Harness sub-agent model selection regression
Type: kb/types/note.md
Current state (as of 2026-07-17)
Codex's multi_agent_v1 spawn surface accepted an explicit model and reasoning_effort. Two retained session traces show workers launched through that surface with model: gpt-5.6-luna; their child turn_context records gpt-5.6-luna at low and medium effort.
The later multi_agent_v2 collaboration surface accepts task identity, context-forking, and message fields but no model field. Its workers inherit the parent model. A direct codex exec --model luna call is not an available fallback under the observed ChatGPT account: the backend rejects Luna as unsupported.
| Surface | Per-worker model selection | Observed result |
|---|---|---|
multi_agent_v1 |
Explicit model field |
Luna workers ran as requested |
multi_agent_v2 |
No model field | Workers inherited gpt-5.6-sol |
codex exec --model luna |
CLI flag exists | Backend rejected the model for the account |
This is a harness regression, not model selection performed by Commonplace. The review system uses model_partition as review identity and freshness scope; selecting a partition does not cause the harness to dispatch that model.
The regression produced false review provenance
Review jobs 6677–6717 were created under the luna partition for seven notes: 41 jobs containing 268 review pairs. Every retained child trace for those jobs records gpt-5.6-sol at maximum effort. The parent nevertheless finalized each job with --model luna, so all 268 generated pair results record both model_partition: luna and runner_model: luna.
The stored provenance therefore contradicts the execution traces. Those results may be read as Sol judgments, but they are not Luna review evidence and must not satisfy a Luna freshness claim. Snapshot freshness cannot repair incorrect execution provenance.
The incident exposed the parent-trust boundary described by ADR 034: finalization validates the model string supplied by the parent against the selected partition, but it does not establish that the worker actually ran that model.
Operational consequence
A model-specific sweep is not executable through the observed multi_agent_v2 surface unless the parent itself runs the required model or the harness restores explicit per-worker selection. A worker label, task name, requested partition, or intended model is not execution evidence. When a trace is available, turn_context.payload.model is the concrete execution record to compare with persisted runner_model.
Until the regression is fixed, jobs whose required partition differs from the inherited worker model should remain unfinalized. Existing jobs 6677–6717 require replacement by genuine Luna executions or explicit retirement of their misattributed Luna evidence.
Relevant Notes:
- Run review batches — procedure: dispatch and provenance rules affected by this harness regression