Building ReproSpec: Portable Evidence Bundles and Offline Replay for Node.js

ners often must choose between setting up a matching VM environment or triaging based on static text logs. Ordinary logs are useful, but they usually do not include a standardized integrity manifest, a portable environment summary, and a documented local replay workflow in one bundle. ReproSpec is an experimental CLI and evidence-bundle format designed to bridge this gap. A ReproSpec bundle contains the following schema layout (reprospec.bundle/v0.1-alpha): manifest.json: Metadata, schema version, and security exceptions.

command.json: Captured command parameters, exit code, and duration. environment.json: Node runtime details, scrubbed environment variables. stdout.txt / stderr.txt: Process logs. integrity.json: SHA-256 hash index of the declared files.

EVIDENCE_CARD.md: A human-readable markdown report card. When you run reprospec capture -- <command>, ReproSpec: Spawns the command and records its output streams. Slices each of stdout and stderr streams individually to enforce a strict 10MB limit. Runs a regex redaction processor to remove supported credential patterns (such as ghp_ tokens) from stdout, stderr, environment keys, and command arguments.

⚠️ Argument Immutability: ReproSpec executes the captured command with its original arguments to ensure program behavior is unchanged. Redaction is applied only to the persisted evidence representation. Verification re-calculates hashes against integrity.json. ⚠️ Important Limit: This only detects changes relative to the SHA-256 hashes recorded in the bundle manifest.

A party able to rewrite the entire bundle could also recompute those hashes. It does not prove author identity or captured trace truthfulness. Immutable Source Snapshot: Capture stores the required source content inside the evidence bundle under snapshot/, with relative paths, sizes, and SHA-256 hashes in snapshot.json and the bundle integrity manifest. Writable Disposable Execution: Before replay, ReproSpec verifies the immutable snapshot and copies it into a new ephemeral execution workspace.

Only that disposable copy is mounted writable, so ordinary build and test commands can create dist, coverage, cache, and temporary files. No Original Checkout Mount: The original checkout and evidence bundle are not mounted into replay. allowWorkspaceMount is disabled and blocked in the Alpha.2 replay path. Output Handling: Replay records stdout, stderr, the exit-code oracle result, and files created, modified, or deleted in the execution workspace.

The workspace and generated files are removed after success, failure, or timeout; generated files are never added back to the evidence snapshot. Dependency Isolation: An ephemeral build container uses npm ci with network access to fetch packages based on package-lock.json. It runs without host credentials, home-directory mounts, Docker socket access, or inherited variables. Offline Replay: The actual replay container runs offline with --network none.

Host Check: The CLI compares git status before and after replay, failing if any host changes occur. During early testing against real repositories, we encountered several portability limits: Vercel's ms package: Replay failed because Jest 30 preset resolution (ts-jest) failed to resolve correctly relative to the isolated workspace root under Node 20. dotenv package: The exact public commit c0e32b8267b69a438bc0cc31345f73b5e2f037db passed the Node.js 20 baseline and Alpha.2 clean-room replay. Its build and test workflow generated dist and .tap content inside the writable ephemeral execution workspace; those files were tracked and discarded without changing the original checkout or immutable bundle snapshot.

Runtime: Node.js 20+ Languages: Pure JavaScript or TypeScript repositories (ordinary single-package structures). Package Manager: npm with standard package-lock.json. Operating Systems: macOS and Linux hosts. npm workspaces and monorepos.

Yarn, pnpm, and non-npm package managers. Runtimes other than Node.js (Python, Go, etc.). Windows hosts. Reuse of native host-compiled binaries inside replay.

Active network connections or remote endpoint access during replay. To capture, verify, and replay an execution: # Capture command execution node bin/reprospec.js capture --output my-bundle -- node -e "console.log('original run')" Output: [reprospec] Capturing: node -e console.log('original run') [reprospec] Working directory: <temporary-workspace> [reprospec] Started at: 2026-07-31T15:46:09.188Z [reprospec] Bundle written to: <temporary-workspace>/my-bundle [reprospec] Exit code: 0 [reprospec] Status: PASS [reprospec] Removed environment variables: <redacted> ✅ Bundle written to: <temporary-workspace>/my-bundle # Verify bundle integrity node bin/reprospec.js verify my-bundle Output: ── ReproSpec Bundle Verification ────────────────────── Bundle: <temporary-workspace>/my-bundle Status: ✅ INTEGRITY_VERIFIED — The bundle manifest and SHA-256 file hashes were verified. File Details: ✅ stdout.txt: OK ✅ stderr.txt: OK ✅ command.json: OK ✅ environment.json: OK ✅ integrity.json: OK ─────────────────────────────────────────────────────── # Replay inside Docker sandbox node bin/reprospec.js replay my-bundle Output: ── ReproSpec Replay Sandbox ─────────────────────────── Bundle: <temporary-workspace>/my-bundle Docker: v29.5.3 ──────────────────────────────────────────────────────── [reprospec] Running replay container: docker run --detach --name reprospec-replay-<id> --network none --user 1000:1000 --memory 1g --cpus 1.0 --pids-limit 128 --security-opt no-new-privileges=true --read-only --env REPROSPEC_REPLAY=1 --env HOME=/tmp -v <temporary-execution-workspace>:/workspace --workdir /workspace --tmpfs /tmp:rw,noexec,nosuid,size=64m --stop-timeout 60 node:20-alpine node -e console.log('original run') ── Replay Result ─────────────────────────────────────── Status: REPLAY_VERIFIED Expected Exit: 0 Container Exit: 0 Oracle Match: ✅ YES Execution Mode: writable-ephemeral-snapshot Files Created: <tracked> Files Modified: <tracked> Files Deleted: <tracked> Workspace Cleanup: ✅ removed Container Output: original run ─────────────────────────────────────────────────────── We are looking for developer feedback on: How you handle test suites requiring local databases or external configurations offline. Caching policies for npm packages to prevent native binary mismatches.

Check out the repository and prerelease: Repository: https://github.com/HenryParker37-VIP/reprospec Prerelease: https://github.com/HenryParker37-VIP/reprospec/releases/tag/v0.1.0-alpha.2 9. AI-Assistance Disclosure Disclosure: This codebase and documentation were prepared with the assistance of agentic AI coding models during development planning, clean-room auditing, and public candidate preparation.