diff --git a/.changeset/manifest-version-automation.md b/.changeset/manifest-version-automation.md
new file mode 100644
index 0000000..a47a563
--- /dev/null
+++ b/.changeset/manifest-version-automation.md
@@ -0,0 +1,7 @@
+---
+'@wolfcola/devtools-extension': patch
+---
+
+Automate manifest version stamping: append CI build number as 4th version
+segment so every Chrome Web Store and Firefox Add-ons upload has a unique,
+always-increasing version.
diff --git a/.github/workflows/publish-extension.yml b/.github/workflows/publish-extension.yml
index 2dbb58b..24a4b9c 100644
--- a/.github/workflows/publish-extension.yml
+++ b/.github/workflows/publish-extension.yml
@@ -21,6 +21,8 @@ jobs:
 
       - name: Build all packages
         run: pnpm build
+        env:
+          BUILD_NUMBER: ${{ github.run_number }}
 
       - name: Zip extension
         working-directory: packages/devtools-extension
@@ -52,10 +54,14 @@ jobs:
 
       - name: Build all packages
         run: pnpm build
+        env:
+          BUILD_NUMBER: ${{ github.run_number }}
 
       - name: Build Firefox extension
         working-directory: packages/devtools-extension
-        run: node build.mjs --target=firefox
+        run: node --experimental-strip-types build.mjs --target=firefox
+        env:
+          BUILD_NUMBER: ${{ github.run_number }}
 
       - name: Zip extension
         working-directory: packages/devtools-extension
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index b16a709..971a8d0 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -38,6 +38,9 @@ jobs:
 
       - name: Build
         run: pnpm build
+        env:
+          BUILD_NUMBER: ${{ github.run_number }}
+          SNAPSHOT: 'true'
 
       - name: Publish npm snapshots
         run: pnpm changeset publish --tag snapshot
@@ -64,7 +67,10 @@ jobs:
       - name: Build Firefox extension
         if: inputs.extension
         working-directory: packages/devtools-extension
-        run: node build.mjs --target=firefox
+        run: node --experimental-strip-types build.mjs --target=firefox
+        env:
+          BUILD_NUMBER: ${{ github.run_number }}
+          SNAPSHOT: 'true'
 
       - name: Zip Firefox extension
         if: inputs.extension
@@ -106,14 +112,17 @@ jobs:
 
       - name: Build
         run: pnpm build
+        env:
+          BUILD_NUMBER: ${{ github.run_number }}
 
       - name: Create release PR or publish
         uses: changesets/action@63a615b9cd06ba9a3e6d13796c7fbcb080a60a0b # v1.8.0
         with:
           publish: pnpm release
           version: pnpm run version
-          title: "chore: version packages"
-          commit: "chore: version packages"
+          title: 'chore: version packages'
+          commit: 'chore: version packages'
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           NPM_CONFIG_PROVENANCE: true
+          BUILD_NUMBER: ${{ github.run_number }}
diff --git a/packages/devtools-extension/build.mjs b/packages/devtools-extension/build.mjs
index 73c89dc..bc3007f 100644
--- a/packages/devtools-extension/build.mjs
+++ b/packages/devtools-extension/build.mjs
@@ -1,6 +1,7 @@
 import { execFileSync } from 'node:child_process';
 import { cpSync, mkdirSync, readFileSync, writeFileSync } from 'node:fs';
 import { createRequire } from 'node:module';
+import { stampVersion } from './src/stamp-version.ts';
 
 const target = process.argv.includes('--target=firefox') ? 'firefox' : 'chrome';
 const cwd = import.meta.dirname;
@@ -67,6 +68,16 @@ cpSync(`${uiDir}/dist/panel.html`, 'dist/panel/panel.html');
 
 // Manifest
 const manifest = JSON.parse(readFileSync('manifest.json', 'utf8'));
+
+// Stamp version: append build number as 4th segment for Chrome Web Store
+const buildNumber = parseInt(process.env.BUILD_NUMBER || '0', 10);
+const isSnapshot = process.env.SNAPSHOT === 'true';
+const stamped = stampVersion(manifest.version, buildNumber, isSnapshot);
+manifest.version = stamped.version;
+if (stamped.version_name) {
+  manifest.version_name = stamped.version_name;
+}
+
 if (target === 'firefox') {
   manifest.background = { scripts: ['background/service-worker.js'], type: 'module' };
   manifest.browser_specific_settings = {
diff --git a/packages/devtools-extension/package.json b/packages/devtools-extension/package.json
index ce53639..0adeb71 100644
--- a/packages/devtools-extension/package.json
+++ b/packages/devtools-extension/package.json
@@ -11,8 +11,8 @@
   "license": "MIT",
   "type": "module",
   "scripts": {
-    "build": "node build.mjs",
-    "build:firefox": "node build.mjs --target=firefox",
+    "build": "node --experimental-strip-types build.mjs",
+    "build:firefox": "node --experimental-strip-types build.mjs --target=firefox",
     "lint": "eslint .",
     "test": "vitest run"
   },
diff --git a/packages/devtools-extension/src/stamp-version.test.ts b/packages/devtools-extension/src/stamp-version.test.ts
new file mode 100644
index 0000000..f95e731
--- /dev/null
+++ b/packages/devtools-extension/src/stamp-version.test.ts
@@ -0,0 +1,59 @@
+import { describe, it, expect } from 'vitest';
+import { stampVersion } from './stamp-version.js';
+
+describe('stampVersion', () => {
+  it('appends build number 0 for local dev', () => {
+    const result = stampVersion('0.1.0', 0, false);
+    expect(result).toEqual({ version: '0.1.0.0' });
+  });
+
+  it('appends build number and snapshot version_name', () => {
+    const result = stampVersion('0.1.0', 23, true);
+    expect(result).toEqual({
+      version: '0.1.0.23',
+      version_name: '0.1.0-snapshot.23',
+    });
+  });
+
+  it('appends build number and release version_name', () => {
+    const result = stampVersion('0.2.0', 24, false);
+    expect(result).toEqual({
+      version: '0.2.0.24',
+      version_name: '0.2.0',
+    });
+  });
+
+  it('handles max valid build number (65535)', () => {
+    const result = stampVersion('1.0.0', 65535, false);
+    expect(result).toEqual({
+      version: '1.0.0.65535',
+      version_name: '1.0.0',
+    });
+  });
+
+  it('throws when build number exceeds 65535', () => {
+    expect(() => stampVersion('1.0.0', 65536, false)).toThrow(
+      'BUILD_NUMBER 65536 exceeds Chrome max of 65535',
+    );
+  });
+
+  it('throws for negative build number', () => {
+    expect(() => stampVersion('1.0.0', -1, false)).toThrow('BUILD_NUMBER -1 must be >= 0');
+  });
+
+  it('does not set version_name when build number is 0', () => {
+    const result = stampVersion('0.1.0', 0, false);
+    expect(result).not.toHaveProperty('version_name');
+  });
+
+  it('does not set version_name when build number is 0 even with snapshot flag', () => {
+    const result = stampVersion('0.1.0', 0, true);
+    expect(result).not.toHaveProperty('version_name');
+  });
+
+  it('throws for NaN build number', () => {
+    expect(() => stampVersion('1.0.0', NaN, false)).toThrow(
+      'BUILD_NUMBER must be an integer, got NaN',
+    );
+  });
+});
diff --git a/packages/devtools-extension/src/stamp-version.ts b/packages/devtools-extension/src/stamp-version.ts
new file mode 100644
index 0000000..61c76a7
--- /dev/null
+++ b/packages/devtools-extension/src/stamp-version.ts
@@ -0,0 +1,26 @@
+export const stampVersion = (
+  baseVersion: string,
+  buildNumber: number,
+  isSnapshot: boolean,
+): { version: string; version_name?: string } => {
+  if (!Number.isInteger(buildNumber)) {
+    throw new Error(`BUILD_NUMBER must be an integer, got ${buildNumber}`);
+  }
+  if (buildNumber < 0) {
+    throw new Error(`BUILD_NUMBER ${buildNumber} must be >= 0`);
+  }
+  if (buildNumber > 65535) {
+    throw new Error(`BUILD_NUMBER ${buildNumber} exceeds Chrome max of 65535`);
+  }
+
+  const version = `${baseVersion}.${buildNumber}`;
+
+  if (buildNumber === 0) {
+    return { version };
+  }
+
+  return {
+    version,
+    version_name: isSnapshot ? `${baseVersion}-snapshot.${buildNumber}` : baseVersion,
+  };
+};