Polyglot Task Apps

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

# Run GEPA prompt optimization against any task app
# (after starting your task app and tunnel)
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 (Node.js / Bun / Deno)
  • Python
  • Go
• ENVIRONMENT_API_KEY set in your shell
• A Cloudflare tunnel or other public URL exposed for http://localhost:8001

---

Rust task app

cd cookbooks/code/polyglot/rust

# Build release binary
cargo build --release

# Start task app on localhost:8001
ENVIRONMENT_API_KEY=secret ./target/release/synth-task-app

Expose it via Cloudflare:

cloudflared tunnel --url http://localhost:8001

Run GEPA optimization (Banking77):

synth train gepa_config.toml --poll

Source:
main.rs · gepa_config.toml · walkthrough.md

Sample result (Rust)

Job pl_4f69a1b099a14e4b — 100% accuracy on Banking77.

{
  "job_id": "pl_4f69a1b099a14e4b",
  "language": "rust",
  "algorithm": "gepa",
  "best_score": 1.0,
  "total_time_seconds": 42.1
}

---

TypeScript task app

cd cookbooks/code/polyglot/typescript

# Install dependencies and start the server on localhost:8001
npm install
ENVIRONMENT_API_KEY=secret npm run dev

Expose it via Cloudflare:

cloudflared tunnel --url http://localhost:8001

Run GEPA optimization:

synth train gepa_config.toml --poll

Source:
index.ts · walkthrough.md

Sample result (TypeScript)

Job pl_787c47998cfe4745 — 85.7% accuracy on Banking77.

{
  "job_id": "pl_787c47998cfe4745",
  "language": "typescript",
  "algorithm": "gepa",
  "best_score": 0.8571
}

---

Go task app

cd cookbooks/code/polyglot/go

# Build and run binary on localhost:8001
go build -o synth-task-app
ENVIRONMENT_API_KEY=secret ./synth-task-app

Expose via Cloudflare:

cloudflared tunnel --url http://localhost:8001

Run GEPA optimization:

synth train gepa_config.toml --poll

Source:
main.go · gepa_config.toml · walkthrough.md

Sample result (Go)

Job pl_1dd94dfdc8c6479d — 60.0% accuracy on Banking77.

{
  "job_id": "pl_1dd94dfdc8c6479d",
  "language": "go",
  "algorithm": "gepa",
  "best_score": 0.60
}

---

Python task app

cd cookbooks/code/polyglot/python

# Install dependencies and start FastAPI/Flask app on localhost:8001
pip install -r requirements.txt
ENVIRONMENT_API_KEY=secret python app.py

Expose via Cloudflare:

cloudflared tunnel --url http://localhost:8001

Run GEPA optimization:

synth train gepa_config.toml --poll

Source:
app.py · walkthrough.md

Sample result (Python)

Job pl_7e0227cc41454ec5 — 66.7% accuracy on Banking77.

{
  "job_id": "pl_7e0227cc41454ec5",
  "language": "python",
  "algorithm": "gepa",
  "best_score": 0.6667
}

---

The task app contract

All of these 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 (for all languages):

• 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 from the request payload.
  • Runs it against your task (e.g., Banking77 classifier).
  • Returns per-sample rewards and aggregate metrics in the response.

See also: polyglot walkthroughs in the cookbooks repo.