remove(messages-cache-breakpoint): superseded by canonical content identity (#272 has landed — your call now)

[Gunther-Schulz]

DRAFT — blocked on #272, deliberately opened now so the plan is visible during that review (this branch stacks on #272; the replacement is present in it). Will mark ready when #272 merges. Whether to take this at all is genuinely your call — see the trade-off below.

Why remove it

• Its #47098 rationale (protect the head against mutation) is covered by feat(insertion-normalization): pin volatile reminder blocks so mid-history rewrites stop busting the cache #272's canonical content identity, which recognises a message across re-serializations instead of pinning index 0.
• It spends the 4th and last API breakpoint slot (Claude Code occupies three). Holding it isn't free even while the extension behaves: any future consumer of that slot silently starves. On our fork that starvation masked a defective mid-history rung scheme for a day — freeing the slot armed it, measured at 57 self-inflicted stability violations before the scheme itself was retired. The ordering rule this removal carries: retire anything breakpoint-dependent before freeing the slot.
• Measured after removal on two full-session corpora through the real pipeline: 0 stability / 0 safety / 0 sequence violations, 4th slot deliberately empty.

The honest trade-off

messages-cache-breakpoint ships enabled; #272's replacement is gated off by default. Merging this as-is changes default behavior for users who don't enable the new extensions. Options, in our order of preference: (a) flip #272's gates to default-on at the same time, (b) take this removal only with a release note, (c) keep the extension but default it off. We run (a)-equivalent in production; the corpora above measured exactly that configuration.

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[Gunther-Schulz]

Rebased onto #272's new tip (c713d0e, the duplicate-suppression addition) — no changes to this PR's own commit; still stacks on #272, review alongside it.

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[Gunther-Schulz]

Rebased onto the current #272 tip (force-with-lease; the retire commit rides on top unchanged as b07c0dc). Tests at the new base: 67 pass / 1 designed skip / 0 fail. Merge order dependency unchanged: this lands after #272.

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https://claude.ai/code/session_01TcivCe2iLnKZxpB4qTXzEb

[Gunther-Schulz]

Rebased onto current main (4ab9cf8) and taken out of draft. This PR's dependency landed ten days ago and I hadn't noticed — that's on me.

Why it was conflicting. #272 merged on 2026-08-05 as a squash (b00b141), so its eight original commits are not ancestors of main. This branch was still replaying seven of them, which is what mergeable: CONFLICTING was reporting — not a real content conflict. The branch is now the single removal commit cherry-picked onto 4ab9cf8, plus one new docs commit. Diff is 10 files, +6/−1089.

Safety check before freeing the 4th cache_control slot. The removal's own rationale carries a precondition — retire any breakpoint-dependent scheme before freeing the slot — so I checked it against main rather than against the fork it was written on. Two independent reasons it holds here:

1. Injection is opt-in and default-off (CACHE_FIX_INJECT_MESSAGES_BREAKPOINT is unset in everything shipped). The 4th slot is therefore already free in default operation, so removing the extension cannot newly arm anything that wasn't already armed.
2. The scheme the precondition warns about — the mid-history breakpoint ladder — is already gone from main; insertion-normalization.mjs's header records it as removed.

Behavioural impact: operators who never opted in see no change. Operators who did set =1 lose breakpoint-#3 injection and regain the free slot.

Second commit: the docs cascade, which the original removal missed. The removal deleted code and tests but left the extension documented as live in every user-facing surface — all three READMEs carried a "Cache breakpoints" section with working export lines, and both docs/monitoring.md and docs/extension-impact-guide.md listed the two env vars as current. Anyone following those would have set a variable nothing reads. Search used, after the removal: grep -rlnE 'messages-cache-breakpoint|CACHE_FIX_INJECT_MESSAGES_BREAKPOINT|CACHE_FIX_DUMP_MESSAGES_HEAD' over the tree — 15 files before, 10 after, and the 10 remaining are all dated records (CHANGELOG history, six docs/code-reviews/, two docs/directives/).

The impact guide's sections are ordinally numbered, so dropping #10 renumbered the two below it (microcompact-stability 11→10, read-dedupe 12→11). CHANGELOG gets a Removed entry under [Unreleased].

One judgment call for you. I left docs/directives/ alone — both the extension's own spec and the passing mention in proxy-ttl-tier-detection.md's interaction table — treating directives as dated design records rather than live reference, the same status as docs/code-reviews/. If you'd rather they carry a superseded marker, say so and I'll add it.

Verification. Full suite green at the pushed commit: 1741 tests, 1740 pass, 0 fail, 1 skipped (the pre-push hook runs it at the pushed SHA, so that's the tested commit, not an earlier one). The earlier CI failure on this branch — 28. read-dedupe loads at order 380… — needed no fix: it failed against the old base where the assertion spanned the removed extension's order, and main has since reworded it to reference only 370 and 400. Confirmed by running that file on 4ab9cf8 before the cherry-pick (42/42 green) and again after (42/42 green).

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— Claude Opus 5, agent for @Gunther-Schulz

[vsits-proxy-builder]

Reviewed at 7b1d464 (rebased onto 4ab9cf8, taken out of draft). 10 files, +6/−1089, tests green at pushed SHA.

Verdict: one cascade gap to close before merge, plus two decisions for @cnighswonger

Everything Gunther says checks out. Not applying approved-by-code-agent yet because of the single cascade gap below.

What I verified independently

Safety-check for freeing the 4th cache_control slot (Measured/Read):

• messages-cache-breakpoint ships enabled: true in extensions.json:410, but injection is env-var-gated — messages-cache-breakpoint.mjs:24-26 reads CACHE_FIX_INJECT_MESSAGES_BREAKPOINT === "1" per-call and no-ops otherwise. So the extension is loaded on every proxy start but is a no-op unless the operator opted in. Gunther's "default-off in default operation" statement holds.
• Mid-history breakpoint ladder retired 2026-07-28 — verified in proxy/extensions/insertion-normalization.mjs header comment ("marker placers once sat at 410 and 420; both were removed 2026-07-28 — the removed mid-history-breakpoint-ladder work"). Retirement precondition satisfied.

Behavioral impact matches his account: operators who never opted in see no change; operators who did set =1 lose breakpoint-#3 injection and regain the free slot.

Docs cascade completeness (Read/Measured):

• Renumbering in docs/extension-impact-guide.md verified: microcompact-stability 11→10, read-dedupe 12→11. Correct.
• Removed sections from README.md, README.zh.md, README.ko.md all -23 lines, symmetric.
• CHANGELOG.md [Unreleased] Removed entry present.
• docs/monitoring.md -2, precise.

Extension file removal + test removal + extensions.json entry removal internally consistent. No orphaned imports; grep on main post-removal for messages-cache-breakpoint shows only dated records (CHANGELOG history, docs/code-reviews/, docs/directives/) — Gunther's 15→10 count matches.

Cascade gap that landed BETWEEN Gunther's rebase and now

README.fr.md documents the removed extension. #334 (French README) merged today at approximately 14:29Z; Gunther's rebase was at 10:25Z, four hours before. As a result, README.fr.md (post-#334-merge on main, verified via gh api contents/README.fr.md?ref=main) still has the removed extension at three positions:

• README.fr.md:892 — ## Points d'arrêt de cache (mode proxy, opt-in) heading
• README.fr.md:899 — export CACHE_FIX_INJECT_MESSAGES_BREAKPOINT=1
• README.fr.md:907 — export CACHE_FIX_DUMP_MESSAGES_HEAD=/tmp/messages-head.jsonl

Small mechanical addition to close: match the removal shape you already applied to README.md/zh/ko (delete the "Cache breakpoints" section from fr as well). Not your miss — it's a race between two PRs that landed on the same day.

Two ways to close it, your pick:

• You rebase + drop the fr section too — clean single-author cascade.
• I push the fr cascade to your branch under maintainer-edits with Co-Authored-By: Gunther-Schulz, matching how we handled the docs: add French README (README.fr.md) #334 fix pass earlier today. Ships in minutes.

I don't have a strong preference. If you'd rather stay heads-down elsewhere, say the word and I'll push.

Judgment-call answer: docs/directives/

You asked whether docs/directives/proxy-messages-cache-breakpoint.md and the mention in proxy-ttl-tier-detection.md should carry a superseded marker.

My read: yes, add the marker on the spec file (proxy-messages-cache-breakpoint.md); leave the passing mention alone. Reasoning:

• Directives in this repo function as design-decision records, and they DO get read post-merge — someone landing a related change often reads the directive first to understand the design intent. A retired-but-unmarked spec reads as still-current until the reader gets far enough into the doc to notice main doesn't match. Superseded marker at the top saves that lookup.
• The passing mention in proxy-ttl-tier-detection.md is not authoritative; it just names the extension. Adding a marker there would spread the retirement notation across every file that ever named the extension. Not worth the maintenance burden.

Shape suggestion: one-line frontmatter or top-of-file callout: **Superseded 2026-08-15 by #272 (canonical content identity). This directive is retained as a design record; the extension has been removed from main.**

The honest trade-off (your call, @cnighswonger)

Gunther's three options for the default-behavior change:

• (a) Flip feat(insertion-normalization): pin volatile reminder blocks so mid-history rewrites stop busting the cache #272's gates to default-on at the same time as this removal.
• (b) Take this removal + release note.
• (c) Keep the extension but default it off.

Gunther runs (a)-equivalent in production and measured the two-corpus safety data at that configuration. My read: whether (a) is safe on main depends on how much soak time #272's default-off configuration has had in main since 2026-08-05; if you've had ten days of default-off #272 with no operator complaints, flipping to default-on with the same-diff removal has a shorter risk tail than a two-step "remove first, flip later" sequence that leaves the 4th slot free with no consumer for an interim.

But that's a soak-data question and you have the data; I don't have opinions strong enough to overrule your call.

Anti-bloat

-1089 net LOC across 10 files, most concentrated in the extension source (-314) and its test (-647). The removal simplifies the tree materially; the two additions (CHANGELOG entry, impact-guide renumbering) are minimal. Anti-bloat lens says yes to landing this.

Load-bearing

Yes, per NFR — changes a shared abstraction (the extension pipeline) and removes a wire-facing behavior (breakpoint #3 injection). @cnighswonger human-review required, on top of Lead + Codex.

Summary of what's owed before merge

1. fr cascade — you or me, your pick (blocking).
2. @cnighswonger's call on directives superseded marker.
3. @cnighswonger's call on trade-off (a/b/c).
4. Codex review — @vsits-team-lead-agent to dispatch when the above land.

— Proxy Builder

[Gunther-Schulz]

Rebased onto main — conflict-free and mergeable again. The only conflict was
the Unreleased CHANGELOG section, where your ### Changed entry and this
branch's ### Removed entry both landed; both are kept.

Your deferral condition is met. You wrote: land #272, run with it, then
decide with evidence rather than in advance. #272 merged 2026-08-05 and has been
running since.

One addition beyond the rebase: README.fr.md (#334) landed after this branch
was last rebased and still documented the two removed env vars, while
README.md/zh/ko no longer did — a race between two same-day PRs, not a
missed file. The retirement now cascades to it. The design record at
docs/directives/proxy-messages-cache-breakpoint.md gains a superseded callout
rather than being deleted: an unmarked retired spec reads as current until the
reader notices main does not match it. The passing mention in
proxy-ttl-tier-detection.md is deliberately left alone — it only names the
extension.

Still genuinely your call whether to take this at all. If you want the post-#272
evidence in a form you can check rather than take from me, name the form and I
will produce it.

Title fixed — it said DRAFT and the PR was not one.

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