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Logging helps you debug your app during local development and after deployment. In fal serverless, anything your app writes to stdout or stderr is captured in runner logs.

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Runner Logs vs Request Logs

  • Runner logs are logs produced by a runner process over its lifetime (startup, setup, and request handling).
  • Request logs are the portion of runner logs emitted while a specific request is being processed.
Request logs (the logs visible to an end user for a request) are a time-scoped subset of runner logs.

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Basic Logging with print

For quick debugging, print() is enough: 
import fal
import sys

class MyApp(fal.App):
    @fal.endpoint("/")
    def run(self) -> dict:
        print("Processing Hello World request...")
        print("Cache miss, loading resources", file=sys.stderr)
        return {"message": "Hello, World!"}
Both messages are captured in logs because both stdout and stderr are collected.

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Application Logging with Python logging

For production apps, use Python’s standard logging module so you can control log levels and message format. 
import logging
import fal

logging.basicConfig(
    level=logging.INFO,
    format="%(asctime)s %(levelname)s %(name)s: %(message)s",
)
logger = logging.getLogger("my_app")

class MyApp(fal.App):
    @fal.endpoint("/")
    def run(self) -> dict:
        logger.info("Started request processing")
        try:
            # Your application logic
            result = {"message": "Hello, World!"}
            logger.info("Request completed successfully")
            return result
        except Exception:
            logger.exception("Request failed")
            raise
Avoid logging secrets, API keys, or large raw payloads. Log IDs, sizes, and high-level status instead.

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Log Sources

When filtering logs by source, you’ll commonly see:
  • run: logs from runners created by fal run
  • gateway: logs from runners serving deployed apps
  • deploy: logs emitted by the deployment process itself when using fal deploy

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Where to View Logs