Capability tools: filesystem & shell

The Signals & Sorcery agent surface includes a small set of capability tools that reach outside the music app: they read and write files on your machine and run external commands like ffmpeg. They're how the in-app chat-plugin agent fulfils requests like:

"What files are in my ~/Music folder?"
"Make a recording checklist at ~/Desktop/checklist.md with these items."
"Convert this WAV to MP3 with ffmpeg."
"Install ffmpeg."

Every capability tool is gated by a per-call consent dialog. The agent never accesses your filesystem or runs a shell command without you explicitly approving each operation in a modal.

When the agent calls a capability tool, you see a native dialog showing:

What the agent wants to do ("Read directory", "Write file", "Run shell command").
A short reason the agent provided.
The exact details: full path, args list, byte counts, timeout, cwd. Args are shown as a list (["-i", "in.wav", "out.mp3"]), never concatenated into a shell string.

You click Allow or Cancel. Cancel returns a structured failure to the agent; it can ask you a different way or move on.

No persistent allowlist in v1. Every call prompts every time. We may add an "always allow" toggle later based on real-world friction; for now, simplicity wins.

Tools

`fs_list_directory`

List a directory's contents.

sas fs list --path ~/Music
sas run fs_list_directory -p path=~/Documents -p depth=2

Input	Notes
`path`	Required. `~` and `~/` expand to your home directory.
`depth`	Optional integer 1–3, default 1.
`maxEntries`	Optional cap, default 1000.

Returns { resolvedPath, entries: [{ name, type, size?, modifiedMs? }], truncated }.

`fs_read_file`

Read a text file.

sas fs read --path ~/Desktop/notes.md
sas run fs_read_file -p path=~/notes.md -p maxBytes=1000000

Input	Notes
`path`	Required. Tilde-expanded.
`maxBytes`	Optional cap, default 1 MB. Files larger than this are rejected without prompting.

Returns { resolvedPath, size, content }.

`fs_search`

Recursive name/glob search.

sas fs search --root-path ~/Music --name-pattern "*.wav"
sas run fs_search -p rootPath=~/Documents -p namePattern=todo

Input	Notes
`rootPath`	Required. Tilde-expanded.
`namePattern`	Required. Supports `*` and `?` globs; otherwise case-insensitive substring.
`extensions`	Optional `["wav","mp3"]` allowlist.
`maxResults`	Optional cap, default 100.

Returns { resolvedRoot, matches: [{ path, size, modifiedMs }], truncated }.

`fs_write_file`

Create, overwrite, or append a text file. Atomic write: produces a .tmp sibling then renames.

sas fs write --path ~/Desktop/todo.md --content "..."
sas run fs_write_file -p path=~/notes.txt -p content="hello" -p mode=append

Input	Notes
`path`	Required. Tilde-expanded. Parent directories created if missing.
`content`	Required string. (For binary writes, use a different tool.)
`mode`	Optional, `"overwrite"` (default) or `"append"`.

If the target file exists, the consent dialog warns explicitly that the file will be overwritten and its current contents lost, with the existing file size shown.

Returns { resolvedPath, bytesWritten, mode, replacedSize? }.

`shell_exec`

Run an external command. Uses execFile-style invocation: args are passed positionally to the OS, never through a shell interpreter. This means there is no shell-injection vector: even if the agent tries to pass ; rm -rf / as an arg, it lands as a literal arg to the named command, which almost certainly errors out.

sas run shell_exec --json '{"command":"ffmpeg","args":["-version"]}'
sas run shell_exec --json '{"command":"brew","args":["install","ffmpeg"]}'

Input	Notes
`command`	Required. Executable name (on `PATH`) or absolute path.
`args`	Optional `string[]`. No shell parsing.
`cwd`	Optional working directory. Tilde-expanded.
`timeoutMs`	Default 30 s; max 10 min. Process killed (`SIGKILL`) on timeout.
`maxOutputBytes`	Default 1 MB; stdout/stderr beyond this is truncated and a `truncated: true` flag returned.

Returns { command, args, cwd, exitCode, stdout, stderr, durationMs, truncated }.

A non-zero exit code is reported with success: true; the agent has the exit code and decides whether to retry. Only timeouts and spawn failures return success: false.

Built-in safety: the deny list

A short pre-consent denylist refuses obviously-malicious commands before any dialog fires:

rm -rf / or rm -rf ~
dd of=/dev/sda (and any raw block device)
mkfs* (any filesystem reformat tool)
The classic fork bomb (:(){ :|:& };:)

If you ever need to run one of these legitimately, you'll have to do it outside the chat-plugin.

Tools the v1 surface deliberately omits

Not included	Why
`fs_delete`	No good undo path without a trash-folder mechanism. The agent doesn't get to delete files in v1.
`package_install` (wrapper)	Already covered by `shell_exec("brew", ["install", "X"])`. Keeping a single consent surface is cleaner than two parallel paths.
Persistent "always allow" allowlist	v1 design choice: always-prompt. Will revisit based on real friction.

For a read:

┌────────────────────────────────────────────────────────────┐
│  Read directory                                            │
│                                                            │
│  The chat-plugin agent wants to list the contents of a     │
│  directory on your machine.                                │
│                                                            │
│    path: /Users/you/Music                                  │
│    depth: 1                                                │
│    maxEntries: 1000                                        │
│                                                            │
│              [ Cancel ]   [ Allow ]   ← default            │
└────────────────────────────────────────────────────────────┘

For a shell call, the dialog uses warning styling and Cancel is the default button; accidental Enter denies:

┌────────────────────────────────────────────────────────────┐
│  ⚠  Run shell command                                       │
│                                                            │
│  The chat-plugin agent wants to run an external command on │
│  your machine.                                             │
│                                                            │
│    command: ffmpeg                                         │
│    args: ["-i","input.wav","output.mp3"]                   │
│    cwd: (current process cwd)                              │
│    timeoutMs: 30000                                        │
│                                                            │
│              [ Cancel ]  ← default     [ Allow ]           │
└────────────────────────────────────────────────────────────┘

Discovery from your agent

The capability tools are part of the default /api/v1/actions curated surface. They're project-scoped, so the chat-plugin's scene-default view (?scope=scene) doesn't list them by default; the chat-plugin internally queries without a scope filter, so it sees them too.

# Confirm they're registered
sas list-actions | grep -E 'fs_|shell_exec'

If you're writing your own agent integration, just call these like any other action. The consent dialog will pop on the user's machine; your agent receives the structured result when they decide.

# Capability tools: filesystem & shell

# The consent contract

# Tools

# fs_list_directory

# fs_read_file

# fs_search

# fs_write_file

# shell_exec

# Built-in safety: the deny list

# Tools the v1 surface deliberately omits

# How the consent dialog feels

# Discovery from your agent