STL API overview - Symmetry Docs

The Symmetry Tax Logic (STL) API takes a completed payroll calculation and returns an AI-generated, plain-language explanation of how each tax was computed. The endpoint reference in this section is generated from the live OpenAPI specification.

Looking for concepts, integration steps, and sample clients? Start with the Symmetry Tax Logic API guide under AI tools. This section is the endpoint-level reference for the same service.

Base URL

The STL API is a hosted service that Symmetry runs for you.

Environment	Base URL
Production	`https://api.symmetry.com/stl`

Endpoints are versioned under /v1 (for example, https://api.symmetry.com/stl/v1/explain).

Authentication

All /v1/* endpoints require a Bearer token. You exchange your Symmetry API key for a short-lived access token, then send that token on each STL request:

Exchange your API key for a token

Call GET /authentication/login on the Symmetry API with your API key in the api-key header. The response includes an accessToken.

Call STL with the Bearer token

Send the token in the Authorization header as Bearer <accessToken> on every /v1/* request.

POST /v1/explain HTTP/1.1
Host: api.symmetry.com
Authorization: Bearer your_access_token_here
Content-Type: application/json
Accept: application/json

The STL API expects a Bearer token — unlike the STE Hosted Web API, which takes the raw API key as the Authorization value. Don’t reuse the same header format across the two services.

Endpoints

Method	Path	Purpose
`POST`	`/v1/explain`	Explain 1–10 taxes for a single paycheck, given a PayCalc context or STE session.
`POST`	`/v1/experimental/compare`	Compare two paychecks and return a narrative of the meaningful differences.
`GET`	`/v1/version`	Return the current STL API version.
`GET`	`/v1/health`, `/health`	Service health and readiness checks.

The full request/response schemas — including ExplainInput, ExplainOutput, and the PayCalcContext shape — are documented in the Endpoints pages in this section.

Try it with live Swagger

For interactive, authenticated calls against the running service, use the hosted Swagger UI.

STL Swagger

Live API explorer for the Symmetry Tax Logic API.

​Base URL

​Authentication

​Endpoints

​Try it with live Swagger

STL Swagger

Base URL

Authentication

Endpoints

Try it with live Swagger