Add ephemeral turn context support

Introduce app-server ephemeralContext, render it as additional_context_for_this_turn fragments, and strip it correctly across contextual-user diffing and compaction flows.

Regenerate app-server schema fixtures and add coverage for model-visible layout, compaction, oversize validation, and resumed/forked context reconstruction.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/protocol/src/protocol.rs

@@ -44,6 +44,7 @@ use crate::plan_tool::UpdatePlanArgs;
 use crate::request_permissions::RequestPermissionsEvent;
 use crate::request_permissions::RequestPermissionsResponse;
 use crate::request_user_input::RequestUserInputResponse;
+use crate::user_input::EphemeralContext;
 use crate::user_input::UserInput;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use schemars::JsonSchema;
@@ -80,6 +81,8 @@ pub const USER_INSTRUCTIONS_OPEN_TAG: &str = "<user_instructions>";
 pub const USER_INSTRUCTIONS_CLOSE_TAG: &str = "</user_instructions>";
 pub const ENVIRONMENT_CONTEXT_OPEN_TAG: &str = "<environment_context>";
 pub const ENVIRONMENT_CONTEXT_CLOSE_TAG: &str = "</environment_context>";
+pub const EPHEMERAL_CONTEXT_OPEN_TAG: &str = "<additional_context_for_this_turn>";
+pub const EPHEMERAL_CONTEXT_CLOSE_TAG: &str = "</additional_context_for_this_turn>";
 pub const COLLABORATION_MODE_OPEN_TAG: &str = "<collaboration_mode>";
 pub const COLLABORATION_MODE_CLOSE_TAG: &str = "</collaboration_mode>";
 pub const REALTIME_CONVERSATION_OPEN_TAG: &str = "<realtime_conversation>";
@@ -209,6 +212,10 @@ pub enum Op {
     UserInput {
         /// User input items, see `InputItem`
         items: Vec<UserInput>,
+        /// Turn-scoped context that is visible to the model but is not part of
+        /// the durable turn baseline.
+        #[serde(default, skip_serializing_if = "Vec::is_empty")]
+        ephemeral_context: Vec<EphemeralContext>,
         /// Optional JSON Schema used to constrain the final assistant message for this turn.
         #[serde(skip_serializing_if = "Option::is_none")]
         final_output_json_schema: Option<Value>,
@@ -4053,6 +4060,7 @@ mod tests {
     fn user_input_serialization_omits_final_output_json_schema_when_none() -> Result<()> {
         let op = Op::UserInput {
             items: Vec::new(),
+            ephemeral_context: Vec::new(),
             final_output_json_schema: None,
         };
 
@@ -4070,6 +4078,7 @@ mod tests {
             op,
             Op::UserInput {
                 items: Vec::new(),
+                ephemeral_context: Vec::new(),
                 final_output_json_schema: None,
             }
         );
@@ -4089,6 +4098,7 @@ mod tests {
         });
         let op = Op::UserInput {
             items: Vec::new(),
+            ephemeral_context: Vec::new(),
             final_output_json_schema: Some(schema.clone()),
         };
 
@@ -4105,6 +4115,33 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn user_input_serialization_includes_ephemeral_context_when_present() -> Result<()> {
+        let op = Op::UserInput {
+            items: Vec::new(),
+            ephemeral_context: vec![EphemeralContext {
+                title: "Context from my editor".to_string(),
+                text: "## Active file: src/main.rs".to_string(),
+            }],
+            final_output_json_schema: None,
+        };
+
+        let json_op = serde_json::to_value(op)?;
+        assert_eq!(
+            json_op,
+            json!({
+                "type": "user_input",
+                "items": [],
+                "ephemeral_context": [{
+                    "title": "Context from my editor",
+                    "text": "## Active file: src/main.rs",
+                }],
+            })
+        );
+
+        Ok(())
+    }
+
     #[test]
     fn user_input_text_serializes_empty_text_elements() -> Result<()> {
         let input = UserInput::Text {

codex-rs/protocol/src/user_input.rs

@@ -6,6 +6,14 @@ use ts_rs::TS;
 /// Conservative cap so one user message cannot monopolize a large context window.
 pub const MAX_USER_INPUT_TEXT_CHARS: usize = 1 << 20;
 
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, TS, JsonSchema)]
+pub struct EphemeralContext {
+    /// Human-readable title for additional context sent with one turn.
+    pub title: String,
+    /// Free-form text payload for additional context sent with one turn.
+    pub text: String,
+}
+
 /// User input
 #[non_exhaustive]
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, TS, JsonSchema)]