Inspect Codex Traces with jsonl-viewer

Melvin Vivas · X post · 2026-09-06 · Open on X

Topics: AI Dev Tools & Productivity, AI Agents, Tool Use & MCP · Level: beginner

Summary

Codex saves its session traces as JSONL files. The creator's open-source jsonl-viewer lets you open and inspect them to see what the agent did.

Key points

• Codex session traces are stored as JSONL.
• jsonl-viewer can open these traces for inspection.
• The same tool also views and edits fine-tuning datasets in JSONL format.

Resources mentioned

• donvito/jsonl-viewer · repo · github.com · free
  Open-source tool for viewing and editing .jsonl files (fine-tuning datasets) and agent traces from Codex, Claude Code, Pi and Hermes.
  Also in: jsonl-viewer: Live Traces for Codex Orchestrator and Subagents (Melvin Vivas on X · notes), See live Codex traces with the jsonl-viewer tool (Melvin Vivas on X · notes), Melvin Vivas's open-source AI tools, built with Codex (Melvin Vivas on X · notes), Analyzing Codex Traces with jsonl-viewer (Melvin Vivas on X · notes) and 12 more

Try this

• Open your Codex trace files in jsonl-viewer to review what the agent did.