> ## Documentation Index
> Fetch the complete documentation index at: https://docs.usesynth.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Optimizers (GEPA)

> Run GEPA prompt optimization on your tasks from the Stack cockpit.

<Note>
  Accurate for Stack **`0.1.0`** and later unless a section cites a newer release.
  See [Stack Changelog](/stack/changelog).
</Note>

Stack runs **GEPA** prompt optimization locally and surfaces the loop in the cockpit,
so you can optimize a prompt against your own task and inspect every step as a receipt.

## What GEPA does

GEPA iteratively proposes and evaluates prompt variants against a task's training and
held-out sets, keeping changes that improve the score. Stack drives the optimizer
service, runs the rollouts, and records the result as a StackEval packet you can
replay.

## Run a built-in task

Stack ships StackEval GEPA tasks you can run end-to-end:

| Task                   | What it optimizes                          |
| ---------------------- | ------------------------------------------ |
| `banking77-local-gepa` | intent classification on the Banking77 set |
| `crafter-local-gepa`   | an agent policy prompt on Crafter          |

Prepare and launch a task from the cockpit harness:

```bash theme={null}
cd ~/Documents/GitHub/stack
./bin/stackeval harness prepare banking77-local-gepa --preset smoke
```

This brings up `stackd` and the Stack TUI, scaffolds a StackEval packet
(`initial_prompt.txt`, `metadata.json`, trace pointers), and starts the optimization
loop. Progress, candidate scores, and the final improved prompt are written to the
packet's trace directory and shown in the cockpit.

## Inspect the run

Every optimizer run is grounded in stackd receipts:

```bash theme={null}
./bin/stackeval harness status   --capture-pane -o "$STACKEVAL_PACKET/harness.debug.json"
./bin/stackeval harness export-thread --packet-dir "$STACKEVAL_PACKET" -o "$STACKEVAL_PACKET/harness.export.json"
```

The export is the auditable record of what the optimizer tried, what scored, and the
prompt it shipped.

## Hosted optimizers

The same loop can run on Synth's hosted optimizer service when you are signed in. See
the `hosted-gepa` Codex skill for connecting a Stack session to hosted runs.