Hold the Weights Fixed: The Ablation Nobody Has Run In-House
Two metrics moved together in the Harvey results, and that pairing is what tells you which failure mode the training actually repaired.
Coverage moved as far as the pass rate did
The most diagnostic number in the Harvey/Baseten results is the one nobody quotes. Under GRPO on Qwen3.5-122B-A10B, held-out rubric pass rate went from 30% to 63% and document coverage went from 62% to 96%, per AINews's account. Both moving together points at a search-and-recall failure that the reinforcement learning repaired. The agent was not reading enough of the data room. Formatting cleanup does not move coverage, and a single scalar pass rate would have hidden the mechanism entirely.
| Intervention | Movement | Delta | What it costs |
|---|---|---|---|
| Standard tool loop to recursive root/sub-agent harness | 23% to 62% mean rubric pass | +39pp | Engineering only, zero training compute |
| Self-distilled SFT, in-harness (GLM-5.2) | 46% to 60% | +14pp | Rollout collection plus one SFT run |
| GRPO, in-harness (Qwen3.5-122B-A10B) | 30% to 63% on held-out rooms | +33pp | RL infrastructure plus rubric design |
| Same GRPO run, retrieval side effect | 62% to 96% document coverage | +34pp | Same run |
Where the write-ups agree, and where they stop
Daily Dose of Data Science and MIT Technology Review both land where Harvey does: the measurable gain sits in the scaffolding, not the weights. Daily Dose of Data Science documents the post-training half collapsing to a config-level task. Qwen3-4B-Base loaded through Unsloth, LoRA applied, GRPO via HuggingFace TRL's GRPOConfig/GRPOTrainer, trained on Open R1 Math with four deterministic reward functions (exact format, approximate format, answer check, number check) on a single GPU. No critic network, no preference labels; the reward signal is four string comparisons. MIT Technology Review supplies the selection rule: the pattern works where a programmatic oracle already exists, query-execution equivalence for text-to-SQL, unit tests for transforms, schema validation for extraction, and it breaks on summarization and judgment, where LLM-as-judge is not an oracle.
Rigor runs one way here. The 23%-to-62% figure is a mean across models on a synthetic benchmark, LAB Diligence, with no per-model breakdown, no variance, a vendor-authored rubric and no independent replication. A swing that size is also what a badly configured baseline tool loop produces, and nothing published rules that out. The zero-label tutorial is thinner. It calls the result a reasoning powerhouse and reports no pass@1, no held-out split and, critically, no SFT control arm on the same data, which leaves the RL gain confounded with the format conditioning the system prompt supplies on its own. The recipes are public; the ablations that would let anyone attribute the gain are not.
What this changes in the harness
Harness is a factor in these results, not a constant. A model x harness factorial with fixed seeds and at least three repeats per cell is the only design that separates orchestration from weights; it reports main effects and the interaction separately, and it puts a number on how much of the last quality win was orchestration. Pass rate and coverage belong in two columns, since Harvey's data shows they move independently and one number cannot triage three failure surfaces. A frozen golden set and a pinned baseline model are the conditions under which a quality change can still be attributed to a cause. Practitioners are already reporting that they had not finished migrating off one model before the next release landed.
The largest measured lever in these results was the scaffolding, not the weights. Most production agent evals are measuring their own harness ceiling and calling it model quality.
What to do
Run your current agent benchmark as a model x harness grid this sprint — standard tool loop versus recursive delegation, fixed seeds, three repeats per cell — reporting per-rubric pass rate and tool/document coverage as separate columns
Instrument per-component reward logging, mean completion length and KL-to-reference before the first GRPO run, and add an SFT control arm on identical data
Pick your single highest-volume long-horizon task with an existing programmatic verifier and pilot in-harness GRPO on an open-weight base this quarter, evaluating only on held-out task instances