Bug Report: UnicodeEncodeError on Windows when saving traces

[tristanbob]

Bug Report: UnicodeEncodeError on Windows when saving traces

Summary

langsmith-fetch traces fails with UnicodeEncodeError on Windows when printing success messages to console, even though traces are successfully saved.

Environment

• OS: Windows 11 (Build 26200.7462)
• Python: 3.13.2544.0
• langsmith-fetch: 0.3.1
• Console encoding: cp1252 (Windows default)

Command

uv run langsmith-fetch traces ./traces --limit 2 --no-progress

Error

UnicodeEncodeError: 'charmap' codec can't encode character '\u2713' in position 2: character maps to <undefined>
  in "C:\Program Files\WindowsApps\...\Lib\encodings\cp1252.py", line 19, in encode

Error location: langsmith_cli/cli.py, line 833

click.echo(f"  \u2713 Saved {trace_id} to {safe_filename} ({summary})")

Issue

The checkmark character \u2713 (✓) cannot be encoded in Windows cp1252 encoding when writing to console.

Impact

• Traces are successfully fetched and saved despite the error
• Error appears cosmetic but exits with code 1, which may break CI/CD pipelines
• The --no-progress flag does not prevent this error

Workaround

None currently - users must ignore the error messages.

Suggested Fix

Use ASCII-safe characters or handle encoding errors gracefully:

# Option 1: ASCII fallback
try:
    click.echo(f"  \u2713 Saved {trace_id} to {safe_filename} ({summary})")
except UnicodeEncodeError:
    click.echo(f"  Saved {trace_id} to {safe_filename} ({summary})")

# Option 2: Use click's color support which handles encoding
click.secho(f"  Saved {trace_id} to {safe_filename} ({summary})", fg='green')

# Option 3: Detect Windows and use ASCII
import sys
checkmark = "\u2713" if sys.platform != "win32" else "✓"  # Will use fallback

Reproduction

1. Run on Windows with default console (cp1252 encoding)
2. Execute: langsmith-fetch traces ./output --limit 1
3. Observe error despite successful trace save

Expected Behavior

Command should complete successfully (exit code 0) with appropriate console output.

Actual Behavior

Command exits with code 1 due to Unicode encoding error, even though traces are saved successfully.