---
title: "Start Experiment"
method: POST
path: "/sessions/{session_id}/experiments"
tags: ["experiments"]
---

# Start Experiment

`POST /sessions/{session_id}/experiments`

## Request

- The document declares no server URL.
- Auth: none declared

Start once per request ID; a busy server rejects instead of queueing work.

## Path parameters

- `session_id` string, required

## Request body

- StartExperiment: Validated search controls and a retry-safe caller identity.
  - `gepa_budget` 'light' | 'medium' | 'heavy', nullable
  - `max_metric_calls` integer, nullable
  - `threads` integer
  - `reflection_minibatch_size` integer
  - `max_candidate_proposals` integer, nullable
  - `validation_ratio` number
  - `trace_search` boolean
  - `request_id` string, required

## Response `202`

Successful Response

- object

## Other responses

- `422`: Validation Error

## Changes

- **2026-10-28** `76f3727ccdcf`: 2 warning
  - the `validation_ratio` request property's max was set to `0.50`
  - the `validation_ratio` request property's min was set to `0.10`
- **2026-10-28** `c248371b9401`: 1 info
  - endpoint added

[Operation history](https://skmtc.dev/qredence/apis/fleet-rlm/changes/sessions/:session_id/experiments/post.md)

---

[API](https://skmtc.dev/qredence/apis/fleet-rlm.md) - [All operations](https://skmtc.dev/qredence/apis/fleet-rlm/llms.txt) - [OpenAPI document](https://skmtc.dev/qredence/apis/fleet-rlm/revisions/a1192f9fece1?raw)