Add structured logging to MCP transport

pkg/server/api/mcp/mcputils/logger.go

@@ -0,0 +1,116 @@
+// Copyright (c) 2025-2026 Probo Inc <hello@probo.com>.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+package mcputils
+
+import (
+	"context"
+	"log/slog"
+	"slices"
+
+	"go.gearno.de/kit/log"
+)
+
+type (
+	slogHandler struct {
+		logger *log.Logger
+		attrs  []slog.Attr
+		groups []slogGroup
+	}
+
+	slogGroup struct {
+		name  string
+		attrs []slog.Attr
+	}
+)
+
+// NewSlogLogger adapts the application logger for dependencies that use slog.
+func NewSlogLogger(logger *log.Logger) *slog.Logger {
+	return slog.New(&slogHandler{logger: logger})
+}
+
+func (h *slogHandler) Enabled(context.Context, slog.Level) bool {
+	return true
+}
+
+func (h *slogHandler) Handle(ctx context.Context, record slog.Record) error {
+	attrs := slices.Clone(h.attrs)
+	groupedAttrs := make([]slog.Attr, 0, record.NumAttrs())
+	record.Attrs(func(attr slog.Attr) bool {
+		groupedAttrs = append(groupedAttrs, attr)
+		return true
+	})
+
+	for i := len(h.groups) - 1; i >= 0; i-- {
+		groupAttrs := append(slices.Clone(h.groups[i].attrs), groupedAttrs...)
+		groupedAttrs = []slog.Attr{
+			slog.Group(h.groups[i].name, attrsToAny(groupAttrs)...),
+		}
+	}
+
+	h.logger.Log(ctx, record.Level, record.Message, append(attrs, groupedAttrs...)...)
+
+	return nil
+}
+
+func (h *slogHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	child := h.clone()
+	if len(child.groups) == 0 {
+		child.attrs = append(child.attrs, attrs...)
+	} else {
+		i := len(child.groups) - 1
+		child.groups[i].attrs = append(child.groups[i].attrs, attrs...)
+	}
+
+	return child
+}
+
+func (h *slogHandler) WithGroup(name string) slog.Handler {
+	if name == "" {
+		return h
+	}
+
+	child := h.clone()
+	child.groups = append(child.groups, slogGroup{name: name})
+
+	return child
+}
+
+func (h *slogHandler) clone() *slogHandler {
+	child := &slogHandler{
+		logger: h.logger,
+		attrs:  slices.Clone(h.attrs),
+		groups: slices.Clone(h.groups),
+	}
+	for i := range child.groups {
+		child.groups[i].attrs = slices.Clone(child.groups[i].attrs)
+	}
+
+	return child
+}
+
+func attrsToAny(attrs []slog.Attr) []any {
+	values := make([]any, len(attrs))
+	for i, attr := range attrs {
+		values[i] = attr
+	}
+
+	return values
+}

pkg/server/api/mcp/mcputils/logger_test.go

@@ -0,0 +1,70 @@
+// Copyright (c) 2025-2026 Probo Inc <hello@probo.com>.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+package mcputils_test
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.gearno.de/kit/log"
+	"go.probo.inc/probo/pkg/server/api/mcp/mcputils"
+)
+
+func TestNewSlogLogger_PreservesRecord(t *testing.T) {
+	t.Parallel()
+
+	var output bytes.Buffer
+	logger := log.NewLogger(
+		log.WithLevel(log.LevelDebug),
+		log.WithName("mcp.transport"),
+		log.WithOutput(&output),
+	)
+	slogLogger := mcputils.NewSlogLogger(logger).
+		With("component", "streamable").
+		WithGroup("request").
+		With("method", "POST").
+		WithGroup("details")
+
+	slogLogger.ErrorContext(context.Background(), "transport failed", "retryable", true)
+
+	var entry map[string]any
+	err := json.Unmarshal(output.Bytes(), &entry)
+	require.NoError(t, err)
+
+	assert.Equal(t, "ERROR", entry["level"])
+	assert.Equal(t, "transport failed", entry["msg"])
+	assert.Equal(t, "mcp.transport", entry["name"])
+	assert.Equal(t, "streamable", entry["component"])
+	assert.Equal(
+		t,
+		map[string]any{
+			"method": "POST",
+			"details": map[string]any{
+				"retryable": true,
+			},
+		},
+		entry["request"],
+	)
+}

pkg/server/api/mcp/v1/v1_handler.go

@@ -98,7 +98,7 @@ func NewMux(
 			Stateless: true,
 			// SessionTimeout: 30 * time.Minute,
 			EventStore: eventStore,
-			Logger:     nil, // TODO put logger here
+			Logger:     mcputils.NewSlogLogger(logger.Named("transport")),
 		},
 	)
 	protectedHandler := http.NewCrossOriginProtection().Handler(handler)