One-line verdict: A genuine modern REST API for construction accounting — whose keys Foundation itself hands you, during an onboarding it runs. Openness: 3 / 5 — rated 3 because Foundation documents a real REST API with an OpenAPI spec, a published rate limit, and worked examples, but access is team-issued and review-gated, with most object-level writes still unverified.
Rung 1 — Direct connection, by invitation. Foundation’s own developer documentation describes the Foundation Accounting API as “programmatic access to Foundation’s Accounting Product… built on modern REST principles,” versioned by path, with downloadable OpenAPI (v3) specifications. This is a real API. What you cannot do is sign yourself up for it. The documentation is explicit: “The API Support team administrates these subscription keys by providing a primary and secondary key during onboarding,” spec access arrives during a “Commercial Evaluation phase,” the access you’re granted “may only be granted limited access” depending on the permissions the Foundation team grants, and an Integration Review is part of the onboarding process — including a review of your auth security.
Authentication is three layers deep, all documented: a subscription key (scoped, separate keys for sandbox and production), a bearer token through OpenID Connect (Azure AD B2C — client-credentials for background apps, authorization-code for user-facing ones; “all authentication methods require the user to log in to authorize an integration before changing FOUNDATION data”), and a connection GUID identifying which client’s books you’re touching, obtained from the Core Catalog with a client ID Foundation provides. Your redirect address must be registered with the API Support Team.
Verified at the worked-example level: Cost Codes — read, with a documented GET returning paged cost-code records from the job-costing API. The Accounts Payable Vendors API product and path are documented as the versioning example — the product exists; its individual create/update verbs were not verified in our pass and are flagged below. One limit is published with unusual honesty: 500 requests per minute, per customer — applied to the customer, not the integration — with rate-limit headers documented, and a warning that repeated limit violations may be treated as a security threat and the integration blocked.
Events and bridges: no webhooks are documented in the Foundation developer docs reviewed, and no Zapier or Make app was found. Foundation’s own integration ecosystem is its sister products (ProjectHQ, WorkMax, SafetyHQ, and kin) — vendor syncs, not bridge apps.
Hidden gate: Foundation holds the keys — literally: their support team has to onboard you and hand you a subscription key before the API answers at all, and they review your integration before it runs. Start that conversation first; every timeline hangs off it.
If your books live in FOUNDATION, this API is a professional tool with a professional front door: plan an onboarding, a review, and a scoped key — not a signup form. Contractors bringing in a real integration partner will find the documentation and the rate limits refreshingly straight. Contractors wanting a quick automation should connect at the edges (your CRM, your field system) and let Foundation stay the system of record it was bought to be.
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