System Tools

Built-in agent capabilities like hanging up the call, enabled per agent

System tools are capabilities built into the platform. There is nothing to create or host. Each agent simply enables the ones it needs.

Tool	Key	What it does
Hang up call	hang_up_call	Lets the agent end the call itself.

Unlike webhook tools, system tools are not workspace resources shared across agents. The on/off state is stored per agent.

Toggle in the Builder

Open your agent’s Tools page in the Builder. The System tools card holds the Hang up call switch.

Toggle via the API

Over the REST API, system tool switches are part of the agent’s configuration, in the same tools section that holds its attached webhook tools. See Configuration for reading and updating an agent’s config.

Farewell

The agent hangs up by calling the built-in _end_call tool, and the call ends once the agent finishes speaking. Choose what callers hear before that:

Mode	Behavior
Generated (default)	The agent writes a short goodbye into the tool call’s message, and the caller hears it.
Fixed message	The agent ends with the exact text you provide.
Off	The call ends as soon as the agent finishes its current reply, with no extra goodbye.

In every mode, anything the agent says in the same reply as the _end_call call is spoken first. In Generated mode, message is said only when that reply says nothing, so callers never hear two goodbyes.

In the Builder, these options appear as Farewell under Hang up call once the switch is on. Over the API, the mode is tools.system_tools.hang_up_call_farewell_mode (generated, fixed, or off) and the fixed text is tools.system_tools.hang_up_call_farewell_message (up to 1,000 characters). The message accepts the same {{placeholders}} as the system prompt (see Dynamic variables). A fixed mode with an empty message falls back to Generated.

curl --request PATCH "https://api.fish.audio/v1/agent/agents/$AGENT_ID/config" \
  --header "Authorization: Bearer $FISH_API_KEY" \
  --header "Content-Type: application/json" \
  --data '{
    "tools": {
      "system_tools": {
        "hang_up_call": true,
        "hang_up_call_farewell_mode": "fixed",
        "hang_up_call_farewell_message": "Thanks for calling Acme. Goodbye!"
      }
    }
  }'

Questions before hanging up

Turn on Don’t hang up on a question to stop the agent from hanging up on a question. With it on, if the message the caller would hear contains a question mark, for example “Is there anything else I can help you with?”, the _end_call call fails and the call stays open. The agent asks the question in its reply, waits for the answer, and calls _end_call again after that.

The switch is under Hang up call on the Tools page and appears when Farewell is Generated. Over the API, set tools.system_tools.hang_up_call_refuse_questions to true:

curl --request PATCH "https://api.fish.audio/v1/agent/agents/$AGENT_ID/config" \
  --header "Authorization: Bearer $FISH_API_KEY" \
  --header "Content-Type: application/json" \
  --data '{ "tools": { "system_tools": { "hang_up_call_refuse_questions": true } } }'

The switch has no effect with a Fixed message or Off. Agent tests and simulations follow the same setting.

Transfer call

_transfer_call has no switch of its own: it is enabled automatically once the agent has a transfer destination configured, and only on phone sessions (web sessions have no call to transfer). See Transfers for configuring destinations and everything else about transfers.

Going further

Tools overview

How webhook, client, and system tools fit together.

Webhook tools

Call your own HTTP endpoints from a conversation.

Client tools

Run tool calls inside your own app via the SDK.

Voice & language

Configure the agent’s voice and speaking language.