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diff --git a/packages/exec-backend/src/backend.ts b/packages/exec-backend/src/backend.ts
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+/**
+ * ExecBackend — the transport-agnostic spawn + minimal filesystem surface.
+ *
+ * Tools (tool-shell, tool-read-file, tool-write-file, tool-edit-file) program
+ * against THIS abstraction instead of `node:fs` / `node:child_process` directly.
+ * Two implementations exist:
+ *
+ * - `LocalExecBackend` — wraps today's node calls (behavior-identical).
+ * - `SshExecBackend` — wraps ssh2 `exec` + `sftp` (added later by the `ssh`
+ *   package; not this package's concern — but THIS interface is the seam it
+ *   implements).
+ *
+ * The surface is deliberately SMALL (only what the bundled tools use) so a
+ * remote implementation is tractable. New operations are added here, not ad hoc.
+ *
+ * Resolved per-call from `ToolExecuteContext.computerId` via the injected
+ * `ExecBackendResolver` (see `./service.js`). `computerId` undefined → local.
+ *
+ * Error contract: `readFile`/`stat`/`readdir`/`writeFile` throw node:fs-style
+ * errors carrying a `.code` property (e.g. `"ENOENT"`) so the tools' existing
+ * error branches work unchanged. `exists` never throws (returns `false` on
+ * missing). The SshExecBackend maps ssh2 errors onto these same shapes.
+ */
+
+/** A spawned process's result. Mirrors tool-shell's `SpawnResult` exactly. */
+export interface ExecResult {
+	readonly exitCode: number | null;
+	readonly timedOut: boolean;
+	readonly aborted: boolean;
+}
+
+/** Parameters for spawning a shell command. Mirrors tool-shell's `SpawnShell` params. */
+export interface SpawnParams {
+	readonly command: string;
+	readonly cwd: string;
+	readonly signal: AbortSignal;
+	readonly timeout: number;
+	readonly onOutput: (data: string, stream: "stdout" | "stderr") => void;
+}
+
+/** Stat result — the subset read_file / write_file / edit_file need. */
+export interface StatResult {
+	readonly isFile: boolean;
+	readonly isDirectory: boolean;
+}
+
+/** A directory entry — the subset read_file lists. */
+export interface DirEntry {
+	readonly name: string;
+	readonly isDirectory: boolean;
+}
+
+/**
+ * The execution backend: spawn + a minimal filesystem surface.
+ * Tools program against THIS, never against `node:fs`. Resolved per-call from
+ * `ToolExecuteContext.computerId` via the injected resolver.
+ */
+export interface ExecBackend {
+	/** Run a shell command, streaming stdout/stderr. The shell-tool seam. */
+	readonly spawn: (params: SpawnParams) => Promise<ExecResult>;
+
+	// --- filesystem (the read_file / write_file / edit_file surface) ---
+
+	/** Read a file as utf8 text. Throws node:fs-style errors with `.code`. */
+	readonly readFile: (path: string) => Promise<string>;
+
+	/** Write utf8 text to a file. Throws on failure (e.g. missing parent dir). */
+	readonly writeFile: (path: string, content: string) => Promise<void>;
+
+	/** Stat a path. Throws node:fs-style errors with `.code` (e.g. `"ENOENT"`). */
+	readonly stat: (path: string) => Promise<StatResult>;
+
+	/** List directory entries. Throws node:fs-style errors with `.code`. */
+	readonly readdir: (path: string) => Promise<readonly DirEntry[]>;
+
+	/** Check existence without throwing (returns `false` when the path is missing). */
+	readonly exists: (path: string) => Promise<boolean>;
+}