diff --git a/sev_verify/guest_vsock.py b/sev_verify/guest_vsock.py
index 6604a95d..9fff3b70 100644
--- a/sev_verify/guest_vsock.py
+++ b/sev_verify/guest_vsock.py
@@ -14,6 +14,7 @@
 import re
 import shlex
 import socket
+import subprocess
 import time
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
@@ -142,9 +143,15 @@ def wait_for_guest(
     *,
     timeout: float | None = None,
     poll_interval: float = 2.0,
+    process: subprocess.Popen | None = None,
 ) -> None:
     """
     Block until the guest vsock agent responds or timeout is reached.
+
+    Pass ``process`` (the QEMU handle) to stop waiting the moment QEMU exits.
+    Without it, a guest that never starts — or one the firmware refuses to
+    launch — costs the full boot timeout and is then reported as an
+    indistinguishable "agent not ready", discarding the reason QEMU gave.
     """
     boot_timeout = timeout if timeout is not None else profile.vsock_boot_timeout
     deadline = time.monotonic() + boot_timeout
@@ -156,6 +163,11 @@ def wait_for_guest(
             return
         except GuestVsockError as exc:
             last_error = str(exc)
+            if process is not None and process.poll() is not None:
+                raise GuestVsockError(
+                    f"QEMU exited with code {process.returncode} before the "
+                    f"guest became ready"
+                ) from exc
             time.sleep(poll_interval)
 
     raise GuestVsockError(
@@ -169,12 +181,20 @@ def check_guest_ready(
     *,
     timeout: float | None = None,
     poll_interval: float = 2.0,
+    process: subprocess.Popen | None = None,
 ) -> tuple[bool, str]:
     """
     Return whether the guest vsock agent responds.
+
+    ``process`` is forwarded to :func:`wait_for_guest`; see its docstring.
     """
     try:
-        wait_for_guest(profile, timeout=timeout, poll_interval=poll_interval)
+        wait_for_guest(
+            profile,
+            timeout=timeout,
+            poll_interval=poll_interval,
+            process=process,
+        )
     except GuestVsockError as exc:
         return False, str(exc)
 
diff --git a/sev_verify/vm_profile.py b/sev_verify/vm_profile.py
index ed500ac5..d042efb3 100644
--- a/sev_verify/vm_profile.py
+++ b/sev_verify/vm_profile.py
@@ -228,10 +228,20 @@ def vm_launch(
             # Import here to avoid circular import with guest_vsock.
             from .guest_vsock import check_guest_ready
 
-            booted, boot_error = check_guest_ready(self)
+            # Passing the process lets the wait abort as soon as QEMU dies,
+            # instead of polling vsock until the boot timeout expires against
+            # a VM that no longer exists.
+            booted, boot_error = check_guest_ready(self, process=process)
             if not booted:
                 ok = False
                 message = f"Guest did not boot: {boot_error}"
+                if process.poll() is not None:
+                    # QEMU's own diagnosis is far more useful than "agent not
+                    # ready" — for a rejected launch it names the firmware
+                    # error, e.g. "SNP_LAUNCH_FINISH ... 'Bad measurement'".
+                    stderr_tail = _read_guest_errors(self.guest_error_log).strip()
+                    if stderr_tail:
+                        message = f"{message}\nQEMU stderr:\n{stderr_tail}"
             elif message == "VM launch verified":
                 message = "VM launched and guest booted"