diff --git a/scripts/lib/missing-dependency.js b/scripts/lib/missing-dependency.js
index 7292cd9d5..d75334714 100644
--- a/scripts/lib/missing-dependency.js
+++ b/scripts/lib/missing-dependency.js
@@ -1,15 +1,38 @@
 'use strict';
 
-// Production dependencies declared in package.json's "dependencies" field.
-// `npm install` never runs when ECC is installed via the Claude Code plugin
-// marketplace (a plain git clone), so these can be missing at runtime even
-// though the code that needs them is fine.
-const RUNTIME_DEPENDENCY_VERSIONS = {
-  ajv: '8.20.0',
-  'sql.js': '1.14.2',
-  'js-yaml': '4.3.1',
-  '@iarna/toml': '2.2.5',
-};
+const fs = require('fs');
+const path = require('path');
+
+// Runtime dependencies that can be missing when ECC is installed via the
+// Claude Code plugin marketplace (a plain git clone, so `npm install` never
+// runs), even though the code that needs them is fine. Versions are read
+// straight from package.json's "dependencies" field instead of a second
+// hardcoded copy, so this can't silently drift out of sync with what's
+// actually declared there.
+const TRACKED_DEPENDENCIES = ['ajv', 'sql.js', 'js-yaml', '@iarna/toml'];
+
+function loadRuntimeDependencyVersions() {
+  try {
+    const packageJsonPath = path.join(__dirname, '..', '..', 'package.json');
+    const declared = JSON.parse(fs.readFileSync(packageJsonPath, 'utf8')).dependencies || {};
+
+    const versions = {};
+    for (const name of TRACKED_DEPENDENCIES) {
+      const declaredVersion = declared[name];
+      if (declaredVersion) {
+        versions[name] = declaredVersion.replace(/^[\^~]/, '');
+      }
+    }
+    return versions;
+  } catch {
+    // package.json isn't reachable from here for some reason. Fall back to
+    // an empty map rather than crash — describeMissingDependencyError()
+    // just won't be able to suggest a pinned version in that case.
+    return {};
+  }
+}
+
+const RUNTIME_DEPENDENCY_VERSIONS = loadRuntimeDependencyVersions();
 
 function describeMissingDependencyError(error) {
   if (!error || error.code !== 'MODULE_NOT_FOUND') {
@@ -18,17 +41,21 @@ function describeMissingDependencyError(error) {
 
   const match = /Cannot find module '([^']+)'/.exec(error.message || '');
   const moduleName = match && match[1];
-  const pinnedVersion = moduleName && RUNTIME_DEPENDENCY_VERSIONS[moduleName];
 
-  if (!pinnedVersion) {
+  if (!moduleName || !TRACKED_DEPENDENCIES.includes(moduleName)) {
     return null;
   }
 
+  const pinnedVersion = RUNTIME_DEPENDENCY_VERSIONS[moduleName];
+  const installCommand = pinnedVersion
+    ? `npm install --no-save ${moduleName}@${pinnedVersion}`
+    : `npm install --no-save ${moduleName}`;
+
   return (
     `Missing dependency '${moduleName}'. ECC's production dependencies aren't installed ` +
     '(this happens when ECC was installed via the Claude Code plugin marketplace, which ' +
     'clones the repo but never runs npm install). Run "npm install" from the ECC repo ' +
-    `root, or install just this package with "npm install --no-save ${moduleName}@${pinnedVersion}".`
+    `root, or install just this package with "${installCommand}".`
   );
 }
 
diff --git a/tests/scripts/plugin-install-without-node-modules.test.js b/tests/scripts/plugin-install-without-node-modules.test.js
index b89ab74e2..6f7d5ce26 100644
--- a/tests/scripts/plugin-install-without-node-modules.test.js
+++ b/tests/scripts/plugin-install-without-node-modules.test.js
@@ -30,11 +30,31 @@ function test(name, fn) {
 }
 
 function copyRuntimeFiles(destDir) {
-  for (const entry of ['scripts', 'schemas', 'manifests']) {
+  for (const entry of ['scripts', 'schemas', 'manifests', 'package.json']) {
     fs.cpSync(path.join(REPO_ROOT, entry), path.join(destDir, entry), { recursive: true });
   }
 }
 
+// Node's module resolution walks up the directory tree looking for
+// node_modules, so if any ancestor of pluginDir happened to have one, a
+// require('ajv') from inside pluginDir could resolve there instead of
+// hitting the MODULE_NOT_FOUND path this test exists to exercise. Confirm
+// the fixture is actually isolated before trusting any of the results below.
+function assertNoNodeModulesInAncestry(dir) {
+  let current = dir;
+  while (true) {
+    if (fs.existsSync(path.join(current, 'node_modules'))) {
+      throw new Error(
+        `Fixture is not isolated: ${path.join(current, 'node_modules')} exists, so this test ` +
+        'would resolve dependencies from there instead of exercising the missing-dependency path.'
+      );
+    }
+    const parent = path.dirname(current);
+    if (parent === current) break;
+    current = parent;
+  }
+}
+
 function run(scriptRelativePath, args, cwd) {
   try {
     const stdout = execFileSync('node', [path.join(cwd, scriptRelativePath), ...args], {
@@ -63,6 +83,10 @@ function runTests() {
   const pluginDir = fs.mkdtempSync(path.join(os.tmpdir(), 'ecc-plugin-install-'));
 
   try {
+    if (test('fixture has no node_modules anywhere in its ancestor chain', () => {
+      assertNoNodeModulesInAncestry(pluginDir);
+    })) passed++; else failed++;
+
     copyRuntimeFiles(pluginDir);
 
     if (test('install-plan.js --list-profiles runs without ajv installed', () => {