Go version

go1.23.3 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE='on'
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/tjozsa/Library/Caches/go-build'
GOENV='/Users/tjozsa/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/tjozsa/go/pkg/mod'
GONOPROXY=''
GONOSUMDB='code.cfops.it,github.com/cloudflare,go.cfdata.org'
GOOS='darwin'
GOPATH='/Users/tjozsa/go'
GOPRIVATE=''
GOPROXY='https://athens.cfdata.org,direct'
GOROOT='/opt/homebrew/opt/go/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/opt/homebrew/opt/go/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.23.4'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/tjozsa/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOARM64='v8.0'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/Users/tjozsa/cf-repos/authnz-publisher/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/81/zyf3xcqd6wv1nh70f4pspr900000gp/T/go-build1369939013=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

When implementing a custom slog.Handler, the middleware pattern breaks in two significant ways. First, calling With() on a logger completely bypasses any custom Handle implementation, falling back to only the wrapped handler. This means any middleware logic in custom handlers stops working as soon as attributes are added via With().

Second, even if we try to work around the first issue by creating a new handler from the existing one, the attributes set through With() become inaccessible via Record.Attrs(). While these attributes still appear in the final log output through the wrapped handler, middleware can no longer access them programmatically.

These behaviors prevent proper implementation of logging middleware that needs to process all records with their complete context, such as forwarding logs to external services like Sentry.

What did you see happen?

To Reproduce

Here's a minimal example demonstrating both issues:

package main

import (
	"context"
	"fmt"
	"log/slog"
	"os"
)

type TestHandler struct {
	slog.Handler
}

func (h *TestHandler) Handle(ctx context.Context, r slog.Record) error {
	attrs := []string{}
	r.Attrs(func(a slog.Attr) bool {
		attrs = append(attrs, a.Key)
		return true
	})
	// Print captured attributes
	fmt.Printf("Custom Handle called! Captured attributes:%+v\n", attrs)
	return nil
}

func main() {
	h := &TestHandler{Handler: slog.NewTextHandler(os.Stdout, nil)}
	log := slog.New(h)

	// Case 1: Direct attributes
	log.Error("direct attributes", "key1", "value1")
	// Terminal output:
	// Custom Handle called! Captured attributes: [key1]
	// time=2025-01-03T11:39:52.504Z level=ERROR msg="with attributes" key2=value2

	logWithAttr := log.With("key2", "value2")
	logWithAttr.Error("with attributes")
	// Issue 1. The custom Handle implementation is no longer called
	// Terminal output:
	// time=2025-01-03T11:35:41.412Z level=ERROR msg="with attributes" key2=value2

	log = slog.New(&TestHandler{Handler: logWithAttr.Handler()})
	log.Error("missing attributes")
	// Issue 2. Attributes are not inherited between Handlers
	// Terminal output:
	// Custom Handle called! Captured attributes:[]

	log = log.With("key3", "value3")
	log.Error("correctly logging attributes")
	// Terminal output:
	//time=2025-01-03T13:28:21.189Z level=ERROR msg="correctly logging attributes" key2=value2 key3=value3
}

What did you expect to see?

These issues make it impossible to implement proper logging middleware that needs to:

Process all log records consistently, regardless of how they were created
Access the complete set of attributes for each record
Forward complete log context to external services

For example, when implementing Sentry integration, we cannot reliably capture all log records or their full context, leading to incomplete error reporting.