Stability AI Free code examples in Python and JavaScript

Nothing to install on modern Node — fetch, FormData and Blob are all built in. Server-side only.

# Node 18+ has fetch, FormData and Blob built in.
export STABILITY_API_KEY="sk-your-key"

DO NOT SET Content-Type. fetch computes the multipart boundary from the FormData, and setting the header by hand omits it — the request then fails with an error about a missing prompt, which is a lie.

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

const form = new FormData();
form.set("prompt", "A studio photograph of a brass microphone on a dark desk");
form.set("model", "sd3.5-large");
form.set("output_format", "png");

const response = await fetch(
  "https://api.stability.ai/v2beta/stable-image/generate/sd3",
  {
    method: "POST",
    headers: {
      authorization: `Bearer ${process.env.STABILITY_API_KEY}`,
      accept: "image/*",
      // No content-type — fetch sets it, with the boundary.
    },
    body: form,
  },
);

if (!response.ok) throw new Error(await response.text());

await writeFile("microphone.png", Buffer.from(await response.arrayBuffer()));

STRENGTH IS THE WHOLE PARAMETER, and mode must be set the moment an image is attached. Leaving mode out with a file present is a 400 that names neither.

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

const sketch = await readFile("sketch.png");

const form = new FormData();
form.set("image", new Blob([sketch], { type: "image/png" }), "sketch.png");
form.set("prompt", "A brass microphone, studio lighting, photographic");
form.set("model", "sd3.5-large");
form.set("mode", "image-to-image");   // required once image is present
form.set("strength", "0.55");         // 0 keeps the input, 1 ignores it
form.set("output_format", "png");

const response = await fetch(
  "https://api.stability.ai/v2beta/stable-image/generate/sd3",
  {
    method: "POST",
    headers: {
      authorization: `Bearer ${process.env.STABILITY_API_KEY}`,
      accept: "image/*",
    },
    body: form,
  },
);

if (!response.ok) throw new Error(await response.text());
await writeFile("rendered.png", Buffer.from(await response.arrayBuffer()));

Sequential, not Promise.all: Stability rate-limits and a 429 costs a 60-second timeout, which is slower than just doing them in order.

const sketch = await readFile("sketch.png");

for (const strength of [0.35, 0.45, 0.55, 0.65, 0.75]) {
  const form = new FormData();
  form.set("image", new Blob([sketch], { type: "image/png" }), "sketch.png");
  form.set("prompt", "A brass microphone, studio lighting, photographic");
  form.set("model", "sd3.5-large");
  form.set("mode", "image-to-image");
  form.set("strength", String(strength));
  form.set("seed", "42");             // hold it fixed or this proves nothing
  form.set("output_format", "png");

  const response = await fetch(
    "https://api.stability.ai/v2beta/stable-image/generate/sd3",
    {
      method: "POST",
      headers: {
        authorization: `Bearer ${process.env.STABILITY_API_KEY}`,
        accept: "image/*",
      },
      body: form,
    },
  );

  if (!response.ok) throw new Error(await response.text());
  await writeFile(`strength-${strength}.png`,
    Buffer.from(await response.arrayBuffer()));
}

With accept: image/* a success is bytes and a failure is JSON. Calling .arrayBuffer() on the failure gives you an unreadable buffer instead of the message, so branch on response.ok first.

async function generate(form) {
  const response = await fetch(
    "https://api.stability.ai/v2beta/stable-image/generate/sd3",
    {
      method: "POST",
      headers: {
        authorization: `Bearer ${process.env.STABILITY_API_KEY}`,
        accept: "image/*",
      },
      body: form,
    },
  );

  if (response.ok) return Buffer.from(await response.arrayBuffer());

  // Errors come back as JSON whatever the accept header says.
  const detail = await response.json().catch(() => ({}));

  // 403 is content moderation, not auth — retrying will not help.
  // 429 is rate limiting and carries a 60-second cool-off.
  throw new Error(`Stability ${response.status}: ${JSON.stringify(detail)}`);
}