diff --git a/src/vm.ts b/src/vm.ts
index ed100ca..49b8d4c 100644
--- a/src/vm.ts
+++ b/src/vm.ts
@@ -108,7 +108,15 @@ export async function status(): Promise<"running" | "stopped" | "missing"> {
 
 /** Launch claude in the container at the given workdir. */
 export async function claude(workdir: string, prompt?: string): Promise<void> {
-  const args = ["container", "exec", "-it", "--user", USER, "--workdir", containerPath(workdir), CONTAINER_NAME, CLAUDE_BIN, "--dangerously-skip-permissions"]
+  const cwd = containerPath(workdir)
+  const systemPrompt = [
+    "You are running inside a sandlot container (Apple Container, ubuntu:24.04).",
+    `Your working directory is ${cwd}, a git worktree managed by sandlot.`,
+    "The host's ~/dev is mounted read-only at /host.",
+    "The host's ~/.sandlot is mounted at /sandlot.",
+  ].join("\n")
+
+  const args = ["container", "exec", "-it", "--user", USER, "--workdir", cwd, CONTAINER_NAME, CLAUDE_BIN, "--dangerously-skip-permissions", "--append-system-prompt", systemPrompt]
   if (prompt) args.push(prompt)
   const proc = Bun.spawn(args, { stdin: "inherit", stdout: "inherit", stderr: "inherit" })
   await proc.exited