Analyzing Codex Traces with jsonl-viewer

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

Topics: AI Dev Tools & Productivity, AI Agents, Tool Use & MCP, Fine-tuning & Model Customization · Level: intermediate

Summary

The creator shares jsonl-viewer, an open-source tool he built for viewing and editing .jsonl files. He suggests using it to understand and analyze Codex session traces, which are stored as JSONL. It also works for fine-tuning and training datasets.

Key points

• Codex traces are stored as JSONL files.
• jsonl-viewer lets you view and edit .jsonl files.
• It works for agent traces and for fine-tuning/training datasets.
• Reading traces helps you see what a coding agent actually did.

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), jsonl-viewer: Inspect Coding Agent Traces (Codex, Claude Code, Pi, Hermes) (Melvin Vivas on X · notes) and 12 more

Try this

• Use jsonl-viewer to open and inspect your Codex trace files.