Turn Any Text into a Two-Voice Podcast

const apiKey = process.env.OPENROUTER_API_KEY;

if (!apiKey) {
  throw new Error("Set OPENROUTER_API_KEY first.");
}

const source = `Tides are caused mostly by the Moon. Its gravity pulls hardest on the side
of the Earth facing it, so the ocean bulges toward the Moon. On the far side the
pull is weaker than at the Earth's center, and that difference stretches the
ocean outward into a second bulge. As the Earth turns, most coastlines pass
through both bulges, which is why they see two high tides and two low tides
roughly every 25 hours.`;

const scriptResponse = await fetch(
  "https://openrouter.ai/api/v1/chat/completions",
  {
    method: "POST",
    headers: {
      Authorization: `Bearer ${apiKey}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      model: "google/gemini-3.8-flash",
      messages: [
        {
          role: "system",
          content:
            "Write a short podcast dialogue between a host and a guest about the text the user sends. " +
            "Use 6 to 8 turns, alternate speakers, and keep each turn under 40 words. " +
            "Give each turn a few words of delivery direction, such as 'curious' or 'laughing'.",
        },
        { role: "user", content: source },
      ],
      response_format: {
        type: "json_schema",
        json_schema: {
          name: "podcast_script",
          strict: true,
          schema: {
            type: "object",
            properties: {
              turns: {
                type: "array",
                items: {
                  type: "object",
                  properties: {
                    speaker: { type: "string", enum: ["host", "guest"] },
                    text: { type: "string" },
                    direction: { type: "string" },
                  },
                  required: ["speaker", "text", "direction"],
                  additionalProperties: false,
                },
              },
            },
            required: ["turns"],
            additionalProperties: false,
          },
        },
      },
    }),
  },
);

if (!scriptResponse.ok) {
  throw new Error(await scriptResponse.text());
}

type Turn = { speaker: "host" | "guest"; text: string; direction: string };
const completion = await scriptResponse.json();
const { turns } = JSON.parse(completion.choices[0].message.content) as {
  turns: Turn[];
};

for (const turn of turns) {
  console.log(`${turn.speaker.padEnd(5)} (${turn.direction}): ${turn.text}`);
}

host  (curious and upbeat): Welcome back! Today we're talking tides. Most people know the Moon causes them, but why do we get two high tides a day instead of just one?
guest (eagerly explaining): It comes down to ocean bulges. The Moon's gravity pulls hardest on the ocean facing it, but it also creates a second bulge on the exact opposite side.
host  (perplexed and amused): Wait, on the far side too? How does the Moon pull water away from itself?
guest (animated and clear): It pulls the Earth's center harder than the far water! Because the pull is weaker out there, the ocean stretches outward, forming that second bulge.
host  (nodding with realization): Ah, so the Earth basically spins right beneath both of these bulges every day?
guest (warm and conclusive): Exactly! Coastlines rotate through both, which is why we usually see two high and two low tides roughly every twenty-five hours.

const voices = { host: "Kore", guest: "Puck" } as const;

const speechResponse = await fetch(
  "https://openrouter.ai/api/v1/audio/speech",
  {
    method: "POST",
    headers: {
      Authorization: `Bearer ${apiKey}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      model: "google/gemini-3.8-flash-lite-tts",
      response_format: "pcm",
      input: turns.map((turn) => ({
        text: turn.text,
        voice: voices[turn.speaker],
        instructions: turn.direction,
      })),
    }),
  },
);

if (!speechResponse.ok) {
  throw new Error(await speechResponse.text());
}

console.log(`Content-Type: ${speechResponse.headers.get("content-type")}`);
console.log(`Generation ID: ${speechResponse.headers.get("x-generation-id")}`);

Content-Type: audio/pcm;rate=24000;channels=1
Generation ID: gen-tts-1791408882-gsi0JmqAdk574VPMDHMH

import { writeFile } from "node:fs/promises";

function pcmToWav(data: Buffer, sampleRate = 24_000, channels = 1) {
  const bitsPerSample = 16;
  const byteRate = (sampleRate * channels * bitsPerSample) / 8;
  const header = Buffer.alloc(44);

  header.write("RIFF", 0);
  header.writeUInt32LE(36 + data.length, 4);
  header.write("WAVE", 8);
  header.write("fmt ", 12);
  header.writeUInt32LE(16, 16);
  header.writeUInt16LE(1, 20);
  header.writeUInt16LE(channels, 22);
  header.writeUInt32LE(sampleRate, 24);
  header.writeUInt32LE(byteRate, 28);
  header.writeUInt16LE((channels * bitsPerSample) / 8, 32);
  header.writeUInt16LE(bitsPerSample, 34);
  header.write("data", 36);
  header.writeUInt32LE(data.length, 40);

  return Buffer.concat([header, data]);
}

const pcm = Buffer.from(await speechResponse.arrayBuffer());
await writeFile("podcast.wav", pcmToWav(pcm));

const seconds = pcm.length / (24_000 * 2);
console.log(`Saved podcast.wav (${seconds.toFixed(1)} seconds)`);

Saved podcast.wav (48.7 seconds)