docs(signals): async rules — the 2026-10-09/10 rulings in F/D

[ryansolid]

The async rules as reviewed 2026-10-09/10, carried into RULES-FUNDAMENTALS.md / RULES-DERIVED.md (the rules are the artifact; no separate rulings doc).

• F-2 / D-2 / D-6: first loads in a branch a change mounts count; a memo born held is loading. <Loading> is how a change escapes the hold. The direction rule's time half (drafted as F-8) is withdrawn.
• F-3 / F-4: a boundary on its fallback; only a hold on the arming frame hides a fresh boundary's fallback.
• F-6: a probe (isPending/latest) alone holds nothing; a plain read beside it is a read (Adding combined isPending breaks atomic publication #3442).
• F-9 / D-23 / D-24: no committed value, no verdict — isPending of a first load or born-held memo throws when owned, false unowned; latest is its pending value if it has one.
• D-4 / D-12: membership is decided by the render effect that reads a derivation, not where the memo was created; an unread memo holds nothing ([2.0] Mounting a memo during a held action can crash a render callback #3802).
• D-16: a header that only calls isPending/latest does not hold a re-arm.
• D-25: a node a hold's fetch is waiting on is that hold's work, not a verdict reader (isPending/latest on a superseded transition depend on whether the parked async memo also reads them #3884).
• Statuses point at the implementing PR (carve branch) and its tests; OPEN-QUESTIONS.md keeps only the U section (U-1/U-2 built, U-3 noted).

Implemented by #3977.

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https://claude.ai/code/session_01LVm3wseHt9wRTY8LfPiHmQ

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Size (brotli, eager entry chunk)

scenario	head	vs base	minified vs base	minified vs recorded	cap		lazy chunks (not counted)
signals: core floor (createSignal/Memo/Effect/Root/flush)	7.44 KB	0 B	0 B	+23 B	7.45 KB	✅
signals: + createStore	14.70 KB	0 B	0 B	+13 B	14.71 KB	✅
signals: + isPending/latest	9.73 KB	0 B	0 B	+21 B	9.73 KB	✅
app: render + one signal (the simple-app floor)	9.94 KB	0 B	0 B	−48 B	9.94 KB	✅
app: hydrating (no stores) with Show/For/Loading/Errored/lazy	17.70 KB	0 B	0 B	−664 B	17.93 KB	✅	lazy-page.js 0.04 KB
app: hydrating + every store primitive family	29.09 KB	0 B	0 B	−686 B	29.27 KB	✅	lazy-page.js 0.04 KB
app: CSR with Show/For/Loading/Errored/lazy	12.94 KB	0 B	0 B	−61 B	12.98 KB	✅	lazy-page.js 0.04 KB
app: CSR, observe tier (same app on the observe artifacts)	14.53 KB	0 B	0 B	−44 B	14.56 KB	✅	lazy-page.js 0.04 KB
app: CSR, observe tier + attribution engine enabled	28.82 KB	0 B	0 B	+12 B	28.89 KB	✅	lazy-page.js 0.04 KB
app: compiled floor (one template, one text hole, one delegated click)	10.13 KB	0 B	0 B	−65 B	10.13 KB	✅
app: compiled CSR (JSX todo app: spread/merge/omit, events, class/style, keyed For, Show, Loading + lazy, store)	25.44 KB	0 B	0 B	−38 B	25.46 KB	✅	stats.js 0.18 KB
app: compiled hydrating (the same JSX todo app through hydrate(), compiled hydratable)	31.19 KB	0 B	0 B	−657 B	31.39 KB	✅	stats.js 0.20 KB
frames: eager client consumer (frames client + transport, lazy codec)	11.43 KB	0 B	0 B	0 B	11.43 KB	✅
page: base server components (hydrating + dynamic + frames + sf reference)	33.90 KB	0 B	0 B	−2325 B	34.33 KB	✅	assets.js 0.78 KB, bind.js 1.83 KB, decode.js 6.23 KB, lazy-page.js 0.04 KB, regions.js 0.80 KB, trace.js 8.79 KB, wire.js 0.93 KB
page: live server components (base + live/GET + action + isPending/latest)	37.67 KB	0 B	0 B	−2325 B	38.15 KB	✅	assets.js 0.78 KB, bind.js 1.83 KB, decode.js 6.23 KB, lazy-page.js 0.04 KB, regions.js 0.80 KB, trace.js 8.78 KB, wire.js 0.93 KB
page: compiled base server components (the base page as JSX: templates with class/style/attributes/events, For/Show; no spread)	34.73 KB	0 B	0 B	−3490 B	35.66 KB	✅	assets.js 0.78 KB, bind.js 1.82 KB, decode.js 6.23 KB, regions.js 0.80 KB, sc-comments.js 0.20 KB, trace.js 8.76 KB, wire.js 0.93 KB
page: compiled live server components (the compiled base page + live/GET + action + isPending/latest)	40.30 KB	0 B	0 B	−3451 B	41.21 KB	✅	eager (counted): web.js 21.26 KB; assets.js 0.78 KB, bind.js 1.84 KB, decode.js 6.23 KB, regions.js 0.79 KB, sc-comments.js 0.19 KB, trace.js 8.76 KB, wire.js 0.93 KB
page: base + router (base page + @solidjs/router: createRouter, two routes, preload, useNavigate)	41.19 KB	0 B	0 B	−2331 B	41.79 KB	✅	assets.js 0.78 KB, bind.js 1.84 KB, decode.js 6.23 KB, lazy-page.js 0.04 KB, regions.js 0.80 KB, server.js 1.02 KB, serverForms.js 3.59 KB, trace.js 8.80 KB, wire.js 0.94 KB
page: live + router (live page + @solidjs/router: createRouter, two routes, preload, useNavigate)	46.99 KB	0 B	0 B	−2288 B	47.49 KB	✅	eager (counted): client.js 27.48 KB; assets.js 0.78 KB, bind.js 1.85 KB, decode.js 6.23 KB, lazy-page.js 0.04 KB, regions.js 0.81 KB, server.js 1.02 KB, serverForms.js 3.32 KB, trace.js 8.78 KB, wire.js 0.94 KB
server: floor (getRequestEvent + isServer)	1.33 KB	0 B	0 B	0 B	1.34 KB	✅
server: renderToString (the server-render floor)	20.40 KB	0 B	0 B	+4 B	20.42 KB	✅

_{Bundled with Rolldown (what Vite ships), brotli q11, decimal KB. A scenario fails only when it is over its brotli cap and its minified size is more than 20 B over the minified recorded with the cap; over the cap within that allowance is brotli layout noise and passes with a warning. Caps and their recorded minified in scripts/size/scenarios.js; the floor and page caps in floor-caps.json are frozen (lower only, or Size-Exception: in the PR body). npm run ratchet lowers caps per RC; it never raises one (scripts/size/README.md).}

[coveralls]

Coverage Report for CI Build 38121964477

Coverage remained the same at 77.429%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	1321
Covered Lines:	1079
Line Coverage:	81.68%
Relevant Branches:	1036
Covered Branches:	746
Branch Coverage:	72.01%
Branches in Coverage %:	Yes
Coverage Strength:	26.99 hits per line

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💛 - Coveralls

[codspeed]

Merging this PR will not alter performance

⚠️ 2 benchmarks spent significant time in system calls

System calls cannot be consistently instrumented, so they are not included in the measure, which understates the real cost. Please switch to the Walltime instrument to accurately measure system calls.

Measurement and system calls

• readAllowed
• propagation:diamond

✅ 195 untouched benchmarks

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_{Comparing docs/async-fundamentals (8378501) with next (a497d4b)}

[ryansolid]

Review at 33d5ead. Good to merge: it merges cleanly with #3977, and rules-index.test.ts / spec-async-semantics.test.ts pass on the combined tree. Three non-blocking notes:

1. It documents fix(signals): carve async toward the F/D rules; redo #3937, #3954, #3895 #3977's behaviour. The text cites carve/born-in-col2, carve/probe-not-holder and carve/reader-position, says "on next it still pins the old screen" (D-4) and "next still shows child 2 beside parent 1 1" (U-1), and points at tests/fresh-boundary-born-held-reader.test.ts, which only exists in fix(signals): carve async toward the F/D rules; redo #3937, #3954, #3895 #3977. Those lines go stale the moment fix(signals): carve async toward the F/D rules; redo #3937, #3954, #3895 #3977 lands; worth a follow-up pass to replace the branch names with the PR number.

2. "isPending throws like its read" is not what fix(signals): carve async toward the F/D rules; redo #3937, #3954, #3895 #3977 ships. For a memo born held, read inside an owner, the engine returns false, joins the reader to the hold, and the pass continues past the probe (checked in a render effect and a memo; unowned is false, as documented). fix(signals): carve async toward the F/D rules; redo #3937, #3954, #3895 #3977's own description says "the owner joins the hold". D-23's guard example (if (!isPending(x)) save(x())) argues against exactly that false. Affected: F-9, the "First load" vocabulary entry, D-2, D-12, D-23, D-24, S-3, S-14. Either the wording becomes "joins the hold" or the engine throws.

3. S-3's "Effects ran" column disagrees with its pin. tests/born-held.test.ts (unchanged by fix(signals): carve async toward the F/D rules; redo #3937, #3954, #3895 #3977) still has the direct <span>{x()}</span> effect run with 0 at the mount and again at the commit. The table matches a Show-driven mount, where nothing runs until the commit, not the direct mount the pin covers.

— Claude via Claude Code

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[ryansolid]

Addressed in 813d6a21:

1. Branch names replaced with fix(signals): carve async toward the F/D rules; redo #3937, #3954, #3895 #3977; statuses no longer describe next.

2. S-3: the pin is a direct mount — a note under the table gives its screen and effect runs (the direct binding shows 0, then 1 at the commit, D-3); the click-mounted table stands, unpinned.

3. ("throws" vs "joins the hold") is a ruling — raised with the maintainer; wording or engine follows that.

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[ryansolid]

Item 2, ruled: "throws" existed to make the hold or the boundary trigger, and born held already does. The engine stays as #3977 ships it; the wording now says what happens — inside an owner, isPending of a memo born held holds its reader as its read does (caught by a boundary that has not shown content, or held with the change) and the reader runs at the commit; a first load's NotReadyError is unchanged; unowned is false. Updated in F-9, D-2, D-12, D-23, D-24, S-3, S-14 (8378501d, plus undo an accidental reformat).

The guard D-23 cares about is pinned in #3977: packages/web/test/ispending-guard-born-held.spec.tsx — <input disabled={isPending(data)}> is never seen enabled before the commit (held branch, or the fallback under a fresh Loading).

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