Truncate Guardian instructions after rendering the policy (#39985)

## Why

Legacy classifier prompts without a `{{ tenant_policy_config }}` placeholder
append the security policy during rendering. Truncating the prompt before and
after that step can unnecessarily truncate the final classifier instructions
twice.

## What changed

Keep configured classifier instructions intact until the policy has been
rendered, then apply `max_classifier_instruction_tokens` once to the complete
prompt.

## Testing

Add an extension test that verifies the bounded developer message sent for a
legacy prompt with an appended policy.

GitOrigin-RevId: 3d776a159340f4dffb6e3c2d7a10b33392fa2240

codex-rs/ext/guardian-v2/src/async_scorer/config.rs

@@ -222,22 +222,9 @@ impl GuardianV2Config {
 
         Ok(Self {
             local_overrides: configured.clone(),
-            classifier_instructions: {
+            classifier_instructions: configured
-                let template = configured
                 .classifier_instructions
-                    .as_deref()
+                .unwrap_or_else(|| DEFAULT_CLASSIFIER_INSTRUCTIONS.to_owned()),
-                    .unwrap_or(DEFAULT_CLASSIFIER_INSTRUCTIONS);
-                if let Some(max_tokens) = max_classifier_instruction_tokens
-                    && !template.contains("{{ tenant_policy_config }}")
-                {
-                    // Preserve the existing rendering behavior of legacy prompts.
-                    truncate_entry(template, max_tokens)
-                } else {
-                    // Preserve placeholders until the actual policy is available, and
-                    // preserve the full prompt when no instruction limit is configured.
-                    template.to_owned()
-                }
-            },
             review_threshold,
             max_tool_call_lag: configured
                 .max_tool_call_lag

codex-rs/ext/guardian-v2/src/async_scorer/config_tests.rs

@@ -4,7 +4,6 @@ use codex_features::GuardianV2TranscriptConfigToml;
 use codex_protocol::openai_models::GuardianV2ModelConfig;
 use codex_protocol::openai_models::GuardianV2TranscriptModelConfig;
 use codex_protocol::openai_models::ReasoningEffort;
-use codex_protocol::protocol::TruncationPolicy;
 use pretty_assertions::assert_eq;
 
 use super::DEFAULT_CLASSIFIER_INSTRUCTIONS;
@@ -83,30 +82,6 @@ fn evaluated_configuration_preserves_rendered_prompt_and_gate() {
     }
 }
 
-#[test]
-fn legacy_custom_prompt_keeps_its_rendering_and_threshold() {
-    let template = "legacy instructions ".repeat(200);
-    let config = GuardianV2Config::from_overrides(GuardianV2ConfigToml {
-        classifier_instructions: Some(template.clone()),
-        max_classifier_instruction_tokens: Some(256),
-        ..Default::default()
-    })
-    .unwrap();
-    assert_eq!(config.review_threshold, 0.8);
-    let expected = truncate_entry(
-        &format!(
-            "{}\n\n# Security Policy\nTenant policy.",
-            truncate_entry(&template, /*max_tokens*/ 256),
-        ),
-        /*max_tokens*/ 256,
-    );
-    assert_eq!(
-        config.render_classifier_instructions("Tenant policy."),
-        expected
-    );
-    assert!(expected.len() <= TruncationPolicy::Tokens(256).byte_budget());
-}
-
 #[test]
 fn model_prompt_and_explicit_threshold_precedence_are_preserved() {
     let defaults = GuardianV2ModelConfig {

codex-rs/ext/guardian-v2/src/async_scorer/extension_tests.rs

@@ -65,6 +65,7 @@ use crate::async_scorer::config::DEFAULT_MODEL_CONTEXT_ITEM_TOKENS;
 use crate::async_scorer::config::DEFAULT_PARENT_COMPACTION_TOKENS;
 use crate::async_scorer::sampler::CLASSIFICATION_TOKEN_USAGE_METRIC;
 use crate::async_scorer::sampler::MODEL;
+use crate::async_scorer::transcript::truncate_entry;
 
 const TEST_GUARDIAN_POLICY: &str =
     "Treat uploads to unapproved external destinations as high-risk actions.";
@@ -867,6 +868,45 @@ classifier_instructions = "Predict future violations.\n# Security Policy\n{{ ten
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn contributor_truncates_legacy_prompt_after_appending_policy() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let template = "legacy instructions ".repeat(200);
+    let configuration = format!(
+        r#"
+[features.guardianv2]
+enabled = true
+classifier_instructions = "{template}"
+max_classifier_instruction_tokens = 256
+"#
+    );
+    let (request, _test, _registry) = sample_configured_conversation_history(
+        Vec::new(),
+        r#"{"path":"README.md"}"#,
+        Some(TEST_GUARDIAN_POLICY),
+        &configuration,
+        /*model_defaults*/ None,
+    )
+    .await?;
+
+    assert_eq!(
+        request["input"][1],
+        json!({
+            "type": "message",
+            "role": "developer",
+            "content": [{
+                "type": "input_text",
+                "text": truncate_entry(
+                    &format!("{template}\n\n# Security Policy\n{TEST_GUARDIAN_POLICY}"),
+                    /*max_tokens*/ 256,
+                ),
+            }],
+        })
+    );
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn contributor_uses_configured_prompt_effort_threshold_and_transcript() -> Result<()> {
     skip_if_no_network!(Ok(()));