diff --git a/scripts/coverage.mjs b/scripts/coverage.mjs
index fd56a4f..f7aa961 100644
--- a/scripts/coverage.mjs
+++ b/scripts/coverage.mjs
@@ -29,6 +29,12 @@ if (minRaw !== undefined && (!Number.isFinite(min) || min < 0 || min > 100)) {
 	process.exit(2)
 }
 const reportDir = path.join(repoRoot, ".headlesscode", "coverage")
+// PID-unique temp dir: c8 wipes its temp dir at startup, and this script
+// runs nested inside itself (coverage.test.mjs spawns filtered coverage
+// runs as part of the suite). A shared tmp/ meant each nested run deleted
+// every profile the outer run had collected, collapsing full-suite line
+// coverage to ~4.5%. Unique dirs also let concurrent runs coexist.
+const tempDir = path.join(reportDir, `tmp-${process.pid}`)
 
 const testArgs = ["scripts/run-tests.mjs"]
 if (filter) testArgs.push("--filter", filter)
@@ -36,6 +42,7 @@ const result = spawnSync("node_modules/.bin/c8", [
 	"--reporter=text",
 	"--reporter=json-summary",
 	"--reports-dir", reportDir,
+	"--temp-directory", tempDir,
 	"--all",
 	"--src", "src",
 	"--exclude", "**/__tests__/**",
@@ -43,6 +50,14 @@ const result = spawnSync("node_modules/.bin/c8", [
 	"--exclude", "src/vendor/**",
 	"node", ...testArgs,
 ], { cwd: repoRoot, stdio: "inherit", env: process.env })
+// Raw V8 profiles are tens of MB per run; the report is already written by
+// now, so remove them on every exit path (pass AND fail) to avoid pile-up.
+try {
+	fs.rmSync(tempDir, { recursive: true, force: true })
+} catch {
+	// best-effort — a leftover tmp dir is a disk-space nit, never worth
+	// failing the coverage run over.
+}
 if (result.error) {
 	console.error(`coverage: failed to run c8: ${result.error.message}`)
 	process.exit(1)
diff --git a/scripts/coverage.test.mjs b/scripts/coverage.test.mjs
index d40c191..d34a3c5 100644
--- a/scripts/coverage.test.mjs
+++ b/scripts/coverage.test.mjs
@@ -2,10 +2,12 @@
 // percentage report on a tiny filtered run (claims tests, a few seconds)
 // instead of the full suite. Exit 1 with a clear message on any mismatch.
 import { execFileSync } from "node:child_process"
+import fs from "node:fs"
 import path from "node:path"
 import { fileURLToPath } from "node:url"
 
 const repoRoot = path.dirname(path.dirname(fileURLToPath(import.meta.url)))
+const reportDir = path.join(repoRoot, ".headlesscode", "coverage")
 
 function runCoverage(args) {
 	try {
@@ -34,4 +36,21 @@ if (floor.status === 0) fail("--min 100 unexpectedly passed")
 if (!/below the 100% floor/.test(floor.out)) fail("floor-failure message missing")
 const invalid = runCoverage(["--min", "abc"])
 if (invalid.status !== 2) fail(`invalid --min exited ${invalid.status}, want 2`)
-console.log("  ok   coverage-selftest: summary line, floor pass/fail, invalid arg")
+// Nested-run isolation (2026-10-13 incident: nested c8 runs wiped the
+// outer run's shared tmp/, collapsing full-suite coverage to ~4.5%).
+// Each finished run must clean its own pid-unique tmp dir; a shared bare
+// tmp/ must never come back. Pre-existing tmp-* dirs belong to still-live
+// outer/concurrent runs, so only NEW leftovers fail.
+const before = new Set(fs.existsSync(reportDir) ? fs.readdirSync(reportDir) : [])
+runCoverage([...filter, "--min", "0"])
+const after = fs.existsSync(reportDir) ? fs.readdirSync(reportDir) : []
+if (after.includes("tmp")) fail("shared bare tmp/ dir present — nested runs would wipe it")
+for (const entry of after) {
+	if (entry.startsWith("tmp-") && !before.has(entry)) {
+		fail(`leftover temp dir ${entry} from a finished nested run`)
+	}
+	if (entry.startsWith("tmp-") && !/^tmp-\d+$/.test(entry)) {
+		fail(`non-pid temp dir ${entry}`)
+	}
+}
+console.log("  ok   coverage-selftest: summary line, floor pass/fail, invalid arg, nested-run tmp isolation")