Handlers are supposed to avoid logging empty attributes.

• "- If an Attr's key and value are both the zero value, ignore the Attr."

Handlers should not log groups (or subgroups) without fields, whether or not they have non-empty names.

• "- If a group has no Attrs (even if it has a non-empty key), ignore it."

Handlers should expand groups named "" (the empty string) into the enclosing log record.

• "- If a group's key is empty, inline the group's Attrs."

Documentation on building a handler suggests testing using slog/slogtest, part of the Go release since 1.21. While the verification suite includes tests patterned after those in slogtest, an additional single test invokes the slogtest testing sequence (involving multiple tests). Since this is a separate package, all error messages are returned at once. This is the only warning that affects TestSlogTest and all of its error messages.

• "To verify that your handler follows these rules and generally produces proper output, use the testing/slogtest package."
• slog/slogtest

Handlers should not log WithGroup() groups with no fields, whether or not they have non-empty names.

• "- If a group has no Attrs (even if it has a non-empty key), ignore it."

Handlers should not log the basic time field if it is zero.

• "- If r.Time is the zero time, ignore the time."

The field name of the "message" key should be msg.

• Constant values are defined for slog/log
  
• Field values are defined for the JSONHandler.Handle() implementation

If HandlerOptions.ReplaceAttr is provided it should be honored by the handler. However, documentation on implementing handler methods seems to suggest it is optional.

• Behavior defined for slog.HandlerOptions
  
• "You might also consider adding a ReplaceAttr option to your handler, like the one for the built-in handlers."

Some handlers return source data as a single string on the caller field in the format <file>:<line> where <file> and <line> correspond to the File and Line fields of the source data group and Function is not provided.

• Behavior defined for JSONHandler.Handle()
• Definition of source data record

Some handlers (e.g. slog.JSONHandler) will output multiple occurrences of the same field name if the logger is called with multiple instances of the same field, generally by using WithAttrs and then the same fields in the eventual log call (e.g. Info). This behavior is currently under debate with no resolution at this time (2024-01-15) and a release milestone of (currently unscheduled) Go 1.23, (whereas Go Release 1.22 is currently expected in February 2024).

The slog.JSONHandler uses nanoseconds for time.Duration but some other handlers use seconds.

• Go issue 59345: Nanoseconds is a recent change with Go 1.21

Each JSON log record contains the logging level of the log statement as a string. Different handlers provide that string in uppercase or lowercase. Documentation for slog.Level says that its String() and MarshalJSON() methods will return uppercase but UnmarshalJSON() will parse in a case-insensitive manner.

The slog.JSONHandler uses nanoseconds for time.Time but some other handlers use milliseconds. This does not apply to the basic time field, only attribute fields. I can't find any supporting documentation or bug on this but Go issue 59345 (see previous warning) may have fixed this as well in Go 1.21.