The Writer API

The Writer API is a session-free way to work against your book with code. Where the MCP connector is for pointing an AI client at your manuscript, the API is for your own scripts — an import/export round-trip, a deterministic transform, a small tool you (or an LLM you brief) writes to do one job against your book.

• Base URL — https://app.writecalliope.ink/api/v1
• Auth — a personal key sent as Authorization: Bearer cal_…
• The contract — /api/v1/openapi.json (OpenAPI 3.1) and /api/v1/llms.txt (a brief written for an LLM to read)

The cost model

The API shares Calliope’s economics exactly — the same ledger, the same rules as the web app and MCP:

• read — free. Outline, chapters, search, the Map, snippets.
• ai — spends your credits. Detect, consistency check, synopsis generation. Drawn from your credits (or your subscription allowance). No subscription needed.
• write — needs a subscription. Snippets, structure, the Map — and your chapter prose.

Every endpoint in the reference is tagged with its tier, and the descriptions say plainly when a call costs something.

The one difference from MCP: it can write prose

This is the API’s distinguishing power, and it deserves to be understood before you use it. Unlike MCP — which is always an LLM on the other end and may never touch your body text — the Writer API can overwrite chapter prose.

That’s not a hole in the augmentation promise; it respects why the promise exists. An MCP client is by definition an AI, so a prose-write tool there would be “an AI writing your book.” A key, by contrast, might be a deterministic script with no model at all, and there are real, honest prose workflows that aren’t authorship:

• round-tripping your book to another editor and back;
• deterministic transforms — normalising quotation marks, fixing a spelling across the whole manuscript.

Calliope can’t see what’s on the other end of a key, so rather than block those real workflows to guard against a risk it can’t detect, it allows prose writes and states the boundary plainly: the API is your own programmatic hand, not an agent authoring for you. There is no prose generation endpoint — the API only persists text your script supplies; it never calls a model to write.

Prose writes have a real cost to your book’s annotations, too, so they get their own page. Read it before you script prose.

Where to go next

1. Setup: your API key — create a key and make your first call (including handing the job to an LLM).
2. Writing prose safely — the prose endpoints, and what a plain-text write costs your mentions and marginalia.
3. Errors & conventions — auth, tiers, and the stable error shape.
4. Endpoint reference — every endpoint, generated from the OpenAPI contract.