[diegosouzapw]

Closes #14110

Root cause

PR #13295 changed errorPathRedaction.ts's ambiguity branch from
failClosedAmbiguity || hasFilesystemEvidence to failClosedAmbiguity only,
and the call-site mustFailClosed at redactUnquotedAbsolutePathSpans only
covered Windows/file-URI/known-POSIX-root candidates. For an unquoted POSIX
path whose root is not in POSIX_FILESYSTEM_ROOTS (e.g. /custom/...,
/data/... in a container) followed by ambiguous prose, findUnquotedPathEnd
returned -1 and the fallback left the whole path in the sanitized error
message in clear text instead of redacting it to <path> — regressing
HR#12 (never leak err.message/paths).

Owner decision

The issue explicitly framed this as non-delegable between (a) fail-closed on
unknown-root ambiguity, or (b) redact-token-preserve-prose with a stronger
route/path discriminator. Owner decision (2026-09-21, recorded in the
plan-file frontmatter): (a) fail-closed — extend mustFailClosed in
redactUnquotedAbsolutePathSpans (open-sse/utils/errorPathRedaction.ts) to
also cover an unknown-root POSIX candidate once findUnquotedPathEnd returns
-1.

This intentionally also fail-closes a bare, unanchored API route in prose
(route-shaped and filesystem-shaped candidates are lexically indistinguishable
with no second anchor) — the accepted trade-off, since HR#12 makes a possible
path leak worse than over-redacting a route. The narrower #13144 guarantee —
a route immediately followed by another absolute-path span (e.g. "... Use POST /v1/y instead.") keeps its trailing prose — is unaffected and
re-asserted as a co-located regression test.

Fix

open-sse/utils/errorPathRedaction.ts: mustFailClosed in
redactUnquotedAbsolutePathSpans now also includes isPosixPath (the
already-computed unknown-root POSIX candidate flag), so an unresolved
ambiguous tail after such a candidate fails closed instead of falling through
to clear text.

Regression test

New: tests/unit/issue-14110-unknown-root-path-exposed.test.ts

RED (pre-fix, git apply -R on the fix):

✖ sanitizeErrorMessage must not expose an unknown-root filesystem path followed by prose (237.33587ms)
  AssertionError [ERR_ASSERTION]: expected the unknown-root path to be redacted, got: Provider failed at /custom/internal secret directory
    actual: 'Provider failed at /custom/internal secret directory'
    expected: /\/custom\/internal|secret directory/

GREEN (post-fix): both assertions in that file pass — the unknown-root path is
redacted, and the #13144 trailing-prose guarantee is unaffected.

Also reactivated the #14110-SUSPENDED assertion in
tests/unit/fixtures/error-public-boundaries-hardening.fixture.ts (added by
PR #14101), which documents the exact same case inline.

Gates run

• node --import tsx/esm --test on the touched/new files (30 assertions) — pass
• node --import tsx/esm --test on the full error-sanitization test family (12
  files, 121 assertions covering routes/tunnels/skills/MITM/provider-validation
  error boundaries) — pass, no regressions
• npm run typecheck:core — exit 0
• node scripts/check/check-open-sse-typecheck.mjs — 0 errors, within frozen baseline
• npx eslint --suppressions-location config/quality/eslint-suppressions.json <changed files> — exit 0
• node scripts/check/check-file-size.mjs — OK, no file over its frozen cap
• node scripts/check/check-complexity-ratchets.mjs --base-ref origin/release/v3.8.51 — OK, 0 new violations (base: 5 cyclomatic / 4 cognitive, unchanged)
• node scripts/check/check-changelog-integrity.mjs — OK

Existing tests

Ran every test file referencing errorPathRedaction/redactErrorPaths/sanitizeErrorMessage
in the error-sanitization family. One existing test
(tests/unit/error-path-redaction-route-truncation-13144.test.ts) had its
"separator evidence alone does not license swallowing the line" assertion
updated: it encoded the pre-#14110 contract (a bare unanchored route in prose
survives verbatim), which the owner's fail-closed decision explicitly
supersedes. The sibling test in the same file (the actual #13144 regression
guard — a route followed by trailing prose with another absolute-path
anchor) was left untouched and still passes, so #13144 is not regressed.

Notes for the auditor

This PR resumes a previous worker's session that was cut mid-run by a usage
limit (no commits existed yet, work was uncommitted). I audited the diff it
left against the plan-file and the owner's binding decision before committing
it as-is — no scope changes were needed; the diff matched the plan exactly
(fail-closed via isPosixPath, reactivated suspended assertion, new + aligned
tests).

Plan-file: _tasks/pipeline/bugs/2-implementing/14110-13295-exposes-unknown-root-filesystem-paths-in-clear-text-when.plan.md