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Debug a failing AI agent from the outside in: first prove that the microVM launches and the guest reaches a usable boot state, then verify the agent service, and only afterward investigate model responses and tool logic. Capture the guest serial console, the virtual-machine monitor (VMM) log, and host and guest journal entries before changing application code. Reproduce the smallest possible run—a single prompt, one model request, and one tool call—in both a known-good and failing VM. This separates boot, isolation, networking, credentials, resource, timeout, and agent-code failures instead of blending them into one vague error.
Why does an AI agent fail only inside a microVM?
A microVM adds several failure boundaries that do not exist when the same process runs directly on a host. The model client, tool implementation, and prompt may be unchanged, while the guest has a different kernel, root filesystem, user identity, mounts, network namespace, DNS configuration, cgroup limits, clock, and security policy. A failure that looks like an agent bug may therefore occur before the agent process starts or at the first operation that crosses the guest boundary.
Use a layered model when debugging:
- Host and VMM: the launcher, Firecracker configuration, jailer, file permissions, sandbox policy, and host capabilities.
- Guest boot: the kernel image, command line, root device, init system, virtio devices, and serial-console setup.
- Agent process: service startup, environment variables, working directory, mounts, packages, user permissions, and subprocesses.
- Model and tools: request serialization, endpoint reachability, credentials, tool execution, response parsing, and state carried between steps.
- Policy and timing: retries, watchdogs, cgroup throttling, request budgets, cancellation, and policy decisions that can terminate an otherwise healthy run.
The first observable symptom tells you which boundary to test. Do not begin by rewriting prompts if the guest never reaches a login prompt, and do not rebuild a kernel when a model response was already received successfully.
Where in the stack does the failure first appear?
Classify the earliest failure, not the final error shown by a supervisor. The following map gives the fastest starting point.
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| Earliest observation | Most likely boundary | First evidence to inspect |
|---|---|---|
| VM exits before any serial output or login prompt | VMM launch, kernel path, root filesystem, device setup, or console configuration | VMM log, launch response, kernel image and command line, guest serial-console file |
| Kernel messages appear, but init or the service never starts | Guest root filesystem, init configuration, mounts, permissions, or resource limits | Guest serial output and journalctl for the boot and service unit |
| Agent starts, but every external call times out | Network namespace, route, DNS, NAT or tap setup, proxy, egress policy, endpoint trust, or clock | Interface and route state, resolver settings, proxy variables, TLS errors, and timed request logs |
| A tool starts and returns an error | Tool binary, credentials, filesystem access, subprocess policy, or tool implementation | Tool exit code and stderr, effective user and environment, mount mode, and policy decision |
| A tool succeeds, then the answer is wrong or rejected | Model response handling, parsing, prompt state, retries, or post-tool policy | Model and tool spans, response payload metadata, parser status, retry reason, and policy result |
| Only large prompts or concurrent runs fail | Memory, request-size, serialization, file-descriptor, process, or timeout limits | OOM and throttling records, payload-size measurements, concurrency, and per-step latency |
Record the boundary and a timestamp in the incident notes. Every later log query and reproduction should use that same time window.
What should I collect before changing application code?
Preserve the first evidence while the failure is reproducible. Early boot output and VMM errors are often lost when a VM is immediately destroyed or relaunched.
Capture the guest serial console and VMM log
Configure the guest kernel console to a serial device that the VMM redirects to a host file or logging stream. Firecracker’s documentation covers guest-kernel configuration and redirecting the guest serial console; use that output to inspect kernel and early-userspace messages: Firecracker documentation. Preserve the file with timestamps and a run identifier rather than truncating it on each launch.
Enable the VMM API and operational logs at the highest useful verbosity for the reproduction. Save the launch request, response, VM identifier, Firecracker version, vCPU count, memory size, kernel image identifier, root filesystem identifier, network configuration, mounts, and environment passed to the guest. Redact secrets, but do not remove names or hashes needed to prove that two runs used the same artifact.
Query host and guest journals by boot, unit, priority, and time
journalctl can narrow a large journal to the exact boot, service, priority, or interval that contains the failure. The systemd manual documents these filters: journalctl manual.
# Host: current boot, warnings and errors, newest entries first
journalctl -b -p warning..emerg -r
# Host: one VMM or launcher unit in a bounded time window
journalctl -u microvm-launcher.service --since "2026-10-26 12:00:00" --until "2026-10-26 12:05:00"
# Guest: previous boot, kernel and init messages
journalctl -b -1 -k
# Service-specific failures with timestamps
journalctl -u agent.service -p err..alert --since "10 minutes ago"
Use the equivalent guest commands from an interactive shell or a captured guest journal. If the guest cannot boot far enough to write a journal, the serial console is the primary source. Keep the exact command line, output, and timezone in the incident bundle.
Record the immutable run context
- Firecracker and host-kernel versions.
- Guest kernel image, command-line parameters, root filesystem image, and image checksums or identifiers.
- vCPU, memory, file-descriptor, process, and cgroup limits.
- Virtio devices, jailer directories, ownership, seccomp or sandbox settings, and required host capabilities.
- Tap or network-namespace names, routes, NAT, DNS, proxy variables, and egress policy.
- Guest mounts and their read/write modes.
- Agent version, installed packages, model name, tool versions, and policy configuration.
- Run ID, request IDs, timestamps, and the exact input or a privacy-safe input hash.
How do I build a deterministic reproduction?
Replace the production workflow with one fixed input, one model request, and one tool call. Pin the model and tool versions, disable unrelated concurrency, and record request IDs and timestamps. The goal is not to simulate every user path; it is to create a failure that can be compared byte-for-byte and event-for-event.
- Define the minimal case. Use a short prompt that requires exactly one known tool invocation. Fix tool arguments and remove optional follow-up steps.
- Pin inputs and artifacts. Use the same guest image, kernel, agent build, model identifier, tool binary, and configuration in both runs. Record checksums or immutable IDs.
- Run a known-good control. Execute the case in a VM that completes successfully. Save serial output, VMM logs, journals, model metadata, tool exit status, and trace identifiers.
- Run the failing case. Change one boundary at a time—network policy, mount, resource limit, image, or timeout—rather than changing several settings together.
- Diff events, not just final text. Compare boot completion, service start, DNS lookup, connection establishment, request duration, tool process start, exit code, retries, and policy decisions.
- Vary one suspected trigger. Test payload size, tool permission, egress access, concurrency, timeout budget, and preserved state independently. A trigger that disappears when one variable changes identifies the next layer to inspect.
Keep the successful control available. It prevents a “fix” that merely changes the input or hides the original failure.
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How do I verify the guest boot and kernel path?
Boot failures must be solved before agent debugging. A VM that never reaches init cannot read credentials, resolve DNS, or execute a tool.
Kernel and command line
Confirm that the configured kernel image exists, is readable by the launcher or jailer, and matches the image used by the control run. Check the root device name, init path, console parameter, and any required early-userspace arguments. A wrong root device can produce a kernel panic or an init failure that looks like a silent launch.
Look for kernel panics, inability to mount the root filesystem, missing virtio drivers, read-only remounts, and messages about an unavailable console. If there is no serial output at all, verify the console parameter and host redirection before concluding that the kernel is dead.
Init, mounts, and service startup
Once the kernel is running, determine whether init completed and whether the agent unit was scheduled. A unit may fail because its executable is absent, its working directory is not mounted, its user lacks permission, or a dependency is not ready. Query the unit status and its journal in the guest, then inspect the same unit definition and environment in the known-good VM.
Do not infer success from a running PID alone. Confirm that the process is the expected binary, has the intended user ID and working directory, and can read its configuration and certificates.
Which device, jailer, and filesystem checks matter?
MicroVMs intentionally expose a minimal device set and often run the VMM or guest process under a jailer. Missing one required file or capability can prevent startup or make a later tool call fail.
Devices and jailer configuration
- Compare the virtio block, network, entropy, and console devices between the successful and failing configurations.
- Verify jailer directories, ownership, mode bits, and paths after any chroot or directory change.
- Check seccomp or sandbox policy for denied system calls and confirm the launcher has the capabilities it actually requires.
- Inspect VMM logs for device-creation, file-open, permission, and API validation errors.
Filesystem and certificates
List every mount from inside the guest and verify that it exists at the path the agent expects. Confirm read/write mode, available space, and ownership. Configuration files, CA certificates, model settings, and tool binaries are common omissions when an image is assembled for a microVM.
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A host path being present does not prove that the guest can see it. Test the exact path as the agent’s effective user, and test both reading and writing when the workflow requires temporary files, caches, sockets, or tool output.
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How do I debug network, DNS, credentials, and TLS failures?
When the agent starts but external calls fail, test the guest’s complete path to the endpoint rather than only checking that the host has internet access.
Network namespace and routing
Inside the guest, inspect the interface state, address, default route, and resolver configuration. On the host, verify the tap or network-namespace attachment, NAT rules, forwarding, and egress policy. A missing route and a policy-denied route can produce similar application timeouts but require different fixes.
Test DNS resolution separately from TCP or TLS. If the name does not resolve, inspect the resolver address and search domain. If resolution succeeds but connection establishment fails, check route, firewall, NAT, proxy, and endpoint allow-list rules. If TCP connects but TLS fails, verify the guest clock and CA trust store.
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Proxy variables and credentials
Print the effective environment from the running guest process, not just the launcher shell. Confirm that proxy variables point to a reachable proxy and that no stale bypass rule sends a required endpoint through an inaccessible path. Check that credentials are present at the expected path or variable, readable by the service user, and not removed by the jailer or service manager.
Record only credential presence, source, and version—not secret values. A missing credential should produce an authentication error; a timeout usually points earlier in the network path, although an intermediary can conceal the distinction.
How do resource limits and timeouts cause agent failures?
Compare limits in the failing and successful VMs before increasing them. Memory, vCPU, file descriptors, process counts, and cgroups can alter behavior without changing application code.
Memory and CPU
Look in the guest and host journals for OOM kills, allocation failures, and cgroup throttling. A large prompt can increase serialization and model-client memory, while a tool may create subprocesses or temporary files. Correlate the failure timestamp with memory pressure rather than assuming the model endpoint rejected the request.
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Check open-file and process limits when failures appear only under concurrency. A tool that cannot open a socket, pipe, or temporary file may return a generic error to the agent. Verify available space and inode availability for request bodies, tool output, and logs.
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Wall-clock and watchdog budgets
Write down the timeout at every layer: tool subprocess, model client, agent step, supervisor, and VM watchdog. A shorter outer timeout can terminate a healthy inner request. Ensure the guest clock is usable for TLS validation and that timestamps from host, guest, and tracing systems can be compared. When retries are enabled, include backoff and the total budget, not only the per-attempt timeout.
How can I prove whether the agent process or a tool is at fault?
Inspect the process from inside the guest using the same identity and environment as the service. Log the effective user ID, working directory, installed package versions, relevant environment keys, and each tool subprocess exit code and stderr.
Agent startup checks
- Verify the executable path and interpreter, including any virtual environment or runtime loader.
- Print a sanitized environment and confirm model endpoint, region, proxy, certificate, and tool paths.
- Check that the service can read its configuration and write only to intended directories.
- Confirm that the agent’s clock, locale, and encoding match assumptions in request signing and parsing.
Tool-call checks
Run the tool directly with a fixed argument under the service user, then run it through the agent with the same argument. Compare exit status, stdout, stderr, duration, files created, and network access. A direct success followed by an agent failure points to serialization, argument escaping, policy checks, or response parsing. A direct failure keeps the investigation at the tool, filesystem, credentials, or sandbox boundary.
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Use distributed traces so one run can be followed from the host supervisor into the guest agent and back through model and tool operations. OpenTelemetry describes propagating context such as trace and span identifiers across logs, metrics, and traces: OpenTelemetry documentation.
One span per meaningful operation
Create a span for each model request, tool invocation, retry, policy decision, and final response. Add attributes for run ID, VM ID, input or prompt hash, model name, tool name, latency, status, and error class. Record request IDs returned by endpoints when available.
Propagate context across the VM boundary
Pass trace context from the host supervisor to the guest agent through an explicit, authenticated channel or environment mechanism. The guest should attach the incoming context to model and tool spans. If a tool launches a subprocess, propagate a child context or at least the run ID so its logs can be joined to the parent span.
Protect secrets while retaining diagnostic value
Never put API keys, authorization headers, full prompts containing sensitive data, or tool payload secrets in span attributes. Store a stable hash, schema version, sizes, and identifiers instead. Keep enough metadata to distinguish a timeout, denial, parse error, retry exhaustion, and policy rejection without exposing private content.
What do common failure patterns mean?
Use the symptom pattern to choose the next test, then confirm it with the captured evidence.
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| Pattern | Likely explanation | Confirm with |
|---|---|---|
| No serial output | VM launch failure, wrong kernel or image, bad console parameters, or an early VMM error | VMM log, launch API response, kernel path and checksum, console redirection |
| Kernel boots but init fails | Root filesystem, init configuration, mount, permission, or required device problem | Serial kernel/init messages and guest boot journal |
| Agent starts; all external calls time out | Route, DNS, NAT, proxy, egress policy, endpoint reachability, or clock/TLS trust | Separate DNS, TCP, TLS, and authenticated-request tests |
| Only large prompts fail | Memory pressure, request-size limit, serialization cost, or insufficient tool/model timeout | Payload-size and memory measurements with one variable changed |
| Retries multiply cost or latency | Wrong retry classification, non-idempotent tool, missing backoff, or a deterministic error being retried | Retry spans, original error class, idempotency rules, and total elapsed time |
When should I change the microVM, the agent, or the tool boundary?
Change the smallest layer that explains the evidence. If boot logs show a missing root device, fix the VM configuration; if the agent has a valid model response but rejects the parsed tool result, fix agent handling; if a tool requires a capability that the sandbox intentionally withholds, redesign the tool boundary rather than weakening isolation globally.
Fix configuration when the guest cannot provide a required primitive
Correct the kernel command line, device, mount, route, DNS, certificate store, ownership, or cgroup setting when the known-good comparison shows that the primitive is absent. Re-run the minimal case after each change and preserve the new logs.
Fix application behavior when the primitive works
Update parsing, state management, retry classification, idempotency, or timeout budgeting when traces prove that the request reached the endpoint and the tool completed. Keep the microVM configuration unchanged so the application fix is independently verifiable.
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Move a capability outside the guest only deliberately
If a tool cannot safely or reliably run inside the guest, place a narrowly scoped broker on the host or another service. Define the protocol, authentication, allowed arguments, response limits, and audit trail. Moving a call outside the microVM trades isolation for reachability, so document the changed threat model and retain trace context across the boundary.
What should a reusable microVM agent debugging runbook contain?
Turn the workflow into a checklist that an on-call engineer can execute without guessing.
- Write the first failing timestamp and classify the earliest boundary.
- Save guest serial output and VMM logs before relaunching or deleting the VM.
- Export host and guest journal entries by boot, unit, priority, and time range.
- Record versions, image identifiers, kernel command line, vCPU, memory, limits, devices, mounts, network settings, and sanitized environment.
- Run one fixed prompt, one model request, and one tool call in a known-good VM.
- Run the same case in the failing VM and diff lifecycle events and trace spans.
- Test boot, filesystem, network, credentials, resources, and timeouts in that order.
- Confirm tool behavior directly under the service identity, then through the agent.
- Classify retries and policy decisions using trace context rather than final user-visible text.
- Apply one change, repeat the deterministic case, and archive the evidence with the result.
This order prevents expensive model-level investigation when the guest never had a usable network or filesystem, while still exposing failures that occur after a successful tool call.
Frequently Asked Questions
Can I debug a microVM agent without an interactive login shell?
Yes. Redirect the guest serial console, collect VMM and host logs, and have the agent emit sanitized startup diagnostics and trace IDs. A reproducible one-prompt run can be analyzed entirely from those artifacts, even when SSH or a login service is unavailable.
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Run separate tests for name resolution, route and TCP connection, TLS handshake, and authenticated application request. Resolution failure implicates the resolver path; successful resolution and TCP with TLS errors point to trust or clock; a successful TLS handshake with an HTTP error reaches the endpoint and should be debugged at the protocol or credential layer.
What information should I remove before sharing debug logs?
Remove tokens, cookies, authorization headers, private keys, full sensitive prompts, and tool arguments containing secrets. Retain timestamps, run and trace IDs, image identifiers, status and error classes, payload sizes, model and tool names, and stable hashes so events can still be correlated.
Why can retries make a microVM failure appear intermittent?
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