I2533 — The incremental derive becomes the default after a shadow period proves it equal, and one flag returns to the full derive

✕BLOCKED wave 8 · p2 · implementation profile · runs alone

Part of a-derivation-computed-once-is-reused — A derivation computed once is reused by every worktree and by CI, and the incremental path becomes the default on evidence.

Objective

Move scripts/derive to the incremental path by default in guarded steps: a shadow mode that runs both and compares, a recorded comparison on the real repository, the switch of the default, and a tested way back.

Why

campaigns/majordomus-incremental-derive-prompt-pack/12_12_ROLLOUT_PROVE.md asks for a rollout in sequence, opt-in, then shadow differential, then default where supported, with a retained --full and a documented and tested rollback, and for before and after figures with sample count, commit, cache state and platform. The differential case of I2527 uses stand-in producers; whether the real 17 targets and the real site generator are declared completely is only shown by comparing the two paths on this repository's own tree.

Current state

After I2525 the incremental path exists behind scripts/derive --incremental and the default is the full sequence. No record compares the two on the real tree.

Desired state

scripts/derive --shadow runs the incremental path into a staging copy and the full path into the tree, and fails naming every file that differs. Shadow runs on the real repository are recorded as evidence for a set of real changes: no-op, one rule, one site page, one schema, one generator source, a fresh worktree, a cold and a warm store. When none differs, scripts/derive runs incrementally by default on the platforms recorded, scripts/derive --full runs the present sequence, the pre-commit and CI checks are unchanged, and docs/DERIVE.md states the rollback. The measurements sit beside the baseline of I2521.

Scope

• scripts/derive
• apps/majordomus-cli/src/derivation/run.rs
• apps/majordomus-cli/src/commands/derive.rs
• apps/majordomus-cli/src/offload
• test/cases/400_derivation_is_one_graph.sh
• test/cases
• docs/DERIVE.md
• docs/PERFORMANCE.md
• docs/OFFLOAD.md
• docs/CLAIMS.yaml
• justfile
• .ai/repo/project/issues/I2533.yaml

Out of scope

• Removing the full derive, scripts/derive-check or the stage order as a fallback
• A release or a deployment; those follow the release procedure and the owner's acceptance
• Switching the default on a platform with no recorded shadow comparison

Dependencies

• I2527✕BLOCKED An incremental derive equals a full derive byte for byte, and an omitted dependency turns a gate red
• I2529✕BLOCKED A node's outputs are kept under its key and restored only after every digest is verified
• I2531✕BLOCKED Two worktrees deriving the same key compute it once, and the machine lock covers the write into a tree, not the computation
• I2532✕BLOCKED CI reuses derivation entries keyed by the graph, and still fails on a stale committed file whether or not the cache was restored

Acceptance criteria

• scripts/derive --shadow exits non-zero and names the file when the two paths differ, shown by a fixture with an omitted edge
• Recorded shadow runs on the real tree show no differing file for each of the eight kinds of change, each with its commit
• With the default switched, scripts/derive --full leaves the same bytes as scripts/derive on an unchanged tree
• A case switches back to the full path and passes without the store directory present
• majordomus offload derive produces the same staged files as before the switch
• The before and after durations are published with commit, machine, store state and sample count, and no figure is published for a scenario that was not run

Validation

• bin/majordomus plan validate
• MJ_TEST_JOBS=2 bash test/run.sh 400_derivation_is_one_graph 51_derived_artifacts_committed 1020_a_derive_sees_what_the_commit_will
• MJ_TEST_JOBS=2 bash test/run.sh <the case this issue adds>
• cargo test --manifest-path apps/majordomus-cli/Cargo.toml --lib offload
• scripts/derive-check

Evidence required

• shadow_runs_on_the_real_tree
• default_enabled_with_rollback_case
• before_and_after_measured

Evidence

None recorded. Every token above needs a command or an artifact behind it before this issue can be completed; narrative is refused.

Timeline

started

—

verified

—

completed

—

Those three fields, the evidence above and the state of the dependencies are all the status is made of. There is no status field to disagree with them.

Canonical record: .ai/repo/project/issues/I2533.yaml. Read it back with majordomus plan show I2533.