File scope

Without it, the SDK's file tools can reach the whole working directory, and keeping the agent out of your own files would rest on the prompt alone. The file scope gate is a PreToolUse hook, registered whenever the session has file tools, that checks every Read, Write, Edit and Grep call against the folders in the session config.

The scope

Scope	Built from
Writable	knowledgeDir, extraWritableDirs
Searchable (Grep)	knowledgeDir, sourcesDir, extraWritableDirs
Read-only	sourcesDir (only changes the wording of the denial)
Denied	deniedPaths

Relative paths are resolved against projectDir. A path is inside a folder when it's the folder itself or anything under it (case-insensitively on Windows).

The rules

Tool	Allowed when	Denied with
Read	the file isn't in a denied path	"<path>" is off limits: it can't be read.
Write, Edit	the file is inside a writable folder and not denied	read-only folder: "<path>" is read-only: originals are never modified — write your own notes inside knowledge/ instead.; anywhere else: "<path>" can't be written: writing is only allowed inside knowledge/.
Grep	its path (the project directory if omitted) is inside a searchable folder, isn't denied and doesn't contain a denied path	Grep only searches inside knowledge/, sources/ — pass one of them (or a folder inside) as "path".
Glob and every other tool	always (not judged here)

Notes:

• Read is allowed anywhere but denied paths. The agent can read the project's files (to learn from them), but not the ones you protected.
• Grep is scoped because it prints file contents: a Grep over the whole project could leak a denied file's lines. For the same reason a folder that merely contains a denied file can't be searched.
• Glob isn't scoped: it lists names, not contents.
• A call without a path is left for the tool itself to reject.
• The rules apply to subagents too: the hook runs for every call in the session.

Examples

const config: BaseSessionConfig = {
  mode: "guided",
  projectDir: "/work/course",
  knowledgeDir: "/work/course/knowledge",
  sourcesDir: "/work/course/sources",
  extraWritableDirs: ["/work/course/drafts"],
  deniedPaths: ["/work/course/agent.config.json"],
};

Call	Result
Write knowledge/index.md	allowed
Edit drafts/newsletter.md	allowed
Edit sources/syllabus.pdf	denied: read-only
Write README.md	denied: not writable
Read syllabus.md	allowed
Read agent.config.json	denied: off limits
Grep "exam" in sources/	allowed
Grep "exam" (no path)	denied: the project root isn't searchable
Grep "password" in .	denied

Using it outside buildSessionOptions()

createFileScopeGate(scope) builds the hook, and checkFileScope(scope, toolName, input) gives the decision as a string (the denial reason) or undefined, handy for tests:

import { checkFileScope, type FileScope } from "@falkenslab/agent-kit";

const scope: FileScope = {
  projectDir: "/work",
  writableDirs: ["/work/notes"],
  searchableDirs: ["/work/notes"],
  readOnlyDirs: [],
  deniedPaths: ["/work/.env"],
};

checkFileScope(scope, "Write", { file_path: "notes/a.md" }); // undefined: allowed
checkFileScope(scope, "Read", { file_path: ".env" }); // '".env" is off limits: it can't be read.'