core: wrap token budget window context (#29494)

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.

Before this change, the window identifiers were injected as an untagged
developer text fragment:

```text
Thread id <THREAD_ID>.
First context window id: <FIRST_WINDOW_ID>
Current context window id: <WINDOW_ID>
Previous context window id: <PREVIOUS_WINDOW_ID>
```

After this change, the same payload is wrapped as a context-window
block:

```text
<context_window>
Thread id: <THREAD_ID>
First context window id: <FIRST_WINDOW_ID>
Current context window id: <WINDOW_ID>
Previous context window id: <PREVIOUS_WINDOW_ID>
</context_window>
```

This adds shared `CONTEXT_WINDOW_*_TAG` protocol constants, updates
`TokenBudgetContext` to render with those 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

@@ -1,5 +1,7 @@
 use super::ContextualUserFragment;
 use codex_protocol::ThreadId;
+use codex_protocol::protocol::CONTEXT_WINDOW_CLOSE_TAG;
+use codex_protocol::protocol::CONTEXT_WINDOW_OPEN_TAG;
 use uuid::Uuid;
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -39,7 +41,7 @@ impl ContextualUserFragment for TokenBudgetContext {
     }
 
     fn type_markers() -> (&'static str, &'static str) {
-        ("", "")
+        (CONTEXT_WINDOW_OPEN_TAG, CONTEXT_WINDOW_CLOSE_TAG)
     }
 
     fn body(&self) -> String {
@@ -47,7 +49,7 @@ impl ContextualUserFragment for TokenBudgetContext {
         let first_window_id = self.first_window_id;
         let window_id = self.window_id;
         let mut lines = vec![
-            format!("Thread id {thread_id}."),
+            format!("Thread id: {thread_id}"),
             format!("First context window id: {first_window_id}"),
             format!("Current context window id: {window_id}"),
         ];
@@ -57,7 +59,7 @@ 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

@@ -15,6 +15,7 @@ use codex_protocol::models::is_image_open_tag_text;
 use codex_protocol::models::is_local_image_close_tag_text;
 use codex_protocol::models::is_local_image_open_tag_text;
 use codex_protocol::protocol::COLLABORATION_MODE_OPEN_TAG;
+use codex_protocol::protocol::CONTEXT_WINDOW_OPEN_TAG;
 use codex_protocol::protocol::MULTI_AGENT_MODE_OPEN_TAG;
 use codex_protocol::protocol::REALTIME_CONVERSATION_OPEN_TAG;
 use codex_protocol::protocol::SKILLS_INSTRUCTIONS_OPEN_TAG;
@@ -36,6 +37,7 @@ const CONTEXTUAL_DEVELOPER_PREFIXES: &[&str] = &[
     "<personality_spec>",
     // Keep recognizing token-budget wrappers persisted by older versions.
     "<token_budget>",
+    CONTEXT_WINDOW_OPEN_TAG,
     "<rollout_budget>",
 ];
 

codex-rs/core/src/event_mapping_tests.rs

@@ -15,6 +15,8 @@ use codex_protocol::models::ReasoningItemContent;
 use codex_protocol::models::ReasoningItemReasoningSummary;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::models::WebSearchAction;
+use codex_protocol::protocol::CONTEXT_WINDOW_CLOSE_TAG;
+use codex_protocol::protocol::CONTEXT_WINDOW_OPEN_TAG;
 use codex_protocol::protocol::SKILLS_INSTRUCTIONS_OPEN_TAG;
 use codex_protocol::user_input::UserInput;
 use pretty_assertions::assert_eq;
@@ -39,6 +41,20 @@ 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: format!(
+            r#"{CONTEXT_WINDOW_OPEN_TAG}
+Thread id: 00000000-0000-0000-0000-000000000000
+{CONTEXT_WINDOW_CLOSE_TAG}"#
+        ),
+    }];
+
+    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

@@ -4,6 +4,8 @@ use codex_config::types::McpServerTransportConfig;
 use codex_core::config::TokenBudgetConfig;
 use codex_features::Feature;
 use codex_model_provider_info::built_in_model_providers;
+use codex_protocol::protocol::CONTEXT_WINDOW_CLOSE_TAG;
+use codex_protocol::protocol::CONTEXT_WINDOW_OPEN_TAG;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::Op;
 use core_test_support::PathBufExt;
@@ -34,10 +36,11 @@ use std::time::Duration;
 const CONFIGURED_CONTEXT_WINDOW: i64 = 128_000;
 
 fn token_budget_contexts(request: &ResponsesRequest) -> Vec<String> {
+    let context_window_prefix = format!("{CONTEXT_WINDOW_OPEN_TAG}\nThread id: ");
     request
         .message_input_texts("developer")
         .into_iter()
-        .filter(|text| text.starts_with("Thread id "))
+        .filter(|text| text.starts_with(&context_window_prefix))
         .collect()
 }
 
@@ -47,7 +50,7 @@ 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}}))?$"
+            r"^{CONTEXT_WINDOW_OPEN_TAG}\nThread id: {thread_id}\nFirst context window id: ([0-9a-f-]{{36}})\nCurrent context window id: ([0-9a-f-]{{36}})(?:\nPrevious context window id: ([0-9a-f-]{{36}}))?\n{CONTEXT_WINDOW_CLOSE_TAG}$"
         ),
         text,
     );
@@ -191,7 +194,7 @@ 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$"
+            r"^{CONTEXT_WINDOW_OPEN_TAG}\nThread id: {thread_id}\nFirst context window id: ([0-9a-f-]{{36}})\nCurrent context window id: ([0-9a-f-]{{36}})\nmanual history hint for thread {thread_id}\nunstructured notes/thread_hint fixture result\n{CONTEXT_WINDOW_CLOSE_TAG}$"
         ),
         &token_budgets[0],
     );

codex-rs/protocol/src/protocol.rs

@@ -92,8 +92,8 @@ use crate::permissions::default_read_only_subpaths_for_writable_root;
 pub use crate::request_permissions::RequestPermissionsArgs;
 pub use crate::request_user_input::RequestUserInputEvent;
 
-/// Open/close tags for special user-input blocks. Used across crates to avoid
+/// Open/close tags for special context blocks. Used across crates to avoid duplicated hardcoded
-/// duplicated hardcoded strings.
+/// strings.
 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>";
@@ -110,6 +110,8 @@ pub const MULTI_AGENT_MODE_OPEN_TAG: &str = "<multi_agent_mode>";
 pub const MULTI_AGENT_MODE_CLOSE_TAG: &str = "</multi_agent_mode>";
 pub const REALTIME_CONVERSATION_OPEN_TAG: &str = "<realtime_conversation>";
 pub const REALTIME_CONVERSATION_CLOSE_TAG: &str = "</realtime_conversation>";
+pub const CONTEXT_WINDOW_OPEN_TAG: &str = "<context_window>";
+pub const CONTEXT_WINDOW_CLOSE_TAG: &str = "</context_window>";
 pub const USER_MESSAGE_BEGIN: &str = "## My request for Codex:";
 
 // TODO(anp): Replace `TurnEnvironmentSelection` with `PathUri` once path URIs carry environment