diff --git a/.changeset/all-mangos-send.md b/.changeset/all-mangos-send.md
new file mode 100644
index 00000000..bec58241
--- /dev/null
+++ b/.changeset/all-mangos-send.md
@@ -0,0 +1,5 @@
+---
+'@openai/agents-realtime': patch
+---
+
+fix: #495 Realtime session config falls back to legacy format when voice is set
diff --git a/packages/agents-realtime/src/clientMessages.ts b/packages/agents-realtime/src/clientMessages.ts
index 3b68a36f..14bc0d55 100644
--- a/packages/agents-realtime/src/clientMessages.ts
+++ b/packages/agents-realtime/src/clientMessages.ts
@@ -133,6 +133,12 @@ export type RealtimeSessionConfigCommon = {
 export type RealtimeSessionConfigDefinition = RealtimeSessionConfigCommon & {
   outputModalities?: ('text' | 'audio')[];
   audio?: RealtimeAudioConfig;
+  /**
+   * TODO: We'll eventually migrate to audio.output.voice instead of this property.
+   * Until we fully migrate to audio.output.voice for all session implementations,
+   * using this top-level voice property helps with backwards compatibility.
+   */
+  voice?: string;
 };
 
 // Deprecated config (legacy) — cannot be mixed with new fields
@@ -175,7 +181,6 @@ function isDeprecatedConfig(
 ): config is Partial<RealtimeSessionConfigDeprecated> {
   return (
     isDefined('modalities', config) ||
-    isDefined('voice', config) ||
     isDefined('inputAudioFormat', config) ||
     isDefined('outputAudioFormat', config) ||
     isDefined('inputAudioTranscription', config) ||
@@ -193,6 +198,25 @@ export function toNewSessionConfig(
   config: Partial<RealtimeSessionConfig>,
 ): Partial<RealtimeSessionConfigDefinition> {
   if (!isDeprecatedConfig(config)) {
+    const inputConfig = config.audio?.input
+      ? {
+          format: normalizeAudioFormat(config.audio.input.format),
+          noiseReduction: config.audio.input.noiseReduction ?? null,
+          transcription: config.audio.input.transcription,
+          turnDetection: config.audio.input.turnDetection,
+        }
+      : undefined;
+
+    const requestedOutputVoice = config.audio?.output?.voice ?? config.voice;
+    const outputConfig =
+      config.audio?.output || typeof requestedOutputVoice !== 'undefined'
+        ? {
+            format: normalizeAudioFormat(config.audio?.output?.format),
+            voice: requestedOutputVoice,
+            speed: config.audio?.output?.speed,
+          }
+        : undefined;
+
     return {
       model: config.model,
       instructions: config.instructions,
@@ -202,25 +226,13 @@ export function toNewSessionConfig(
       providerData: config.providerData,
       prompt: config.prompt,
       outputModalities: config.outputModalities,
-      audio: config.audio
-        ? {
-            input: config.audio.input
-              ? {
-                  format: normalizeAudioFormat(config.audio.input.format),
-                  noiseReduction: config.audio.input.noiseReduction ?? null,
-                  transcription: config.audio.input.transcription,
-                  turnDetection: config.audio.input.turnDetection,
-                }
-              : undefined,
-            output: config.audio.output
-              ? {
-                  format: normalizeAudioFormat(config.audio.output.format),
-                  voice: config.audio.output.voice,
-                  speed: config.audio.output.speed,
-                }
-              : undefined,
-          }
-        : undefined,
+      audio:
+        inputConfig || outputConfig
+          ? {
+              input: inputConfig,
+              output: outputConfig,
+            }
+          : undefined,
     };
   }
 
diff --git a/packages/agents-realtime/test/realtimeVoiceConfigRegression.test.ts b/packages/agents-realtime/test/realtimeVoiceConfigRegression.test.ts
new file mode 100644
index 00000000..b8432afb
--- /dev/null
+++ b/packages/agents-realtime/test/realtimeVoiceConfigRegression.test.ts
@@ -0,0 +1,82 @@
+import { describe, it, expect } from 'vitest';
+import { toNewSessionConfig } from '../src/clientMessages';
+import { RealtimeAgent } from '../src/realtimeAgent';
+import { RealtimeSession } from '../src/realtimeSession';
+import { OpenAIRealtimeBase } from '../src/openaiRealtimeBase';
+import type { RealtimeClientMessage } from '../src/clientMessages';
+
+const TELEPHONY_AUDIO_FORMAT = { type: 'audio/pcmu' as const };
+
+class CapturingTransport extends OpenAIRealtimeBase {
+  status: 'connected' | 'disconnected' | 'connecting' | 'disconnecting' =
+    'disconnected';
+  mergedConfig: any = null;
+  events: RealtimeClientMessage[] = [];
+
+  async connect(options: { initialSessionConfig?: any }) {
+    this.mergedConfig = (this as any)._getMergedSessionConfig(
+      options.initialSessionConfig ?? {},
+    );
+  }
+
+  sendEvent(event: RealtimeClientMessage) {
+    this.events.push(event);
+  }
+
+  mute() {}
+  close() {}
+  interrupt() {}
+
+  get muted() {
+    return false;
+  }
+}
+
+describe('Realtime session voice config regression', () => {
+  it('drops GA audio formats when top-level voice is present', () => {
+    const converted = toNewSessionConfig({
+      voice: 'alloy',
+      audio: {
+        input: { format: TELEPHONY_AUDIO_FORMAT },
+        output: { format: TELEPHONY_AUDIO_FORMAT },
+      },
+    });
+
+    expect(converted.audio?.input?.format).toEqual(TELEPHONY_AUDIO_FORMAT);
+    expect(converted.audio?.output?.format).toEqual(TELEPHONY_AUDIO_FORMAT);
+    expect(converted.audio?.output?.voice).toBe('alloy');
+  });
+
+  it('resets audio formats when connecting a session for an agent with voice configured', async () => {
+    const transport = new CapturingTransport();
+    const agent = new RealtimeAgent({
+      name: 'voice-agent',
+      instructions: 'Respond cheerfully.',
+      voice: 'alloy',
+    });
+
+    const session = new RealtimeSession(agent, {
+      transport,
+      model: 'gpt-realtime',
+      config: {
+        audio: {
+          input: { format: TELEPHONY_AUDIO_FORMAT },
+          output: {
+            format: TELEPHONY_AUDIO_FORMAT,
+            voice: 'marin',
+          },
+        },
+      },
+    });
+
+    await session.connect({ apiKey: 'dummy-key' });
+
+    expect(transport.mergedConfig?.audio?.input?.format).toEqual(
+      TELEPHONY_AUDIO_FORMAT,
+    );
+    expect(transport.mergedConfig?.audio?.output?.format).toEqual(
+      TELEPHONY_AUDIO_FORMAT,
+    );
+    expect(transport.mergedConfig?.audio?.output?.voice).toBe('marin');
+  });
+});