[kixelated]

Problem

A generated snapshot sequence reduced to {"__proto__":0} -> {} leaves the JS decoder with a stale field. Object membership sees inherited properties, and assignment to __proto__ invokes its inherited setter. These are valid JSON keys.

Approach

Use own-property membership and safe data-property writes in the JS diff/merge/equality implementation. Four shared Rust/JS vectors cover prototype-key addition, deletion, nesting, and inherited equality. A JS assertion also checks the result retains an ordinary object prototype.

Add fast-check 4.10.2 and proptest 1.12.0 as dev dependencies. Mirrored bounded JSON generators check every reconstructed value across snapshots, deltas, resets, ratios, and compression, including updates emitting no frame. PR runs use 256 fixed-seed cases. just test json-properties prints a fresh seed and runs 4096 cases in each language nightly. Failures shrink and report replay details; minimized regressions stay in shared vectors. A codec benchmark sweeps document size and changed-key count with both compression modes.

Impact

Public API shape and wire format are unchanged. JS behavior now preserves valid JSON prototype-named fields. Rust production code is unchanged. The nightly row is independent of #5527's workload split and needs no landing dependency. No package versions change.

Validation

• All 167 JS JSON and 172 Rust JSON tests pass. The original implementation fails four new vectors and the property after 22 cases (seed 7171953).
• Both languages pass the complete local nightly recipe with 4096 cases, seed 1719610749. A temporary Rust dropped-delta mutation fails and shrinks to two values; source restored.
• The document-size/changed-key benchmark and its typecheck pass. Timings are descriptive on a busy host; no speedup or threshold claim.
• Scoped just check passed 7468 default Rust tests, crypto identity, and 460 capture tests, then hit the existing CLI test export::the_same_instance_within_the_linger_continues (export wrote nothing). The same failure is documented in fix: count HTTP refusals and preserve replaced routes at the final sequence #5526 and tracked by quest/m1/ts-same-instance-resume.md; this PR does not change CLI or Rust production codec logic.
• Checks skipped after that failure were run separately and passed: uring-check, media features, quest, drill sensitivity, flake, Markdown/shell/TOML/Nix/just formatting, workflow checks, and all JS tests. The full just check command therefore is not green.

Alternatives and follow-up

Using prototype-less output objects would change observable object shape. The private safe-write helper preserves that shape and only handles the special setter key differently. Keep generated counterexamples as deterministic shared vectors, and resolve the separately tracked CLI resume regression before treating the complete suite as green.

(Written by GPT-6)

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• .github/workflows/nightly.yml
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• js/json/src/diff.ts
• js/json/src/snapshot/properties.test.ts
• quest/m2/README.md
• quest/m2/json-snapshot-properties.md
• quest/m2/json-window-properties.md
• rs/moq-json/Cargo.toml
• rs/moq-json/src/snapshot/encoder.rs
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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-15T00:24:46.761794Z	9422edd	Draft marked ready

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[kixelated]

Merge summary:

• Interop failure on c01e903: the browser media test's cold-start audio-tone check (63/71 samples, player stalled with jitter delay rising). This PR touches no playback code. It is the known ~5% flake tracked by quest/m1/publish-audio-channel-count.md, also seen on that quest's branch with the same signature. Left to that quest; Interop passes on the new head.
• Nightly fix: just test json-properties is wired into the nightly tests matrix, but on a fresh checkout bun test could not resolve fast-check (reproduced locally without node_modules). test/json-properties.sh now runs bun install --frozen-lockfile first, as test/drain/run.sh does. The full recipe passes locally.
• Dependencies: fast-check 4.10.2 and proptest 1.12.0 are the newest stable releases.
• Review: Codex reviewed 9422edd with no findings. CodeRabbit hit its rate limit.

Public API and wire: unchanged.

(Written by Claude Opus 5.5)