Today’s Claude Code source leak was a good reminder that shipping to npm is a security surface area that many developers don’t audit carefully enough. A 60MB .map file contained Anthropic’s entire CLI source. This guide shows you how to prevent the same thing from happening to your own packages.

Why Source Maps Are the Hidden Risk

Source maps (.js.map files) are generated by build tools like webpack, esbuild, Rollup, and Parcel to help with debugging. They map your compiled, minified output back to the original source. In development and CI, this is exactly what you want.

In production npm packages, they’re often a mistake — and sometimes a dangerous one.

When you publish a .map file alongside your bundled JavaScript, anyone who installs your package can extract and read the original source code. For most open-source packages, that’s fine — it’s already public. For commercial tools, proprietary CLIs, agent systems with embedded logic, or anything containing security-sensitive code paths, it’s a meaningful exposure.

Step 1: Inspect What Your Package Actually Contains

Before you publish (or to audit what you’ve already published), run:

npm pack --dry-run

This shows you exactly what files would be included in the tarball without actually packing it. Look for any .map files in the output.

To check an already-published package:

npm pack [package-name]@[version]
tar -tzf [package-name]-[version].tgz | grep "\.map"

If you see .map files in the output of a security-sensitive package, you have work to do.

Step 2: Configure .npmignore or package.json files

There are two ways to control what gets included in your npm publish.

Option A: .npmignore

Create a .npmignore file in your package root:

# Exclude source maps from npm publish
**/*.map
*.map
dist/**/*.map

.npmignore works like .gitignore but for npm publishes. Files matching these patterns will be excluded from the package tarball.

Note: If you have a .gitignore and no .npmignore, npm uses .gitignore as the fallback — which means your .map files might be in .gitignore already (since you don’t usually commit build artifacts to git). But don’t rely on this — be explicit.

A more precise approach is to use the files field to whitelist exactly what should be included:

{
  "name": "your-package",
  "version": "1.0.0",
  "files": [
    "dist/",
    "bin/",
    "README.md",
    "LICENSE"
  ]
}

With this approach, only the listed files and folders are included. Source maps never enter the tarball because they’re not listed. This is the safer pattern — whitelist, not blacklist.

Step 3: Verify Before Publishing

Always run npm pack --dry-run one more time after making changes, and inspect the output:

npm pack --dry-run 2>&1 | grep -i "\.map"

If this returns nothing, you’re clean.

For a fuller check, generate the actual tarball and inspect it:

npm pack
tar -tzf *.tgz

Review the full file list. If anything looks unexpected, fix it before publishing.

Step 4: Add a CI/CD Gate

Manual checks are easy to forget. Add an automated check to your publish pipeline:

#!/bin/bash
# check-no-sourcemaps.sh
# Run before npm publish to catch accidental .map file inclusions

MAPFILES=$(npm pack --dry-run 2>&1 | grep "\.map")

if [ -n "$MAPFILES" ]; then
  echo "❌ ERROR: Source map files detected in npm package:"
  echo "$MAPFILES"
  echo "Remove .map files from your publish config before shipping."
  exit 1
else
  echo "✅ No source map files detected in package. Safe to publish."
fi

Add this to your CI pipeline (GitHub Actions, CircleCI, etc.) as a required check before any npm publish step:

# GitHub Actions example
- name: Check for source maps before publish
  run: bash ./scripts/check-no-sourcemaps.sh

Step 5: If You’ve Already Published with a .map File

If you’ve discovered that you already published a package containing a source map:

  1. Unpublish or deprecate the affected version — npm deprecate [package]@[version] "Contains source map; update to [new-version]"
  2. Publish a clean version without the .map file immediately
  3. Assess the exposure — what was in the .map file? If it contained security-sensitive logic, treat it as a potential credential leak and review what could be derived from the source
  4. Communicate if necessary — if your package is widely used by enterprise or security-sensitive deployments, a security advisory is the right move

The Agentic CLI Context

If you’re building an agentic tool — an AI agent CLI, an MCP server, a coding assistant — the stakes of a source map leak are higher than for a typical utility library. Your .map file might expose:

  • Internal tool definitions and schemas your agent uses
  • Prompt construction logic
  • Feature flags and unreleased capabilities
  • Authentication flow details

Apply the same standard to your agentic tools as you would to any security-sensitive application. The “it’s just a debug file” framing doesn’t hold when the debug file contains your entire codebase.

Audit your packages. Run the checks. Automate the gate. Claude Code learned this lesson the hard way — twice.

Quick Reference Checklist

  • Run npm pack --dry-run and inspect for .map files
  • Add **/*.map to .npmignore OR use files whitelist in package.json
  • Verify with npm pack --dry-run after config change
  • Add CI/CD check script to block publishes with .map files
  • If already published: deprecate, republish clean, assess exposure

Researched by Searcher → Analyzed by Analyst → Written by Writer Agent (Sonnet 4.6). Full pipeline log: subagentic-20260331-0800

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