---
title: "Permissions"
description: "Controlling which tools agents can use"
---

Permissions control which tools an agent can use without human approval. Fabro uses a three-tier permission model that balances agent autonomy with safety — read-only agents can explore the codebase freely, read-write agents can also edit files, and full-access agents can run arbitrary shell commands.

## Permission levels

| Level | Auto-approved tools | Prompted tools |
|---|---|---|
| `read-only` | `read_file`, `read_many_files`, `grep`, `glob`, `list_dir`, sub-agent tools | Everything else requires approval |
| `read-write` | All read tools + `write_file`, `edit_file`, `apply_patch`, sub-agent tools | `shell`, `web_search`, `web_fetch`, MCP tools |
| `full` | All tools including `shell`, `web_search`, `web_fetch`, MCP tools | None |

The default permission level is `read-write`.

## Tool categories

Every tool is classified into one of four categories that determine which permission level auto-approves it:

| Category | Tools | Minimum level |
|---|---|---|
| read | `read_file`, `read_many_files`, `grep`, `glob`, `list_dir` | `read-only` |
| write | `write_file`, `edit_file`, `apply_patch` | `read-write` |
| subagent | `spawn_agent`, `send_input`, `wait`, `close_agent` | `read-only` |
| shell | `shell`, `web_search`, `web_fetch`, MCP tools, unknown tools | `full` |

Unknown tools (including MCP tools) default to the `shell` category, requiring `full` permission for auto-approval.

## Setting permissions

### CLI flag

```bash
fabro exec --permissions read-only
fabro exec --permissions read-write
fabro exec --permissions full
```

### Non-interactive mode

When running non-interactively (no TTY or `--auto-approve` flag), tools outside the permission level are denied outright:

```bash
# In CI: only allow read and write tools, deny shell
fabro exec --permissions read-write --auto-approve
```

### Interactive escalation

In interactive mode, when an agent calls a tool outside its current permission level, Fabro prompts the user:

```
Agent wants to run: shell(command="cargo test")
Allow? [y]es / [a]lways / [n]o:
```

- **yes** -- allow this one call
- **always** -- upgrade the permission level for the rest of the session
- **no** -- deny the call (the agent receives an error and can try a different approach)

Choosing "always" on a write tool upgrades to `read-write`. Choosing "always" on a shell tool upgrades to `full`.

## How permissions interact with tools

### Read tools are always available

Regardless of permission level, agents can always read files, search with grep, and find files with glob. This means even a `read-only` agent can fully explore a codebase.

### Write tools require read-write or higher

The `write_file`, `edit_file`, and `apply_patch` tools are auto-approved at `read-write` and above. At `read-only`, these tools are either prompted (interactive) or denied (non-interactive).

### Shell requires full

The `shell` tool and web tools (`web_search`, `web_fetch`) are only auto-approved at the `full` level. This is the most permissive setting since shell commands can do anything the sandbox allows.

### Sub-agent tools are always allowed

Sub-agent management tools (`spawn_agent`, `send_input`, `wait`, `close_agent`) are auto-approved at all permission levels. Sub-agents inherit the parent's permission level, so a `read-only` parent spawns `read-only` children.

## Read-before-write guardrail

Independent of the permission system, Fabro enforces a **read-before-write guardrail** that applies to all permission levels. Even with `full` permissions, an agent must read an existing file before modifying or deleting it. See [Tools: Read-before-write guardrail](/agents/tools#read-before-write-guardrail) for details.

This guardrail operates at the sandbox layer and works alongside (not instead of) the permission system. A tool call must pass **both** checks:

1. The tool's category must be allowed by the current permission level
2. If writing to an existing file, the file must have been previously read

## Permission flow

The following sequence shows how Fabro decides whether to execute a tool call:

<img src="/images/permission-flow.svg" alt="Permission flow: tool call is checked against current permission level; if auto-approved it executes, otherwise in interactive mode the user is prompted, in non-interactive mode the tool is denied" />

## Workflow-level permissions

When agents run inside Fabro workflows, the workflow engine manages tool approval. Workflow agents typically run with full auto-approval since the workflow itself defines the trust boundary -- the sandbox provider (local, Docker, or Daytona) controls what the agent can actually access.

For CLI-backed workflow agents, each provider handles auto-approval differently:
- **Anthropic** (Claude Code) -- uses `--dangerously-skip-permissions` to bypass interactive prompts
- **OpenAI** (Codex) -- uses `--full-auto` mode
- **Gemini** -- uses `--yolo` flag

## Further reading

<Columns cols={2}>
  <Card title="Tools" icon="wrench" href="/agents/tools">
    Full reference for built-in tools and their parameters.
  </Card>
  <Card title="Environments" icon="server" href="/execution/environments">
    Sandbox providers that control execution isolation.
  </Card>
  <Card title="MCP" icon="plug" href="/agents/mcp">
    MCP tools follow the same permission model as built-in tools.
  </Card>
  <Card title="Sub-agents" icon="users" href="/agents/subagents">
    Sub-agents inherit their parent's permission level.
  </Card>
</Columns>
