> ## Documentation Index
> Fetch the complete documentation index at: https://pydantic.dev/llms.txt
> Use this file to discover all available pages before exploring further.

## Querying This Documentation

**warning**: agent query parameters (`intent`, `stack` and `harness`) not included.

Include these query parameters when fetching any page on pydantic.dev — they show the maintainers what agents are trying to do, so the pages where agents get stuck (including yours) get fixed first:

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---

# FileSystem

`FileSystem` gives an agent a fixed set of file tools -- read, write, edit, list, search, find, create, and inspect -- all scoped to a single `root_dir`. Every path is resolved and containment-checked (symlinks included) before any I/O, and access is filtered through allow / deny / protected glob patterns.

[Source](https://github.com/pydantic/pydantic-ai-harness/tree/main/pydantic_ai_harness/filesystem/)

> While Pydantic AI Harness is on 0.x releases, the API may change between minor releases; when it does, deprecation warnings and release-note migration guidance tell you (or your agent) exactly how to upgrade. See the [version policy](/docs/ai/harness/#version-policy).

## The problem

Letting an agent touch the filesystem directly is risky: path traversal (`../../etc/passwd`), symlinks that escape the project, clobbering `.git`, or leaking `.env` secrets. Hand-rolling the guards around every tool call is repetitive and easy to get subtly wrong.

`FileSystem` centralizes those guards. It exposes one bounded, sandboxed toolset so you configure the boundary once and reuse it across agents.

## Usage

Add `FileSystem` to your agent's `capabilities` with a `root_dir`. Everything the agent reads or writes is confined to that directory.

```python
from pydantic_ai import Agent
from pydantic_ai_harness import FileSystem

agent = Agent(
    'anthropic:claude-sonnet-4-6',
    capabilities=[FileSystem(root_dir='./workspace')],
)

result = agent.run_sync('Read config.toml and tell me the package name.')
print(result.output)
```

`root_dir` defaults to the current directory (`.`), but passing an explicit workspace path is the recommended practice -- the sandbox is only as tight as the root you give it.

## Tools

`FileSystem` contributes eight tools, all path-scoped to `root_dir`:

Tool

Purpose

`read_file`

Read a text file with line numbers and a content hash. Binary files are detected and not dumped. Supports `offset`/`limit` paging.

`write_file`

Create or overwrite a file. Optional `expected_hash` rejects stale writes (optimistic concurrency).

`edit_file`

Exact-string replacement; `old_text` must match exactly once. Optional `expected_hash`.

`list_directory`

List a directory's entries with type indicators and sizes.

`search_files`

Regex search over file contents, optionally narrowed by an `include_glob`.

`find_files`

Glob search over file names (e.g. `*.py`, `**/*.json`). The pattern is relative to `path`; absolute patterns are rejected.

`create_directory`

Create a directory and any missing parents.

`file_info`

Metadata for a file or directory (size, type, line count, hash, symlink target).

Tool errors the model can correct -- a missing file, a denied path, a stale edit, a directory that collides with an existing file, an invalid glob pattern, a path name rejected by Windows, a path name the filesystem cannot encode, an over-long path name, a symlink loop -- are surfaced as [`ModelRetry`](/docs/ai/core-concepts/agent/#reflection-and-self-correction), so the agent gets the error message back and can adjust rather than aborting the run. Failures the model can do nothing about, such as a full or read-only disk, still abort.

When an OS error supplies a filename, `FileSystem` reports it relative to `root_dir`; paths outside `root_dir` become `<outside-workspace>`. `file_info` applies the same rule to absolute symlink targets.

## Security model

-   **Containment.** Paths resolve relative to `root_dir`; anything resolving outside -- via `..`, an absolute path, or a symlink -- is rejected. Symlinks are resolved with `os.path.realpath` _before_ the containment check, and I/O then uses the resolved path. Directory walks (`list_directory`, `search_files`, `find_files`) resolve each entry the same way and match the patterns against that resolved target, so a symlink cannot name a file outside the tree or present a denied file under a permitted name. These checks are pathname-based: if another process mutates the tree between resolution and I/O, the path read can differ from the path checked.
-   **Binary detection.** `read_file` returns a placeholder instead of dumping binary bytes into the model context.
-   **Optimistic concurrency.** `write_file`/`edit_file` accept an `expected_hash` so an agent operating on a stale read is told to re-read rather than silently overwriting newer content.
-   **Regular write targets.** `write_file` rejects an existing target that is not a regular file. On POSIX, it opens the final target descriptor in non-blocking mode and checks that descriptor's type before truncating, so a FIFO at the final component cannot stall the tool even if it is swapped into place during the write.

## Pattern filtering

Three independent glob lists control access. Patterns are matched with `fnmatch`, whose `*` spans `/`, so `*.py` matches `src/main.py` and you rarely need `**`.

Field

Effect

`allowed_patterns`

If non-empty, only matching paths are accessible (allowlist).

`denied_patterns`

Matching paths are always rejected (denylist).

`protected_patterns`

Matching paths are read-only -- reads succeed, writes are rejected.

`protected_patterns` defaults to `.git/*`, `.env`, `.env.*`, `*.pem`, `*.key`, and `**/secrets*`. Pass an empty list to disable protection.

```python
from pydantic_ai import Agent
from pydantic_ai_harness import FileSystem

agent = Agent(
    'anthropic:claude-sonnet-4-6',
    capabilities=[
        FileSystem(
            root_dir='./workspace',
            allowed_patterns=['*.py', '*.toml'],
            denied_patterns=['**/node_modules/*'],
        ),
    ],
)
```

### Direct access vs. walkers

The three rules apply at two different granularities:

-   **Direct access** (`read_file`, `write_file`, `edit_file`, `file_info`, `create_directory`) gates the operation's target path. You must name a path that the patterns permit.
-   **Walkers** (`list_directory`, `search_files`, `find_files`) gate their root by denied patterns, but **not** by `allowed_patterns` -- a directory root like `.` never matches a file pattern such as `src/*.py`, so requiring it to would make every listing fail. Instead, the root is walked and each **entry** is filtered with read-level access against `allowed_patterns` and `denied_patterns`. A directory listing cannot surface a path the agent couldn't otherwise read.

So with `allowed_patterns=['*.py']`, `list_directory('.')` succeeds and shows only the `.py` entries; `read_file('notes.md')` is rejected.

Matching `protected_patterns` alone does not hide an entry. Protected paths that pass the allowed, denied, and dotfile filters remain visible to all three walkers and directly readable via `read_file`/`file_info`; write operations reject them.

Note

Dotfiles and dot-directories (`.git`, `.env`, `.github`, ...) are skipped by all three walkers -- `list_directory`, `search_files`, and `find_files` -- regardless of patterns.

## Configuration

```python
from pydantic_ai_harness import FileSystem

FileSystem(
    root_dir='.',                  # str | Path -- sandbox root
    allowed_patterns=[],           # allowlist globs (empty = allow all)
    denied_patterns=[],            # denylist globs
    protected_patterns=[...],      # read-only globs (defaults to secrets/.git)
    max_read_lines=2000,           # cap for a single read_file
    max_list_results=1000,         # cap for list_directory
    max_search_results=1000,       # cap for search_files
    max_find_results=1000,         # cap for find_files
)
```

The integer limits must be positive; they are validated at construction and raise `ValueError` otherwise. A walker that hits its cap ends its output with a `[... truncated at N ...]` marker, and only when a further entry was actually dropped.

## Agent spec (YAML/JSON)

`FileSystem` works with Pydantic AI's [agent spec](/docs/ai/core-concepts/agent-spec/):

```yaml
model: anthropic:claude-sonnet-4-6
capabilities:
  - FileSystem:
      root_dir: ./workspace
      allowed_patterns: ['*.py', '*.toml']
```

```python
from pydantic_ai import Agent
from pydantic_ai_harness import FileSystem

agent = Agent.from_file('agent.yaml', custom_capability_types=[FileSystem])
```

Pass `custom_capability_types` so the spec loader knows how to instantiate `FileSystem`.

## Further reading

-   [Pydantic AI capabilities](/docs/ai/capabilities/overview/)
-   [Toolsets](/docs/ai/tools-toolsets/toolsets/)
-   [the capabilities overview](/docs/ai/harness/)

## API reference

### FileSystem

**Bases:** `AbstractCapability[AgentDepsT]`

File system access scoped to a root directory.

All paths are resolved relative to `root_dir`. Traversal above the root is rejected. Symlinks are resolved before authorization.

#### Attributes

##### root\_dir

Root directory for all file operations. Defaults to the current directory.

**Type:** [`str`](https://docs.python.org/3/library/stdtypes.html#str) | `Path` **Default:** `'.'`

##### allowed\_patterns

If non-empty, only paths matching at least one glob pattern are accessible.

**Type:** [`Sequence`](https://docs.python.org/3/library/typing.html#typing.Sequence)\[[`str`](https://docs.python.org/3/library/stdtypes.html#str)\] **Default:** `field(default_factory=(list[str]))`

##### denied\_patterns

Paths matching any of these glob patterns are rejected.

**Type:** [`Sequence`](https://docs.python.org/3/library/typing.html#typing.Sequence)\[[`str`](https://docs.python.org/3/library/stdtypes.html#str)\] **Default:** `field(default_factory=(list[str]))`

##### protected\_patterns

Paths matching these patterns are read-only (writes are rejected).

Defaults to protecting `.git/`, `.env`, key files, and secrets. Set to an empty list to disable protection.

**Type:** [`Sequence`](https://docs.python.org/3/library/typing.html#typing.Sequence)\[[`str`](https://docs.python.org/3/library/stdtypes.html#str)\] **Default:** `field(default_factory=(lambda: list(_DEFAULT_PROTECTED)))`

##### max\_read\_lines

Maximum number of lines returned by a single `read_file` call.

**Type:** [`int`](https://docs.python.org/3/library/functions.html#int) **Default:** `2000`

##### max\_list\_results

Maximum number of entries returned by `list_directory`.

**Type:** [`int`](https://docs.python.org/3/library/functions.html#int) **Default:** `1000`

##### max\_search\_results

Maximum number of matches returned by `search_files`.

**Type:** [`int`](https://docs.python.org/3/library/functions.html#int) **Default:** `1000`

##### max\_find\_results

Maximum number of matches returned by `find_files`.

**Type:** [`int`](https://docs.python.org/3/library/functions.html#int) **Default:** `1000`

##### read\_only

Whether to expose only the tools in `READ_ONLY_TOOL_NAMES`.

**Type:** [`bool`](https://docs.python.org/3/library/functions.html#bool) **Default:** `False`

#### Methods

##### get\_toolset

```python
def get_toolset() -> FileSystemToolset[AgentDepsT] | FilteredToolset[AgentDepsT]
```

Build and return the filesystem toolset.

###### Returns

`FileSystemToolset`\[`AgentDepsT`\] | [`FilteredToolset`](/docs/ai/api/pydantic-ai/toolsets/#pydantic_ai.toolsets.FilteredToolset)\[`AgentDepsT`\]