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🌐 API & Frontend Communication

This section explains how the SvelteKit frontend communicates with the FastAPI backend, ensuring type safety and consistency across the stack.

🏗️ Architecture

LibreFolio uses a strict OpenAPI-first approach (generated from code) to synchronize the backend and frontend.

graph LR
    Backend[FastAPI Backend] -- "Generates" --> OpenAPI[openapi.json]
    OpenAPI -- "openapi-zod-client" --> TSClient[generated.ts]
    TSClient -- "Imports" --> Frontend[SvelteKit Frontend]

    Frontend -- "Calls" --> TSClient
    TSClient -- "Validates (Zod)" --> TSClient
    TSClient -- "HTTP Request" --> Backend

🔄 The Synchronization Workflow

The synchronization process is automated via dev.py:

  1. Backend Definition: API endpoints and Pydantic models are defined in Python (backend/app/api/).
  2. Schema Export: ./dev.py api schema starts a temporary backend process to export the openapi.json file.
  3. Client Generation: ./dev.py api client uses openapi-zod-client to read the JSON schema and generate a TypeScript client (frontend/src/lib/api/generated.ts).

One-step sync

Use ./dev.py api sync to run both steps (schema export + client generation) in a single command. This is the recommended workflow after any backend API change.

💻 CLI Commands

# Export OpenAPI schema only
./dev.py api schema

# Generate TypeScript client from existing schema
./dev.py api client

# Both in one step (recommended)
./dev.py api sync

⚡ Generated Client Features

The generated client provides:

  • TypeScript Interfaces: Matching the Pydantic models (e.g., AssetRead, TransactionCreate).
  • Zod Schemas: Runtime validation schemas for API responses.
  • API Functions: Typed functions for each endpoint (e.g., api.getAssets()).

🖥️ Usage in Frontend

In the SvelteKit frontend, developers import the generated client to make API calls.

import { api } from '$lib/api';

async function loadPortfolio() {
    // 'data' is fully typed as PortfolioResponse
    const data = await api.getPortfolio();
    return data;
}

This ensures that if the backend API changes (e.g., a field is renamed), the frontend build will fail with a type error, preventing runtime crashes.

📜 Contract Rules

Three rules the pipeline depends on. Breaking any of them fails the frontend build, not a backend test — which is why they are written down here.

1. Every endpoint declares response_model

Every API endpoint declares an explicit response_model Pydantic schema: the generated client is only as typed as the OpenAPI schema, and an endpoint without a model silently degrades to unknown. The audit (08) closed the last stragglers; the deliberate exceptions are endpoints that do not return JSON documents — the SSE stream (GET /assets/provider/search/stream), binary file downloads (FileResponse under /uploads/file/…, /brim/files/…/download), and a handful of legacy dict returns.

2. New discriminated-union members register in fix-openapi-discriminators.mjs

Pydantic discriminated unions (Annotated[A | B, Field(discriminator="kind")]) generate correct OpenAPI, but openapi-zod-client exports each member as const Member: z.ZodType<Member> = … — the exported z.ZodType<T> annotation hides the ZodObject methods that z.discriminatedUnion needs, and the generated client fails to compile. The post-processor frontend/scripts/fix-openapi-discriminators.mjs strips that annotation for a hardcoded list of schemas (40 today), letting TypeScript infer the concrete type while keeping the exported alias. It runs as part of npm run generate-api (which ./dev.py api sync / ./dev.py api client wrap) and throws if a registered schema is not found exactly once — a stale entry fails loud, never silently.

When you add a new member to a discriminated union, add its schema name to discriminatedSchemas in frontend/scripts/fix-openapi-discriminators.mjs, then re-run ./dev.py api sync.

3. Discriminator fields carry an explicit enum

The discriminator field of each union member must pin its value with Field(json_schema_extra={"enum": [...]}):

class SchedulerLogCurrentPriceEntry(BaseModel):
    job: Literal["current_price"] = Field(json_schema_extra={"enum": ["current_price"]})

Without the extra, the discriminator is emitted without an enum constraint and the generated TypeScript client fails to compile (real incident, 03/09 — the scheduler log union). This pattern is used across schemas/signals.py, schemas/ai_export_runtime.py, schemas/risk.py, schemas/risk_scenarios.py, and schemas/settings.py.

🗜️ Response Compression

backend/app/main.py installs Starlette's GZipMiddleware on the whole application (minimum_size=1024, compresslevel=6), so it covers API JSON as well as the SvelteKit bundle and the documentation served by the same process.

  • A response is compressed only when the request's Accept-Encoding header contains gzip. Browsers send it and decompress transparently, so frontend code never handles compressed bytes.
  • A compressed response carries Content-Encoding: gzip. Its Content-Length is the compressed size; a streamed response drops the header.
  • Vary: Accept-Encoding is added to every response eligible for compression — 1 KiB or more, or streamed — whether or not the client asked for gzip, so a cache keeps the two variants apart.
  • Left untouched: bodies under 1 KiB sent in one piece, responses that already declare a Content-Encoding, 206 Partial Content range replies, Server-Sent Events (text/event-stream, such as GET /assets/provider/search/stream), AVIF, GIF, JPEG, PNG, and WebP images, WOFF and WOFF2 fonts, audio, video, and gzip or zip archives. SVG is text and is compressed.
  • A body chunk of 128 KiB or more is compressed in a worker thread, off the event loop.

curl sends no Accept-Encoding by default and therefore receives the uncompressed body. With --compressed it asks for gzip and decompresses transparently:

# Print the response headers of the compressed variant (Content-Encoding, Vary)
curl -s --compressed -D - -o /dev/null http://localhost:6040/api/v1/openapi.json

📡 Notable Endpoints

POST /api/v1/assets/prices/current — Bulk Current Price

Returns the current price of each asset in a list: the assigned provider's latest quote, or the last close stored in the database when no provider answers. The response is designed for the frontend's live-price polling.

Request body: List[int] — asset IDs.

Response (FACurrentPriceResponse, a BaseBulkResponse): one results item per requested ID, in request order; success_count, the number of items that carry a value; and errors, for operation-level errors, which this endpoint leaves empty — a per-asset problem goes in the item's error, and a failure of the whole call is an HTTP 500.

{
  "results": [
    {
      "asset_id": 1,
      "value": "123.45",
      "currency": "EUR",
      "as_of_date": "2026-04-10",
      "source": "provider:justetf",
      "error": null
    },
    {
      "asset_id": 2,
      "value": "98.32",
      "currency": "USD",
      "as_of_date": "2026-04-09",
      "source": "db:last_known",
      "error": null
    },
    {
      "asset_id": 3,
      "value": null,
      "currency": null,
      "as_of_date": null,
      "source": null,
      "error": "No price data available"
    }
  ],
  "success_count": 2,
  "errors": []
}

value is a decimal string, and as_of_date is a date (YYYY-MM-DD), not a timestamp.

Resolution strategy (per asset, all assets in parallel — get_current_prices_bulk in backend/app/services/asset_sources/price_query.py):

  1. If a provider is assigned, ask its get_current_value() — at most five calls at a time, ten seconds each — and answer with source: "provider:<code>". A provider answer is cached for two minutes. What "current" means is up to the provider: JustETF answers EUR with a real-time gettex quote, kept up to date by a WebSocket feed per ISIN, and otherwise (other currencies, or no quote) with the latest daily quote of its chart API; Yahoo Finance reads regularMarketPrice from ticker.info (then currentPrice or previousClose).
  2. Fallback: with no usable provider, or when its call fails or times out, return the latest close stored in PriceHistory, with source: "db:last_known".
  3. With no stored price either, value is null and error is No price data available (Asset not found for an unknown ID).

Side effect: the call is not read-only. A provider quote dated today creates today's PriceHistory row (open, high, low and close all equal to the quote) or extends it (high and low widened, open set if missing, close replaced). A database fallback is never written back.

This endpoint is used by the Assets list (inline live prices on cards and table rows) and by the asset detail page (price summary and chart head) — see Live Prices.