Production re-embed sweeps lack a causal non-convergence test

Context

PR #16876 adds a direct embedChunks() repetition witness for #16875, but exact-head falsification found that the test never invokes the production sweep selector in VectorService.embed(). The loop arm passes the full corpus manually on every sweep; the control arm manually filters remaining before calling the lower-level method.

Two independent mutations left the new 4/4 cases green: making production VectorService.embed() throw unconditionally, and removing the failing collection's throw. The opposite outcomes are therefore caused by different test-owned caller logic, not by the production persistence/selection composition the prose claims.

Live latest-open sweep: checked the latest 20 open issues at 2026-08-10T12:11:21.358Z; no equivalent found. The all-state A2A claim sweep over the latest 30 messages found no overlapping lane claim.

The Problem

A lower-level repetition witness is useful, but it cannot convict or protect the deployed sweep. The production question is whether VectorService.embed() re-selects persisted versus unpersisted chunks across successive sweeps. A test that supplies its own selected set can remain green while the production selector is deleted, inverted, or bypassed.

The current pair also confounds the independent variable: the loop case always resubmits the full corpus, while the control case computes a shrinking set in the test. Persistence is not the only thing that differs.

The Architectural Reality

ai/services/knowledge-base/VectorService.mjs owns both the production sweep selection in embed() and the lower-level provider/write work in embedChunks(). The merged #16867 provider-activity ledger owns the shared reembedRatio observation; a test-local submission counter is not the deployed consumer.

Structure-map V-B-A confirms VectorService.mjs remains in the existing ai/services/knowledge-base owner folder. This ticket adds no new production module.

The Fix

Replace or supplement the direct-embedChunks() pair with a production-bound sweep fixture through VectorService.embed():

• both arms use the same production selection and caller flow;
• only the persistence outcome varies;
• repeated work is asserted through the shared reembedRatio projection shipped by #16867; and
• shared AiConfig leaves are isolated with the sanctioned snapshot/config-child primitive rather than Object.assign on the live provider.

Decision Record impact

none — test authority and isolation within the existing Knowledge Base service boundary.

Acceptance Criteria

• Both failure and convergence arms invoke the real VectorService.embed() production selector for every sweep.
• The two arms share identical corpus, selection, and calling logic; only collection persistence outcome differs.
• Making VectorService.embed() unreachable or throwing causes the new witness to fail.
• Removing the persistence failure collapses the repetition witness rather than leaving it green.
• The failure arm drives the shared #16867 reembedRatio above the clean control; no test-local counter substitutes for the public observation.
• The converging arm reaches a no-work next sweep through production selection.
• Knowledge Base config mutation uses the sanctioned isolation helper or a per-test config child and restores exact prior state.
• Named red mutations prove selection bypass, persistence removal, and observer bypass independently.

Out of Scope

• Changing VectorService retry, persistence, or vector-retention behavior.
• Reopening #16875; this is a linked successor because PR #16876 resolves that leaf.
• Diagnosing why a real vector store rejects writes.

Avoided Traps

• Hand-computing remaining. That replaces the production selector with the test's answer.
• Calling only embedChunks(). That proves lower-level repetition, not sweep composition.
• Counting only local submissions. It bypasses the cross-process observer the operator actually consumes.

Related

Related: #16780 · #16875 · PR #16876 · PR #16867

Origin Session ID: 878f05af-2c4e-4da2-a5c2-9e4af666fcb8

Retrieval Hint: VectorService production sweep selection persistence non-convergence reembedRatio

Authored by Emmy (GPT-5.6 Sol Ultra, Codex).