Built-in Tools

14 tools are available by default when running without a config file (env-based mode). With a config file, built-in tools are added alongside any MCP tools defined in mcp_servers.

Tool Reference

Tool	Description
`bash`	Execute bash commands. Working directory persists between calls. Default timeout: 120s, max: 600s.
`read`	Read a file with line numbers. Detects binary files. Max size: 256 KB.
`write`	Write content to a file. Creates parent directories. Read-before-write guard.
`edit`	Replace an exact string in a file (must appear exactly once). Read-before-write guard.
`patch`	Apply unified diff patches to one or more files. Single-pass hunk application.
`glob`	Find files matching a glob pattern. Skips hidden files.
`grep`	Search file contents with regex. Uses `rg` when available, falls back to built-in.
`list`	List directory contents as an indented tree. Skips common build artifacts.
`webfetch`	Fetch content from a URL via HTTP GET. Supports text, markdown, HTML. Max: 5 MB.
`websearch`	Search the web via Exa AI. Requires `EXA_API_KEY`.
`todowrite`	Write/replace the full todo list. Only 1 item in progress at a time.
`todoread`	Read the current todo list.
`skill`	Load skill definitions from `SKILL.md` files.
`question`	Ask the user structured questions (only available when `on_question` callback is set).

Cross-Agent Coordination

Blackboard

The Blackboard is a shared Key -> Value store for squad agents (those dispatched via form_squad). Sub-agents receive three additional tools:

Tool	Description
`blackboard_read`	Read a value by key from the shared blackboard
`blackboard_write`	Write a key-value pair to the shared blackboard
`blackboard_list`	List all keys currently in the blackboard

After each sub-agent completes, its result is automatically written to the blackboard under the key "agent:{name}".

Delegation Tools

The orchestrator agent has two tools for dispatching work to sub-agents:

Tool	Description
`delegate_task`	Dispatch independent parallel subtasks to sub-agents
`form_squad`	Dispatch collaborative subtasks that share a Blackboard

Structured Output

Set response_schema in config (or .structured_schema() on the builder) to constrain an agent’s output format. When configured:

A synthetic __respond__ tool is injected into the agent’s tool set
The agent calls __respond__ to produce structured JSON
The result is available in AgentOutput::structured

This works in both standalone and Restate execution paths.

Human-in-the-Loop (HITL)

The --approve flag enables interactive approval before each tool execution round:

The CLI prompts the user before tool calls are executed
Denied tools receive error results so the LLM can adjust and retry
In the Restate path, approval uses per-turn promise keys for durable waiting

Streaming

The on_text callback receives text deltas as they arrive from the LLM. Both Anthropic and OpenRouter providers implement SSE streaming. Sub-agents do not stream — only the orchestrator or top-level agent streams text.

Agent Events

Structured AgentEvent variants are emitted via the OnEvent callback:

Event	Description
`RunStarted`	Agent run has begun
`TurnStarted`	A new reasoning turn started
`LlmResponse`	LLM returned a response
`ToolCallStarted`	A tool call is about to execute
`ToolCallCompleted`	A tool call finished
`ApprovalRequested`	Waiting for HITL approval
`ApprovalDecision`	HITL approval/denial received
`SubAgentsDispatched`	Orchestrator dispatched sub-agents
`SubAgentCompleted`	A sub-agent finished its task
`ContextSummarized`	Context was compacted via summarization
`RunCompleted`	Agent run completed successfully
`GuardrailDenied`	A guardrail blocked the action
`GuardrailWarned`	A guardrail issued a warning
`RunFailed`	Agent run failed with an error
`RetryAttempt`	Provider retrying after transient error
`DoomLoopDetected`	Identical tool calls detected across turns
`SessionPruned`	Old tool results were pruned
`AutoCompactionTriggered`	Context overflow triggered auto-compaction
`ModelEscalated`	Cascade provider escalated to a higher tier
`BudgetExceeded`	Token budget exhausted

Use --verbose / -v to emit events as JSON to stderr.

Cost Tracking

estimate_cost(model, usage) -> Option<f64> returns estimated USD cost for known models (Claude 4, 3.5, and 3 generations, including OpenRouter aliases). The estimate accounts for cache read/write token rates. Cost is displayed in CLI output after each run.

Custom Tools

Implement the Tool trait and register with an agent:

use heartbit::{Tool, ToolDefinition, ToolOutput, Error};
use serde_json::Value;
use std::pin::Pin;
use std::future::Future;

pub struct MyTool;

impl Tool for MyTool {
    fn definition(&self) -> ToolDefinition {
        ToolDefinition {
            name: "my_tool".into(),
            description: "Does something useful".into(),
            input_schema: serde_json::json!({
                "type": "object",
                "properties": {
                    "input": { "type": "string", "description": "The input value" }
                },
                "required": ["input"]
            }),
        }
    }

    fn execute(
        &self,
        input: Value,
    ) -> Pin<Box<dyn Future<Output = Result<ToolOutput, Error>> + Send + '_>> {
        Box::pin(async move {
            let input_str = input["input"].as_str().unwrap_or_default();
            Ok(ToolOutput::success(format!("Processed: {input_str}")))
        })
    }
}

// Register with an agent:
// AgentRunner::builder(provider).tools(vec![Arc::new(MyTool)]).build()