A client tool runs in the caller's browser or app via the widget or SDK, not on ElevenLabs' servers. The agent emits a call over the event stream and the customer's own code executes it. This is different from a webhook tool (ElevenLabs calls an HTTP endpoint). All API calls use xi-api-key: $API_KEY against https://api.elevenlabs.io.

1. Gather current state first

Before creating, make these reads (in parallel where independent):

GET /v1/convai/agents/$AGENT_ID?branch_id=$BRANCH_ID to confirm the agent, its LLM, and whether it is a workflow agent (workflow agents scope tools per node via additional_tool_ids, so where you attach the new tool matters; the workflow is in this config).
GET /v1/convai/tools to avoid creating a duplicate and to reuse an existing client tool if one fits.

2. Create the tool

POST /v1/convai/tools

with the tool config discriminated by the client type. The fields that cause failures:

architect-create-client-tool

1. Gather current state first

2. Create the tool