Polyglot Task Apps

Prompt optimization with task apps in Rust, Go, TypeScript, and Python

# Run GEPA prompt optimization against any task app
synth train gepa_config.toml --poll

Task apps can be written in any language as long as they implement the Synth task app contract (/health and /rollout).

Results Summary (Banking77, GEPA)

Language	Job ID	Accuracy	Source
Rust	pl_4f69a1b099a14e4b	100%	main.rs
TypeScript	pl_787c47998cfe4745	85.7%	index.ts
Python	pl_7e0227cc41454ec5	66.7%	app.py
Go	pl_1dd94dfdc8c6479d	60.0%	main.go

All jobs optimize prompts for the same Banking77 task app; only the implementation language changes.

Prerequisites

• Synth CLI installed (synth on your PATH)
• Task app contract implemented in one of: Rust, TypeScript, Python, Go
• ENVIRONMENT_API_KEY set in your shell
• A Cloudflare tunnel or other public URL exposed for http://localhost:8001

The Task App Contract

All task apps implement the same HTTP contract:

• GET /health — basic health check used by Synth to discover and monitor the app
• POST /rollout — evaluate a batch of prompt candidates and return rewards/metrics

OpenAPI spec:

• GitHub: task_app.yaml
• Raw: https://raw.githubusercontent.com/synth-laboratories/synth-ai/main/synth_ai/contracts/task_app.yaml

Key rules:

• ENVIRONMENT_API_KEY must match the X-API-Key header on /rollout requests
• Your /rollout handler reads the prompt candidate, runs it against your task, and returns rewards

See also: polyglot walkthroughs in the cookbooks repo