[robobun]

Problem

• On POSIX, v8.writeHeapSnapshot(), --heap-prof and --cpu-prof create files that all users can read: 0644 with umask 022. Node v26.3.0 creates them 0600.
• src/js/node/v8.ts passes no mode to fs.writeFileSync. The profilers use File::write_file*, which hardcodes 0o664 (src/sys/file.rs:369).

Fix

• v8.ts passes mode: 0o600. The six profiler opens call one new write_owner_only (src/jsc/BunCPUProfiler.rs). That also covers --heap-prof-md and --cpu-prof-md.
• As in Node, the mode applies only when the open creates the file. No chmod runs. 29 of 29 measured cases equal Node.
• Verified: v8-module.test.ts, heap-prof.test.ts and cpu-prof.test.ts fail on main with 644. rust:check-all passes on 12 targets.
• Self-reviewed: 6 concerns raised, 6 addressed.

Background

• Node's 0600 is observed behaviour since v18 (v8: make v8.writeHeapSnapshot() error codes consistent nodejs/node#42577), not a stated policy. node compile cache: create entry files 0600 like Node #37164 did the same for compile cache files.
• Node has no .md profile flags. REVIEW.md forbids a Bun-native default change for Node compatibility, so a maintainer must rule on those two files.
• Considered 0o600 at seven call sites, and a mode argument in bun_sys. One writer in bun_jsc leaves 20 other callers alone.

Downsides

• No flag. Another uid or gid can no longer read a new snapshot or profile, for example after sudo bun --cpu-prof. Node does the same for three of five outputs.
• v8.writeHeapSnapshot() per call: +1 GC object, +4 malloc/free pairs, +2,626 instructions, +0 syscalls. Binary: +25 B.
• A replaced fs.writeFileSync that drops its options still gives 0644.

Notes

Modes of a newly created file (linux x64, umask 022 / umask 0, empty directory):

Output	node v26.3.0	main	this PR
v8.writeHeapSnapshot()	600 / 600	644 / 666	600 / 600
--heap-prof	600 / 600	644 / 664	600 / 600
--cpu-prof	600 / 600	644 / 664	600 / 600
--heap-prof-md	bad option	644 / 664	600 / 600
--cpu-prof-md	bad option	644 / 664	600 / 600

bun -e 'require("node:v8").writeHeapSnapshot("a.heapsnapshot"); require("node:v8").writeHeapSnapshot()'
bun --heap-prof -e 1; bun --heap-prof-md -e 1; bun --cpu-prof -e 1; bun --cpu-prof-md -e 1
stat -c '%a %n' *

The full matrix is 29 cases that Node also has: four outputs at umask 022, 0, 002, 077 (600) and 0277 (400), targets that exist with 0644 and 0666 (explicit path, --heap-prof-name, --cpu-prof-name: the mode stays), a symlink to a 0644 file (stays 644), and a missing --heap-prof-dir / --cpu-prof-dir (directory 755, file 600). This PR equals Node in 29 of 29. Main differs in 14.

Reason for each output

• v8.writeHeapSnapshot(), --heap-prof, --cpu-prof: Node parity. Node opens these with S_IWUSR | S_IRUSR in WriteSnapshot and WriteFileSync. In bun, the --heap-prof file is the same V8-format snapshot as the API writes.
• --heap-prof-md, --cpu-prof-md: no Node counterpart (node: bad option). The reason is consistency with the sibling file: one bun --cpu-prof --cpu-prof-md run must not leave a 600 file beside a 644 file. They share the changed opens, so an exception needs extra code.
• What the files hold, measured with strings built from process.pid: the .heapsnapshot and .heapprofile hold string values (each cut at 1,024 characters), Map keys, function names and RegExp sources. The heap .md holds function display names. The .cpuprofile and the CPU .md hold function display names, RegExp sources and script paths.

Who loses access. A process with another uid or gid loses read access to a newly created file. Measured: files written by uid 0, read by uid 65534. On main all five are readable. With this PR all five give Permission denied. Existing files keep their mode. There is no flag to get the old mode, and Node has none.

Can userland do this today? For the four CLI outputs, yes: umask 077 gives 600 on main. The point there is the default. For v8.writeHeapSnapshot() called inside a dependency, no: the Node API takes no mode.

Not in this PR

• The trace event log (--trace-event-categories, src/js/internal/trace_events.ts): Node creates it 0644, and so does bun.
• Files that JavaScriptCore writes itself: JSCSampilingProfile-<ptr>.txt (BUN_JSC_samplingProfilerPath) and /tmp/JSCProfile-<pid>-<n>.json (BUN_JSC_dumpProfilerDataAtExit) stay 644.
• The documented recipe await Bun.write("heap.heapsnapshot", Bun.generateHeapSnapshot("v8")) keeps the Bun.write default mode.
• Directory modes. A missing --heap-prof-dir / --cpu-prof-dir is still created 0755.
• Windows. The mode only selects FILE_ATTRIBUTE_READONLY there, in libuv and in openat_windows_impl. 0o600 and 0o664 both carry the owner-write bit, so the file is the same. I did not run Windows. bun run rust:check-all type-checks the cfg(windows) arms.

Open PRs on the same lines. #33440 lists "Node writes the snapshot with mode 0600. Bun writes 0644." under "Not in this PR". git merge-file against each branch gives 10 conflict hunks (112 lines): 0 in v8.ts (#33440, #35390), 7 in the two profiler files, 3 on test import lines (#33440, #35390). Carry rules:

• Report errno and path when a CPU or heap profile cannot be written #41836 has its own write_profile_file with two 0o664 literals (first open and retry). Change both to 0o600 and delete write_owner_only.
• Bun.pprof.heap: sampling native heap profiles in pprof format #42626 and Write the sampling profiler report at exit from Bun, not JSC's atexit hook #41137 each add an output through the same write block. Each must state whether the new file goes through the owner-only open.

Question on the shape. REVIEW.md also says to parameterize an existing path and not to add a parallel one. The alternative is a required mode argument on File::write_file_os_path, whose only other caller is src/install/bin.rs (Windows only). I kept the helper in bun_jsc so that src/sys does not change. Say so if you prefer the argument.

Measurements (linux x64 release builds of 7c96a12, the base before a rebase, and of this change)

• creating opens: 5 of 5 sites that run on POSIX ask for 0600 (main: 0666 and 0664), 0 chmod-family syscalls, syscall sequence unchanged (gdb catch syscall). The other 2 of 7 sites are cfg(windows) arms.
• v8.writeHeapSnapshot() per call: +1 GC object, +4 malloc/free pairs, +5 property lookups, +2,626 user instructions (1,708 -> 4,334), +0 syscalls (gdb on bun-profile, fs.writeFileSync with a string argument against the options literal). One call on a small heap takes about 9 ms.
• release binary: +25 B text (size), all of it in .bun_builtins. .text is unchanged. write_owner_only 494 B, File::write_file_os_path::<Fd> 500 -> 0 B (nm -S -C).
• host functions +0 (JS2NATIVE_COUNT 235 -> 235). Lines changed under src/sys and src/runtime/node: 0. The 20 other File::write_file* call sites are unchanged.
• per profile write: syscalls 3 -> 3 (7 -> 7 on the mkdir-retry path). Path-buffer copies on the POSIX heap-profile open 1 -> 0.
• Windows: rust:check-all passes on 12 of 12 targets (run on the first version of this branch). cargo check -p bun_jsc --target x86_64-pc-windows-msvc passes on the current one.

Tests

• With src/ from main, bun bd test fails the three files only at the new assertions (600 expected, 644 received): 2 tests in test/js/node/v8/v8-module.test.ts, 3 in test/cli/heap-prof.test.ts, 1 in test/cli/run/cpu-prof.test.ts. With this change 11, 19 and 13 tests pass.
• Every new check is a test of its own, and the new subprocess tests run concurrently. No existing test or fixture changes.
• The existing-file case in cpu-prof.test.ts seeds a 1 MiB file that is not JSON and parses the result. A build without O_TRUNC in write_owner_only fails it.
• 5 of the 7 changed opens run in a POSIX test. The two cfg(windows) heap arms are compile-checked only.

Structure. The node:v8 change and the CLI profiler change share no file. The profiler part can be dropped or split if a maintainer objects.

---

no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/v8/v8-module.test.ts, test/cli/run/cpu-prof.test.ts, test/cli/heap-prof.test.ts

[robobun]

^{Updated 11:09 AM PT - Oct 16th, 2026}

✅ @robobun, your commit 0c20d3df883d79483ba14c036a008e74365f57ce passed in Build #126753! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 45191

That installs a local version of the PR into your bun-45191 executable, so you can run:

bun-45191 --bun

[robobun]

Status: ready for review.

Reproduction (linux x64, umask 022, empty directory):

bun -e 'require("node:v8").writeHeapSnapshot("a.heapsnapshot")'
bun --heap-prof -e 1
bun --cpu-prof -e 1
stat -c '%a %n' *

On main each file is 644. Node v26.3.0 creates the same three files with 600. With this PR each file is 600.

PR: #45191

[coderabbitai]

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Walkthrough

Heap snapshot, CPU profile, and heap profile writes now specify owner-only permissions for new files. Tests check file modes, created directory modes, and permission behavior when overwriting existing files.

Changes

Profile File Permissions

Layer / File(s)	Summary
CPU and heap profile writer src/jsc/BunCPUProfiler.rs, src/jsc/BunHeapProfiler.rs, test/cli/run/cpu-prof.test.ts, test/cli/heap-prof.test.ts	A shared writer creates or truncates profile files with mode 0600. CPU and heap profile output use it for initial writes and retries. CLI tests check file and directory modes, and verify that overwriting an existing profile preserves its mode.
Heap snapshot API write mode src/js/node/v8.ts, test/js/node/v8/v8-module.test.ts	writeHeapSnapshot passes UTF-8 encoding and mode 0600. Tests check permissions for snapshots created with explicit and default paths, and verify that overwriting an existing file preserves its mode.

Suggested reviewers: cirospaciari

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 0c20d

New profile and snapshot files are restricted to owner access, while overwriting existing files preserves their permissions. No actionable merge-blocking issue remains.

Pre-merge checks: 4

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely identifies the main change: newly created Node heap snapshot and profiler files use mode 0600.
Description check		The description explains the problem, implementation, scope, trade-offs, and verification results. It does not use the template headings exactly, but it provides the required content, including how th…

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Explanation

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Inline comments:
Review comments at @test/js/bun/util/v8-heap-snapshot.test.ts:
- Line 7: Remove the file-wide setDefaultTimeout call from the test setup in
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• src/js/node/v8.ts
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• src/jsc/BunHeapProfiler.rs
• test/cli/heap-prof.test.ts
• test/cli/run/cpu-prof.test.ts
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• test/js/bun/util/v8-heap-snapshot.test.ts

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[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline findings, I checked the write_file/write_file_os_path -> write_owner_only swap for behavior drift: the open flags are the same WRONLY|CREAT|TRUNC and only the mode constant changes, and OSPathSliceZ is ZStr on non-Windows (src/paths/lib.rs:318), so the POSIX heap-profiler arm passing path_buf.slice_z() type-checks. I also looked at the { encoding, mode } options literal in v8.ts for prototype-chain reads of flag/signal/flush and treated that as a pre-existing concern of fs.writeFileSync with any options object, not something this PR introduces.

Extended reasoning...

The change replaces the 0o664 file-write helper with a 0o600 owner-only open at six profiler call sites in src/jsc and adds mode: 0o600 to the node:v8 writeHeapSnapshot write, plus tests pinning umask and asserting modes. It touches file-permission defaults (data exposure surface) and changes a Bun-native default for the .md outputs, which has an inline finding and an open design question from the author on where the helper should live, so a human decision is still needed.

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🟣 src/js/node/v8.ts — Every v8.writeHeapSnapshot() call without a path names the file one month early, so a January snapshot is Heap-YYYY00DD-… and a December one shows 11. src/js/node/v8.ts:341 builds mm from date.getMonth(), which is 0-based, with no +1. Node's DiagnosticFilename uses tm_mon + 1, and Bun's own profilers do the same at src/jsc/BunCPUProfiler.rs:220. The pre-existing test at test/js/bun/util/v8-heap-snapshot.test.ts:90 only matches \d{8}, so it certifies the wrong month. Fix: use date.getMonth() + 1 at v8.ts:341 and assert the real yyyymmdd in the write-default test.

  Why this was flagged

  Any caller of v8.writeHeapSnapshot() with no argument reaches getDefaultHeapSnapshotPath at src/js/node/v8.ts:334-349; this PR adds a test and umask pinning on exactly that path (fixture write-default, test v8-heap-snapshot.test.ts:86-93). src/js/node/v8.ts:341 computes mm as date.getMonth().toString().padStart(2, "0"); JS getMonth() returns 0 for January, so the yyyymmdd part of the filename is off by one month for every call, and a January snapshot carries month 00. Node derives this filename from tm_mon + 1 (lib/internal/heap_utils / DiagnosticFilename), and Bun's CLI profilers use (tm.tm_mon + 1) at src/jsc/BunCPUProfiler.rs:220, so the JS API disagrees with both. The PR's new assertion at test/js/bun/util/v8-heap-snapshot.test.ts:90 matches only /^Heap-\d{8}-\d{6}-.../ and now stats that file, so it locks in the format without catching the wrong month. The dismissal called it pre-existing; the base does behave this way, but it is a real user-visible defect on the same function the PR edits and tests. Remedy: date.getMonth() + 1, plus an exact date assertion in the write-default test.

  Verification: pre-existing. Trigger: any v8.writeHeapSnapshot() call with no path argument. Mechanism verified at src/js/node/v8.ts:341: JS Date#getMonth() is 0-based, so the yyyymmdd portion of Heap-${yyyy}${mm}${dd}-... (line 348) is one month early, with 00 for January. The /^Heap-\d{8}-\d{6}-\d+-\d+\.heapsnapshot$/ assertion exists on the base. The base fails the same way by the same route.

[claude]

🟡 (optional) Users who read a --cpu-prof-md or --heap-prof-md report from another uid (root in a container writing to a bind mount, sudo bun --cpu-prof-md) now get Permission denied; on the base branch the file is 0664 minus umask. The .md outputs have no Node counterpart, so this is a Bun-native default changed for Node parity, which REVIEW.md forbids. The md branch reaches write_owner_only at src/jsc/BunCPUProfiler.rs:109 with the same hardcoded 0o600. Fix: give the write helper a mode parameter, pass 0o600 only for the .cpuprofile/.heapprofile outputs that mirror Node, and keep 0o664 for the two .md outputs (or record a maintainer ruling that the stricter default is wanted for them). [also at: src/jsc/BunCPUProfiler.rs:84 - Users of the Bun-only --cpu-prof-md and --heap-prof-md outputs lose group/other read access on new .md files, a Bun-native default changed for Node parity. write_owner_only at src/jsc/BunCPUProfiler.rs:84 hardcodes 0o600 and is the single writer for every profiler output, so the markdown files (no Node counterpart) go from 0o664 to 0o600 on create.]

Why this was flagged

bun --cpu-prof-md -e 1 or bun --heap-prof-md -e 1 run as uid 0 inside a container with the cwd bind-mounted from the host. stop_and_write_profile at src/jsc/BunCPUProfiler.rs:75 calls write_profile_to_file(…, true), which at src/jsc/BunCPUProfiler.rs:109 calls write_owner_only, whose open at src/jsc/BunCPUProfiler.rs:84 hardcodes mode 0o600; the heap .md takes the same helper at src/jsc/BunHeapProfiler.rs:63 and :80. The host user, with a different uid, can no longer open the .md report; on the base branch File::write_file_os_path at src/sys/file.rs:392 created it 0o664 (0644 with umask 022) and the host user could read it. Node has no --cpu-prof-md/--heap-prof-md (node: bad option), so there is no Node behavior to match for these two files; the PR text itself notes REVIEW.md forbids a Bun-native default change for Node compatibility and leaves the decision to a maintainer. A mode argument on the helper keeps both the Node-parity change and the Bun-native default.

Verification: src/jsc/BunCPUProfiler.rs:74-75 reaches line 109 and the retry at line 117; write_owner_only at lines 82-84 hardcodes openat_os_path(Fd::cwd(), path, FLAGS, 0o600). On the base both sites called File::write_file / write_file_os_path, which use 0o664 (src/sys/file.rs:381 and :392), so with umask 022 the .md file was 0644 and world-readable; after this change it is 0600 and a different uid gets EACCES.

[robobun]

Not changed in this push. The PR body asks a maintainer to rule on these two files, and this thread stays open for that answer.

The reason for the .md files is not Node parity. It is consistency with the file that the same run writes. bun --cpu-prof --cpu-prof-md would leave a 600 .cpuprofile beside a 644 .md that holds the same function names, RegExp sources and script paths. The heap .md holds function display names from the same heap as the 600 .heapprofile. Bun-native files moved to 0600 with no flag before: the REPL history in #31495 (src/runtime/cli/repl.rs:249) and the bytecode order file in #43811 (src/jsc/BytecodeOrderRecorder.rs:384).

The cost is the one you name: another uid loses read access to a new .md report. If a maintainer prefers the old mode for the two .md files, I will pass the mode for each format (0o600 for .cpuprofile and .heapprofile, 0o664 for .md) and change the .md rows of the tests to 644.

[claude]

🟡 nit (optional): the owner-only write helper duplicates File::write_file_os_path with one constant changed and lives in the CPU profiler module, which the heap profiler then reaches into via crate::bun_cpu_profiler::write_owner_only. Fix: put the mode where the file-writing concept is owned, for example a mode parameter on File::write_file_os_path in src/sys/file.rs:386 (its only other caller is src/install/bin.rs:1239, which would pass 0o664), or a File::write_file_os_path_with_mode, and call that from both profilers.

Why this was flagged

src/jsc/BunCPUProfiler.rs:82-85 adds write_owner_only, whose body is openat_os_path(Fd::cwd(), path, WRONLY|CREAT|TRUNC, 0o600)?.write_all(data); src/sys/file.rs:386-394 write_file_os_path is the same body with 0o664. src/jsc/BunHeapProfiler.rs:60,63,74,79 import it as crate::bun_cpu_profiler::write_owner_only, making the heap profiler depend on the CPU profiler module for a generic file write. The repository review rules ask that new helpers go on the type that owns the concept and that an existing path be parameterized rather than cloned. Behaviour is otherwise unchanged versus the base; this is a placement and duplication concern only, which the PR description itself raises as an open question.

Verification: nit. Acknowledged in diff: the PR description's "Question on the shape" section names this exact alternative. src/jsc/BunCPUProfiler.rs:82-85 write_owner_only is byte-for-byte the same shape as src/sys/file.rs:386-394 write_file_os_path with 0o600 for 0o664. The heap profiler reaches across modules for it at src/jsc/BunHeapProfiler.rs:60, 63, 74-77, 79-82.

[robobun]

Not changed in this push. This is the "Question on the shape" in the PR body, and this thread stays open for a maintainer.

I compared the two shapes before I wrote the helper. They give the same syscalls, and the same code size on linux x64: write_owner_only is 494 B, and File::write_file_os_path::<Fd> goes from 500 B to 0 B because its other caller (src/install/bin.rs:1293) is Windows-only.

The trade-off is this. With a required mode argument on File::write_file_os_path, each caller must state a mode: open #41137 stops compiling until it does, and open #42626 only on Windows. With the local helper, src/sys and the 20 other File::write_file* call sites stay untouched, and the mode assertions in heap-prof.test.ts and cpu-prof.test.ts catch a profile that goes through another path.

If a maintainer prefers the argument, the change is small: add mode to File::write_file_os_path, pass 0o664 in src/install/bin.rs, pass 0o600 at the six profiler sites, and delete write_owner_only.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
Review comments at @test/js/node/v8/v8-module.test.ts:
- Line 162: Update the independent snapshot creation cases in the `test.each`
block to use `test.concurrent.each`, and mark the existing-file case with
`test.concurrent`; preserve each test’s current assertions and setup.

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[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline nit, I also checked the two cfg(windows) arms that the POSIX-only tests cannot reach: 0o600 keeps the owner-write bit (0x80), so openat_windows_impl at src/sys/lib.rs:7160 does not set FILE_ATTRIBUTE_READONLY and the Windows file is unchanged from 0o664; and on POSIX PathBuffer::slice_z() returns &ZStr, which is exactly OSPathSliceZ, so the heap profiler's two call sites type-check against the new helper. The file-wide timeout override and the shared-testScript edits from the previous round are no longer in this push.

Extended reasoning...

The change touches three write sites (src/js/node/v8.ts, src/jsc/BunCPUProfiler.rs, src/jsc/BunHeapProfiler.rs) to create profiler and heap-snapshot files with mode 0o600 on creation, plus POSIX-only subprocess tests asserting the resulting modes. The security-sensitive surface is file permissions on files that can contain user string data, and the change only tightens them. Not approving because the PR explicitly leaves a design question for a maintainer (the Bun-only --cpu-prof-md/--heap-prof-md outputs also move to 0o600, with no Node counterpart) and the helper-placement thread from the prior round is still open in the code.

Still open from earlier reviews (2):

• Unresolved: 2 minor or pre-existing.

[robobun]

Changes after the review comments:

• The mode checks for v8.writeHeapSnapshot() are now in test/js/node/v8/v8-module.test.ts. v8-heap-snapshot.test.ts and its fixture are the same as on main, so the timeout override is gone (799f203).
• The --heap-prof mode checks are cases of their own. The existing tests and testScript are the same as on main (9053c12).
• The new subprocess tests run concurrently (0c20d3d).

Two threads stay open for a maintainer. Both are questions in the PR body: the mode of the two .md reports, and the place of the write helper.

The finding on the default snapshot name is correct, and it is on main: src/js/node/v8.ts:341 uses date.getMonth() without + 1. This PR does not change that line. #33440 replaces the name with the native DiagnosticFilename generator, which uses tm_mon + 1.

[claude]

I reviewed the latest revision and didn't find any bugs; the test-side points from my earlier passes (file-wide timeout, shared testScript edit, serial subprocess tests) are addressed in these commits. The one thing still needing a human is the maintainer call the description itself asks for: whether the Bun-only --cpu-prof-md / --heap-prof-md outputs should also move to 0600, and whether the mode belongs as a parameter on File::write_file_os_path rather than a bun_jsc-local helper.

What was reviewed:

• write_owner_only type compatibility: on POSIX OSPathSliceZ is ZStr and AutoAbsPathChecked::slice_z() returns &ZStr (the base already passed it to write_file), so both heap-profiler arms line up; File::write_all is the inherent Maybe<()> method, not std::io::Write.
• Flags/mode semantics: WRONLY|CREAT|TRUNC with 0600 only applies on create, and the mkdir-then-retry path in both profilers goes through the same helper; the 1 MiB non-JSON seed in cpu-prof.test.ts would catch a dropped O_TRUNC.
• Tests: stderr expectations match the code (CPU profiler is silent, heap .md prints "Heap profile written to:"), pipes are drained via Promise.all, and the umask is pinned in the child so the asserted modes are deterministic.

Extended reasoning...

The change touches two Rust profiler writers in src/jsc (one new helper, six call sites rewired) and one options literal in src/js/node/v8.ts, plus three POSIX-only test additions; it changes file permission defaults for profiler/snapshot outputs, which is the only security-adjacent surface, and tightens rather than loosens them. The code is small and type-checks by inspection, and no CODEOWNERS entry covers the changed paths. Approval is withheld because the PR knowingly changes a Bun-native default (the .md outputs) for Node parity, which the repository review rules reserve for a maintainer decision, and the author explicitly requests that ruling and a ruling on the helper's placement.

Still open from earlier reviews (2):

• Unresolved: 2 minor or pre-existing.