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Custom functions are Clay functions your team builds in the Clay UI, then runs from agents, the CLI, MCP, or the API.

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What custom functions are good for

  • Team-specific enrichment waterfalls.
  • Account scoring and qualification.
  • Inbound routing and territory logic.
  • CRM hygiene and validation.
  • Repeatable research workflows.
  • Internal tools that need the same Clay logic your GTM team already trusts.

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Runs and batch runs

The key distinction is where the function is built:
  • Build and manage the function in the Clay UI.
  • Run and batch-run it programmatically from agents, the CLI, MCP, or API.

Encode Clay functions into business systems

Call custom functions from backend services, queues, and internal tools.

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Expose a function

To call a custom function:
  1. Open Functions.
  2. Create or select a function.
  3. Open Details.
  4. Enable the integrations you need, such as API or MCP.
  5. Copy the t_... function id.
  6. Prefix it with function: when you call it from the plugin, CLI, MCP, or API.
Custom function routine ids use the format function:t_.... Each access setting enables a separate way to call the function: An agent can use either the CLI or MCP, so choose access based on its connection. Enabling one does not enable the others. Workspace and role permissions still apply.

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Access a published function through MCP

Publishing a function does not automatically enable MCP access. Enable MCP access in the function’s settings before calling it through an MCP client. API access alone is not sufficient. Call list_subroutines to find functions available to the connected MCP client. Use the returned id as subroutine_id when running a function through MCP.

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A published function is missing or cannot run by id

MCP access is required both to list a function and to run it by id. Supplying a known id does not bypass that requirement. If a function is missing from list_subroutines or a run returns a not-found error:
  1. Confirm the MCP client is connected to the workspace that contains the function.
  2. Check that MCP access is enabled in the function’s settings.
  3. If access cannot be changed, ask a workspace admin to check the integration and role access settings.
  4. Call list_subroutines again and use an id returned by that call.
A not-found error does not distinguish a nonexistent function from one unavailable to the connected client.