diff --git a/dev/src/utils/logger.ts b/dev/src/utils/logger.ts
index afc437153..0b36dbbe7 100644
--- a/dev/src/utils/logger.ts
+++ b/dev/src/utils/logger.ts
@@ -69,7 +69,11 @@ export class AdkLogger implements Logger {
       },
       level: 'error',
       format: winston.format.combine(...formats),
-      transports: [new winston.transports.Console()],
+      transports: [
+        // Winston matches stderrLevels against the raw level name on
+        // info[LEVEL], so these stay lowercase despite the upper-casing format.
+        new winston.transports.Console({stderrLevels: ['warn', 'error']}),
+      ],
     });
   }
 
diff --git a/dev/test/utils/logger_test.ts b/dev/test/utils/logger_test.ts
new file mode 100644
index 000000000..2b387b15e
--- /dev/null
+++ b/dev/test/utils/logger_test.ts
@@ -0,0 +1,134 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import {Console} from 'node:console';
+import {Writable} from 'node:stream';
+import {stripVTControlCharacters} from 'node:util';
+import {afterEach, beforeEach, describe, expect, it} from 'vitest';
+
+import {LogLevel} from '@google/adk';
+import {AdkLogger, AdkLoggerOptions} from '../../src/utils/logger.js';
+
+/** The options `AdkApiServer` builds its logger with. */
+const API_SERVER_OPTIONS: AdkLoggerOptions = {
+  label: 'ADK API Server',
+  timestamp: true,
+  colorize: {level: true},
+  printFormat: (info) =>
+    `${info.level}: [${info.label}] ${info.timestamp} ${info.message}`,
+};
+
+/** The options the ADK CLI builds its logger with. */
+const CLI_OPTIONS: AdkLoggerOptions = {
+  label: 'ADK CLI',
+  colorize: {all: true},
+};
+
+/**
+ * A stream that keeps everything written to it. Winston's Console transport
+ * writes to `console._stdout` / `console._stderr`, so a test reads a record by
+ * giving `Console` streams that the test owns.
+ */
+class CaptureStream extends Writable {
+  text = '';
+
+  override _write(
+    chunk: Buffer,
+    _encoding: string,
+    done: (error?: Error | null) => void,
+  ): void {
+    this.text += chunk.toString();
+    done();
+  }
+}
+
+const realConsole = globalThis.console;
+
+describe('AdkLogger', () => {
+  let stdout: CaptureStream;
+  let stderr: CaptureStream;
+
+  beforeEach(() => {
+    stdout = new CaptureStream();
+    stderr = new CaptureStream();
+    globalThis.console = new Console(stdout, stderr);
+  });
+
+  afterEach(() => {
+    globalThis.console = realConsole;
+  });
+
+  it('writes an error record to stderr', () => {
+    new AdkLogger({label: 'test'}).error('boom');
+
+    expect(stderr.text).toContain('boom');
+    expect(stdout.text).toBe('');
+  });
+
+  it('writes a warning record to stderr', () => {
+    new AdkLogger({label: 'test'}).warn('careful');
+
+    expect(stderr.text).toContain('careful');
+    expect(stdout.text).toBe('');
+  });
+
+  it('writes an info record to stdout', () => {
+    new AdkLogger({label: 'test'}).info('hello');
+
+    expect(stdout.text).toContain('hello');
+    expect(stderr.text).toBe('');
+  });
+
+  it('writes a debug record to stdout', () => {
+    const logger = new AdkLogger({label: 'test'});
+    logger.setLogLevel(LogLevel.DEBUG);
+
+    logger.debug('trace me');
+
+    expect(stdout.text).toContain('trace me');
+    expect(stderr.text).toBe('');
+  });
+
+  it('keeps the record layout on stderr', () => {
+    new AdkLogger(API_SERVER_OPTIONS).error('kaboom');
+
+    expect(stripVTControlCharacters(stderr.text)).toMatch(
+      /^ERROR: \[ADK API Server\] .+ kaboom/,
+    );
+  });
+
+  it('still suppresses a record below the log level', () => {
+    const logger = new AdkLogger({label: 'test'});
+    logger.setLogLevel(LogLevel.ERROR);
+
+    logger.warn('quiet');
+
+    expect(stderr.text).toBe('');
+    expect(stdout.text).toBe('');
+
+    logger.error('loud');
+
+    expect(stderr.text).toContain('loud');
+    expect(stdout.text).toBe('');
+  });
+
+  it('still suppresses debug and info at the default log level', () => {
+    const logger = new AdkLogger({label: 'test'});
+
+    logger.debug('too chatty');
+    logger.setLogLevel(LogLevel.WARN);
+    logger.info('also chatty');
+
+    expect(stdout.text).toBe('');
+    expect(stderr.text).toBe('');
+  });
+
+  it('routes a colorized CLI error to stderr', () => {
+    new AdkLogger(CLI_OPTIONS).error('cli failure');
+
+    expect(stderr.text).toContain('cli failure');
+    expect(stdout.text).toBe('');
+  });
+});