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import { access, lstat, readlink, realpath, stat, writeFile } from "node:fs/promises";
import { dirname, resolve, sep } from "node:path";
import type { ToolContract, ToolResult } from "@dispatch/kernel";
interface ValidatedArgs {
readonly path: string;
readonly content: string;
readonly overwrite: boolean;
}
export type OverwriteDecision = "create" | "overwrite" | { readonly error: string };
/** Pure: decide the action based on file existence and the overwrite flag. */
export function decideOverwrite(fileExists: boolean, overwrite: boolean): OverwriteDecision {
if (!fileExists && !overwrite) return "create";
if (fileExists && !overwrite) {
return { error: "Error: File already exists; set overwrite: true to replace it." };
}
if (fileExists && overwrite) return "overwrite";
return { error: "Error: overwrite: true but the file does not exist." };
}
/** Pure: check that a resolved absolute path is within the workdir (prefix check). */
export function isPathWithinWorkdir(resolvedPath: string, workdir: string): boolean {
const normalizedWorkdir = workdir.endsWith(sep) ? workdir : workdir + sep;
return resolvedPath === workdir || resolvedPath.startsWith(normalizedWorkdir);
}
/** Pure: validate and coerce args from the model. */
export function validateArgs(args: unknown): ValidatedArgs | { readonly error: string } {
if (args === null || args === undefined || typeof args !== "object") {
return { error: "Error: Arguments must be an object." };
}
const obj = args as Record<string, unknown>;
const rawPath = obj.path;
if (typeof rawPath !== "string" || rawPath.length === 0) {
return { error: 'Error: Missing or invalid "path" parameter (must be a non-empty string).' };
}
const rawContent = obj.content;
if (typeof rawContent !== "string") {
return {
error: 'Error: Missing or invalid "content" parameter (must be a string).',
};
}
let overwrite = false;
if (obj.overwrite !== undefined) {
if (typeof obj.overwrite !== "boolean") {
return { error: 'Error: Invalid "overwrite" parameter (must be a boolean).' };
}
overwrite = obj.overwrite;
}
return { path: rawPath, content: rawContent, overwrite };
}
/**
* Factory: create a write_file ToolContract bound to a working directory.
* The working directory is injected so the tool is testable.
*/
export function createWriteFileTool(workingDirectory: string): ToolContract {
const workdir = resolve(workingDirectory);
return {
name: "write_file",
description:
"Write a whole file to disk. " +
"By default, creates a new file; errors if it already exists. " +
"Set overwrite: true to replace an existing file (errors if the file does not exist). " +
"Parent directories are NOT auto-created — the parent must already exist.",
parameters: {
type: "object",
properties: {
path: {
type: "string",
description: "Path to the file, relative to the working directory.",
},
content: {
type: "string",
description: "The full content to write to the file.",
},
overwrite: {
type: "boolean",
description:
"When false/unset: creates a new file (errors if it already exists). " +
"When true: replaces an existing file (errors if it does not exist).",
default: false,
},
},
required: ["path", "content"],
},
concurrencySafe: false,
async execute(args: unknown, ctx): Promise<ToolResult> {
const validated = validateArgs(args);
if ("error" in validated) {
return { content: validated.error, isError: true };
}
const { path: relPath, content, overwrite } = validated;
const effectiveBase = ctx.cwd ? resolve(ctx.cwd) : workdir;
const resolvedPath = resolve(effectiveBase, relPath);
if (!isPathWithinWorkdir(resolvedPath, effectiveBase)) {
return {
content: `Error: Path "${relPath}" is outside the working directory.`,
isError: true,
};
}
// Symlink hardening: realpath the parent directory and the base, then re-check.
let realParent: string;
let realBase: string;
try {
const parentDir = dirname(resolvedPath);
[realParent, realBase] = await Promise.all([realpath(parentDir), realpath(effectiveBase)]);
} catch (err: unknown) {
const code = (err as NodeJS.ErrnoException).code;
if (code === "ENOENT") {
return {
content: `Error: Parent directory for "${relPath}" does not exist.`,
isError: true,
};
}
return {
content: `Error resolving path: ${err instanceof Error ? err.message : String(err)}`,
isError: true,
};
}
const realResolvedPath = realParent + sep + resolvedPath.split(sep).at(-1);
if (!isPathWithinWorkdir(realResolvedPath, realBase)) {
return {
content: `Error: Path "${relPath}" is outside the working directory.`,
isError: true,
};
}
// If the resolved path itself is a symlink, verify the target is contained.
try {
const linkStat = await lstat(resolvedPath);
if (linkStat.isSymbolicLink()) {
const linkTarget = await readlink(resolvedPath);
const resolvedTarget = resolve(dirname(resolvedPath), linkTarget);
if (!isPathWithinWorkdir(resolvedTarget, realBase)) {
return {
content: `Error: Path "${relPath}" is outside the working directory.`,
isError: true,
};
}
}
} catch (err: unknown) {
const code = (err as NodeJS.ErrnoException).code;
if (code !== "ENOENT") {
return {
content: `Error checking path: ${err instanceof Error ? err.message : String(err)}`,
isError: true,
};
}
}
// Check existence.
let fileExists = false;
try {
await access(resolvedPath);
fileExists = true;
} catch (err: unknown) {
const code = (err as NodeJS.ErrnoException).code;
if (code !== "ENOENT") {
return {
content: `Error checking file: ${err instanceof Error ? err.message : String(err)}`,
isError: true,
};
}
}
// Pure decision.
const decision = decideOverwrite(fileExists, overwrite);
if (typeof decision === "object") {
return { content: decision.error, isError: true };
}
// Verify it's not a directory.
if (fileExists) {
const pathStat = await stat(resolvedPath);
if (pathStat.isDirectory()) {
return {
content: `Error: "${relPath}" is a directory, not a file.`,
isError: true,
};
}
}
// Write the file.
try {
await writeFile(resolvedPath, content, "utf8");
} catch (err: unknown) {
return {
content: `Error writing file: ${err instanceof Error ? err.message : String(err)}`,
isError: true,
};
}
const action = decision === "create" ? "Created" : "Overwrote";
return { content: `${action} "${relPath}" (${content.length} bytes).` };
},
};
}
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