feat: add tag filtering to traces command

[Harry Nguyen (Hazzng)]

feat(tags): add tag-based trace filtering to traces command

Summary

Add the ability to filter traces by tag(s) using --tag/-t option on the traces command. This enables users to fetch traces that have been tagged in LangSmith, . Add the output handling for fetching traces so if message is [] then fetch all output to be compatible with more agent outputs (not all agents just output messages)

Problem

Users needed a way to fetch traces by their tags from LangSmith. The existing traces command only supported time-based filtering (--last-n-minutes, --since) but had no way to filter by tags assigned to traces.

Solution

Extended the traces command with:

• --tag/-t TAG option (can be repeated for multiple tags)
• New fetch_traces_by_tags() fetcher function
• Fallback to return full outputs dict for custom agents without standard message format

Changes

File	Description
src/langsmith_cli/fetchers.py	Added fetch_traces_by_tags() function (+147 lines); Fixed message extraction to fall back to full outputs for custom agents
src/langsmith_cli/cli.py	Added --tag options; Added tag filtering logic for both directory and stdout modes
tests/test_fetchers.py	Added TestFetchTracesByTags class with 6 unit tests
tests/test_cli.py	Added TestTracesCommandTagOption class with 5 CLI tests
tests/conftest.py	Added tag-related test constants and fixtures
README.md	Added tag filtering documentation and examples

Usage Examples

# Fetch traces with a single tag
langsmith-fetch traces --tag production --limit 10

# Fetch traces with multiple tags (OR logic - any tag matches)
langsmith-fetch traces --tag prod,staging --limit 10

# Combine with existing options
langsmith-fetch traces --tag production --last-n-minutes 60 --project-uuid <uuid>

Testing

• All 86 tests pass (uv run pytest tests/ -v)
• Manual testing with real LangSmith API (tag: 20251225_023631)
• Verified custom agent outputs are returned correctly

Breaking Changes

None - this is a purely additive feature.