOutput {
         result: RESULT.to_string(),
-        output_hint: Some(OUTPUT_HINT.to_string()),
     };
 
     let ResponseInputItem::FunctionCallOutput {
@@ -73,15 +71,10 @@ fn generated_output_returns_image_input_and_output_hint() {
     };
     assert_eq!(
         content_items,
-        vec![
-            FunctionCallOutputContentItem::InputImage {
-                image_url: format!("data:image/png;base64,{RESULT}"),
-                detail: Some(DEFAULT_IMAGE_DETAIL),
-            },
-            FunctionCallOutputContentItem::InputText {
-                text: OUTPUT_HINT.to_string(),
-            },
-        ]
+        vec![FunctionCallOutputContentItem::InputImage {
+            image_url: format!("data:image/png;base64,{RESULT}"),
+            detail: Some(DEFAULT_IMAGE_DETAIL),
+        }]
     );
 }
 

codex-rs/ext/image-generation/src/tool.rs

@@ -3,6 +3,7 @@ use codex_api::ImageEditRequest;
 use codex_api::ImageGenerationRequest;
 use codex_api::ImageQuality;
 use codex_api::ImageUrl;
+use codex_extension_api::ExtensionTurnItem;
 use codex_extension_api::FunctionCallError;
 use codex_extension_api::ToolCall;
 use codex_extension_api::ToolExecutor;
@@ -11,6 +12,7 @@ use codex_extension_api::ToolOutput;
 use codex_extension_api::ToolPayload;
 use codex_extension_api::ToolSpec;
 use codex_extension_api::parse_tool_input_schema;
+use codex_protocol::items::ImageGenerationItem;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::DEFAULT_IMAGE_DETAIL;
 use codex_protocol::models::FunctionCallOutputBody;
@@ -84,6 +86,15 @@ impl ToolExecutor<ToolCall> for ImageGenerationTool {
     async fn handle(&self, call: ToolCall) -> Result<Box<dyn ToolOutput>, FunctionCallError> {
         let args = parse_args(&call)?;
         let request = request_for_action(&args, call.conversation_history.items())?;
+        call.turn_item_emitter
+            .emit_started(ExtensionTurnItem::ImageGeneration(ImageGenerationItem {
+                id: call.call_id.clone(),
+                status: "in_progress".to_string(),
+                revised_prompt: None,
+                result: String::new(),
+                saved_path: None,
+            }))
+            .await;
         let response = match request {
             ImageRequest::Generate(request) => self.backend.generate(request).await,
             ImageRequest::Edit(request) => self.backend.edit(request).await,
@@ -96,14 +107,16 @@ impl ToolExecutor<ToolCall> for ImageGenerationTool {
                 "image generation returned no image data".to_string(),
             ));
         };
-        let output_hint = call
-            .turn_item_emitter
-            .image_generation_completed(call.call_id.clone(), args.prompt, result.clone())
+        call.turn_item_emitter
+            .emit_completed(ExtensionTurnItem::ImageGeneration(ImageGenerationItem {
+                id: call.call_id.clone(),
+                status: "completed".to_string(),
+                revised_prompt: Some(args.prompt),
+                result: result.clone(),
+                saved_path: None,
+            }))
             .await;
-        Ok(Box::new(GeneratedImageOutput {
-            result,
-            output_hint,
-        }))
+        Ok(Box::new(GeneratedImageOutput { result }))
     }
 }
 
@@ -284,7 +297,6 @@ fn imagegen_tool_spec() -> ToolSpec {
 
 struct GeneratedImageOutput {
     result: String,
-    output_hint: Option<String>,
 }
 
 impl ToolOutput for GeneratedImageOutput {
@@ -298,17 +310,12 @@ impl ToolOutput for GeneratedImageOutput {
         true
     }
 
-    /// Returns generated bytes and persisted-artifact context for model follow-up.
+    /// Returns generated bytes for model follow-up.
     fn to_response_item(&self, call_id: &str, _payload: &ToolPayload) -> ResponseInputItem {
-        let mut content = vec![FunctionCallOutputContentItem::InputImage {
+        let content = vec![FunctionCallOutputContentItem::InputImage {
             image_url: format!("data:image/png;base64,{}", self.result),
             detail: Some(DEFAULT_IMAGE_DETAIL),
         }];
-        if let Some(output_hint) = &self.output_hint {
-            content.push(FunctionCallOutputContentItem::InputText {
-                text: output_hint.clone(),
-            });
-        }
         ResponseInputItem::FunctionCallOutput {
             call_id: call_id.to_string(),
             output: FunctionCallOutputPayload {