Create a session.

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.