---
title: "Tools"
description: "Define typed actions the agent can call, and gate sensitive ones on human approval."
url: /tools
---

A tool is a typed action the agent can call, such as hitting an API, running a query, or writing a file. The action stays in code you control. Tools run in your app runtime with full access to `process.env`, not in the [sandbox](/docs/sandbox).

## Define a tool

The filename is the tool name the model sees. A file at `agent/tools/get_weather.ts` is exposed as `get_weather`.

```ts title="agent/tools/get_weather.ts"
import { defineTool } from "eve/tools";
import { z } from "zod";

export default defineTool({
  description: "Get the current weather for a city.",
  inputSchema: z.object({ city: z.string().min(1) }),
  async execute({ city }, ctx) {
    return { city, condition: "Sunny", temperatureF: 72 };
  },
});
```

A tool definition needs:

- a filename slug under `agent/tools/`, the model-facing name.
- a `description`: what the tool does, written for the model.
- an `inputSchema`: a Zod schema (or any Standard Schema, or a plain JSON Schema object). Required. For no input, pass `z.object({})`. Zod and Standard Schema infer the `input` type in `execute`. Plain JSON Schema types it as `Record<string, unknown>`.
- an `execute(input, ctx)`: the implementation. May be sync or async.

When a tool returns structured data, add an optional `outputSchema`. With Zod or Standard Schema it also types the `execute` return.

### The `ctx` parameter

`execute` gets a `ctx` carrying the runtime accessors:

- `ctx.session`: session metadata, turn, auth, parent lineage.
- `ctx.getSandbox()`: the live [sandbox](/docs/sandbox) handle.
- `ctx.getSkill(id)`: read a packaged [skill](/docs/skills)'s metadata and files.

Running in the app runtime is what lets a tool import shared code from `lib/`, read `process.env`, and take part in eve’s durable pause/resume model.

eve never runs authored tools during discovery. The model sees descriptors first, and only what it actually calls gets executed. Completed steps never re-run; eve replays the recorded result. A step interrupted mid-execution re-runs, so make non-idempotent side effects like charges or emails idempotent, or gate them with approval.

## Gate a tool on human approval

A tool can require a person to sign off before it runs. Set `approval` with the helpers from `eve/tools/approval`:

```ts title="agent/tools/refund_charge.ts"
import { defineTool } from "eve/tools";
import { always } from "eve/tools/approval";
import { z } from "zod";

export default defineTool({
  description: "Refund a charge.",
  inputSchema: z.object({ chargeId: z.string(), amount: z.number() }),
  approval: always(), // or once() / never() / a policy
  async execute(input) {
    return refund(input);
  },
});
```

Approval is one half of eve's [human-in-the-loop](./human-in-the-loop) model — the page covers the `always/once/never` helpers, input-dependent policies, and how a gated call pauses and resumes durably.

## Shape what the model sees with `toModelOutput`

By default the model sees the full `execute` return. When a tool returns rich data a channel needs for rendering but the model only needs the gist, project it down with `toModelOutput`:

```ts
toModelOutput(output) {
  return { type: "text", value: `Report for ${output.domain}: score ${output.score}.` };
},
```

`toModelOutput` receives the full, typed `execute` return and only affects the model. Channel event handlers and hooks still get the full output on `action.result`, so a channel can render rich platform output (Slack Block Kit, say) the model never sees. Return `{ type: "text", value }` for a summary, or `{ type: "json", value }` for a smaller object.

Tool outputs must be JSON-serializable. Return plain objects, arrays, strings, numbers, booleans, or `null`; convert values like `Date`, `Map`, `Set`, `NaN`, and cyclic objects before returning them from `execute` or from a `{ type: "json" }` `toModelOutput`.

Do not return secrets, credentials, unnecessary personal data, or unbounded sensitive content from tools. Filter, minimize, and redact tool outputs before returning them.

## What to read next

- [Human-in-the-loop](./human-in-the-loop): gate a tool on approval, or have the agent ask a question
- [Skills](/docs/skills): on-demand procedures the model loads when relevant
- [Default harness](/docs/concepts/default-harness): the built-in tools and how to override or disable them
- [Dynamic capabilities](/docs/guides/dynamic-capabilities): tools whose set is resolved per session with `defineDynamic`
- [Auth & route protection](/docs/guides/auth-and-route-protection): authenticate a tool to an external service
