Live Mastra Code instrumentation

Run the real TUI against a real model, have an agent type the prompts, and log timestamps at each layer. Comparing the layers step by step shows where timing or ordering goes wrong: at the provider, in the network wrapper, in stream conversion, in the run engine, or in the UI reducer.

Pair this with debugging-difficult-bugs: instrument, reproduce, read the log before fixing, and clean up afterwards. Afterwards, turn the finding into deterministic checked-in coverage (unit tests plus a TUI E2E scenario under mastracode/tui/e2e/tui/). A live run is evidence, not a regression test.

Cost and consent

The TUI uses the user's configured model and stored credentials (for example a Claude Max OAuth login), so every prompt spends their quota. Use short prompts. Tell the user which model you ran and roughly how many sessions.

1. Add temporary diagnostics

Append one JSON line per event to <cwd>/debug-token-rate.jsonl (or a similarly named file) with appendFileSync. Rules:

• Metadata only: timestamps, event types, ids, lengths, and token counts. Never log prompt, response, or tool content.
• Tag every record with a unique build marker (for example 'my-bug-diagnostics-v1'). That lets you confirm the build you ran contains the instrumentation and find every leftover when cleaning up.
• Wrap each append in try {} catch {} so logging can never break the run.
• Take the timestamp before writing. A synchronous append adds a little latency to the path you are measuring, which is fine for millisecond-scale timing. If you are measuring anything finer, push records onto an in-memory array and write them out once when the run ends.
• Log at the earliest and latest point of each layer you care about, so the timestamps can be lined up per step.

Useful hook points, from outermost to innermost:

For other providers, find the equivalent fetch option or wrapper in mastracode/sdk/src/providers/.

2. Build and verify the instrumentation shipped

pnpm install --frozen-lockfile   # fresh worktrees only
pnpm build:mastracode            # ~50s once deps are built
rg -l "my-bug-diagnostics-v1" mastracode/tui/dist packages/core/dist mastracode/sdk/dist

If rg finds nothing in a dist, the run will not produce that layer's records.

3. Drive the TUI in tmux

Start the TUI in a throwaway git repo so the agent's tool calls default to scratch files. Setting the working directory is not a sandbox: do not grant the session extra allowed paths, and keep prompts pointed at the scratch repo.

mkdir -p /tmp/mc-repro && git -C /tmp/mc-repro init -q
tmux new-session -d -s mcrepro -x 200 -y 50 -c /tmp/mc-repro \
  "node $PWD/mastracode/tui/dist/cli.js"
sleep 14   # startup

Send each prompt, then send Enter as a separate key press, then wait and read the screen:

tmux send-keys -t mcrepro "/new"; sleep 1; tmux send-keys -t mcrepro Enter; sleep 3
tmux send-keys -t mcrepro "Please do these one at a time as separate tool calls (not in parallel): write 3 files named a1.md through a3.md, each with a ~100 word paragraph about a different tree, writing a sentence of explanation before each tool call. Then run ls -la with execute_command and summarize in three sentences."
sleep 1; tmux send-keys -t mcrepro Enter
sleep 50
tmux capture-pane -t mcrepro -p | grep -v '^\s*$' | tail -3   # status line is last

Tips:

• Pick prompts that exercise the path under test. A multi-step, one-tool-at-a-time prompt produces a mix of thinking, text, and tool-argument steps.
• Loop the prompt several times, starting each with /new, because intermittent behavior needs a sample. Record what the screen showed each time so you can check the log against what the user would have seen.
• Rebuild and restart the tmux session after every code change. The TUI runs from dist.

4. Analyze per step

Group the log records by step (for example, split at each usage.computed or step-finish record) with a short python3 script. For each step, compare durations across layers: request → headers → first content → last content at the network layer, and the same span at the provider, engine, and UI layers.

• Spans equal across layers mean the pipeline adds no distortion, so the behavior comes from the provider.
• A span that shrinks or grows between two layers is where the bug lives.
• To test a hypothesis, change one variable (for example send accept-encoding: identity), rebuild, and rerun.

Report the evidence as a small table (normal steps vs. anomalous steps), along with the model used, the number of runs and steps, and what the data cannot prove.

5. Clean up

tmux kill-session -t mcrepro
rg -n "my-bug-diagnostics" mastracode packages --glob '!**/dist/**'   # must print nothing
git diff                                                               # read it: only intended changes remain
rm -f debug-token-rate.jsonl /tmp/mc-repro/debug-token-rate.jsonl

Also revert any experiment toggles (like changed request headers) and confirm with rg before saying they are gone. Rebuild so dist no longer contains the instrumentation.