Log formats

Which log fields agentmeter reads for Claude Code and Codex CLI, and what is assumed.

Both formats are not documented officially and change between versions. Parsers skip unknown fields and lines and count what could not be read.

Claude Code

Location: ~/.claude/projects/<project>/<session>.jsonl, sub-agents in subagents/.

Assistant entries carry usage with fresh input, output, cache reads and cache writes.
The same response can appear more than once, so entries are deduplicated by message id and request id.
Compactions are system entries with compactMetadata (trigger and tokens before).
Entries marked as sidechain belong to a sub-agent.

Location: ~/.codex/sessions/ and ~/.codex/archived_sessions/.

token_count events carry cumulative totals and the last call, plus the model’s context window.
Input includes cached input; output includes reasoning. Checked on one author’s rollouts (3,453 events, versions 0.130 to 0.165): total_tokens is always input plus output, and cached input is never larger than input.
Repeated events with unchanged totals occur, so deltas of the cumulative total are used. Summing the last call instead would count them twice.
Replayed parent history in sub-agent rollouts is not counted twice.
Cache writes are read only where the field exists (newer versions) and carries a value above 0; otherwise they are “not measured”, never zero.
Compactions appear as compacted entries with a response id. The trigger and the size after compaction are not logged, so rules that need them report “not measurable”.
Older rollouts have no token_count events and are reported as files without usage.
Compressed rollouts (.jsonl.zst) are read as a stream; damaged files are counted and reported.
The version in session_meta is shown but not used to pick a format, because a resumed session keeps the original value.

Both agents can export OpenTelemetry data, which is the only documented and stable interface. Reading it is a possible later addition.