API & MCP reference

Janus API v1

Overview

8 AI tools

Images, edits, voice, music, video, playable games, text assist and atlas packing — the same models as Janus Studio.

11 MCP tools

A Streamable HTTP MCP server. Your agent generates assets and games, and gets back URLs it can use immediately.

One library

Everything you make by API, MCP or in the browser lands in the same library, spending the same credits.

Base URL: https://janusengine.com/api/v1
MCP endpoint: https://janusengine.com/api/mcp
Format: JSON in, JSON out (UTF-8)
Auth: Authorization: Bearer jns_live_…
CORS: Open — call it from any origin
Billing: Credits · 100 credits = $1

Quick start

1
Create a key
Open Account → API keys. The key is shown once — store it as JANUS_API_KEY.
2
Call a tool
POST to /api/v1/tools/image.generate. The response carries the asset URL and your new balance.
3
Use the result
Asset URLs are public and CORS-open — load them in your game, or pass the asset_id to another tool.

curl

curl https://janusengine.com/api/v1/tools/image.generate \
  -H "Authorization: Bearer jns_live_YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{"prompt": "a brave knight puppy with a tiny shield, game sprite", "model": "nb2-lite"}'

Send your key as a bearer token on every request: Authorization: Bearer jns_live_…. Keys look like jns_live_ followed by 32 letters and digits. A key acts as your account — same credits, same library, same rate limit as Janus Studio in the browser.

We store only a SHA-256 hash of each key, so a key is shown exactly once, when you create it. Lost one? Revoke it and make another — revocation takes effect immediately. Keep keys server-side and out of source control; never ship them in a browser bundle.

Keys made before you sign up work right away and move with you to your account when you create it. Sign up to keep credits across devices and get bonus credits.

Check a key

curl https://janusengine.com/api/v1/me -H "Authorization: Bearer jns_live_YOUR_KEY"

Model Context Protocol

MCP server

Janus speaks MCP over Streamable HTTP at https://janusengine.com/api/mcp, authenticated with the same API key. Add it once, then ask your agent for what you need — “make a pixel-art coin sprite and a jump sound, then build a platformer that uses them” — and it calls the tools, gets URLs back, and wires them into your project.

Run in your terminal. Add --scope user to make it available in every project.

Claude Code

claude mcp add --transport http janus https://janusengine.com/api/mcp \
  --header "Authorization: Bearer jns_live_YOUR_KEY"

Tools

generate_image3–16 cr

Use this to create game art from a text prompt — characters, enemies, props, item icons, tiles, backgrounds, key art. For sprites, set style to "Game sprite on a flat #00FF00 green screen" so the background can be keyed out. Pass reference_ids (asset ids) to keep a character consistent. Returns the image URL and asset_id.

Parameters

edit_image3–16 cr

Use this to change an existing image by asset_id with a plain-English instruction (recolor, add/remove details, new pose, new background). Saves the result as a new asset. To edit a local file, call upload_asset first.

Parameters

generate_speech1–2 cr

Use this to voice game dialogue, barks, tutorials or narration. Choose one of 17 voices and an optional performance direction ("whispering", "like a game announcer"). Returns a WAV URL.

Parameters

generate_music10–25 cr

Use this to compose original game music — menu themes, level loops, boss tracks. Describe genre, tempo, instruments and mood; include "loopable" for background music. Returns an audio URL.

Parameters

start_video40–150 cr

Use this to render a short video with native audio (trailer shot, cutscene, animated key art) from a prompt, or animate a still by passing image_id. Returns a job_id right away; renders take 1–3 minutes — call get_job with the job_id every ~10 seconds until status is succeeded or failed. Credits are refunded automatically if the render fails.

Parameters

get_jobFree

Use this to check a video render started with start_video. Returns status (queued, rendering, succeeded, failed) and, when succeeded, the video URL. Free.

build_game5–20 cr

Use this to generate a complete, playable single-file HTML5 browser game from a description (Phaser for 2D, three.js for 3D, or plain canvas). To change a game you built earlier, pass its asset id as base_game_id and describe the change. Optionally pass your own sprite/sound URLs as assets. Returns the playable game URL, its asset_id and a short summary (not the source — fetch the URL if you need the HTML).

Parameters

pack_atlasFree

Use this to pack animation frames (asset ids, in order) into one PNG sprite sheet with edge extrusion, plus a Bevy TextureAtlasLayout JSON (at the sheet URL + ?variant=atlas).

Parameters

upload_assetFree

Use this to add a local file (base64) to your Janus library so other tools can use it — e.g. upload a sprite, then edit_image it or pack_atlas it. Images, audio, video, GLB, JSON. Max 40 MB. Free.

list_assetsFree

Use this to find things you already made — images, audio, video, games — newest first, optionally filtered by kind. Returns asset ids and URLs. Free.

get_creditsFree

Use this to check the account's credit balance before an expensive run, and see what each tool costs. 1 credit = $0.01. Free.

How results come back

Each tool returns a short text summary with the asset URL and asset_id, a resource_link content block pointing at the file, and structuredContent with the same fields as JSON. build_game returns the playable URL and a summary — not the full source — to keep your agent’s context small; fetch the URL when you want the HTML. The server is stateless: no session to manage, every request stands alone.

Raw JSON-RPC (any client)

curl https://janusengine.com/api/mcp \
  -H "Authorization: Bearer $JANUS_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"generate_image","arguments":{"prompt":"a sleepy kitten on a bookshelf, game sprite","model":"nb2-lite"}}}'

Catalog

GET/api/v1/catalogPublic

A machine-readable list of every tool: id, description written for coding agents, JSON input schema, default model and price table, plus links to the matching Janus Studio tools. No key needed — point an agent at it and it can discover everything on its own.

Response (abridged)

{
  "api": "janus-studio",
  "version": 1,
  "credits_per_dollar": 100,
  "tools": [
    {
      "id": "image.generate",
      "endpoint": "POST /api/v1/tools/image.generate",
      "mcp_tool": "generate_image",
      "input_schema": {
        "type": "object",
        "properties": {
          "prompt": {
            "type": "string"
          },
          "model": {
            "enum": [
              "nb2-lite",
              "nb2",
              "nb-pro"
            ]
          }
        },
        "required": [
          "prompt"
        ]
      },
      "default_model": "nb2",
      "pricing": {
        "unit": "credits per run",
        "default": 8,
        "models": {
          "nb2-lite": 3,
          "nb2": 8,
          "nb-pro": 16
        }
      }
    },
    "…"
  ]
}

Account

GET/api/v1/meAPI key

Your credit balance and the key in use. Check it before an expensive run.

Response

{
  "credits": 1250,
  "credits_usd": 12.5,
  "account": {
    "type": "user",
    "name": "Ada",
    "email": "ada@example.com"
  },
  "key": {
    "id": "key_3f9a1c0d7e2b4a55",
    "name": "Claude Code",
    "prefix": "jns_live_iD5Hup"
  }
}

Tools

POST/api/v1/tools/{tool_id}API key

Run one AI tool. The body is the tool’s input (validated against its schema; a bad body returns 400 with an issues list). On success you get the asset — with an absolute url — plus cost (credits charged) and credits (your balance after). Failed generations are never charged.

Generate image

image.generateMCP: generate_image

Use this to create a new image (character, enemy, prop, item icon, tile, background, key art) from a text prompt. For sprites, add the style "Game sprite on a flat #00FF00 green screen" and chroma-key it. Pass reference_ids to keep a character consistent across images. Every image carries SynthID provenance and IPTC AI-marking.

POST/api/v1/tools/image.generateAPI key

nb2-lite · Nano Banana 2 Lite · 3 cr ($0.03)nb2 · Nano Banana 2 · 8 cr ($0.08)defaultnb-pro · Nano Banana Pro · 16 cr ($0.16)

promptrequired
string
What to draw. Be specific: subject, pose, framing, lighting, palette, background.(1–4,000 chars)
modeloptional
"nb2-lite" | "nb2" | "nb-pro" · default "nb2"
nb2-lite (3 cr, fastest), nb2 (8 cr, best value), nb-pro (16 cr, highest fidelity and text rendering).
styleoptional
string
Optional style directive appended to the prompt, e.g. "pixel art, 32x32, limited palette" or "Game sprite on a flat #00FF00 green screen" (then chroma-key it).(≤ 400 chars)
aspect_ratiooptional
"1:1" | "2:3" | "3:2" | "3:4" | "4:3" | "4:5" | "5:4" | "9:16" | "16:9" | "21:9"
Output aspect ratio. Omit for the model default.
image_sizeoptional
"1K" | "2K" | "4K"
Output resolution tier (nb2 / nb-pro).
reference_idsoptional
string[]
Up to 4 asset ids from your library to use as visual references (character consistency, style matching).(≤ 4 items)

Parameter	Type	Description
`prompt` required	`string`	What to draw. Be specific: subject, pose, framing, lighting, palette, background.(1–4,000 chars)
`model` optional	`"nb2-lite" \| "nb2" \| "nb-pro"` default "nb2"	nb2-lite (3 cr, fastest), nb2 (8 cr, best value), nb-pro (16 cr, highest fidelity and text rendering).
`style` optional	`string`	Optional style directive appended to the prompt, e.g. "pixel art, 32x32, limited palette" or "Game sprite on a flat #00FF00 green screen" (then chroma-key it).(≤ 400 chars)
`aspect_ratio` optional	`"1:1" \| "2:3" \| "3:2" \| "3:4" \| "4:3" \| "4:5" \| "5:4" \| "9:16" \| "16:9" \| "21:9"`	Output aspect ratio. Omit for the model default.
`image_size` optional	`"1K" \| "2K" \| "4K"`	Output resolution tier (nb2 / nb-pro).
`reference_ids` optional	`string[]`	Up to 4 asset ids from your library to use as visual references (character consistency, style matching).(≤ 4 items)

Returns: An image asset: asset_id, url (PNG), width, height, model_label, credits (your new balance), cost. In the Studio: Image Forge, Sprite Forge, Tile Forge, Material Forge, Background Remover.

Request

curl https://janusengine.com/api/v1/tools/image.generate \
  -H "Authorization: Bearer $JANUS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"prompt":"a brave knight puppy with a tiny shield, side view, game sprite","model":"nb2-lite","style":"Game sprite on a flat #00FF00 green screen"}'

Response

{
  "asset_id": "9712608cb5c04801",
  "mime": "image/png",
  "url": "https://janusengine.com/api/assets/9712608cb5c04801",
  "width": 1024,
  "height": 1024,
  "model_label": "Nano Banana 2 Lite",
  "prompt": "a brave knight puppy with a tiny shield, side view, game sprite",
  "content_credentials": "SynthID",
  "iptc_digital_source_type": "http://cv.iptc.org/newscodes/digitalsourcetype/trainedAlgorithmicMedia",
  "latency_ms": 2744,
  "cost": 3,
  "credits": 94
}

Edit image

image.editMCP: edit_image

Use this to modify an image you already have (by asset id): recolor, re-pose, add or remove details, change the background, make a variant. The original is untouched; the edit is saved as a new asset.

POST/api/v1/tools/image.editAPI key

nb2-lite · Nano Banana 2 Lite · 3 cr ($0.03)nb2 · Nano Banana 2 · 8 cr ($0.08)defaultnb-pro · Nano Banana Pro · 16 cr ($0.16)

asset_idrequired
string
The image to edit (from your library, a previous result, or an upload).
instructionrequired
string
The change to make, in plain English, e.g. "make the armor gold and add a red cape".(1–2,000 chars)
modeloptional
"nb2-lite" | "nb2" | "nb-pro" · default "nb2"
nb2-lite (3 cr, fastest), nb2 (8 cr, best value), nb-pro (16 cr, highest fidelity and text rendering).

Parameter	Type	Description
`asset_id` required	`string`	The image to edit (from your library, a previous result, or an upload).
`instruction` required	`string`	The change to make, in plain English, e.g. "make the armor gold and add a red cape".(1–2,000 chars)
`model` optional	`"nb2-lite" \| "nb2" \| "nb-pro"` default "nb2"	nb2-lite (3 cr, fastest), nb2 (8 cr, best value), nb-pro (16 cr, highest fidelity and text rendering).

Returns: A new image asset (parent_id = the source). In the Studio: Magic Edit, Background Remover.

Request

curl https://janusengine.com/api/v1/tools/image.edit \
  -H "Authorization: Bearer $JANUS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"asset_id":"404aac0f25644665","instruction":"make the shield gold and add a red cape","model":"nb2-lite"}'

Response

{
  "asset_id": "c81e3a0f5d2b4e17",
  "parent_id": "404aac0f25644665",
  "mime": "image/png",
  "url": "https://janusengine.com/api/assets/c81e3a0f5d2b4e17",
  "width": 1024,
  "height": 1024,
  "model_label": "Nano Banana 2 Lite",
  "cost": 3,
  "credits": 91
}

Generate speech

speechMCP: generate_speech

Use this to voice dialogue, barks, tutorials or narration. Pick a voice and optionally a direction ("whispering", "like a game announcer"). Returns a WAV file.

POST/api/v1/tools/speechAPI key

standard · Standard · 2 cr ($0.02)defaultlite · Lite · 1 cr ($0.01)

textrequired
string
The line(s) to speak.(1–5,000 chars)
voiceoptional
"Puck" | "Kore" | "Charon" | … (17) · default "Puck"
One of 17 voices. Puck (upbeat), Kore (firm), Charon (informative), Fenrir (excitable), Aoede (breezy), Leda (youthful), Zephyr (bright)…
directionoptional
string
Performance direction, e.g. "whispering", "like a game announcer", "cheerful".(≤ 300 chars)
modeloptional
"standard" | "lite" · default "standard"
standard (2 cr) or lite (1 cr).

Parameter	Type	Description
`text` required	`string`	The line(s) to speak.(1–5,000 chars)
`voice` optional	`"Puck" \| "Kore" \| "Charon" \| … (17)` default "Puck"	One of 17 voices. Puck (upbeat), Kore (firm), Charon (informative), Fenrir (excitable), Aoede (breezy), Leda (youthful), Zephyr (bright)…
`direction` optional	`string`	Performance direction, e.g. "whispering", "like a game announcer", "cheerful".(≤ 300 chars)
`model` optional	`"standard" \| "lite"` default "standard"	standard (2 cr) or lite (1 cr).

Returns: An audio asset (audio/wav). In the Studio: Voice Studio.

Request

curl https://janusengine.com/api/v1/tools/speech \
  -H "Authorization: Bearer $JANUS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"text":"Level complete! Nap time, everyone.","voice":"Puck","direction":"cheerful game announcer","model":"lite"}'

Response

{
  "asset_id": "7d2f9a41c0be4a55",
  "mime": "audio/wav",
  "url": "https://janusengine.com/api/assets/7d2f9a41c0be4a55",
  "voice": "Puck",
  "direction": "cheerful game announcer",
  "cost": 1,
  "credits": 90
}

Generate music

musicMCP: generate_music

Use this to make a soundtrack: menu themes, level loops, boss music, stingers. Describe genre, tempo, instruments and mood; say "loopable" for background music. Returns an audio file.

POST/api/v1/tools/musicAPI key

clip · Lyria clip · 10 cr ($0.10)defaultpro · Lyria full track · 25 cr ($0.25)

promptrequired
string
Genre, tempo (bpm), instruments, mood and structure. Say "loopable" for background music.(1–2,000 chars)
modeloptional
"clip" | "pro" · default "clip"
clip (10 cr): a short loopable clip. pro (25 cr): a full-length track.

Parameter	Type	Description
`prompt` required	`string`	Genre, tempo (bpm), instruments, mood and structure. Say "loopable" for background music.(1–2,000 chars)
`model` optional	`"clip" \| "pro"` default "clip"	clip (10 cr): a short loopable clip. pro (25 cr): a full-length track.

Returns: An audio asset. In the Studio: Music Studio.

Request

curl https://janusengine.com/api/v1/tools/music \
  -H "Authorization: Bearer $JANUS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"prompt":"cheerful chiptune level theme, 140 bpm, square leads, loopable","model":"clip"}'

Response

{
  "asset_id": "e4b8c6d2a9f04b31",
  "mime": "audio/mpeg",
  "url": "https://janusengine.com/api/assets/e4b8c6d2a9f04b31",
  "prompt": "cheerful chiptune level theme, 140 bpm, square leads, loopable",
  "cost": 10,
  "credits": 80
}

Start video render

videoasyncMCP: start_video

Use this to render a short video shot with native audio, from a prompt or by animating a still (image_id). Renders take 1–3 minutes: this returns a job_id immediately — poll GET /api/v1/jobs/{job_id} (or the get_job MCP tool) every ~10 s. Credits are charged up front and refunded automatically if the render fails.

POST/api/v1/tools/videoAPI key

lite · Veo 3.1 Lite · 40 cr ($0.40)fast · Veo 3.1 Fast · 60 cr ($0.60)defaultstandard · Veo 3.1 · 150 cr ($1.50)

promptrequired
string
The shot: subject, action, camera move, mood. Native audio is generated too.(1–2,000 chars)
modeloptional
"lite" | "fast" | "standard" · default "fast"
lite (40 cr), fast (60 cr), standard (150 cr, highest quality).
image_idoptional
string
Optional asset id of a still image to animate (image-to-video).
aspect_ratiooptional
"16:9" | "9:16" · default "16:9"
16:9 (landscape trailer) or 9:16 (vertical/mobile).

Parameter	Type	Description
`prompt` required	`string`	The shot: subject, action, camera move, mood. Native audio is generated too.(1–2,000 chars)
`model` optional	`"lite" \| "fast" \| "standard"` default "fast"	lite (40 cr), fast (60 cr), standard (150 cr, highest quality).
`image_id` optional	`string`	Optional asset id of a still image to animate (image-to-video).
`aspect_ratio` optional	`"16:9" \| "9:16"` default "16:9"	16:9 (landscape trailer) or 9:16 (vertical/mobile).

Returns: A job: job_id, status (queued → rendering → succeeded | failed). On success the job carries the video asset url (video/mp4). In the Studio: Video Studio.

Request

curl https://janusengine.com/api/v1/tools/video \
  -H "Authorization: Bearer $JANUS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"prompt":"slow push-in on a sleepy kitten on a bookshelf at golden hour","model":"lite","aspect_ratio":"16:9"}'

Response

{
  "job_id": "5b0c2e7d9a1f4c3e",
  "status": "queued",
  "cost": 40,
  "credits": 40
}

Build game

code.gameMCP: build_game

Use this to generate a complete, playable single-file HTML5 game (Phaser, three.js or plain canvas) from one sentence, or to change an existing one by passing its HTML as current_html. Pass assets to use your own sprites and sounds. The game is saved to your library; its url plays it in any browser.

POST/api/v1/tools/code.gameAPI key

fast · Fast · 5 cr ($0.05)defaultpro · Pro · 20 cr ($0.20)

promptrequired
string
The game to build — or, when iterating, the change to make.(1–4,000 chars)
current_htmloptional
string
The current game's full HTML, to iterate on it. Omit to build a new game.(≤ 200,000 chars)
engineoptional
"phaser" | "three" | "canvas" · default "phaser"
phaser (2D), three (3D) or canvas (no library).
modeloptional
"fast" | "pro" · default "fast"
fast (5 cr) or pro (20 cr, stronger reasoning for complex games).
assetsoptional
{ url, role }[]
Your own sprites/sounds for the game to load instead of procedural art.(≤ 12 items)

Parameter	Type	Description
`prompt` required	`string`	The game to build — or, when iterating, the change to make.(1–4,000 chars)
`current_html` optional	`string`	The current game's full HTML, to iterate on it. Omit to build a new game.(≤ 200,000 chars)
`engine` optional	`"phaser" \| "three" \| "canvas"` default "phaser"	phaser (2D), three (3D) or canvas (no library).
`model` optional	`"fast" \| "pro"` default "fast"	fast (5 cr) or pro (20 cr, stronger reasoning for complex games).
`assets` optional	`{ url, role }[]`	Your own sprites/sounds for the game to load instead of procedural art.(≤ 12 items)

Returns: A game asset (text/html): asset_id, url (playable), html (the full source), engine. In the Studio: Game Genie.

Request

curl https://janusengine.com/api/v1/tools/code.game \
  -H "Authorization: Bearer $JANUS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"prompt":"a cozy endless runner where a puppy hops over garden gnomes","engine":"phaser","model":"fast"}'

Response

{
  "asset_id": "f50d6cd0c54a461c",
  "mime": "text/html",
  "url": "https://janusengine.com/api/assets/f50d6cd0c54a461c",
  "engine": "phaser",
  "html": "<!DOCTYPE html>\n<html>… the full game …</html>",
  "cost": 5,
  "credits": 35
}

Text assist

text.assist

Use this for quick text help: turn a rough idea into a strong image or music prompt, brainstorm game names or ideas, or plan a PBR material. Returns text (some tasks return JSON in the text).

POST/api/v1/tools/text.assistAPI key

fast · Fast · 1 cr ($0.01)defaultpro · Pro · 3 cr ($0.03)

promptrequired
string
Your idea or question.(1–4,000 chars)
taskoptional
"enhance-image-prompt" | "enhance-music-prompt" | "material" | "name-ideas" | "game-ideas" | "freeform" · default "freeform"
What kind of help you want.
modeloptional
"fast" | "pro" · default "fast"
fast (1 cr) or pro (3 cr).

Parameter	Type	Description
`prompt` required	`string`	Your idea or question.(1–4,000 chars)
`task` optional	`"enhance-image-prompt" \| "enhance-music-prompt" \| "material" \| "name-ideas" \| "game-ideas" \| "freeform"` default "freeform"	What kind of help you want.
`model` optional	`"fast" \| "pro"` default "fast"	fast (1 cr) or pro (3 cr).

Returns: { task, text }. In the Studio: Image Forge, Music Studio, Material Forge, Game Genie.

Request

curl https://janusengine.com/api/v1/tools/text.assist \
  -H "Authorization: Bearer $JANUS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"prompt":"spooky but cute haunted kitchen level","task":"enhance-image-prompt"}'

Response

{
  "task": "enhance-image-prompt",
  "text": "A cozy haunted kitchen at night, game level art: copper pots glowing faintly…",
  "cost": 1,
  "credits": 34
}

Pack atlas

pack.atlasMCP: pack_atlas

Use this to pack animation frames (asset ids, in order) into one PNG sprite sheet with edge extrusion. Free. The Bevy TextureAtlasLayout JSON is at the result url + ?variant=atlas.

POST/api/v1/tools/pack.atlasAPI key

Free

asset_idsrequired
string[]
Frame asset ids, in animation order. Mixed sizes are normalised to the largest frame.(1–256 items)
columnsoptional
integer
Grid columns (default: near-square).(1–64)
extrudeoptional
integer · default 2
Edge-extrusion pixels per frame (prevents texture bleeding).(0–8)
characteroptional
string
Optional character name recorded in the atlas JSON.(≤ 80 chars)

Parameter	Type	Description
`asset_ids` required	`string[]`	Frame asset ids, in animation order. Mixed sizes are normalised to the largest frame.(1–256 items)
`columns` optional	`integer`	Grid columns (default: near-square).(1–64)
`extrude` optional	`integer` default 2	Edge-extrusion pixels per frame (prevents texture bleeding).(0–8)
`character` optional	`string`	Optional character name recorded in the atlas JSON.(≤ 80 chars)

Returns: An image asset (the sheet). Atlas JSON at `${url}?variant=atlas`. In the Studio: Atlas Packer, Sprite Forge.

Request

curl https://janusengine.com/api/v1/tools/pack.atlas \
  -H "Authorization: Bearer $JANUS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"asset_ids":["a1b2c3d4e5f60718","b2c3d4e5f6071829"],"extrude":2}'

Response

{
  "asset_id": "6e991d4210fd41f1",
  "mime": "image/png",
  "url": "https://janusengine.com/api/assets/6e991d4210fd41f1",
  "bevy": {
    "schema": "janus.texture_atlas/v1",
    "engine": "bevy",
    "tile_size": [
      1024,
      1024
    ],
    "columns": 2,
    "rows": 1,
    "frame_count": 2
  },
  "cost": 0,
  "credits": 34
}

Jobs

GET/api/v1/jobs/{job_id}API key

Video renders run in the background. video returns a job_id straight away; poll this endpoint every ~10 seconds. status moves queued → rendering → succeeded or failed. Most renders finish in 1–3 minutes. A failed render is refunded automatically, exactly once.

Succeeded

{
  "job_id": "5b0c2e7d9a1f4c3e",
  "status": "succeeded",
  "asset_id": "a7c1e9f03b5d4e28",
  "url": "https://janusengine.com/api/assets/a7c1e9f03b5d4e28",
  "credits": 40
}

Failed (refunded)

{
  "job_id": "5b0c2e7d9a1f4c3e",
  "status": "failed",
  "error": "no video returned (possibly filtered)",
  "refunded": true,
  "credits": 80
}

Assets

GET/api/v1/assets?kind=image&limit=50API key

POST/api/v1/assetsAPI key

GET lists your library, newest first. Filter with kind (image, audio, video, game, file), tool and limit (max 200).

POST uploads a file so tools can use it — for example, upload your own sprite, then pass its asset_id to image.edit or pack.atlas. Send {data_base64, mime, filename?, prompt?}; up to 40 MB. Images, audio, MP4, GLB, JSON and ZIP are accepted.

Asset URLs (https://janusengine.com/api/assets/{id}) are public, stable and CORS-open, so games can load them directly. Add ?download=1 to force a download, or ?variant=atlas on a packed sheet for its atlas JSON.

Upload

curl https://janusengine.com/api/v1/assets \
  -H "Authorization: Bearer $JANUS_API_KEY" \
  -H "Content-Type: application/json" \
  -d "{\"mime\": \"image/png\", \"filename\": \"hero.png\", \"data_base64\": \"$(base64 < hero.png)\"}"

Response

{
  "assets": [
    {
      "id": "9712608cb5c04801",
      "kind": "image",
      "mime": "image/png",
      "tool": "image-gen",
      "prompt": "a brave knight puppy with a tiny shield",
      "created_at": "2026-10-13 00:09:37",
      "url": "https://janusengine.com/api/assets/9712608cb5c04801"
    }
  ],
  "count": 1
}

Errors

Errors are JSON with a stable machine-readable error code and a human message. Some carry extra fields (issues, needed, credits).

402 Payment Required

{
  "error": "insufficient_credits",
  "message": "This run costs 16 credits and you have 9.",
  "needed": 16,
  "credits": 9
}

Status	error	Meaning
400	bad_request	The body failed validation — `issues` lists each problem.
401	missing_api_key / invalid_api_key	No key, or a revoked or mistyped key.
402	insufficient_credits	Not enough credits for this run. `needed` and `credits` say by how much.
403	forbidden	You referenced an asset that belongs to another account.
404	unknown_op / asset_not_found / not_found	No such tool, asset or job.
413	bad_size	Upload is empty or over 40 MB.
422	prompt_blocked	The prompt names protected IP (a franchise, an artist's style, a real person). Describe it in your own words. Not charged.
422	generation_refused	The model declined the request (safety filters). Not charged.
429	rate_limited	More than 20 AI calls in a minute. Back off and retry.
502	generation_failed	The model provider failed. You are not charged — retry.
503	capacity / service_unavailable	Generation is temporarily unavailable. Retry later.

Rate limits

AI calls are limited to 20 per minute per account, shared across the browser, the REST API and MCP (a token bucket, so short bursts are fine). Over the limit you get 429 rate_limited — wait a few seconds and retry with exponential backoff. Polling jobs, listing assets and reading the catalog are not metered.

Need more throughput for a pipeline or a studio? Talk to us.

Pricing

Pay as you go with credits — 100 credits = $1. The API, MCP and Janus Studio share one balance. Free browser tools (pixel snap, chroma key, voxels, 3D-to-2D, SFX) cost nothing.

Tool	Model	Credits	USD
image.generate	`nb2-lite` · Nano Banana 2 Lite	3	$0.03
	`nb2` · Nano Banana 2	8	$0.08
	`nb-pro` · Nano Banana Pro	16	$0.16
image.edit	`nb2-lite` · Nano Banana 2 Lite	3	$0.03
	`nb2` · Nano Banana 2	8	$0.08
	`nb-pro` · Nano Banana Pro	16	$0.16
speech	`standard` · Standard	2	$0.02
	`lite` · Lite	1	$0.01
music	`clip` · Lyria clip	10	$0.10
	`pro` · Lyria full track	25	$0.25
video	`lite` · Veo 3.1 Lite	40	$0.40
	`fast` · Veo 3.1 Fast	60	$0.60
	`standard` · Veo 3.1	150	$1.50
code.game	`fast` · Fast	5	$0.05
	`pro` · Pro	20	$0.20
text.assist	`fast` · Fast	1	$0.01
	`pro` · Pro	3	$0.03
pack.atlas	`default` · Free	0	$0.00

Credit packs

$10

1,000 credits

$25

2,750 credits

+10% bonus

$50

6,000 credits

+20% bonus

Buy credits on the pricing page.

Janus API & MCP

Overview

Quick start

Authentication

MCP server

Tools

Catalog

Account

Tools

Generate image

Edit image

Generate speech

Generate music

Start video render

Build game

Text assist

Pack atlas

Jobs

Assets

Errors

Rate limits

Pricing

Credit packs