Design and wire eval instrumentation

Contract: references/interaction-contract.md. Calibration, templates, provenance: references/trace-schema.md.

Trigger

• "What should we log?" / "Design a trace schema." / "Set up tracing before evals."
• Failures that cannot be reconstructed, scored, or sliced after the fact.
• An evidence map with signals nothing currently records.

Do

1. Work backward from each required decision to the fields needed to reconstruct it. Rule: if a variable matters to interpretation or a release decision, it needs a field.
2. Design the record shape and the span tree together (reference.md) — one span per LLM call, tool call, and scorer call, nested to mirror actual control flow.
3. Add the four fields always discovered too late: the raw input artifact, a per-item status kept separate from the score, the fully resolved config including serving path and tool manifest, and a stable grouping key on every trace that belongs to a larger unit of work — a conversation, session, or thread.
4. Mark each field required or optional with its retention and privacy handling, and scrub sensitive values at emission rather than in post-processing.
5. Verify by round trip: run one item, open the trace, confirm you could diagnose a seeded failure from it alone. Fix the instrumentation, not the eval, if you cannot.

Avoid

• Do not log sensitive data because it might be useful someday — every field is a retention commitment.
• Do not confuse chain-of-thought collection with behavioral evidence.
• Do not let the score double as the status field.
• Do not decide what the evidence means here.

Check

• A past real failure is reconstructable, scoreable, sliceable, and attributable to the module that first erred, from these fields alone.
• Every stratification variable in the dataset plan exists as a field.
• Status separate from score; resolved config present; raw artifact attached for non-text.
• Privacy, PII handling, and retention stated per sensitive field.

Risk

• Missing fields make later questions permanently unanswerable — you cannot retroactively log a subgroup variable, offline or in production.
• Mis-nested spans look complete while making root-cause attribution impossible.
• Renamed provider options are dropped silently; without the resolved config, "the feature didn't help" and "it never applied" are indistinguishable.

Braintrust

Of the four objects (references/platform-mechanics.md §1), this stage owns traces — the one everything else consumes, and the only one you cannot backfill.

Field placement decides what the platform can ever do: scores in native scores (0–1); tokens/latency/cost in native metrics, or they cannot be charted or gated on; slicing variables in record metadata, or subgroup analysis needs re-labeling; scorer name/version in span metadata; raw non-text input as a span Attachment (input.audio = Attachment(...)), which renders inline so "listen to the actual clip" is one click. Note experiments surface attachments more prominently than datasets — if human review of raw artifacts is part of the loop, make an experiment the review surface.

A stratum not in metadata is a stratum you cannot report, with no retroactive fix. Include scenario_id wherever items are variants of one scenario, so clustered standard errors stay computable.

Instrumentation also fixes the evaluation scopes available later. Evaluators run against a span, a trace, or a group of traces joined by a metadata key — so multi-turn behavior is only measurable if something like conversation_id, session_id, or thread_id was emitted at the time. Same no-retroactive-fix rule as any other stratum, with a sharper edge: a grouping key that is present but inconsistently populated is worse than one that is absent, because group-scoped evaluation will silently drop the traffic where it is null and report a confident number over the remainder. Assert its coverage the way you assert any other transform invariant.