HOT_SCOPE_FANOUT
5+/50+/500+ scopes hot from one root cause (attribution enabled).
What you saw
- Kind:
perf - Severity:
warn(code that works but is structurally wrong or expensive)
The console report opens with [HOT_SCOPE_FANOUT] and the message, then an in line naming the owners that enclose the subject, root first, and an at line with the source location when the code was compiled with one.
In a captured artifact the same finding is an event with code: "HOT_SCOPE_FANOUT", ownerPath, and loc.
Diagnostics explains each line.
Why it fires
Many scopes went hot from ONE root cause (named in the message) — the aggregate form of HOT_SCOPE_RERUNS, emitted so a single culprit doesn't produce one warning per victim.
These fire only while attribution is enabled and describe cost, not incorrect behavior. The numbers in the message are measurements, not guesses.
How to fix it
Fix the cause, not the scopes: if consumers ask keyed questions of it (every row vs. one selected id), invert it into a store used as a map keyed by id; if it's a per-frame value, gate it behind a slower derivation.
In tests
captureArtifact from @solidjs/diagnostics runs a scenario with the diagnostics channel open and returns the events it saw.
expectNoDiagnostics(artifact) fails while HOT_SCOPE_FANOUT is among them, with the events as evidence; after the fix, the code is gone from artifact.diagnostics.
expectRerunBudget(artifact, max, { scope }) bounds how often a named scope may re-run, and expectNoWaste(artifact) fails on re-runs whose result was unchanged.
Both read the attribution records, which captureArtifact collects unless attribution: false is passed.
Check for regressions in tests shows the loop and the Vitest matcher form.
Learn more
- Diagnostics: every code, with its kind, severity, and trigger.
- Debugging reactivity: how to read a report and what each group of codes points at.
- Attribution: the engine whose records explain a re-run, a hold, or a waterfall.