# M5.5 — verl training loop

| Field | Value |
|---|---|
| Phase | M5 — Post-training |
| Size | L — 3+ days |
| Status | ⬜ Not started |
| Flags | — |
| Spec | inlined below |
| Blocks | M5.3, M5.4 |

## Goal

Train the LoRA that makes the update and exit gates better than prompting alone.

## Facts (inlined — no spec read needed)

Python, using verl (`github.com/volcengine/verl`), which is what the paper used.
Lives outside the Rust workspace; the JSONL corpus is the only interface.

Paper hyperparameters, Table 3 — start here rather than guessing:

```
chunk size            5000
max prompt length     8192
max response length   2048
clip ratio            0.20
learning rate         1e-6
temperature (train)   1.0      top_p 1.0
temperature (val)     1.0      top_p 0.7
train batch size      128
rollout N             16
mini batch size       128
LR warmup steps       20
```

Advantage, paper §3.2.2 — the part most likely to be implemented wrong:

```
Â_traj = r_traj_g − mean over the GROUP of trajectories
Â_turn = r_update_{g,t} − mean over turns AT STEP t across groups
      = α·Â_traj + (1−α)·Â_turn        α = 0.9
```

Two distinct baselines. `Â_turn` is normalised across groups **at the same `t`**,
and the group size at step `t` can differ from the trajectory group size, because
trajectories that exited early have fewer turns.

α=0.9 is the paper's default and the ablation (Figure 8) explains why: at α=1
there is no update-gate reward and accuracy on evidence-free chunks collapses —
the model updates indiscriminately, which is exactly the failure this whole
system exists to avoid.

Expect instability. The paper's own limitations section says the extra rewards
"reduce training stability, requiring a smaller off-policy degree and longer
convergence time."

## Steps

1. `training/` directory, Python, `uv`-managed. System Python is 3.9.6; this
   needs 3.11+.
2. Corpus loader for M5.3's format.
3. Configure verl for LoRA on Qwen2.5-3B-Instruct, rank 16–32.
4. Implement the three rewards and the two-baseline advantage, α configurable.
5. Log per step: update accuracy split by evidence-present and evidence-free,
   exact-exit ratio, format correctness, mean response length, validation reward.
6. Hold out a validation split by **project**, not by trajectory — same-project
   trajectories share vocabulary and leak.
7. Export the adapter, version it, publish where M5.4 can mount it.

## Acceptance

- Training runs to convergence on the validation reward.
- Both advantage terms are computed with their own baselines.
- Update accuracy on evidence-free chunks does not collapse.
- The adapter loads in M5.4's server.

## Verify

**Harness:** pytest for reward and advantage maths; a short training run for the
loop itself.

**Integration test** — `training/tests/test_rewards.py`:
1. `a1_r_update_signs` — matching label +1, mismatching −1.
2. `a2_r_exit_bands` — exact 0, early −0.75, late −0.5.
3. `a3_r_format_strict` — any unparsed turn zeroes the whole trajectory.
4. `a4_traj_baseline` — `Â_traj` uses the group mean; hand-computed fixture.
5. `a5_turn_baseline_at_step_t` — `Â_turn` normalises across groups at the same
   `t`; a fixture with unequal trajectory lengths must not misalign. This is the
   assertion that catches the most likely implementation error.
6. `a6_alpha_mix` — α=1 yields pure trajectory advantage; α=0 pure turn.
7. `a7_alpha_1_degenerates` — train 50 steps at α=1 on a fixture; assert
   evidence-free accuracy drops relative to α=0.9, reproducing the paper's
   Figure 8b.
8. `a8_validation_split_by_project` — assert no project appears in both splits.
9. `a9_adapter_loads` — export, mount in M5.4, assert a completion returns 200.

**Command:** `uv run pytest training/tests -v`

**False pass:**
- Normalising `Â_turn` over the whole batch rather than per step `t`. It trains,
  loss goes down, and the turn-level signal is diluted into noise — assertion 5
  is the only thing that catches it.
- Splitting validation by trajectory. Same-project trajectories share phrasing
  and file paths, so validation reward looks excellent and generalisation is
  untested.
- Skipping assertion 7 as "too slow". It is the only end-to-end evidence that the
  update reward is wired to anything.

## Traps

- Tuning α before the rewards are verified. Every α is wrong if `r_update`'s sign
  is flipped, and the symptom looks identical.
- Training on labels whose κ was never measured. The policy learns the labeler,
  and there is no held-out signal that would reveal it — M5.2 exists for this and
  M5.3 refuses to export without it.

---

Background: [DESIGN.md](../DESIGN.md) — P6 · paper §3.2.2, Table 3, Fig 8