Warnings

Table of contents

• Warnings
  • Warnings In Fastify
  • Fastify Warning Codes
    • FSTWRN001
    • FSTWRN002
  • Fastify Deprecation Codes

Warnings

Warnings In Fastify

Fastify uses Node.js's warning event API to notify users of deprecated features and coding mistakes. Fastify's warnings are recognizable by the FSTWRN and FSTDEP prefixes. When encountering such a warning, it is highly recommended to determine the cause using the --trace-warnings and --trace-deprecation flags. These produce stack traces pointing to where the issue occurs in the application's code. Issues opened about warnings without this information will be closed due to lack of details.

Warnings can also be disabled, though it is not recommended. If necessary, use one of the following methods:

• Set the NODE_NO_WARNINGS environment variable to 1
• Pass the --no-warnings flag to the node process
• Set no-warnings in the NODE_OPTIONS environment variable

For more information on disabling warnings, see Node's documentation.

Disabling warnings may cause issues when upgrading Fastify versions. Only experienced users should consider disabling warnings.

Fastify Warning Codes

Code	Description	How to solve	Discussion
FSTWRN001	The specified schema for a route is missing. This may indicate the schema is not well specified.	Check the schema for the route.	#4647
FSTWRN002	The %s plugin being registered mixes async and callback styles, which will result in an error in fastify@5.	Do not mix async and callback style.	#5139

Fastify Deprecation Codes

Deprecation codes are supported by the Node.js CLI options:

• --no-deprecation
• --throw-deprecation
• --trace-deprecation

Code	Description	How to solve	Discussion