core: wrap token budget window context

Token-budget initial context carries thread and context-window lineage that the model should treat as one structured context-window block. Wrapping it in `<context_window>` makes that boundary explicit while preserving the existing window id content.

This updates `TokenBudgetContext` to render with `<context_window>` markers, treats the new wrapper as contextual developer content when mapping history, and refreshes the token-budget request-shape assertions and snapshot.

Verification:
- `just test -p codex-core token_budget`
- `just test -p codex-core recognizes_context_window_as_contextual_developer_content`

codex-rs/core/src/context/token_budget_context.rs

@@ -2,6 +2,9 @@ use super::ContextualUserFragment;
 use codex_protocol::ThreadId;
 use uuid::Uuid;
 
+const CONTEXT_WINDOW_OPEN_TAG: &str = "<context_window>";
+const CONTEXT_WINDOW_CLOSE_TAG: &str = "</context_window>";
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub(crate) struct TokenBudgetContext {
     thread_id: ThreadId,
@@ -39,7 +42,7 @@ impl ContextualUserFragment for TokenBudgetContext {
     }
 
     fn type_markers() -> (&'static str, &'static str) {
-        ("", "")
+        (CONTEXT_WINDOW_OPEN_TAG, CONTEXT_WINDOW_CLOSE_TAG)
     }
 
     fn body(&self) -> String {
@@ -57,7 +60,7 @@ impl ContextualUserFragment for TokenBudgetContext {
         if let Some(mcp_result) = &self.mcp_result {
             lines.push(mcp_result.clone());
         }
-        lines.join("\n")
+        format!("\n{}\n", lines.join("\n"))
     }
 }
 

codex-rs/core/src/event_mapping.rs

@@ -36,6 +36,7 @@ const CONTEXTUAL_DEVELOPER_PREFIXES: &[&str] = &[
     "<personality_spec>",
     // Keep recognizing token-budget wrappers persisted by older versions.
     "<token_budget>",
+    "<context_window>",
     "<rollout_budget>",
 ];
 

codex-rs/core/src/event_mapping_tests.rs

@@ -39,6 +39,18 @@ fn recognizes_legacy_token_budget_as_contextual_developer_content() {
     assert!(!has_non_contextual_dev_message_content(&content));
 }
 
+#[test]
+fn recognizes_context_window_as_contextual_developer_content() {
+    let content = vec![ContentItem::InputText {
+        text:
+            "<context_window>\nThread id 00000000-0000-0000-0000-000000000000.\n</context_window>"
+                .to_string(),
+    }];
+
+    assert!(is_contextual_dev_message_content(&content));
+    assert!(!has_non_contextual_dev_message_content(&content));
+}
+
 #[test]
 fn parses_user_message_with_text_and_two_images() {
     let img1 = "https://example.com/one.png".to_string();

codex-rs/core/tests/suite/snapshots/all__suite__token_budget__token_budget_new_context_window_tool_full_context.snap

@@ -1,6 +1,6 @@
 ---
 source: core/tests/suite/token_budget.rs
-assertion_line: 588
+assertion_line: 545
 expression: snapshot
 ---
 Scenario: New context window tool installs fresh full context before the next follow-up request.
@@ -9,7 +9,7 @@ Scenario: New context window tool installs fresh full context before the next fo
 00:message/developer[3]:
     [01] <PERMISSIONS_INSTRUCTIONS>
     [02] <SKILLS_INSTRUCTIONS>
-    [03] Thread id <THREAD_ID>.\nFirst context window id: <FIRST_WINDOW_ID>\nCurrent context window id: <WINDOW_ID>\nPrevious context window id: <FIRST_WINDOW_ID>
+    [03] <context_window>\nThread id <THREAD_ID>.\nFirst context window id: <FIRST_WINDOW_ID>\nCurrent context window id: <WINDOW_ID>\nPrevious context window id: <FIRST_WINDOW_ID>\n</context_window>
 01:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
 02:function_call/update_plan
 03:function_call_output:Plan updated

codex-rs/core/tests/suite/token_budget.rs

@@ -37,7 +37,7 @@ fn token_budget_contexts(request: &ResponsesRequest) -> Vec<String> {
     request
         .message_input_texts("developer")
         .into_iter()
-        .filter(|text| text.starts_with("Thread id "))
+        .filter(|text| text.starts_with("<context_window>\nThread id "))
         .collect()
 }
 
@@ -47,7 +47,7 @@ fn token_budget_window_ids(
 ) -> (String, Option<String>, String) {
     let captures = assert_regex_match(
         &format!(
-            r"^Thread id {thread_id}\.\nFirst context window id: ([0-9a-f-]{{36}})\nCurrent context window id: ([0-9a-f-]{{36}})(?:\nPrevious context window id: ([0-9a-f-]{{36}}))?$"
+            r"^<context_window>\nThread id {thread_id}\.\nFirst context window id: ([0-9a-f-]{{36}})\nCurrent context window id: ([0-9a-f-]{{36}})(?:\nPrevious context window id: ([0-9a-f-]{{36}}))?\n</context_window>$"
         ),
         text,
     );
@@ -191,7 +191,7 @@ async fn token_budget_context_injects_plain_thread_hint_text() -> Result<()> {
     assert_eq!(token_budgets.len(), 1);
     let captures = assert_regex_match(
         &format!(
-            r"^Thread id {thread_id}\.\nFirst context window id: ([0-9a-f-]{{36}})\nCurrent context window id: ([0-9a-f-]{{36}})\nmanual history hint for thread {thread_id}\nunstructured notes/thread_hint fixture result$"
+            r"^<context_window>\nThread id {thread_id}\.\nFirst context window id: ([0-9a-f-]{{36}})\nCurrent context window id: ([0-9a-f-]{{36}})\nmanual history hint for thread {thread_id}\nunstructured notes/thread_hint fixture result\n</context_window>$"
         ),
         &token_budgets[0],
     );