diff --git a/infra/firecracker/build-image.sh b/infra/firecracker/build-image.sh
index 1ab65858..a6e77028 100755
--- a/infra/firecracker/build-image.sh
+++ b/infra/firecracker/build-image.sh
@@ -199,6 +199,32 @@ build_rootfs() {
   sudo cp "${HARNESS_BIN}" "${mnt}/sbin/init"
   sudo chmod 0755 "${mnt}/usr/local/bin/lantern-harness" "${mnt}/sbin/init"
 
+  # A static shell, so the guest can actually run a tool.
+  #
+  # The harness's `shell_exec` tool spawns `/bin/sh -c <command>`; without a
+  # shell in the image every tool call fails with "spawn failed: No such file
+  # or directory", so the in-guest tool path cannot be exercised at all. Must
+  # be STATIC — this rootfs has no libc.
+  #
+  # Optional: when no static busybox is present the image is built without a
+  # shell exactly as before, and only shell_exec is unavailable.
+  if [ -n "${FC_BUSYBOX_BIN:-}" ] && [ -x "${FC_BUSYBOX_BIN}" ]; then
+    BUSYBOX_SRC="${FC_BUSYBOX_BIN}"
+  elif [ -x /bin/busybox ] && file /bin/busybox 2>/dev/null | grep -q "statically linked"; then
+    BUSYBOX_SRC=/bin/busybox
+  else
+    BUSYBOX_SRC=""
+  fi
+  if [ -n "${BUSYBOX_SRC}" ]; then
+    sudo mkdir -p "${mnt}/bin"
+    sudo cp "${BUSYBOX_SRC}" "${mnt}/bin/busybox"
+    sudo chmod 0755 "${mnt}/bin/busybox"
+    sudo ln -sf busybox "${mnt}/bin/sh"
+    log "Installed static shell: ${BUSYBOX_SRC} -> /bin/sh"
+  else
+    log "No static busybox found — image has NO /bin/sh; shell_exec tool calls will fail"
+  fi
+
   # Minimal defaults file (ADR-0004). The manager overrides these via the
   # kernel cmdline (build_boot_args) and the per-VM env contract.
   sudo tee "${mnt}/etc/lantern/harness.toml" >/dev/null <<'TOML'
diff --git a/infra/firecracker/integration-test.sh b/infra/firecracker/integration-test.sh
index 90f3ed00..ad70ab63 100755
--- a/infra/firecracker/integration-test.sh
+++ b/infra/firecracker/integration-test.sh
@@ -301,6 +301,32 @@ else
   skip "VendSecret round-trip PENDING in-guest harness agent (services/harness manager_client.rs is a stub); set EXPECT_VENDSECRET=1 to assert it. Secret-value-never-leaked invariant: enforced."
 fi
 
+# ---------------------------------------------------------------------------
+# 4b. Invoke a tool in the guest via ExecTool.
+#
+# This is the workflow-engine's tool_call path end to end: manager -> in-guest
+# harness exec server -> tool registry -> real execution -> typed result. It can
+# only succeed after the harness has opened its Heartbeat stream (that is what
+# gives the manager the guest address to dial back on), so it also proves the
+# harness is reachable, not just that it started.
+#
+# Needs a shell in the image; build-image.sh installs a static busybox when one
+# is available, and skips this assertion when it did not.
+# ---------------------------------------------------------------------------
+log "Invoking a tool in the guest (ExecTool -> shell_exec)"
+TOOL_RESP=$(grpcurl -cacert "${WORK}/ca.crt" -cert "${WORK}/client.crt" -key "${WORK}/client.key" -servername localhost \
+  -import-path "${PROTO_DIR}" -proto "${PROTO_FILE}" \
+  -d "{\"vm_id\":\"${VM_ID}\",\"run_id\":\"itest-run-001\",\"step_id\":\"itest-step-1\",\"tool_name\":\"shell_exec\",\"args\":{\"command\":\"echo lantern-tool-marker\"},\"idempotency_key\":\"itest-run-001:itest-step-1:1\"}" \
+  "${MANAGER_ADDR}" lantern.v1.RuntimeManager/ExecTool 2>>"${MANAGER_LOG}") || TOOL_RESP=""
+
+if printf '%s' "${TOOL_RESP}" | grep -q "lantern-tool-marker"; then
+  pass "tool invoked in-guest and returned its output (ExecTool -> shell_exec)"
+elif printf '%s' "${TOOL_RESP}" | grep -q "No such file or directory"; then
+  skip "in-guest tool ran but the image has no /bin/sh — install a static busybox to assert execution"
+else
+  fail "ExecTool did not return the tool's output. Response:\n${TOOL_RESP}\nLog:\n$(tail -30 "${MANAGER_LOG}")"
+fi
+
 # ---------------------------------------------------------------------------
 # 5. Tear down.
 # ---------------------------------------------------------------------------