Troubleshooting with Logfire

When a ValidationError is raised, the message tells you what went wrong: which field, which rule, and the value that triggered it. In production, the hard part is usually everything the message can’t show you: where that data came from, how often it happens, and what else your application was doing at the time. By the time you read the log, the payload that failed is often already gone.

Getting started

Troubleshooting validation errors in production is much easier when something records them as they happen, capturing the input alongside the error. Logfire’s Pydantic integration does this: it records each validation as it runs, so you can open the one that failed instead of reconstructing it from logs after the fact.

If you haven’t set it up yet, follow the three-step getting started guide, then instrument your application:

from datetime import date

import logfire

from pydantic import BaseModel

logfire.configure()
logfire.instrument_pydantic(record='failure')  # (1)


class User(BaseModel):
  name: str
  country_code: str
  dob: date


User(name='Anne', country_code='USA', dob='not-a-date')  # (2)

record='failure' records a trace for each failed validation, while still collecting metrics for all of them. Drop it (the default is record='all') if you also want a trace for every successful validation.

This validation fails because dob is not a valid date. Logfire records the input, the error, and the surrounding context, so you can troubleshoot it without adding any logging of your own.

Once instrumented, each failed validation shows up in the live view, recorded with:

• Its input: the exact data passed to validation, so you don’t have to reconstruct the payload from logs or guess what your model received.
• Its context: a span alongside the surrounding request, task, or trace, so you can follow bad data back to its source.
• A queryable history: every failure is stored, so you can ask “which field fails most often?” or “did this error spike after the last deploy?” in SQL.
• No extra logging code: one logfire.instrument_pydantic() call covers all your models; you don’t wrap each validation attempt in a try/except.

Recording the input naturally raises the question of sensitive data, since the failing payload may contain it. The Logfire SDK scrubs common sensitive values (things that look like passwords, tokens, or other secrets) from spans before they leave your machine, and you can extend the rules for your own fields.

Reading the error from the trace

Beyond the plain-language explanation, each failed validation span shows the raw structured errors() list next to the input that produced it: the field path (loc), the machine-readable type, and the offending value, so you can see which field failed and with what value without parsing the rendered message string by hand.

From one failure to the pattern

A single trace tells you about one failure. Often the more useful question is whether it’s a one-off or something recurring. Logfire groups repeated exceptions into issues, so a validation that fails a thousand times shows up as one entry with a count and a first-seen time, rather than a thousand lines to scroll through, which makes it easy to tell a genuine spike from background noise.

Once you know which failures matter, you don’t have to keep watching for them. Logfire alerts run a SQL query on a schedule and notify you (for example, in Slack) when it matches. A rule like “validation failures for this model crossed a threshold” means the next occurrence finds you instead of a user reporting it.

Have Logfire explain the error

Open a failed validation span in Pydantic Logfire and it explains the failure in plain language (currently in beta), reading the structured errors and, for each field, telling you what was expected and what it received, including the messages from your own custom validators. You get to the fix without memorising every error code.

If you debug with an AI coding agent, the Logfire MCP server lets the agent query your telemetry directly, including the input and errors from a failed validation, so it can investigate against your real data instead of guessing.

Learn more

• Pydantic Logfire integration: how to install and configure Logfire with Pydantic.
• Logfire documentation: the full Logfire docs.

For a reference of the individual error types you may encounter, see Validation Errors and Usage Errors.