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Tool: Subagent

Source: R/tool-agent-subagent.R
btw_tool_agent_subagent.Rd

btw_tool_agent_subagent() is a btw tool that enables hierarchical agent workflows. When used by an LLM assistant (like btw_app(), btw_client(), or third-party tools like Claude Code), this tool allows the orchestrating agent to delegate complex tasks to specialized subagents, each with their own isolated conversation thread and tool access.

This function is primarily intended to be called by LLM assistants via tool use, not directly by end users.

How Subagents Work

When an LLM calls this tool:

  1. A new chat session is created (or an existing one is resumed)

  2. The subagent receives the prompt and begins working with only the tools specified in the tools parameter

  3. The subagent works independently, making tool calls until it completes the task

  4. The function returns the subagent's final message text and a session_id

  5. The orchestrating agent can resume the session later by providing the session_id

Each subagent maintains its own conversation context, separate from the orchestrating agent's context. Subagent sessions persist for the duration of the R session.

Tool Access

The orchestrating agent must specify which tools the subagent can use via the tools parameter. The subagent is restricted to only these tools - it cannot access tools from the parent session. Tools can be specified by:

  • Specific tool names: c("btw_tool_files_read_text_file", "btw_tool_files_write_text_file")

  • Tool groups: "files" includes all file-related tools

  • NULL (default): Uses the default tool set from options or btw_tools()

Configuration Options

Subagent behavior can be configured via R options:

  • btw.subagent.client: The ellmer::Chat client or provider/model string to use for subagents. If not set, falls back to btw.client, then to the default Anthropic client.

  • btw.subagent.tools_default: Default tools to provide to subagents when the orchestrating agent doesn't specify tools via the tools parameter. If not set, falls back to btw.tools, then all btw tools from btw_tools(). This is a convenience option for setting reasonable defaults.

  • btw.subagent.tools_allowed: An allowlist of tools that subagents are allowed to use at all. When set, any tools requested (either explicitly via the tools parameter or from defaults) will be filtered against this list. If disallowed tools are requested, an error is thrown. This provides a security boundary to restrict subagent capabilities. If not set, all btw_tools() are allowed.

These options follow the precedence: function argument > btw.subagent.* option > btw.* option > default value. The tools_allowed option acts as a filter on top of the resolved tools, regardless of their source.

Usage

btw_tool_agent_subagent(
  prompt,
  tools = NULL,
  session_id = NULL,
  `_intent` = ""
)

Arguments

prompt

Character string with the task description for the subagent. The subagent will work on this task using only the tools specified in tools. The subagent does not have access to the orchestrating agent's conversation history.

tools

Optional character vector of tool names or tool groups that the subagent is allowed to use. Can be specific tool names (e.g., "btw_tool_files_read_text_file"), tool group names (e.g., "files"), or NULL to use the default tools from btw.subagent.tools_default, btw.tools, or btw_tools().

session_id

Optional character string with a session ID from a previous call. When provided, resumes the existing subagent conversation instead of starting a new one. Session IDs are returned in the result and have the format "adjective_noun" (e.g., "swift_falcon").

_intent

Optional string describing the intent of the tool call. Added automatically by the ellmer framework when tools are called by LLMs.

Value

A BtwSubagentResult object (inherits from BtwToolResult) with:

  • value: The final message text from the subagent

  • session_id: The session identifier for resuming this conversation

See also

btw_tools() for available tools and tool groups

Examples

# This tool is typically called by LLMs via tool use, not directly.
# The examples below show how to configure subagent behavior.

# Configure the client and default tools for subagents
withr::with_options(
  list(
    btw.subagent.client = "anthropic/claude-sonnet-4-20250514",
    btw.subagent.tools_default = "files"
  ),
  {
    getOption("btw.subagent.client")
  }
)
#> [1] "anthropic/claude-sonnet-4-20250514"

# Restrict subagents to only certain tools
withr::with_options(
  list(
    btw.subagent.tools_allowed = c("files", "docs"),
    btw.subagent.tools_default = "files"
  ),
  {
    getOption("btw.subagent.tools_allowed")
  }
)
#> [1] "files" "docs"