diff --git a/packages/playground/blueprints/src/lib/steps/login.ts b/packages/playground/blueprints/src/lib/steps/login.ts
index 2e6bd1632a8..7a1cdf20530 100644
--- a/packages/playground/blueprints/src/lib/steps/login.ts
+++ b/packages/playground/blueprints/src/lib/steps/login.ts
@@ -28,9 +28,12 @@ export type LoginStep = {
 /**
  * Logs in to Playground.
  * Under the hood, this function sets the `PLAYGROUND_AUTO_LOGIN_AS_USER` constant.
- * The `0-auto-login.php` mu-plugin uses that constant to log in the user on the first load.
+ * The `1-auto-login.php` mu-plugin uses that constant to log in the user on the first load.
  * This step depends on the `@wp-playground/wordpress` package because
  * the plugin is located in and loaded automatically by the `@wp-playground/wordpress` package.
+ *
+ * In the CLI, the `--login` flag injects this constant at boot time via
+ * `mergeDefinedConstants` so every worker has it before the server starts.
  */
 export const login: StepHandler<LoginStep> = async (
 	playground,
@@ -39,6 +42,5 @@ export const login: StepHandler<LoginStep> = async (
 ) => {
 	progress?.tracker.setCaption(progress?.initialCaption || 'Logging in');
 
-	// TODO: Make defineConstant apply to all workers
 	playground.defineConstant('PLAYGROUND_AUTO_LOGIN_AS_USER', username);
 };
diff --git a/packages/playground/cli/src/blueprints-v1/blueprints-v1-handler.ts b/packages/playground/cli/src/blueprints-v1/blueprints-v1-handler.ts
index b03fb70ef06..d81a0a267fe 100644
--- a/packages/playground/cli/src/blueprints-v1/blueprints-v1-handler.ts
+++ b/packages/playground/cli/src/blueprints-v1/blueprints-v1-handler.ts
@@ -195,6 +195,7 @@ export class BlueprintsV1Handler {
 			withXdebug: !!this.args.xdebug,
 			nativeInternalDirPath,
 			pathAliases: this.args.pathAliases,
+			constants: mergeDefinedConstants(this.args),
 		});
 		await playground.isReady();
 		return playground;
@@ -237,7 +238,7 @@ export class BlueprintsV1Handler {
 		return isBlueprintBundle(resolvedBlueprint)
 			? resolvedBlueprint
 			: {
-					login: this.args.login,
+					login: this.getEffectiveLogin(),
 					...(resolvedBlueprint || {}),
 					preferredVersions: {
 						php:
@@ -252,4 +253,20 @@ export class BlueprintsV1Handler {
 					},
 				};
 	}
+
+	/**
+	 * When the user explicitly sets PLAYGROUND_AUTO_LOGIN_AS_USER via
+	 * --define, skip the blueprint login step — the boot-time constant
+	 * from mergeDefinedConstants already handles auto-login and the
+	 * blueprint step would overwrite the custom username with 'admin'.
+	 */
+	private getEffectiveLogin(): boolean {
+		if (!this.args.login) {
+			return false;
+		}
+		if (this.args.define?.['PLAYGROUND_AUTO_LOGIN_AS_USER']) {
+			return false;
+		}
+		return true;
+	}
 }
diff --git a/packages/playground/cli/src/blueprints-v1/worker-thread-v1.ts b/packages/playground/cli/src/blueprints-v1/worker-thread-v1.ts
index c28cd4a159e..e1ceb238a26 100644
--- a/packages/playground/cli/src/blueprints-v1/worker-thread-v1.ts
+++ b/packages/playground/cli/src/blueprints-v1/worker-thread-v1.ts
@@ -52,6 +52,7 @@ interface WorkerBootRequestHandlerOptions {
 	withMemcached?: boolean;
 	withXdebug?: boolean;
 	pathAliases?: PathAlias[];
+	constants?: Record<string, string | number | boolean>;
 }
 
 /**
@@ -162,6 +163,7 @@ export class PlaygroundCliBlueprintV1Worker extends PHPWorker {
 			const requestHandler = await bootRequestHandler({
 				siteUrl: options.siteUrl,
 				maxPhpInstances: 1,
+				constants: options.constants,
 				createPhpRuntime: createPhpRuntimeFactory(
 					options,
 					this.fileLockManager!
diff --git a/packages/playground/cli/src/defines.ts b/packages/playground/cli/src/defines.ts
index 790b6daa2dd..7ec41880774 100644
--- a/packages/playground/cli/src/defines.ts
+++ b/packages/playground/cli/src/defines.ts
@@ -172,6 +172,10 @@ function mergeConstants(
 /**
  * Merge all constants from CLI arguments.
  *
+ * When `login` is true, `PLAYGROUND_AUTO_LOGIN_AS_USER` is injected so that
+ * every PHP worker defines the constant at boot time — not just the single
+ * worker that would run the blueprint `login` step via the pool proxy.
+ *
  * @param args - CLI arguments
  * @returns Merged constants
  */
@@ -179,10 +183,17 @@ export function mergeDefinedConstants(args: {
 	define?: Record<string, string>;
 	'define-bool'?: Record<string, boolean>;
 	'define-number'?: Record<string, number>;
+	login?: boolean;
 }): Record<string, string | number | boolean> {
-	return mergeConstants(
+	const merged = mergeConstants(
 		args['define'],
 		args['define-bool'],
 		args['define-number']
 	);
+
+	if (args.login && !Object.hasOwn(merged, 'PLAYGROUND_AUTO_LOGIN_AS_USER')) {
+		merged['PLAYGROUND_AUTO_LOGIN_AS_USER'] = 'admin';
+	}
+
+	return merged;
 }
diff --git a/packages/playground/cli/tests/merge-defined-constants.spec.ts b/packages/playground/cli/tests/merge-defined-constants.spec.ts
new file mode 100644
index 00000000000..2f91963f6d1
--- /dev/null
+++ b/packages/playground/cli/tests/merge-defined-constants.spec.ts
@@ -0,0 +1,61 @@
+import { mergeDefinedConstants } from '../src/defines';
+
+describe('mergeDefinedConstants', () => {
+	it('should merge string, boolean, and number constants', () => {
+		const result = mergeDefinedConstants({
+			define: { API_KEY: 'secret' },
+			'define-bool': { WP_DEBUG: true },
+			'define-number': { LIMIT: 100 },
+		});
+		expect(result).toEqual({
+			API_KEY: 'secret',
+			WP_DEBUG: true,
+			LIMIT: 100,
+		});
+	});
+
+	it('should return empty object when no constants are provided', () => {
+		const result = mergeDefinedConstants({});
+		expect(result).toEqual({});
+	});
+
+	it('should inject PLAYGROUND_AUTO_LOGIN_AS_USER when login is true', () => {
+		const result = mergeDefinedConstants({ login: true });
+		expect(result).toEqual({
+			PLAYGROUND_AUTO_LOGIN_AS_USER: 'admin',
+		});
+	});
+
+	it('should not inject login constant when login is false', () => {
+		const result = mergeDefinedConstants({ login: false });
+		expect(result).toEqual({});
+	});
+
+	it('should not inject login constant when login is undefined', () => {
+		const result = mergeDefinedConstants({});
+		expect(result).toEqual({});
+	});
+
+	it('should not override explicit PLAYGROUND_AUTO_LOGIN_AS_USER from --define', () => {
+		const result = mergeDefinedConstants({
+			define: { PLAYGROUND_AUTO_LOGIN_AS_USER: 'editor' },
+			login: true,
+		});
+		expect(result).toEqual({
+			PLAYGROUND_AUTO_LOGIN_AS_USER: 'editor',
+		});
+	});
+
+	it('should preserve other constants alongside the injected login constant', () => {
+		const result = mergeDefinedConstants({
+			define: { API_KEY: 'secret' },
+			'define-bool': { WP_DEBUG: true },
+			login: true,
+		});
+		expect(result).toEqual({
+			API_KEY: 'secret',
+			WP_DEBUG: true,
+			PLAYGROUND_AUTO_LOGIN_AS_USER: 'admin',
+		});
+	});
+});