---
title: "Create a session."
method: POST
path: "/v0/sessions"
tags: ["Workspaces"]
---

# Create a session.

`POST /v0/sessions`

## Request

- Base URL: `https://api.conductor.build` (the document declares no server; this is the origin it was published from)
- URL: `https://api.conductor.build/v0/sessions`
- Auth: HTTP bearer

Creates a session in an existing workspace and optionally sends its initial message. If the workspace is still initializing, Conductor queues the message and delivers it when the workspace is ready. When a message is included, the response's initialMessage contains its ID and whether it was sent or queued. To retry a create whose response was lost, pass the same sessionId and messageId; the message is deduplicated within that session. Accepted model ids by agent — claude: fable-5-1, fable-5, opus-5-5-1m, opus-5-1m, opus-4-8-1m, opus-4-8, opus-4-7-1m, opus-4-7, opus-4-6-1m, sonnet-5-5-1m, sonnet-5-1m, sonnet-4-6-1m, sonnet-4-6, haiku-5-5-1m, haiku-4-5; codex: gpt-5.5, gpt-5.4, gpt-5.6-sol, gpt-5.6-terra, gpt-5.6-luna, gpt-5.3-codex-spark, gpt-5.3-codex, gpt-5.2-codex, gpt-6-astra, gpt-6.1-sol, gpt-6-sol, gpt-6-luna, gpt-daybreak-blue-latest; cursor: auto, composer-2.5, grok-4.7, grok-4.6, grok-4.5. Accepted effort levels by agent — claude: low, medium, high, xhigh, max, ultracode; codex: none, low, medium, high, xhigh, max, ultra; cursor: low, medium, high, xhigh; codex max and ultra, and claude ultracode, require support from the selected model. Omit effort to use the agent's default — claude: high; codex: high. Models accepting fastMode by agent — claude: opus-5-5-1m, opus-5-1m, opus-4-8-1m, opus-4-8, opus-4-7-1m, opus-4-7, opus-4-6-1m; codex: gpt-5.5, gpt-5.4, gpt-5.6-sol, gpt-5.6-terra, gpt-5.6-luna, gpt-5.3-codex-spark, gpt-5.3-codex, gpt-5.2-codex, gpt-6-astra, gpt-6.1-sol, gpt-6-sol, gpt-6-luna, gpt-daybreak-blue-latest; cursor: auto, composer-2.5, grok-4.7, grok-4.6, grok-4.5. Omit fastMode (or send false) for other models.

## Query parameters

- `channel` 'prod' | 'alpha' | 'alpha-chromium' | 'beta' | 'patch' | 'dev' | 'funhouse'

## Request body

- object
  - `workspaceId` string, required
  - `sessionId` string
  - `name` string
  - `agent` union, required
    - 'claude'
    - 'codex'
    - 'cursor'
    - 'acp'
  - `model` union
    - 'fable-5-1'
    - 'fable-5'
    - 'opus-5-5-1m'
    - 'opus-5-1m'
    - 'opus-4-8-1m'
    - 'opus-4-8'
    - 'opus-4-7-1m'
    - 'opus-4-7'
    - 'opus-4-6-1m'
    - 'sonnet-5-5-1m'
    - 'sonnet-5-1m'
    - 'sonnet-4-6-1m'
    - 'sonnet-4-6'
    - 'haiku-5-5-1m'
    - 'haiku-4-5'
    - 'gpt-5.5'
    - 'gpt-5.4'
    - 'gpt-5.6-sol'
    - 'gpt-5.6-terra'
    - 'gpt-5.6-luna'
    - 'gpt-5.3-codex-spark'
    - 'gpt-5.3-codex'
    - 'gpt-5.2-codex'
    - 'gpt-6-astra'
    - 'gpt-6.1-sol'
    - 'gpt-6-sol'
    - 'gpt-6-luna'
    - 'gpt-daybreak-blue-latest'
    - 'auto'
    - 'composer-2.5'
    - 'grok-4.7'
    - 'grok-4.6'
    - 'grok-4.5'
  - `effort` union
    - 'none'
    - 'low'
    - 'medium'
    - 'high'
    - 'xhigh'
    - 'max'
    - 'ultra'
    - 'ultracode'
  - `fastMode` boolean
  - `messageId` string, uuid
  - `message` string

## Response `201`

Default Response

- object
  - `id` string, required
  - `deepLink` string, required
  - `name` string
  - `model` string
  - `resolvedModel` string
  - `effort` string
  - `fastMode` boolean
  - `archivedAt` string
  - `initialMessage` object
    - `messageId` string, required
    - `state` union, required
      - 'queued'
      - 'sent'
    - `deepLink` string, required

## Other responses

- `400`: Invalid request.
- `401`: Missing or invalid bearer token.
- `404`: Resource not found.
- `default`: Error response.

---

[API](https://skmtc.dev/conductor-build/apis/roundhouse-public-api.md) - [All operations](https://skmtc.dev/conductor-build/apis/roundhouse-public-api/llms.txt) - [OpenAPI document](https://skmtc.dev/conductor-build/apis/roundhouse-public-api/revisions/1f39c7f7f219?raw)