For developers building finance apps

The ledger your finance app doesn't have to build.

Connect bank accounts, keep real double-entry books, and add an AI assistant that answers from them, all through one REST API with OAuth2 sign-in.

Start building Read the API docs
  • checkBalanced journals, every time
  • checkPlaid bank sync
  • checkAI answers from the ledger

Ask your books

Let your users ask their books anything.

Add a conversation to your app through the API. The BigBooks assistant works with dozens of finance tools, covering balances, spending, budgets, cash forecasts, debt payoff and what-if scenarios, and streams its answer straight to your UI.

  • lock_personEvery conversation is scoped to the signed-in user's own books.
  • shieldPersonal details stay protected, even from the AI models doing the work.
  • receipt_longNumbers come from posted journals, not from the model's memory.

What you get

Books that balance, data that stays fresh, access that stays safe.

Three things most finance apps end up building for themselves, done once and exposed through one API.

account_balance

A real double-entry ledger

Every transaction posts as a balanced journal across assets, liabilities, equity, revenue and expenses, so balances, statements and budgets always agree.

sync_alt

Bank data, already categorized

Connect institutions through Plaid or import CSV and PDF statements. Merchants are normalized, transactions categorized and recurring bills detected for you.

hub

Sign-in and AI for your app

Users sign in once with OAuth2 and OpenID Connect. Your app reads and writes through the REST API and can add an AI assistant on top, and both see only the books the user grants.

Also built in

  • summarizeBalance sheets, income statements and cash flow
  • savingsBudgets and month-end projections
  • show_chartInvestment holdings and performance
  • insightsCash forecasts, debt payoff and what-if scenarios
  • groupsHouseholds, advisors and clients sharing books
  • descriptionStatements and documents stored with the books

How it works

From bank feed to answer in three steps.

  1. Connect

    Users link their banks through Plaid or upload statements, and BigBooks sets up the accounts for them.

  2. BigBooks keeps the books

    Transactions post as balanced journals, get categorized and matched to merchants, and roll up into balances and reports on their own.

  3. Build on it

    Read and write through the REST API, add AI conversations to your UI, and let users revoke access at any time.

For developers

Standard OAuth. Plain JSON. No SDK required.

Register a public client, run the PKCE flow against the BigBooks authorization server, and call /v1 with the bearer token. The integrator guide and OpenAPI spec cover the rest.

terminal 
# 1. Trade the PKCE code for a token
curl -X POST https://www.bigbooks.app/oauth2/token \
  -d grant_type=authorization_code \
  -d client_id=$CLIENT_ID \
  -d code=$CODE -d code_verifier=$VERIFIER \
  -d redirect_uri=https://yourapp.example/callback

# 2. Call the API with the access token
curl https://api.bigbooks.app/v1/accounts \
  -H "Authorization: Bearer $ACCESS_TOKEN"

Integrations

Works with what you already use.

Bank data

Plaid CSV and PDF statements

AI models

Anthropic OpenAI Google Gemini DeepSeek

Platform

OAuth2 and OpenID Connect Cloud document storage

On the roadmap: MX, Akoya, Finicity, Yodlee and Amazon Bedrock.

Get started

Ship your finance app on books that balance.

Create a workspace, register an OAuth client and make your first API call.

Create account Read the docs