Add agent rehearsal harness, findings, and per-run evidence for split-loop-with-lambda
Six scripted pi sub-agent runs (3 tool regimes × 2 models) rehearsed the exercise; deterministic gate = solution test + approved file. Key results: mandated CLI triples cost on the strong model (4:06 vs 0:39, PASS both); weak model fails only under the mandate (A+Q), passes when free or optional. Drift documented with code: scorer.updateSelection left inside the extracted method (A+Q, C+Q). Harness in tools/rehearse; summary in rehearsal-findings.md; per-run evidence in rehearsal-runs/; cheat sheet in rehearse-with-agent.md corrected (lambda-typed parameters unsupported).
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# Rehearsal of the exercise with coding agents — full summary
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**Date:** 2026-09-27 · **Harness:** `tools/rehearse/` · **Raw per-run artifacts:** [`rehearsal-runs/`](./)
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· **Exercise:** Split Loop with Lambda (IceCreamForecasting kata, `LongRangeForecasts`)
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Six scripted sub-agent runs did the refactoring under different tool rules and
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two models. Each run got a fresh workspace, a shared AGENTS.md (identical
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design instructions), a 10-minute wall-clock cap, then measurement from the pi
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session log and a deterministic gate: the reference solution's test + approved
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file (`{Strawberry:12, Chocolate:12, Vanilla:17, }` with an `alwaysSunny`
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lambda) dropped in and `mvn test` executed.
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Variants:
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- **A** — `java-refactor` CLI mandated (text edits forbidden except the
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introduce-lambda step and a fallback for step 3)
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- **B** — plain pi tools (read/bash/edit), CLI absent from PATH
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- **C** — CLI available, use *optional*, with a 6-line intro to the relevant
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subcommands
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- **+Q** — same rules on entrim's Qwen/Qwen3.6-35B-A3B instead of
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openrouter's z-ai/glm-5.3-flash
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## Results
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| Run | Model | Rules | Wall clock | Turns | Tool calls | Tokens (sum) | Peak context | Gate |
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|-----|-------|-------|-----------:|------:|-----------:|-------------:|-------------:|------|
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| A | glm-5.3-flash | CLI mandated | 4:06 | 27 | 27 (19 bash, 4 read, 4 edit) | 418,718 | 23.0 K | **PASS** |
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| B | glm-5.3-flash | plain tools | 0:39 | 10 | 10 (5 bash, 1 read, 4 edit) | 62,196 | 7.6 K | **PASS** |
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| C | glm-5.3-flash | CLI optional | 2:37 | 23 | 23 (15 bash, 4 read, 4 edit) | 150,805 | 13.5 K | **PASS** |
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| A+Q | Qwen3.6-35B | CLI mandated | 2:07 | 27 | 28 (17 bash, 8 read, 3 edit) | 352,812 | 22.6 K | **FAIL** |
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| B+Q | Qwen3.6-35B | plain tools | 1:13 | 17 | 17 (7 bash, 5 read, 5 edit) | 260,052 | 29.8 K | **PASS** |
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| C+Q | Qwen3.6-35B | CLI optional | 1:12 | 17 | 17 (7 bash, 5 read, 5 edit) | 149,663 | 16.1 K | **PASS** |
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Matrix (n=1 per cell — indicative, not statistical):
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| | glm-5.3-flash (strong) | Qwen3.6-35B (weak) |
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|---|---|---|
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| **CLI mandated (A)** | PASS | **FAIL** |
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| **CLI optional (C)** | PASS — used it surgically | PASS — ignored it entirely |
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| **Plain tools (B)** | PASS | PASS |
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Measurement notes: token sums are per-request usage added over the whole
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session; entrim reports no prompt-cache reads, so +Q runs re-send full context
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every turn (their sums are inflated relative to openrouter's cached runs —
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e.g. B+Q's 260 K is 17 requests growing 2.3 K → 29.5 K; peak single request
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~30 K, far under the 262 K limit). Peak context = largest single request.
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Cross-provider, compare wall clock / turns / tool calls, not raw token sums.
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## Headlines
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1. **Mandating the tool tripled the strong model's cost** (A vs B: 4:06 vs
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0:39, 419 K vs 62 K tokens) for an equally-passing result. The cost is not
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the refactoring operations — it is the ceremony around them plus one
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genuine coverage gap (see "Why the CLI regime needs so many turns").
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2. **"Available, introduced, not mandated" is the safe default.** C never did
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worse than the alternative on either model: the strong model used the CLI
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surgically (extract dry-run → apply) and edited manually elsewhere; the
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weak model ignored the CLI entirely and simply edited.
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3. **The weak model failed only in the mandated regime.** A+Q produced broken
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code and rationalized the failing tests as "expected". The same model
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passed in B+Q and C+Q with materially correct end states.
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## The drift, documented
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The exercise's step 1 says: *introduce a lambda for the scorer-dependent code
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and move its definition to the top of the method, leaving the invocation where
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it is.* The scorer-dependent code is **both** scorer calls — `updateSelection()`
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and `lookupWeather()`:
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```java
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// PRISTINE (exercise start) — LongRangeForecasts.java, lines 28–35
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List<Boolean> expectedWeather = new ArrayList<>();
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for (Instant interestingDate : interestingDates) {
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scorer.updateSelection(); // ← scorer-dependent, void side effect
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var forecastDate = Instant.parse("2023-04-26T14:00:00.000-07:00");
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long daysForward = ChronoUnit.DAYS.between(forecastDate, interestingDate);
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Boolean lookupWeather = scorer.lookupWeather(daysForward); // ← scorer-dependent, value
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expectedWeather.add(lookupWeather);
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}
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```
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After a correct step 1+2, the extracted method is **static and pure** — its
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only "dependency" is the lambda parameter, and the scorer calls live inside
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the lambda definition in the no-arg overload (reference solution):
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```java
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// CORRECT — reference solution
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public Map<IceCream, Integer> longRangeForecast() {
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return longRangeForecast(daysForward -> {
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scorer.updateSelection(); // scorer-dependent code, inside the lambda
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return scorer.lookupWeather(daysForward);
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});
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}
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private static List<Boolean> collectExpectedWeather(LongFunction<Boolean> dayOffsetToWeather) {
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var expectedWeather = new ArrayList<Boolean>();
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for (Instant interestingDate : DATES) {
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long daysForward = ChronoUnit.DAYS.between(ANCHOR_DATE, interestingDate);
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expectedWeather.add(dayOffsetToWeather.apply(daysForward)); // loop is scorer-free
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}
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return expectedWeather;
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}
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```
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The **drift** (A+Q and C+Q both): lambda-izing only the value-producing call
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and leaving `updateSelection()` as loop machinery, so extraction carried the
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`scorer` field dependency into the new method:
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```java
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// DRIFTED — A+Q / C+Q end state
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private List<Boolean> collectExpectedWeather(List<Instant> dates, LongFunction<Boolean> dayOffsetToWeather) {
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List<Boolean> expectedWeather = new ArrayList<>();
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for (Instant date : dates) {
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scorer.updateSelection(); // ← LEFT BEHIND: method still depends on the field
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var forecastDate = Instant.parse("2023-04-26T14:00:00.000-07:00");
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long daysForward = ChronoUnit.DAYS.between(forecastDate, date);
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expectedWeather.add(dayOffsetToWeather.apply(daysForward));
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}
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return expectedWeather;
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}
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```
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Consequences of the drift: `collectExpectedWeather` is neither static nor
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pure, step 4 (moving it to another class) would be blocked, and the "unwanted
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external dependency" was never actually severed — the split is cosmetic.
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Why the gate still passed: the test injects its own pure lambda, and the stray
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`updateSelection()` only mutates the static `Scorer.flavour` field, which
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nothing in the forecast reads (`lookupWeather(daysForward)` ignores it; the
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field's real consumer is the unrelated `DailyForecasts.salesForecast()`).
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**PASS ≠ drift-free** — the deterministic gate verifies behavior, not design
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intent. Catch it mechanically: `grep -n scorer <extracted-method>` must return
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nothing, or simply try to make the extracted method `static` (it won't
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compile if the field reference remains).
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### The debrief aha hiding in this line
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`updateSelection()` randomly re-rolls `Scorer.flavour` (see `Scorer.java:51–59`)
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— state that belongs to the *daily* forecast, which is the only real consumer
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(`DailyForecasts.java:28–30`, copy-paste residue into the long-range loop).
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Once the loop is split and the lambda extracted, it becomes visible that
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`updateSelection()`'s result is never used — **it is deletable outright**, and
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the approved file does not change (listed in `solution/README.md`). The
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refactoring does not just hide the dependency behind a lambda; it exposes that
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one of the two scorer calls was dead all along.
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## A+Q's failure anatomy (the mandated-CLI run)
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Three stacked problems, all verified from the session log:
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1. **Over-deletion (the actual gate failure).** Removing `String quarter`, it
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deleted BOTH the dead Q1/Q4 branch AND the flavour-reset loop in one edit.
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The reset loop is what populates the result map → forecast returns `{}` →
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`expected {Strawberry:12, Chocolate:12, Vanilla:17} but was <{}>`. The
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reference solution drops the Q1/Q4 branch too — but keeps the reset loop.
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Silent failure: compiles and runs, just semantically empty.
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2. **It never saw its final test behavior.** Its last in-run `mvn -q test`
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(piped through `head -30`) hit a stale-bytecode artifact — incremental
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compilation kept the old test class calling the removed
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`longRangeForecast(String)` — throwing an `Error` at the call site whose
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exception name scrolled off the pipe. It concluded: *"The test fails
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because the old longRangeForecast(String) signature no longer exists —
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which is expected per the task instructions."* The AGENTS.md's "tests may
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fail — that is expected: the code must still compile" gave it a ready-made
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rationalization to stop on red.
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3. **The flaky pristine test camouflaged everything.** `Scorer` is random, so
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mid-run assertion failures looked like expected noise; at one point its
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`received.txt` even showed `{Strawberry:12, Chocolate:12, Vanilla:17}` — a
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lucky random roll coincidentally equal to the solution's approved output,
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sitting in the tree while the real behavior was `{}`.
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## Why the mandated CLI regime needs so many turns (Run A trace)
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Of A's 27 turns: ~8 discovery (reads, `--help` ×2, backup, coordinate
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computation via `cat -n`), ~8 CLI attempts (2× `extract`, 2×
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`decompose-conditional`, `remove-param`, `introduce-param` — the last four all
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hunting in vain for a subcommand that can turn `longRangeForecast(String)` into
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`longRangeForecast(LongFunction<Boolean>)` + overload), 3 manual edits, ~6
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verify rounds (`mvn` ×5 + final diff). The point-based API forces a
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read → compute-coordinates → invoke → confirm cycle per operation, and every
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operation is a bash round-trip. Run B does it in 10 turns: 4 edits, 3 `mvn`,
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1 read, 2 housekeeping.
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C shows the fix: with the tool merely *available* plus a 6-line intro, the
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agent used `extract` (dry-run → one rejected selection → corrected → apply)
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and edited manually for steps 1 and 3, with no shopping detour — 23 turns,
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2:37, and the closest-to-reference diff of all six runs (dates hoisted into
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`collectExpectedWeather`).
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## Tool limitations discovered (worth reflecting into the exercise)
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1. **Lambdas are not first-class.** `introduce-param` cannot introduce a
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lambda expression as a parameter: applied to the step-3 lambda it *inlined
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the expression into the call site* — referencing the private `scorer`
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field from outside the class, non-compiling (observed in A and A+Q; C and
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C+Q avoided it after reading `--help`). The CLI's lambda support is the
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reverse direction: method *references* are converted to lambdas when
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rewriting call sites for parameter changes (golden-master approved), but a
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lambda expression is never the *subject* of an operation. The original
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cheat sheet's claim that "add lambda parameter" maps to
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`introduce-param`/`change-method-signature` was wrong and is corrected in
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`rehearse-with-agent.md`.
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2. **`extract` rejects non-statement selections** with a diagnostic
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("Selection does not contain any complete statements") — correct, safe
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behavior; C recovered from its one mis-selection in a single retry.
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3. **`--dry-run` preview → apply is mandatory hygiene.** Both A and A+Q had
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their broken `introduce-param` results caught by the dry-run diff — the
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only reason they didn't ship non-compiling code. It also doubles every CLI
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round trip, part of the mandated-regime cost.
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4. **Stale-bytecode mvn artifacts can mask real test results** when a public
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signature is removed without `clean` (the old test class still runs and
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throws `NoSuchMethodError`-style errors). Agents doing verification-by-mvn
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should prefer reading the assertion message, not just the exit status.
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## Prompt corrections fed back into the exercise
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- The rehearsal prompt originally omitted the overload step; the reference
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test (`longRangeForecast(LongFunction<Boolean>)`) cannot compile without it.
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The prompt now includes step 3 with the **exact target signature** ("do not
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keep the `String quarter` parameter") — vaguer wording produced
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`longRangeForecast(String, LongFunction)` in run A's first attempt.
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- AGENTS.md must say "tests may fail, must still compile" up front — otherwise
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agents burn turns fighting the flaky pristine test (and A+Q ignored even
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that, see above).
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## Implications for the learning hour
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- Expect the **tool-availability seam** to be where participants lose time:
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the CLI covers extract/rename/move, but the *interesting* mechanics of this
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kata (introduce lambda, slide statement, function-typed parameter) are
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manual. Make that explicit up front rather than letting participants
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discover it by trial.
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- **Time estimate from this rehearsal:** a strong pair with plain editing
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lands ~40 s of agent effort; with the CLI mandated, ~4 minutes. Humans will
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be slower on both, but the *ratio* (tool ceremony ≈ 3–6× on a 55-line file)
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is the honest expectation-setter.
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- The drift (scorer left in the extracted method) is the canonical mistake to
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seed the debrief: ask participants to run `grep scorer` on their extracted
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method. It happened in 2 of 6 runs, on both models, only under manual-edit
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conditions — humans will hit it too.
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## Reproducing
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```bash
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bash tools/rehearse/setup.sh # build + install + pristine + smoke test
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bash tools/rehearse/run-rehearsal.sh A B C # strong model, three regimes
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bash tools/rehearse/run-rehearsal.sh A+Q B+Q C+Q # weak model (entrim Qwen3.6-35B-A3B)
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```
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Raw sessions (jsonl, full transcripts) live in `/tmp/rehearsal/runs/<run>/`
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and are not committed; this directory carries the curated evidence
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(`measure.txt`, `verify.txt`, `diff.patch`, the exact AGENTS.md and prompt per
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run, the agent's final report, and the mvn failure tails).
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