The Tool Desk
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Debug the failure from the outside in: first determine whether the microVM launched and booted, then whether the guest started the agent, then whether the agent could reach its model and tools, and finally whether it handled their responses correctly. Capture the guest serial console and virtual-machine monitor (VMM) logs before changing application code; an early kernel, device, or filesystem failure can prevent the agent from running at all.
This guide applies to microVM setups generally, with Firecracker as a documented example. The central diagnostic question is: at which boundary does the known-good run stop matching the failing run?
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Why does an AI agent fail only inside a microVM?
A microVM changes the environment in which the agent runs. The guest may have a different kernel, root filesystem, network path, mounts, credentials, device set, resource limits, clock behavior, or service environment than the host. An agent can therefore work outside the VM and still fail because a dependency it assumes is available is missing or restricted inside the guest.
Start by locating the first observable failure, rather than treating “the agent failed” as one problem. A VM that never reaches a login prompt is a different case from a running agent whose first tool request times out. The table maps those boundaries to the evidence to inspect first.
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| First failure you can observe | Likely layer | First evidence to check |
|---|---|---|
| The VM exits or produces no usable boot output | Host launch, VMM, kernel, root filesystem, or console setup | VMM logs, launch configuration, and guest serial output |
| The kernel starts, but the guest does not reach its normal service state | Guest init, root filesystem, mounts, or permissions | Serial output and guest boot journal |
| The guest boots, but the agent service does not start | Service configuration, environment, user permissions, resources, or missing files | Guest journal for the service and the service’s effective runtime environment |
| The agent starts, but model or tool calls fail | DNS, routes, egress policy, proxy, credentials, TLS trust, or tool code | Agent error and timing, guest network configuration, and tool exit status |
| Calls succeed, but the final answer is wrong or missing | Response handling, parsing, prompt state, retries, or policy | Correlated model, tool, and policy traces |
These are starting hypotheses, not proof. Follow the timestamps and run identifiers across the VM, agent, and its dependencies to find the earliest divergence.
What logs should I collect first?
Collect evidence before changing code or configuration. Keep timestamps intact, record the exact failing run, and gather logs at both the host and guest boundaries. Firecracker’s documentation covers guest-kernel configuration and redirecting the guest serial console to a host log; that console is especially useful for kernel and early-userspace output that may appear before the agent or guest logging service starts. See the Firecracker documentation.
Capture host and VMM evidence
- Save the Firecracker API and VMM logs at the highest useful verbosity for the reproduction.
- Preserve the guest serial-console output in a host-side file, if the setup supports it. Do not rely on the guest journal alone: it may not be available if boot fails early.
- Record the VM’s start and exit times, exit status, and the exact command or supervisor action used to launch it.
Capture guest and agent evidence
- Collect the guest boot journal and the agent service’s journal for the failing time window.
- Save the agent’s standard output and error, tool subprocess exit codes, and the model or tool request IDs available for the run.
- Record the effective environment, working directory, user ID, installed packages, and relevant configuration as seen from inside the guest. Redact secret values before sharing logs.
Record the configuration that makes the run reproducible
Include the exact kernel command line, root filesystem image identifier, Firecracker version, vCPU and memory settings, network setup, mounts, and relevant environment-variable names and non-secret values. Without these, a second run may not test the same VM even if it uses the same prompt.
How do I use journalctl to narrow down the failure?
journalctl can filter entries by boot, unit, priority, and time. Run it on the host to inspect host services and the VM supervisor; run it inside the guest to inspect guest services and the agent, when the guest booted far enough to retain a journal. The systemd journalctl manual documents these filters.
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For a quick investigation, adapt these examples to the relevant machine and service:
journalctl -b— entries from the current boot. Use this to keep unrelated earlier boots out of the first pass.journalctl -b -u agent.service— entries for one service during the current boot. Substitute the actual unit name.journalctl -b -p warning— warnings and more severe entries from the current boot. If that hides useful context, inspect the complete boot log as well.journalctl -b --since "2026-10-26 10:00:00" --until "2026-10-26 10:05:00"— entries within a bounded time window. Replace the example times with the run’s actual timestamps and timezone.
On the host, first search the supervisor’s unit and the time around VM launch or exit. In the guest, search the boot and agent unit. Compare the journal timestamps with the serial log and agent request times. A guest journal showing nothing is not evidence that nothing failed: the guest may have stopped before the journal was available, or its output may not have been retained.
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Reduce the workflow until it exercises only the failing boundary: one fixed prompt, one model request, and one tool call. Use the same model and tool versions, inputs, and relevant configuration in both a known-good VM and the failing VM; record request IDs and timestamps so you can line up their logs.
- Choose one failing example. Fix the prompt or input and note the expected model response, tool action, and outcome. Avoid running the full multi-step workflow until you know which part matters.
- Pin the moving parts. Record the model and tool versions, VM configuration, agent build, and relevant policy settings for each run. Keep concurrency and other variable inputs controlled.
- Run the smallest case in both environments. First confirm the same input succeeds in the known-good VM; then run it in the failing VM. Preserve each run’s serial, VMM, journal, and agent logs.
- Compare at the first divergence. Check whether both VMs booted, whether the service environment matches, whether requests reached the same endpoint, and whether responses and tool exit codes match. Compare configuration and telemetry rather than changing multiple settings at once.
- Vary one factor at a time. If the minimal case passes, add back one feature—such as larger input, a second tool call, concurrency, or carried state—until the failure returns. The first change that reproduces it narrows the investigation.
This method helps separate an input-size or serialization problem from a permission, network, timeout, concurrency, or state-related failure. Keep one known-good run as a baseline; without one, a change that appears to help may simply have changed the conditions.
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Which microVM boundaries should I check?
Compare the VM’s actual runtime environment with what the agent and its tools assume. Work through boot, devices, files, network, resources, and process state in that order; these checks cover common differences between a host run and an isolated guest.
Boot and kernel
- Verify that the expected kernel image and command-line parameters were supplied, and that the root device points to the intended filesystem.
- Confirm the guest’s init system and expected boot path. Use serial output to identify how far the kernel and early userspace progress.
- Check that the serial console is configured for the guest’s actual console device. A silent console may indicate a console-path mismatch rather than a silent kernel.
Devices, jailer, and sandbox policy
- Confirm that the guest has the virtio devices the workload needs, without assuming a device available on the host is also present in the VM.
- Check ownership and permissions of jailer directories and files, and whether the VMM has the required capabilities.
- Review seccomp and other sandbox restrictions for a denied operation. Distinguish an intentional policy denial from a missing device or ordinary application error.
Filesystem and mounts
- For each expected mount, verify it exists inside the guest at the path the agent uses and has the intended read/write mode.
- Check that certificates, model configuration, and tool binaries are present in the guest filesystem. A host path existing outside the VM does not establish that the guest can see it.
- Compare file ownership and permissions with the agent’s guest user. If a tool reports “not found,” check both the executable path and its required runtime files.
Network and endpoint trust
- Inspect the guest interface and routes, then verify the configured DNS resolver and whether the target hostname resolves from inside the guest.
- Check the tap or NAT setup, egress policy, and any proxy variables the agent expects. A successful host request does not show that the guest has the same route or policy.
- For TLS failures, inspect the guest’s certificate trust configuration and clock behavior as well as endpoint reachability. Connectivity and certificate validation are separate checks.
Resources and process state
- Compare memory and vCPU allocation, file-descriptor and process limits, and cgroup constraints with the agent’s needs. Look in the journal for out-of-memory kills or throttling clues.
- From inside the guest, capture the agent’s effective environment, working directory, user ID, installed packages, and tool subprocess exit codes.
- Check both application timeouts and wall-clock or watchdog limits. A tool may be working but exceed a guest or supervisor deadline.
How do I tell whether the VMM, guest kernel, or agent is broken?
Use the earliest confirmed event in the logs, not the most visible final error. A VM that exits before a login prompt points toward launch or boot; a booted guest without a running agent points toward guest service setup; an agent that runs but fails on a tool call points higher, toward connectivity, credentials, tool behavior, or policy.
| Observed pattern | Where to investigate next | Useful discriminating check |
|---|---|---|
| No serial output | Kernel path, console parameters, wrong image, or VM launch failure | Compare VMM logs and launch configuration; verify the guest console is attached to the expected output path. |
| Kernel output appears, but init fails | Root filesystem, init configuration, mounts, or permissions | Identify the last boot stage in the serial output and check whether required paths exist inside the guest. |
| Agent starts, but every external call times out | Routes, DNS, NAT, proxy, egress policy, or clock/TLS trust | Check name resolution, route and endpoint access from inside the guest; compare the same request from the known-good VM. |
| Only large prompts fail | Memory pressure, request-size limits, serialization, or tool timeout budgets | Reduce input size while holding the tool and VM configuration constant; inspect resource and request errors. |
| Retries increase cost or latency | Retry policy, idempotency, backoff, or a deterministic failure being retried | Trace each attempt and compare error class and timing; determine whether repeated attempts are safe and whether the original error changes. |
Do not assume a timeout identifies the network as the cause. It can also reflect an unreachable endpoint, a stalled subprocess, a blocked request, or a deadline that expired while another layer was waiting. Match the timeout to the component that emitted it and inspect the surrounding events.
How should I trace model calls, tools, and policy decisions?
Instrument the agent so a single run can be followed across the host supervisor, guest, model requests, tool calls, retries, and policy checks. OpenTelemetry describes correlating logs, metrics, and traces with propagated context such as trace and span identifiers; see the OpenTelemetry documentation.
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Give each meaningful operation its own span
Emit a span for each model request, tool invocation, retry, policy decision, and final response. Attach a run ID and VM ID, plus a prompt or input hash, model name, tool name, latency, status, and error class. Use a hash rather than recording a full prompt when the full content is not needed to diagnose the issue.
Propagate context across the host and guest
Pass trace context from the host supervisor into the guest agent and between model and tool operations. When an error appears in one component, the shared context lets you find its parent operation and compare it with the relevant log entries. Preserve timestamps and request IDs as additional correlation points, particularly when a component cannot propagate tracing context.
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Keep diagnostic value without exposing secrets
Do not put credentials, authorization headers, or other secrets in trace attributes or logs. Retain enough non-secret metadata—such as operation name, endpoint category, status, latency, and error class—to establish what failed and when. Apply the same redaction discipline before sharing collected logs.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should I change after I find the failing layer?
Change only the component implicated by the first divergence, then rerun the same minimal case. A targeted fix is easier to validate than a simultaneous change to the kernel, network, agent, and timeout policy.
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- Launch or VMM failure: correct the launch configuration or device and sandbox setup indicated by the VMM logs, then confirm the guest produces the expected console output.
- Kernel or guest boot failure: correct the kernel, command line, root device, init, or mount issue that matches the observed boot stage. Verify the guest reaches its expected service state before testing the agent.
- Agent service failure: correct the guest-specific environment, permissions, missing package or file, or resource constraint. Confirm the service starts under its real guest user and working directory.
- Model or tool connectivity failure: repair the guest’s route, DNS, proxy, egress permission, credentials, or certificate trust as the evidence indicates. Re-test the actual endpoint from within the guest.
- Incorrect final response: inspect response parsing, prompt state, policy decisions, and retry handling using correlated spans. A successful tool call alone does not establish that the agent processed its result correctly.
- Deadline or retry issue: identify which layer’s deadline expired and whether the underlying failure is transient. Check idempotency and backoff before changing retry behavior, since repeated attempts can multiply latency or cost.
After the fix, rerun both the original minimal reproduction and the known-good baseline. Keep the logs and configuration identifiers from both runs so later changes can be checked against the same case.
Frequently Asked Questions
Can I debug a microVM if the guest never boots?
Yes, but the guest journal may not exist or may not be complete. Start with the host-side VMM logs and any serial-console output captured during launch; those can show failures before guest services are available.
Should I log the full prompt and tool output to diagnose failures?
Not by default. A prompt or input hash and operation metadata can help correlate runs while limiting exposure of user data; record full content only when it is necessary, permitted, and handled with appropriate access controls.
Why does a tool work on the host but fail in the guest?
The guest may have a different route, DNS resolver, proxy environment, filesystem view, certificate store, or egress policy. Test the tool’s actual prerequisites from inside the guest rather than using host success as a substitute.
What does a timeout tell me about an agent failure?
It tells you that an operation did not finish within a deadline, not which layer caused the delay. Correlating the timeout with request, subprocess, and VM timestamps helps distinguish a blocked call from a deadline that is simply too short.
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