Account for JSON overhead when truncating MCP tool results (#50470)

## Why

Truncating the preview alone can leave the serialized MCP tool result above its byte budget because JSON escaping and the result wrapper add bytes.

## What changed

Measure the complete serialized replacement and progressively reduce the preview budget until the result fits or the preview is empty.

## Testing

Strengthen the regression test with quotes, backslashes, and newlines. Assert that the serialized result fits within the 1,024-byte budget and that truncating it again returns the existing result unchanged.

GitOrigin-RevId: ebd97b1a6ca71343941a96e472cb313028ac680c

codex-rs/utils/output-truncation/src/lib.rs

@@ -40,8 +40,8 @@ pub fn truncate_text(content: &str, policy: TruncationPolicy) -> String {
     }
 }
 
-/// Replaces an oversized serialized MCP result with a text preview while preserving its error
-/// status and supported bounded metadata.
+/// Replaces an oversized serialized MCP result with a bounded text preview while preserving its
+/// error status and supported bounded metadata.
 pub fn truncate_mcp_tool_result(
     result: &CallToolResult,
     max_bytes: usize,
@@ -53,15 +53,37 @@ pub fn truncate_mcp_tool_result(
         return Cow::Borrowed(result);
     }
 
-    Cow::Owned(CallToolResult {
-        content: vec![serde_json::json!({
-            "type": "text",
-            "text": truncate_text(&serialized, TruncationPolicy::Bytes(max_bytes)),
-        })],
-        structured_content: None,
-        is_error: result.is_error,
-        meta: None,
-    })
+    let meta = None;
+
+    let mut preview_budget = max_bytes;
+    loop {
+        let preview = if preview_budget == 0 {
+            String::new()
+        } else {
+            truncate_text(&serialized, TruncationPolicy::Bytes(preview_budget))
+        };
+        let truncated = CallToolResult {
+            content: vec![serde_json::json!({
+                "type": "text",
+                "text": preview,
+            })],
+            structured_content: None,
+            is_error: result.is_error,
+            meta: meta.clone(),
+        };
+        let Ok(truncated_serialized) = serde_json::to_string(&truncated) else {
+            return Cow::Borrowed(result);
+        };
+        let truncated_len = truncated_serialized.len();
+        if truncated_len <= max_bytes || preview_budget == 0 {
+            return Cow::Owned(truncated);
+        }
+
+        // The preview is serialized as a JSON string, so escaping can make it exceed its raw byte
+        // budget. Scale the next budget by the observed size of the complete replacement.
+        preview_budget = (preview_budget.saturating_mul(max_bytes) / truncated_len)
+            .min(preview_budget.saturating_sub(1));
+    }
 }
 
 /// Applies the existing byte/token budget without changing success metadata or media ordering.

codex-rs/utils/output-truncation/src/truncate_tests.rs

@@ -32,10 +32,11 @@ fn truncate_mcp_tool_result_borrows_small_result() {
 
 #[test]
 fn truncate_mcp_tool_result_compacts_large_result() {
+    let max_bytes = 1024;
     let result = CallToolResult {
         content: vec![serde_json::json!({
             "type": "text",
-            "text": format!("head\n{}\ntail", "x".repeat(5_000)),
+            "text": format!("head\n{}\ntail", "\"\\\n".repeat(5_000)),
         })],
         structured_content: Some(serde_json::json!({"x": "y".repeat(5_000)})),
         is_error: Some(true),
@@ -44,7 +45,7 @@ fn truncate_mcp_tool_result_compacts_large_result() {
         })),
     };
 
-    let truncated = truncate_mcp_tool_result(&result, /*max_bytes*/ 1024).into_owned();
+    let truncated = truncate_mcp_tool_result(&result, max_bytes).into_owned();
 
     let preview = truncated.content[0]["text"]
         .as_str()
@@ -61,10 +62,14 @@ fn truncate_mcp_tool_result_compacts_large_result() {
             meta: None,
         }
     );
-    assert!(preview.len() < 2 * 1024);
+    assert!(serde_json::to_string(&truncated).unwrap().len() <= max_bytes);
     assert!(preview.contains("head"));
     assert!(preview.contains("chars truncated"));
     assert!(preview.contains("meta-tail"));
+
+    let truncated_again = truncate_mcp_tool_result(&truncated, max_bytes);
+    assert!(matches!(truncated_again, std::borrow::Cow::Borrowed(_)));
+    assert_eq!(truncated_again.as_ref(), &truncated);
 }
 
 #[test]