# Saturday API

> Nutrition intelligence for endurance athletes — as an API.

Saturday provides AI-powered nutrition intelligence for endurance athletes. The API calculates personalized fuel, hydration, and electrolyte recommendations based on athlete profiles, activity parameters, and environmental conditions.

## Authentication

All requests require a Bearer token in the Authorization header. API keys use the `sk_test_` (sandbox) or `sk_live_` (production) prefix.

```
Authorization: Bearer sk_test_abc123def456
```

## Core Endpoints

- POST /v1/nutrition/calculate — Calculate fuel/hydration/electrolyte prescription
- POST /v1/nutrition/calculate/batch — Batch calculate for multiple scenarios
- POST /v1/nutrition/calculate/compare — Compare prescriptions across conditions
- POST /v1/athletes — Create an athlete profile
- GET /v1/athletes — List partner's athletes
- POST /v1/athletes/{id}/activities — Create an activity
- POST /v1/athletes/{id}/activities/{activityId}/prescription — Calculate prescription
- POST /v1/ai/conversations — Start AI coaching conversation (SSE streaming)
- POST /v1/webhooks — Register webhook for real-time events

## Safety

Every response includes safety metadata (confidence_score, warnings, max_safe_fluid_ml_per_hr, max_safe_sodium_mg_per_hr, not_instructions). Safety data is NEVER gated behind subscription status. Bad hydration recommendations can cause exercise-associated hyponatremia — a condition that kills athletes.

## Freemium Model

The API is free for partners. Subscribed athletes get exact numbers; free athletes get ranges (teasers). Safety data is always complete regardless of subscription status.

## Documentation

- Full docs: https://api.saturday.fit
- OpenAPI spec: https://api.saturday.fit/openapi.yaml
- MCP server: https://api.saturday.fit/v1/mcp
- Full LLM context: https://api.saturday.fit/llms-full.txt