Harvest

Harvest combines time tracking with invoicing in a single platform, which makes it particularly valuable for service businesses that need to connect tracked time directly to client billing. The API covers the full workflow from time entry through invoice generation and expense management — enabling agents to automate the time-to-invoice pipeline without manual data transfer between separate systems. This combined scope differentiates Harvest from pure time tracking tools.

Authentication uses OAuth 2.0 for user-delegated access and Personal Access Tokens for server-side automation. The API requires both an access token and an Account ID header — teams new to Harvest's API should note this two-part auth requirement. Personal Access Tokens are available from Harvest account settings and provide simpler server-to-server auth without OAuth flow.

Documentation is thorough and well-organized. The v2 API reference is complete with examples for all endpoints. The authentication documentation explains the Account ID requirement clearly. Teams integrating Harvest for billing automation will find the docs cover the full workflow including the invoice creation patterns needed for end-to-end billing.

The API covers time entries, projects, tasks, clients, users, invoices, expenses, and estimates. The invoice creation API is the distinctive capability — agents can generate invoices from tracked time programmatically, applying rates and bill rates defined in Harvest to produce accurate client bills without manual assembly. Combined with time entry creation, this enables end-to-end billing automation for service-based workflows.

Reliability is appropriate for a billing-adjacent service. Invoice operations that generate financial records should use idempotent request patterns where possible. Harvest's API has been stable and consistent, reflecting the product's long track record. Teams building billing automation should implement confirmation workflows — verifying invoice creation rather than assuming API success means the document is ready.