diff --git a/src/capabilities/sandboxed.ts b/src/capabilities/sandboxed.ts
index b00a567..546eaf9 100644
--- a/src/capabilities/sandboxed.ts
+++ b/src/capabilities/sandboxed.ts
@@ -30,7 +30,7 @@ import path from "node:path";
 import fsp from "node:fs/promises";
 import { localFs, localShell } from "./local.js";
 import type { Capabilities, ExecOptions, ExecResult, FsCapability } from "./types.js";
-import { wrapForSandbox, type SandboxSpec } from "../sandbox/sandbox.js";
+import { wrapForSandbox, wrapArgvForSandbox, type SandboxSpec } from "../sandbox/sandbox.js";
 import { modeIsBounded, modeAllowsWrites } from "../sandbox/mode.js";
 
 export interface SandboxedOptions {
@@ -147,7 +147,22 @@ export function createSandboxedCapabilities(opts: SandboxedOptions): Capabilitie
           await wrapped.cleanup?.();
         }
       },
-      exec: localShell.exec,
+      async exec(command: string, args: string[], execOpts: ExecOptions): Promise<ExecResult> {
+        // MED-3: exec used to run raw/unconfined even in bounded modes, so a
+        // mutating program invoked via argv (or a change to grep's args) would
+        // silently escape read-only/workspace-write. Route it through the same
+        // fail-closed confinement as run() so ONE mode governs both shell halves.
+        const wrapped = await wrapArgvForSandbox(
+          { ...opts.spec, cwd: execOpts.cwd },
+          command,
+          args,
+        );
+        try {
+          return await localShell.exec(wrapped.command, wrapped.args, execOpts);
+        } finally {
+          await wrapped.cleanup?.();
+        }
+      },
     },
   };
 }
diff --git a/src/channels/router.ts b/src/channels/router.ts
index 409fc33..902b9c1 100644
--- a/src/channels/router.ts
+++ b/src/channels/router.ts
@@ -4,6 +4,7 @@ import { providerForModel } from "../providers/registry.js";
 import { buildSystemPromptSnapshot, type PromptSnapshot } from "../prompt.js";
 import { reflectOnSession } from "../reflect.js";
 import { SessionStore } from "../sessions/store.js";
+import { untrustedSurfaceMode } from "../sandbox/sandbox.js";
 import type {
   StoredMessage,
   ToolDefinition,
@@ -135,7 +136,9 @@ export class ChannelRouter {
           cwd: this.opts.cwd,
           signal: this.opts.signal,
           log: () => {},
-          sandboxMode: this.opts.sandboxMode,
+          // Channels are remote-origin/untrusted; default to the confined
+          // untrusted-surface mode unless the operator pinned one explicitly.
+          sandboxMode: this.opts.sandboxMode ?? untrustedSurfaceMode(),
         },
         history: ctx.history,
         userMessage: msg.text,
diff --git a/src/heartbeat/runner.ts b/src/heartbeat/runner.ts
index a3e6e73..84629b5 100644
--- a/src/heartbeat/runner.ts
+++ b/src/heartbeat/runner.ts
@@ -19,6 +19,7 @@ import { withFileLock } from "../soul/lock.js";
 import { getAutonomyEnabled } from "../autonomy/state.js";
 import { autonomousSubset, desireReviewSubset } from "../tools/registry.js";
 import { runSubagent } from "../subagent.js";
+import { untrustedSurfaceMode } from "../sandbox/sandbox.js";
 import { recordAutonomyRun, type AutonomyKind } from "../autonomy/runs.js";
 import { recentAgentRecap } from "../orchestrator/recent-recap.js";
 import type { ToolDefinition } from "../types.js";
@@ -202,6 +203,8 @@ async function runHeartbeatInner(opts: {
         signal: opts.signal,
         model: opts.model,
         moodOrigin: `a ${runKind} turn`,
+        // Unattended self-driven run — confine to the untrusted-surface mode. H2.
+        sandboxMode: untrustedSurfaceMode(),
       });
     } catch (err) {
       await recordAutonomyRun({
@@ -306,6 +309,7 @@ export async function runDesireReviewOnce(opts: {
             model: opts.model,
             budgetTokens: 100_000,
             provider: opts.provider,
+            sandboxMode: untrustedSurfaceMode(),
             moodOrigin: "a desire-review turn",
           });
           const text = result.text
diff --git a/src/idle/runner.ts b/src/idle/runner.ts
index 62e409a..97ed6cb 100644
--- a/src/idle/runner.ts
+++ b/src/idle/runner.ts
@@ -3,6 +3,7 @@ import { lisaHome } from "../paths.js";
 import { withFileLock } from "../soul/lock.js";
 import { autonomousSubset } from "../tools/registry.js";
 import { runSubagent } from "../subagent.js";
+import { untrustedSurfaceMode } from "../sandbox/sandbox.js";
 import { recordAutonomyRun, type AutonomyOutcome } from "../autonomy/runs.js";
 import { getAutonomyEnabled } from "../autonomy/state.js";
 import { readIndex } from "../kb/store.js";
@@ -159,6 +160,9 @@ async function runIdleInner(
       cwd: opts.cwd,
       signal: opts.signal,
       model: opts.model,
+      // Unattended: confine to the untrusted-surface mode (defense in depth
+      // atop the tool subset above). H2.
+      sandboxMode: untrustedSurfaceMode(),
       budgetTokens: IDLE_BUDGET_TOKENS || undefined,
       moodOrigin: "an idle turn while the user was away",
     });
diff --git a/src/sandbox/mode.test.ts b/src/sandbox/mode.test.ts
index 92bbf4c..7d12477 100644
--- a/src/sandbox/mode.test.ts
+++ b/src/sandbox/mode.test.ts
@@ -8,7 +8,12 @@ import {
   resolveSandboxMode,
 } from "./mode.js";
 import { buildMacosSeatbeltPolicy } from "./macos.js";
-import { wrapForSandbox } from "./sandbox.js";
+import {
+  wrapForSandbox,
+  wrapArgvForSandbox,
+  sandboxEnforceable,
+  untrustedSurfaceMode,
+} from "./sandbox.js";
 
 /** H2 acceptance (docs/PLAN_HARNESS_ALIGNMENT_v1.0.md §3). */
 
@@ -135,3 +140,45 @@ describe("wrapForSandbox — fail closed, never silently unconfined", () => {
     assert.equal(existsSync(wrapped.args[1]!), false, "policy file removed");
   });
 });
+
+describe("untrusted surfaces + argv confinement (H2 follow-up)", () => {
+  test("untrusted surfaces never inherit danger-full-access where the sandbox is enforceable", () => {
+    if (sandboxEnforceable()) {
+      assert.equal(untrustedSurfaceMode(), "workspace-write");
+    } else {
+      // No OS mechanism to enforce a bounded mode: it can't confine, so it
+      // falls back to the env default (and warns once) rather than break.
+      assert.equal(untrustedSurfaceMode(), resolveSandboxMode());
+    }
+  });
+
+  test("a stricter env pin (read-only) is honoured over the workspace-write cap", () => {
+    process.env.LISA_SANDBOX_MODE = "read-only";
+    assert.equal(untrustedSurfaceMode(), "read-only");
+  });
+
+  test("wrapArgvForSandbox runs the argv directly when unconfined", async () => {
+    const w = await wrapArgvForSandbox(
+      { mode: "danger-full-access" as const, allowNetwork: true, cwd: process.cwd() },
+      "grep",
+      ["-n", "needle", "file.txt"],
+    );
+    assert.equal(w.command, "grep");
+    assert.deepEqual(w.args, ["-n", "needle", "file.txt"]);
+  });
+
+  test("wrapArgvForSandbox confines a bounded mode, or fails closed with no mechanism", async () => {
+    const spec = { mode: "workspace-write" as const, allowNetwork: false, cwd: process.cwd() };
+    if (sandboxEnforceable()) {
+      const w = await wrapArgvForSandbox(spec, "rm", ["-rf", "x"]);
+      assert.notEqual(w.command, "rm", "the program runs under the sandbox launcher, not directly");
+      assert.ok(w.args.includes("rm"), "the argv is wrapped, not dropped");
+      await w.cleanup?.();
+    } else {
+      await assert.rejects(
+        wrapArgvForSandbox(spec, "rm", ["-rf", "x"]),
+        SandboxUnavailableError,
+      );
+    }
+  });
+});
diff --git a/src/sandbox/sandbox.ts b/src/sandbox/sandbox.ts
index 6246e5b..16d0190 100644
--- a/src/sandbox/sandbox.ts
+++ b/src/sandbox/sandbox.ts
@@ -36,9 +36,32 @@ export interface SandboxedCommand {
 export async function wrapForSandbox(
   spec: SandboxSpec,
   shellCommand: string,
+): Promise<SandboxedCommand> {
+  // The shell form: run the command string through a login shell.
+  return wrapProgram(spec, ["/bin/bash", "-lc", shellCommand]);
+}
+
+/**
+ * The argv form of {@link wrapForSandbox}: confine a `command + args` invocation
+ * (no shell) under the same mechanism and the same fail-closed rule. This is
+ * what `ShellCapability.exec` uses so a bounded mode governs BOTH shell halves —
+ * previously `exec` ran unconfined even in `read-only`/`workspace-write`.
+ */
+export async function wrapArgvForSandbox(
+  spec: SandboxSpec,
+  command: string,
+  args: string[],
+): Promise<SandboxedCommand> {
+  return wrapProgram(spec, [command, ...args]);
+}
+
+/** Shared core: wrap a full argv (`program[0]` = executable) for `spec.mode`. */
+async function wrapProgram(
+  spec: SandboxSpec,
+  program: string[],
 ): Promise<SandboxedCommand> {
   if (!modeIsBounded(spec.mode)) {
-    return { command: "/bin/bash", args: ["-lc", shellCommand] };
+    return { command: program[0]!, args: program.slice(1) };
   }
 
   if (process.platform === "darwin") {
@@ -54,7 +77,7 @@ export async function wrapForSandbox(
     await fs.writeFile(tmp, policy, "utf8");
     return {
       command: "/usr/bin/sandbox-exec",
-      args: ["-f", tmp, "/bin/bash", "-lc", shellCommand],
+      args: ["-f", tmp, ...program],
       cleanup: async () => {
         try {
           await fs.unlink(tmp);
@@ -64,7 +87,7 @@ export async function wrapForSandbox(
   }
 
   if (process.platform === "linux" && hasBubblewrap()) {
-    return { command: "bwrap", args: [...bwrapArgs(spec), "/bin/bash", "-lc", shellCommand] };
+    return { command: "bwrap", args: [...bwrapArgs(spec), ...program] };
   }
 
   throw new SandboxUnavailableError(
@@ -77,6 +100,46 @@ export async function wrapForSandbox(
 );
 }
 
+/** True when this host has a mechanism that can actually enforce a bounded mode. */
+export function sandboxEnforceable(): boolean {
+  return (
+    process.platform === "darwin" ||
+    (process.platform === "linux" && hasBubblewrap())
+  );
+}
+
+let warnedUnenforceable = false;
+/**
+ * The mode the unattended / untrusted surfaces (channels, idle, heartbeat,
+ * feed/mail classification) default to. These process the least-trusted input
+ * — an inbound DM, a fetched web page — so they should not inherit the local
+ * user's `danger-full-access`. Where the host can enforce it we cap them at
+ * `workspace-write` (honouring a stricter env pin); where it cannot, bounded
+ * modes would fail closed and silently break autonomy, so we keep the env
+ * default and warn ONCE — a loud "install bwrap" beats a broken heartbeat.
+ * An operator can force fail-closed everywhere with `LISA_SANDBOX_MODE`.
+ */
+export function untrustedSurfaceMode(): SandboxMode {
+  const env = resolveSandboxMode();
+  if (!sandboxEnforceable()) {
+    if (!warnedUnenforceable) {
+      warnedUnenforceable = true;
+      console.error(
+        "[sandbox] untrusted surfaces (channels/idle/heartbeat) run UNCONFINED — no OS " +
+          "sandbox on this host; install bubblewrap (Linux) or set LISA_SANDBOX_MODE to confine them.",
+      );
+    }
+    return env;
+  }
+  // Enforceable: never looser than workspace-write, but honour a stricter pin.
+  return env === "read-only" ? "read-only" : "workspace-write";
+}
+
+/** Test hook — the one-time unenforceable warning. */
+export function _resetUntrustedWarningForTest(): void {
+  warnedUnenforceable = false;
+}
+
 /**
  * bubblewrap invocation for a bounded mode: a read-only bind of the whole
  * filesystem, then the writable paths the mode grants layered on top.
diff --git a/src/tools/task.ts b/src/tools/task.ts
index 13323b1..a601c07 100644
--- a/src/tools/task.ts
+++ b/src/tools/task.ts
@@ -55,7 +55,7 @@ export function createTaskTool(deps: {
       },
       required: ["description", "prompt"],
     },
-    async execute(input) {
+    async execute(input, ctx) {
       const tools =
         input.type === "explore" ? deps.readOnlyToolset() : deps.fullToolset();
       const system =
@@ -67,6 +67,9 @@ export function createTaskTool(deps: {
         cwd: deps.cwd,
         signal: deps.signal,
         model: input.model ?? deps.defaultModel,
+        // A dispatched subagent inherits the parent turn's confinement — it must
+        // not be able to escape the sandbox its caller runs under. H2.
+        sandboxMode: ctx?.sandboxMode,
       });
       return `[subagent: ${input.description} — ${result.toolCallCount} tool calls, ${result.outputTokens} tokens]\n${result.text}`;
     },
diff --git a/src/web/server.ts b/src/web/server.ts
index 220eb29..e6e1569 100644
--- a/src/web/server.ts
+++ b/src/web/server.ts
@@ -3973,6 +3973,9 @@ self.addEventListener('fetch', (event) => {
               // Abort on server shutdown OR this client disconnecting (Stop).
               signal: AbortSignal.any([abort.signal, turnAbort.signal]),
               log: () => {},
+              // Pin the turn to the session's mode, frozen at creation (H2), so
+              // concurrent sessions confine independently of the process env.
+              sandboxMode: chat.session.header.sandboxMode,
             },
             history: modelContext.history,
             userMessage: message,