poimen/tasks/T1.5-context-partition-capture.md

T1.5 — Context partition capture

Field	Value
Phase	P1 — Walking skeleton
Size	S — under 1 day
Status	Not started
Flags	—
Spec	inlined below
Blocks	—

Goal

Record, per model step, which context was packed, which was available but not packed, and which was dropped — as identifiers.

Facts (inlined — no spec read needed)

• Three buckets: packed, available-not-packed, dropped.
• They hold identifiers, not text. That keeps them small enough to inline in the attempt view rather than going by blob reference, which is what lets a verifier read them without paying for prompt text.
• This is the signal behind "retried three times because context was missing X" — the thing a pass rate cannot see and a learning loop needs.
• It is not derivable after the fact from the conversation. If it is not captured at pack time it is gone, which is why its absence is a test failure rather than a gap.

Steps

1. Define ContextPartition { packed: Vec<ContextItemId>, available: Vec<ContextItemId>, dropped: Vec<ContextItemId> }. Ordered collections, so serialization is stable.
2. Capture at the point the prompt is assembled — the packer already knows all three sets; recording them costs a clone of id vectors.
3. Attach the partition to the attempt record and emit it in the same EventLog::commit as the rest of the attempt transition.
4. Surface it on AttemptView.context: Option<ContextPartition> for verifiers (T4.2) and graders.
5. Test: assert every model-step attempt in a completed run carries a partition; a missing one fails the test rather than being tolerated as None.

Acceptance

• Partition present on every model step.
• Not derivable from the conversation, so its absence is a test failure.

Verify

Harness: a workflow with a packer configured to drop known items, so the expected partition is known in advance.

Integration test — tests/it_context_partition.rs:

1. Configure the packer with 10 candidate context items and a budget admitting 6.
2. Run a model step.
3. Assert the recorded partition has packed.len() == 6, available + dropped covering the remaining 4, and that the three sets are disjoint and their union is the full candidate set.
4. Assert dropped is non-empty — a capture that only ever records packed is the common half-implementation and passes any "partition present" check.
5. Iterate every model-step attempt in a completed multi-step run; assert context.is_some() for each. A None fails the test.
6. Assert the partition survives assert_refold_identical.

Command: cargo test -p executor context_partition

False pass:

• Asserting only context.is_some(). An empty partition is Some. Step 3's set arithmetic is the real check.
• Testing on a workflow whose budget admits everything, so dropped is legitimately empty and step 4 cannot fail.

Traps

• Storing the packed context text here. It belongs in the prompt blob (T1.6); inlining text blows broker payload limits and penalizes verifiers that need none of it.
• Capturing only packed. The interesting signal is usually in dropped.

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Background (not required to do this task): rust-agentic-sys.md §10.1 · rust-agentic-task.md