Round 2: when the reply triggers another revision

Agent Determinism Illusions (Part 17)

Where this fits: Part 16 collected four reader-driven revisions — Mike HHI pair-join, Tom Jones position-adjacency, Xiao Man shape-routing (rename_keys), Mike quiet-failure. Before Part 16 shipped, Xiao Man replied to the rename_keys section of the draft with a refinement: not "pick a better anchor," but "remove the anchor from the probe's responsibility." This part is the response — what the refinement predicts, what the experiments on this fixture support, and the methodological rule that falls out.

1. The refinement

Xiao Man's reply (2026-07-30) on the rename_keys cell:

The probe should never re-find what the router already resolved. The mutation suite then becomes: "did we accidentally put lookup responsibility back into the probe?"

Two claims, separable:

• Probe layer: if the router passes the resolved path (e.g., "services is at art['components'] after rename"), the probe stops doing key-name lookup and becomes rename-immune by construction.
• System layer: the anchor doesn't disappear; it relocates from probe to declaration/router. The new anchor layer has its own survival question.

Round 1 said: shape-routing is fragile under rename. Round 2 sharpens: the fix isn't a better anchor, it's a boundary redraw.

2. Path-passing probe — confirmed on fixture

Script: probe-path-passing-redesign-test.py → results-v2/probe-path-passing-redesign.json.

Two probe designs on the same rename_keys population (T3 good artifacts, services → components + inner renames, n=40, seed=7):

Probe-side fix works. The probe becomes a value-checker at a router-resolved coordinate, not a finder.

Implementation leak worth naming. First v2 draft only took the services path from the router; it still hardcoded the inner timeout_ms lookup — and rename_keys also changes timeout_ms → request_timeout_ms. v2 still rejected everything. The probe had to take all renamed keys as router-resolved paths before it became rename-immune. Exactly the failure mode Xiao Man named: "did we accidentally put lookup responsibility back into the probe?"

3. Declaration-anchor survival matrix

Script: declaration-anchor-survival-test.py → results-v2/declaration-anchor-survival.json.

Probe-side fix is one half. The other half: the new anchor layer (declaration/router) has its own survival envelope. Four anchors × eight perturbations; each cell is a single deterministic resolution check, not a stochastic trial:

Perturbations:

• P0 baseline; P1 services→components (in synonym list); P2 services→instances (out-of-decl); P3 add decoy field; P4 inner shape corrupt; P5 add third entry; P6 decoy-with-limits inserted before services (shape clone); P7 inner field rename (port→port_number).

No anchor survives 8/8. Each has a distinct failure signature:

• synonym_list dies on out-of-declaration rename (P2). Narrow but predictable.
• structural dies on shape clone (P6). Can't distinguish services from a decoy that mimics list-of-dicts-with-limits.
• cardinality dies on count change (P5) and shape clone (P6).
• cross_field dies on inner field rename (P7). Semantic-structural breaks under inner synonym rename.

The "wide" anchors (structural, cross_field) trade robustness on outer rename for fragility on shape clone and inner rename. Narrow vs wide is a trade-off, not a monotone improvement. Any "X is more robust than Y" claim must name the attack class.

4. Boundary-leak detector rule

Xiao Man's deeper reframe — mutation suite as architectural-violation detector, not bug-finder:

rename_keys doesn't introduce a defect; it only swaps key names. If the system boundary is clean, rename should be a no-op. If rename triggers failure, someone put lookup where it doesn't belong.

Codified as a fixture-design rule (working-notes/boundary-leak-detector-rule.md):

Any fixture with router/probe or judge/lookup layering must include a set of neutral mutations — rename, position-permute, cardinality-preserve. Neutral mutations introduce no defect by design. Failures under neutral mutation count as boundary leaks, reported independently of catch rate.

Neutral-mutation classes:

The rule is a label, not a framework. Existing fixtures (rename_keys, decoy_nest, cue_erase, cross-model pair-join) already run neutral mutations — they just weren't called that. Future fixtures should declare their neutral-mutation inventory up front and report boundary-leak count as a primary metric, alongside catch rate.

What this rule does not do: replace catch rate. A fixture with zero boundary leaks can still have wrong catch rate. The two metrics are independent.

5. Closing

Round 1 said: depth-from-shape is fragile under rename. Round 2 sharpens:

• Probe layer: anchor can be removed. Path-passing redesign confirmed (n=40, seed=7); probe becomes rename-immune by construction.
• System layer: anchor doesn't vanish, it relocates. Declaration/router is the new anchor site, with its own measurable survival envelope.
• Methodological consequence: neutral mutations are boundary-leak detectors. Future fixtures should report leak count alongside catch rate.

Xiao Man named the architectural principle. The empirical work on this fixture supports it: probe becomes anchor-free; system stays anchor-bound at a different layer; the survival question moves with the anchor.

Probe without anchor, system with anchor at a different layer. That's the relocation.

Series: Agent Determinism Illusions · Scripts: GitHub

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