Tool Creation

Agent objective

Produce a ToolDefinition object + handler, registered with the builder via .withTools({ tools: [...] }). Every field in the definition must be present and typed correctly.

When to load this skill

• Adding a custom tool (API call, database query, file operation, etc.)
• Restricting which built-in tools an agent can use
• Forcing the agent to call a specific tool before completing
• Setting per-tool approval requirements or risk levels

Implementation baseline

import { ReactiveAgents } from "@reactive-agents/runtime";
import { tool } from "@reactive-agents/tools";  // simple factory — no Effect knowledge needed

const lookupUser = tool("lookup-user", "Look up a user record by ID", async (args) => {
  const { userId } = args as { userId: string };
  const user = await db.findUser(userId);
  return { id: user.id, name: user.name, email: user.email };
});

const agent = await ReactiveAgents.create()
  .withProvider("anthropic")
  .withReasoning({ defaultStrategy: "adaptive" })
  .withTools({
    tools: [lookupUser],
    allowedTools: ["lookup-user", "checkpoint"],  // restrict to only these
  })
  .withRequiredTools({ tools: ["lookup-user"], maxRetries: 2 })
  .build();

Key patterns

Two tool factories

// Factory 1: tool() — simple async, no Effect knowledge needed
import { tool } from "@reactive-agents/tools";

const myTool = tool("my-tool", "Description of what the tool does", async (args) => {
  const { param } = args as { param: string };
  return { result: await doSomething(param) };
});

// Factory 2: defineTool() — Effect-native, Schema-inferred typed args
import { defineTool } from "@reactive-agents/tools";
import { Schema } from "effect";

const myTool = defineTool({
  name: "my-tool",
  description: "Description of what the tool does",
  parameters: Schema.Struct({ param: Schema.String }),
  handler: ({ param }) =>
    Effect.tryPromise(() => doSomething(param)),
});

Full ToolDefinition shape (when using low-level API)

import type { ToolDefinition } from "@reactive-agents/tools";

const myTool: ToolDefinition = {
  name: "send-email",          // unique identifier — no spaces
  description: "Send an email to a recipient",
  parameters: [
    { name: "to", type: "string", description: "Recipient email", required: true },
    { name: "subject", type: "string", description: "Email subject", required: true },
    { name: "body", type: "string", description: "Email body text", required: false },
  ],
  returnType: "object",        // "string" | "object" | "array" | "boolean" | "number"
  riskLevel: "medium",         // "low" | "medium" | "high" | "critical"
  timeoutMs: 10_000,           // default varies — set explicitly for network tools
  requiresApproval: false,     // true = agent pauses for human approval before calling
  source: "function",          // "function" for custom tools (not "builtin" or "mcp")
  category: "http",            // "search"|"file"|"code"|"http"|"data"|"custom"|"system"
  isCacheable: false,          // optional — enable for idempotent tools
  cacheTtlMs: 60_000,          // optional — cache duration
};

Built-in tools available

Enable selectively with allowedTools:

Standard tools (enabled by .withTools() with no args): web-search, http-get, file-read, file-write, code-execute

Meta-tools (registered by kernel, available in .withTools()): checkpoint, recall, find, brief, pulse, context-status, final-answer

Opt-in tools (not auto-enabled — must be registered manually): shell-execute — see shell-execution-sandbox skill for registration pattern

// Only allow read-only tools
.withTools({ allowedTools: ["web-search", "http-get", "file-read", "checkpoint"] })

// Add custom tools alongside built-ins
.withTools({
  tools: [myCustomTool],
  allowedTools: ["web-search", "my-custom-tool"],
})

Requiring tool calls before completion

// Agent must call web-search before final-answer is accepted
.withRequiredTools({ tools: ["web-search"], maxRetries: 3 })

// Framework decides which tools are required based on task content
.withRequiredTools({ adaptive: true })

Builder API reference

Method	Key params	Notes
.withTools(opts?)	{ tools?, allowedTools?, adaptive?, resultCompression? }	No args = all built-ins
.withRequiredTools(cfg)	{ tools?: string[], adaptive?: boolean, maxRetries?: number }	Forces tool calls

Pitfalls

• Tool name must be unique — registering a tool with a name that matches a built-in silently overrides the built-in
• timeoutMs default is often too short for network/API tools — always set it explicitly
• withRequiredTools without withTools does nothing — tools must be enabled first
• source: "builtin" is reserved for internal tools; use "function" for all custom tools
• defineTool() handlers must return Effect<unknown, ToolExecutionError> — wrap non-Effect async code with Effect.tryPromise()
• requiresApproval: true on a tool pauses execution until a human responds — requires an approval callback configured elsewhere