[codex] Make realtime startup context truncation deterministic (#18172)

## Summary

- remove the final whole-blob truncation pass from realtime
startup-context assembly
- enforce fixed per-section budgets, including each section heading
- keep the existing per-section caps and raise the overall realtime
startup-context budget to `5300`, matching the sum of those section
budgets
- add focused tests for the new wrapping and section-budget behavior

## Why

The previous flow truncated each section and then middle-truncated the
final combined startup-context blob again. Small input changes could
shift that combined cut point, which made retained context unstable and
caused nondeterministic tests.

## Impact

Startup context now preserves section boundaries and ordering
deterministically. Each section is still budgeted independently, but the
final assembled blob is no longer truncated again as a single opaque
string. To match that design, the overall startup-context token budget
is updated to the sum of the existing section budgets rather than
lowering the section caps.

## Validation

- `cargo +1.93.0 test -p codex-core realtime_context`
- `cargo +1.93.0 test -p codex-core --test all
suite::realtime_conversation::conversation_start_injects_startup_context_from_thread_history
-- --exact`
- `cargo +1.93.0 test -p codex-core --test all
suite::realtime_conversation::conversation_startup_context_current_thread_selects_many_turns_by_budget
-- --exact`
- `cargo +1.93.0 test -p codex-core --test all
suite::realtime_conversation::conversation_startup_context_falls_back_to_workspace_map
-- --exact`
- `cargo +1.93.0 test -p codex-core --test all
suite::realtime_conversation::conversation_startup_context_is_truncated_and_sent_once_per_start
-- --exact`

codex-rs/core/src/realtime_context.rs

@@ -108,9 +108,10 @@ pub(crate) async fn build_realtime_startup_context(
         parts.push(section);
     }
 
-    let context = format_startup_context_blob(&parts.join("\n\n"), budget_tokens);
+    let context = format_startup_context_blob(&parts.join("\n\n"));
     debug!(
         approx_tokens = approx_token_count(&context),
+        requested_budget_tokens = budget_tokens,
         bytes = context.len(),
         has_current_thread_section,
         has_recent_work_section,
@@ -440,31 +441,22 @@ fn format_section(title: &str, body: Option<String>, budget_tokens: usize) -> Op
         return None;
     }
 
-    Some(format!(
-        "## {title}\n{}",
-        truncate_text(body, TruncationPolicy::Tokens(budget_tokens))
-    ))
+    let heading = format!("## {title}\n");
+    let body_budget = budget_tokens.saturating_sub(approx_token_count(&heading));
+    if body_budget == 0 {
+        return None;
+    }
+
+    let body = truncate_realtime_text_to_token_budget(body, body_budget);
+    if body.is_empty() {
+        return None;
+    }
+
+    Some(format!("{heading}{body}"))
 }
 
-fn format_startup_context_blob(body: &str, budget_tokens: usize) -> String {
-    let wrapper = format!("{STARTUP_CONTEXT_OPEN_TAG}\n\n{STARTUP_CONTEXT_CLOSE_TAG}");
-    let mut body_budget = budget_tokens.saturating_sub(approx_token_count(&wrapper));
-
-    loop {
-        let body = truncate_text(body, TruncationPolicy::Tokens(body_budget));
-        let wrapped = format!("{STARTUP_CONTEXT_OPEN_TAG}\n{body}\n{STARTUP_CONTEXT_CLOSE_TAG}");
-        let wrapped_tokens = approx_token_count(&wrapped);
-        if wrapped_tokens <= budget_tokens || body_budget == 0 {
-            return wrapped;
-        }
-
-        let excess_tokens = wrapped_tokens.saturating_sub(budget_tokens);
-        let next_budget = body_budget.saturating_sub(excess_tokens.max(1));
-        if next_budget == body_budget {
-            return wrapped;
-        }
-        body_budget = next_budget;
-    }
+fn format_startup_context_blob(body: &str) -> String {
+    format!("{STARTUP_CONTEXT_OPEN_TAG}\n{body}\n{STARTUP_CONTEXT_CLOSE_TAG}")
 }
 
 async fn format_thread_group(

codex-rs/core/src/realtime_context_tests.rs

@@ -1,6 +1,12 @@
+use super::CURRENT_THREAD_SECTION_TOKEN_BUDGET;
+use super::NOTES_SECTION_TOKEN_BUDGET;
+use super::RECENT_WORK_SECTION_TOKEN_BUDGET;
+use super::STARTUP_CONTEXT_HEADER;
+use super::WORKSPACE_SECTION_TOKEN_BUDGET;
 use super::build_current_thread_section;
 use super::build_recent_work_section;
 use super::build_workspace_section_with_user_root;
+use super::format_section;
 use super::format_startup_context_blob;
 use chrono::TimeZone;
 use chrono::Utc;
@@ -172,20 +178,56 @@ fn current_thread_section_keeps_latest_turns_when_history_exceeds_budget() {
 }
 
 #[test]
-fn startup_context_blob_is_wrapped_in_tags_and_fits_budget() {
-    let body = format!(
-        "Startup context from Codex.\n{}\n{}",
-        "recent work ".repeat(1_200),
-        "workspace tree ".repeat(800),
-    );
+fn startup_context_blob_is_wrapped_in_tags_without_final_truncation() {
+    let body = "Startup context from Codex.\n## Current Thread\nhello";
+    let wrapped = format_startup_context_blob(body);
 
-    let wrapped = format_startup_context_blob(&body, /*budget_tokens*/ 200);
+    assert_eq!(
+        wrapped,
+        "<startup_context>\nStartup context from Codex.\n## Current Thread\nhello\n</startup_context>"
+    );
+}
+
+#[test]
+fn fixed_section_budgets_apply_per_section_without_total_blob_truncation() {
+    let body = [
+        STARTUP_CONTEXT_HEADER.to_string(),
+        format_section(
+            "Current Thread",
+            Some("current thread ".repeat(2_000)),
+            CURRENT_THREAD_SECTION_TOKEN_BUDGET,
+        )
+        .expect("current thread section"),
+        format_section(
+            "Recent Work",
+            Some("recent work ".repeat(3_000)),
+            RECENT_WORK_SECTION_TOKEN_BUDGET,
+        )
+        .expect("recent work section"),
+        format_section(
+            "Machine / Workspace Map",
+            Some("workspace map ".repeat(2_500)),
+            WORKSPACE_SECTION_TOKEN_BUDGET,
+        )
+        .expect("workspace section"),
+        format_section(
+            "Notes",
+            Some("notes ".repeat(500)),
+            NOTES_SECTION_TOKEN_BUDGET,
+        )
+        .expect("notes section"),
+    ]
+    .join("\n\n");
+
+    let wrapped = format_startup_context_blob(&body);
 
     assert!(wrapped.starts_with("<startup_context>\n"));
     assert!(wrapped.ends_with("\n</startup_context>"));
-    assert!(wrapped.contains("Startup context from Codex."));
     assert!(wrapped.contains("tokens truncated"));
-    assert!(wrapped.len().div_ceil(4) <= 200);
+    assert!(wrapped.contains("## Current Thread"));
+    assert!(wrapped.contains("## Recent Work"));
+    assert!(wrapped.contains("## Machine / Workspace Map"));
+    assert!(wrapped.contains("## Notes"));
 }
 
 #[tokio::test]

codex-rs/core/src/realtime_conversation.rs

@@ -63,7 +63,7 @@ const AUDIO_IN_QUEUE_CAPACITY: usize = 256;
 const USER_TEXT_IN_QUEUE_CAPACITY: usize = 64;
 const HANDOFF_OUT_QUEUE_CAPACITY: usize = 64;
 const OUTPUT_EVENTS_QUEUE_CAPACITY: usize = 256;
-const REALTIME_STARTUP_CONTEXT_TOKEN_BUDGET: usize = 5_000;
+const REALTIME_STARTUP_CONTEXT_TOKEN_BUDGET: usize = 5_300;
 const DEFAULT_REALTIME_MODEL: &str = "gpt-realtime-1.5";
 pub(crate) const REALTIME_USER_TEXT_PREFIX: &str = "[USER] ";
 pub(crate) const REALTIME_BACKEND_TEXT_PREFIX: &str = "[BACKEND] ";