diff --git a/.github/workflows/job-compile-and-test.yml b/.github/workflows/job-compile-and-test.yml
index 18299aa63..37868e7f3 100644
--- a/.github/workflows/job-compile-and-test.yml
+++ b/.github/workflows/job-compile-and-test.yml
@@ -34,6 +34,10 @@ jobs:
         with:
           node-version: 24
 
+      - name: Validate Yarn lockfile
+        run: yarn test-yarn-lock && yarn verify-yarn-lock
+        working-directory: Extension
+
       - name: Install Dependencies
         run: yarn install ${{ inputs.yarn-args }}
         working-directory: Extension
diff --git a/Extension/.scripts/verifyYarnLock.mjs b/Extension/.scripts/verifyYarnLock.mjs
new file mode 100644
index 000000000..53f8610ab
--- /dev/null
+++ b/Extension/.scripts/verifyYarnLock.mjs
@@ -0,0 +1,97 @@
+import fs from 'node:fs';
+import path from 'node:path';
+import process from 'node:process';
+import { fileURLToPath, pathToFileURL } from 'node:url';
+
+const dependencySections = ['dependencies', 'devDependencies', 'optionalDependencies'];
+
+function parseLockfileKey(key) {
+    const selectors = [];
+    let selectorStart = 0;
+    let quoted = false;
+    let escaped = false;
+
+    for (let index = 0; index < key.length; index++) {
+        const character = key[index];
+        if (escaped) {
+            escaped = false;
+        } else if (character === '\\' && quoted) {
+            escaped = true;
+        } else if (character === '"') {
+            quoted = !quoted;
+        } else if (character === ',' && !quoted) {
+            selectors.push(key.slice(selectorStart, index));
+            selectorStart = index + 1;
+        }
+    }
+    selectors.push(key.slice(selectorStart));
+
+    return selectors.map(selector => {
+        const trimmedSelector = selector.trim();
+        return trimmedSelector.startsWith('"') ? JSON.parse(trimmedSelector) : trimmedSelector;
+    });
+}
+
+function parseLockfileSelectors(lockfile) {
+    const selectors = new Set();
+    for (const line of lockfile.split(/\r?\n/)) {
+        if (/^[^\s#].*:\s*$/.test(line)) {
+            for (const selector of parseLockfileKey(line.replace(/:\s*$/, ''))) {
+                selectors.add(selector);
+            }
+        }
+    }
+    return selectors;
+}
+
+function getResolutionPackageName(pattern) {
+    const segments = pattern.split('/');
+    const packageName = segments.at(-1);
+    const scope = segments.at(-2);
+    return scope?.startsWith('@') ? `${scope}/${packageName}` : packageName;
+}
+
+function getExpectedSelectors(manifest) {
+    const selectors = [];
+    for (const section of dependencySections) {
+        for (const [packageName, range] of Object.entries(manifest[section] ?? {})) {
+            selectors.push(`${packageName}@${range}`);
+        }
+    }
+    for (const [pattern, range] of Object.entries(manifest.resolutions ?? {})) {
+        selectors.push(`${getResolutionPackageName(pattern)}@${range}`);
+    }
+    return selectors;
+}
+
+function findMissingSelectors(manifest, lockfile) {
+    const lockfileSelectors = parseLockfileSelectors(lockfile);
+    return getExpectedSelectors(manifest)
+        .filter(selector => !lockfileSelectors.has(selector))
+        .sort();
+}
+
+function validateYarnLock(packageJsonPath, yarnLockPath) {
+    const manifest = JSON.parse(fs.readFileSync(packageJsonPath, 'utf8'));
+    const lockfile = fs.readFileSync(yarnLockPath, 'utf8');
+    const missingSelectors = findMissingSelectors(manifest, lockfile);
+    if (missingSelectors.length > 0) {
+        throw new Error(`yarn.lock is missing selectors required by package.json:\n${missingSelectors.map(selector => `  ${selector}`).join('\n')}\nRun yarn install to update yarn.lock.`);
+    }
+}
+
+const invokedUrl = process.argv[1] ? pathToFileURL(path.resolve(process.argv[1])).href : undefined;
+if (invokedUrl === import.meta.url) {
+    const extensionRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
+    const packageJsonPath = process.argv[2] ?? path.join(extensionRoot, 'package.json');
+    const yarnLockPath = process.argv[3] ?? path.join(extensionRoot, 'yarn.lock');
+
+    try {
+        validateYarnLock(packageJsonPath, yarnLockPath);
+    } catch (error) {
+        process.stderr.write(`${error instanceof Error ? error.message : String(error)}\n`);
+        process.exitCode = 1;
+    }
+}
+
+export { findMissingSelectors, getExpectedSelectors, parseLockfileSelectors, validateYarnLock };
diff --git a/Extension/.scripts/verifyYarnLock.test.mjs b/Extension/.scripts/verifyYarnLock.test.mjs
new file mode 100644
index 000000000..ac52202b6
--- /dev/null
+++ b/Extension/.scripts/verifyYarnLock.test.mjs
@@ -0,0 +1,41 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { findMissingSelectors } from './verifyYarnLock.mjs';
+
+test('reports a stale resolution selector', () => {
+    const manifest = { resolutions: { 'fast-uri': '^3.1.5' } };
+    const lockfile = `fast-uri@^3.0.1, fast-uri@^3.1.4:
+  version "3.1.5"
+`;
+
+    assert.deepEqual(findMissingSelectors(manifest, lockfile), ['fast-uri@^3.1.5']);
+});
+
+test('accepts direct, scoped, and nested resolution selectors', () => {
+    const manifest = {
+        dependencies: { '@scope/direct': '^1.0.0' },
+        devDependencies: { 'gulp-typescript': '^5.0.1' },
+        resolutions: {
+            '@scope/resolved': '^2.0.0',
+            'gulp-typescript/**/glob-parent': '^5.1.2',
+            'parent/**/@nested/package': '~3.0.0'
+        }
+    };
+    const lockfile = `"@nested/package@~3.0.0":
+  version "3.0.1"
+
+"@scope/direct@^1.0.0":
+  version "1.0.0"
+
+"@scope/resolved@^2.0.0":
+  version "2.0.0"
+
+glob-parent@^3.1.0, glob-parent@^5.1.2:
+  version "5.1.2"
+
+gulp-typescript@^5.0.1:
+  version "5.0.1"
+`;
+
+    assert.deepEqual(findMissingSelectors(manifest, lockfile), []);
+});
diff --git a/Extension/package.json b/Extension/package.json
index b2715001e..20d10f505 100644
--- a/Extension/package.json
+++ b/Extension/package.json
@@ -7163,6 +7163,8 @@
         "scripts": "ts-node -T .scripts/scripts.ts",
         "show": "ts-node -T .scripts/clean.ts show",
         "clean": "ts-node -T .scripts/clean.ts",
+        "test-yarn-lock": "node --test .scripts/verifyYarnLock.test.mjs",
+        "verify-yarn-lock": "node .scripts/verifyYarnLock.mjs",
         "test": "yarn install && (yarn verify prep --quiet || yarn prep) && (yarn verify compiled --quiet || yarn build) && ts-node -T .scripts/test.ts",
         "code": "yarn install && (yarn verify compiled --quiet || yarn build) && yarn verify binaries && ts-node -T .scripts/code.ts",
         "verify": "ts-node -T .scripts/verify.ts",