diff --git a/packages/core/src/sandbox.test.ts b/packages/core/src/sandbox.test.ts
new file mode 100644
index 0000000..f96f2bc
--- /dev/null
+++ b/packages/core/src/sandbox.test.ts
@@ -0,0 +1,76 @@
+/**
+ * The path guard's job is to deny what is genuinely outside the project — and,
+ * just as importantly, to allow everything inside it. A false deny is not a
+ * safe failure: the model burns turns probing why it was refused and then
+ * routes around the guard, which costs money and produces a worse edit.
+ */
+import {
+  mkdirSync,
+  mkdtempSync,
+  realpathSync,
+  rmSync,
+  symlinkSync,
+  writeFileSync,
+} from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { isPathInside } from "./sandbox";
+
+let root: string;
+
+beforeEach(() => {
+  root = mkdtempSync(join(tmpdir(), "airship-sandbox-test-"));
+  mkdirSync(join(root, "src"));
+  writeFileSync(join(root, "src", "app.ts"), "x");
+});
+
+afterEach(() => {
+  rmSync(root, { force: true, recursive: true });
+});
+
+describe("isPathInside", () => {
+  it("allows a relative path in the project", () => {
+    expect(isPathInside(root, "src/app.ts")).toBe(true);
+  });
+
+  it("allows an absolute path in the project", () => {
+    expect(isPathInside(root, join(root, "src", "app.ts"))).toBe(true);
+  });
+
+  it("allows a file that does not exist yet", () => {
+    expect(isPathInside(root, join(root, "src", "new.ts"))).toBe(true);
+  });
+
+  it("allows the symlink-resolved spelling of an in-project path", () => {
+    // On macOS `mkdtemp` hands back `/var/...` while everything that resolves
+    // the path reports `/private/var/...`. Both name the same file, and a guard
+    // that denies one of them fires on an ordinary project.
+    const viaRealpath = join(realpathSync(root), "src", "app.ts");
+    expect(isPathInside(root, viaRealpath)).toBe(true);
+  });
+
+  it("allows an in-project path reached through a symlinked root", () => {
+    const link = join(tmpdir(), `airship-sandbox-link-${process.pid}`);
+    rmSync(link, { force: true });
+    symlinkSync(root, link);
+    try {
+      expect(isPathInside(link, join(root, "src", "app.ts"))).toBe(true);
+      expect(isPathInside(root, join(link, "src", "app.ts"))).toBe(true);
+    } finally {
+      rmSync(link, { force: true });
+    }
+  });
+
+  it("denies a sibling directory that shares the project's name prefix", () => {
+    expect(isPathInside(root, `${root}-evil/secret.txt`)).toBe(false);
+  });
+
+  it("denies a traversal out of the project", () => {
+    expect(isPathInside(root, "../../etc/hosts")).toBe(false);
+  });
+
+  it("denies an unrelated absolute path", () => {
+    expect(isPathInside(root, "/etc/hosts")).toBe(false);
+  });
+});
diff --git a/packages/core/src/sandbox.ts b/packages/core/src/sandbox.ts
new file mode 100644
index 0000000..c0dedb5
--- /dev/null
+++ b/packages/core/src/sandbox.ts
@@ -0,0 +1,139 @@
+/**
+ * The `--safe` guards, and the Claude hook that installs them.
+ *
+ * The two screens — `screenEdit` and `screenBash` — are the policy, expressed
+ * without reference to any backend. `makeSandboxHook` wraps them as a Claude
+ * `PreToolUse` hook, where a deny hard-blocks the tool because hooks run
+ * ahead of permission rules and `canUseTool`; that is the safety layer which
+ * replaces the reference tools' blanket `--dangerously-skip-permissions`.
+ *
+ * OpenCode has no hook of any kind, but it does emit a permission request and
+ * accept a reply, so its adapter answers each request from these same two
+ * functions. Keeping the policy separate from the hook is what lets one set of
+ * rules — and one set of tests — cover both.
+ */
+import { realpathSync } from "node:fs";
+import { basename, dirname, resolve, sep } from "node:path";
+import type { HookCallback } from "@anthropic-ai/claude-agent-sdk";
+
+export const EDIT_TOOLS = new Set([
+  "Write",
+  "Edit",
+  "MultiEdit",
+  "NotebookEdit",
+]);
+
+const DESTRUCTIVE = [
+  /\brm\s+-[rf]/,
+  /\bgit\s+push\b/,
+  /\bgit\s+reset\s+--hard\b/,
+  /\bmkfs\b/,
+  /\bdd\s+if=/,
+  />\s*\/dev\/(sd|disk)/,
+  /\bsudo\b/,
+  /:\(\)\s*\{/,
+];
+
+function deny(reason: string) {
+  return {
+    hookSpecificOutput: {
+      hookEventName: "PreToolUse" as const,
+      permissionDecision: "deny" as const,
+      permissionDecisionReason: reason,
+    },
+  };
+}
+
+/**
+ * Resolve symlinks so two spellings of the same path compare equal.
+ *
+ * Falls back to the containing directory for a file that does not exist yet,
+ * which is the create case, and to the raw path when even that is missing.
+ */
+function canonical(path: string): string {
+  try {
+    return realpathSync(path);
+  } catch {
+    try {
+      return resolve(realpathSync(dirname(path)), basename(path));
+    } catch {
+      return path;
+    }
+  }
+}
+
+/**
+ * True if `target` resolves to a path at or under `root`.
+ *
+ * Both sides are canonicalized first. Comparing raw strings denies perfectly
+ * legitimate in-project edits whenever the project sits under a symlink — on
+ * macOS `/tmp` and `/var` both are, so `cwd` arrives as `/var/folders/…` while
+ * the agent reports the file as `/private/var/folders/…`. The failure is
+ * expensive rather than loud: the edit is refused, the model burns turns
+ * probing with `pwd -P` and `realpath` to work out why, and eventually routes
+ * around a guard that should never have fired.
+ */
+export function isPathInside(root: string, target: string): boolean {
+  const absRoot = canonical(resolve(root));
+  const abs = canonical(resolve(resolve(root), target));
+  return abs === absRoot || abs.startsWith(absRoot + sep);
+}
+
+/** The verdict shape both screens return. `reason` is user-facing. */
+export interface ScreenResult {
+  allowed: boolean;
+  reason?: string;
+}
+
+const ALLOWED: ScreenResult = { allowed: true };
+
+/** Confine a write to the project directory. */
+export function screenEdit(root: string, filePath: string): ScreenResult {
+  if (isPathInside(root, filePath)) {
+    return ALLOWED;
+  }
+  return {
+    allowed: false,
+    reason: `Refusing to modify a path outside the project: ${filePath}`,
+  };
+}
+
+/** Refuse a shell command that matches a destructive pattern. */
+export function screenBash(command: string): ScreenResult {
+  if (DESTRUCTIVE.some((re) => re.test(command))) {
+    return {
+      allowed: false,
+      reason: `Blocked a potentially destructive command: ${command}`,
+    };
+  }
+  return ALLOWED;
+}
+
+export function makeSandboxHook(cwd: string): HookCallback {
+  const root = resolve(cwd);
+  return (input) => {
+    if (input.hook_event_name !== "PreToolUse") {
+      return Promise.resolve({});
+    }
+    const name = input.tool_name;
+    const ti = (input.tool_input ?? {}) as Record<string, unknown>;
+
+    if (EDIT_TOOLS.has(name) && typeof ti.file_path === "string") {
+      const verdict = screenEdit(root, ti.file_path);
+      if (!verdict.allowed) {
+        return Promise.resolve(deny(verdict.reason ?? "denied"));
+      }
+    }
+
+    if (name === "Bash") {
+      const verdict = screenBash(
+        typeof ti.command === "string" ? ti.command : ""
+      );
+      if (!verdict.allowed) {
+        return Promise.resolve(deny(verdict.reason ?? "denied"));
+      }
+    }
+
+    return Promise.resolve({});
+  };
+}