diff --git a/codex-rs/core/src/exec.rs b/codex-rs/core/src/exec.rs
index 275f2fb856..e71a504c16 100644
--- a/codex-rs/core/src/exec.rs
+++ b/codex-rs/core/src/exec.rs
@@ -312,20 +312,7 @@ async fn exec_windows_sandbox(
         text: stderr_text,
         truncated_after_lines: None,
     };
-    // Best-effort aggregate: stdout then stderr (capped).
-    let mut aggregated = Vec::with_capacity(
-        stdout
-            .text
-            .len()
-            .saturating_add(stderr.text.len())
-            .min(EXEC_OUTPUT_MAX_BYTES),
-    );
-    append_capped(&mut aggregated, &stdout.text, EXEC_OUTPUT_MAX_BYTES);
-    append_capped(&mut aggregated, &stderr.text, EXEC_OUTPUT_MAX_BYTES);
-    let aggregated_output = StreamOutput {
-        text: aggregated,
-        truncated_after_lines: None,
-    };
+    let aggregated_output = aggregate_output(&stdout, &stderr);
 
     Ok(RawExecToolCallOutput {
         exit_status,
@@ -519,6 +506,41 @@ fn append_capped(dst: &mut Vec<u8>, src: &[u8], max_bytes: usize) {
     dst.extend_from_slice(&src[..take]);
 }
 
+fn aggregate_output(
+    stdout: &StreamOutput<Vec<u8>>,
+    stderr: &StreamOutput<Vec<u8>>,
+) -> StreamOutput<Vec<u8>> {
+    let total_len = stdout.text.len().saturating_add(stderr.text.len());
+    let mut aggregated = Vec::with_capacity(total_len.min(EXEC_OUTPUT_MAX_BYTES));
+
+    if total_len <= EXEC_OUTPUT_MAX_BYTES {
+        append_capped(&mut aggregated, &stdout.text, EXEC_OUTPUT_MAX_BYTES);
+        append_capped(&mut aggregated, &stderr.text, EXEC_OUTPUT_MAX_BYTES);
+        return StreamOutput {
+            text: aggregated,
+            truncated_after_lines: None,
+        };
+    }
+
+    let stdout_cap = EXEC_OUTPUT_MAX_BYTES / 3;
+    let mut stdout_take = stdout.text.len().min(stdout_cap);
+    let stderr_cap = EXEC_OUTPUT_MAX_BYTES.saturating_sub(stdout_take);
+    let stderr_take = stderr.text.len().min(stderr_cap);
+    let remaining = stderr_cap.saturating_sub(stderr_take);
+    if remaining > 0 && stdout.text.len() > stdout_take {
+        stdout_take = stdout_take.saturating_add(remaining.min(stdout.text.len() - stdout_take));
+    }
+
+    let total_take = stdout_take.saturating_add(stderr_take);
+    append_capped(&mut aggregated, &stdout.text, stdout_take);
+    append_capped(&mut aggregated, &stderr.text, total_take);
+
+    StreamOutput {
+        text: aggregated,
+        truncated_after_lines: None,
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct ExecToolCallOutput {
     pub exit_code: i32,
@@ -683,20 +705,7 @@ async fn consume_truncated_output(
         Duration::from_millis(IO_DRAIN_TIMEOUT_MS),
     )
     .await?;
-    // Best-effort aggregate: stdout then stderr (capped).
-    let mut aggregated = Vec::with_capacity(
-        stdout
-            .text
-            .len()
-            .saturating_add(stderr.text.len())
-            .min(EXEC_OUTPUT_MAX_BYTES),
-    );
-    append_capped(&mut aggregated, &stdout.text, EXEC_OUTPUT_MAX_BYTES);
-    append_capped(&mut aggregated, &stderr.text, EXEC_OUTPUT_MAX_BYTES * 2);
-    let aggregated_output = StreamOutput {
-        text: aggregated,
-        truncated_after_lines: None,
-    };
+    let aggregated_output = aggregate_output(&stdout, &stderr);
 
     Ok(RawExecToolCallOutput {
         exit_status,
@@ -771,6 +780,7 @@ fn synthetic_exit_status(code: i32) -> ExitStatus {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use pretty_assertions::assert_eq;
     use std::time::Duration;
     use tokio::io::AsyncWriteExt;
 
@@ -846,6 +856,65 @@ mod tests {
         assert_eq!(out.text.len(), EXEC_OUTPUT_MAX_BYTES);
     }
 
+    #[test]
+    fn aggregate_output_prefers_stderr_on_contention() {
+        let stdout = StreamOutput {
+            text: vec![b'a'; EXEC_OUTPUT_MAX_BYTES],
+            truncated_after_lines: None,
+        };
+        let stderr = StreamOutput {
+            text: vec![b'b'; EXEC_OUTPUT_MAX_BYTES],
+            truncated_after_lines: None,
+        };
+
+        let aggregated = aggregate_output(&stdout, &stderr);
+        let stdout_cap = EXEC_OUTPUT_MAX_BYTES / 3;
+        let stderr_cap = EXEC_OUTPUT_MAX_BYTES.saturating_sub(stdout_cap);
+
+        assert_eq!(aggregated.text.len(), EXEC_OUTPUT_MAX_BYTES);
+        assert_eq!(aggregated.text[..stdout_cap], vec![b'a'; stdout_cap]);
+        assert_eq!(aggregated.text[stdout_cap..], vec![b'b'; stderr_cap]);
+    }
+
+    #[test]
+    fn aggregate_output_fills_remaining_capacity_with_stderr() {
+        let stdout_len = EXEC_OUTPUT_MAX_BYTES / 10;
+        let stdout = StreamOutput {
+            text: vec![b'a'; stdout_len],
+            truncated_after_lines: None,
+        };
+        let stderr = StreamOutput {
+            text: vec![b'b'; EXEC_OUTPUT_MAX_BYTES],
+            truncated_after_lines: None,
+        };
+
+        let aggregated = aggregate_output(&stdout, &stderr);
+        let stderr_cap = EXEC_OUTPUT_MAX_BYTES.saturating_sub(stdout_len);
+
+        assert_eq!(aggregated.text.len(), EXEC_OUTPUT_MAX_BYTES);
+        assert_eq!(aggregated.text[..stdout_len], vec![b'a'; stdout_len]);
+        assert_eq!(aggregated.text[stdout_len..], vec![b'b'; stderr_cap]);
+    }
+
+    #[test]
+    fn aggregate_output_rebalances_when_stderr_is_small() {
+        let stdout = StreamOutput {
+            text: vec![b'a'; EXEC_OUTPUT_MAX_BYTES],
+            truncated_after_lines: None,
+        };
+        let stderr = StreamOutput {
+            text: vec![b'b'; 1],
+            truncated_after_lines: None,
+        };
+
+        let aggregated = aggregate_output(&stdout, &stderr);
+        let stdout_len = EXEC_OUTPUT_MAX_BYTES.saturating_sub(1);
+
+        assert_eq!(aggregated.text.len(), EXEC_OUTPUT_MAX_BYTES);
+        assert_eq!(aggregated.text[..stdout_len], vec![b'a'; stdout_len]);
+        assert_eq!(aggregated.text[stdout_len..], vec![b'b'; 1]);
+    }
+
     #[cfg(unix)]
     #[test]
     fn sandbox_detection_flags_sigsys_exit_code() {