diff --git a/pkg/cli/lambda_server__added.go b/pkg/cli/lambda_server__added.go
index 006f4b9d8..fae724ae3 100644
--- a/pkg/cli/lambda_server__added.go
+++ b/pkg/cli/lambda_server__added.go
@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"os"
 	"runtime/debug"
+	"strings"
 	"sync"
 	"time"
 
@@ -22,6 +23,7 @@ import (
 	"github.com/go-jose/go-jose/v4"
 	"github.com/maypok86/otter/v2"
 	"github.com/mitchellh/mapstructure"
+	"github.com/spf13/cast"
 	"github.com/spf13/cobra"
 	"github.com/spf13/viper"
 	"go.uber.org/zap"
@@ -334,7 +336,7 @@ func OptionallyAddLambdaCommand[T field.Configurable](
 			}
 
 			connectorConfig := configStruct.AsMap()
-			effectiveConfig := effectiveLambdaConfig(v, connectorConfig)
+			effectiveConfig := effectiveLambdaConfig(v, connectorConfig, connectorSchema)
 			logLevelConfig, err := lambdaLogLevelConfigFromViper(effectiveConfig)
 			if err != nil {
 				return nil, err
@@ -488,14 +490,48 @@ func cloneViperSettings(v *viper.Viper) *viper.Viper {
 	return cloned
 }
 
-func effectiveLambdaConfig(v *viper.Viper, connectorConfig map[string]any) *viper.Viper {
+func effectiveLambdaConfig(v *viper.Viper, connectorConfig map[string]any, connectorSchema field.Configuration) *viper.Viper {
 	effectiveConfig := cloneViperSettings(v)
+	stringMapFields := lambdaStringMapFields(connectorSchema)
 	for key, value := range connectorConfig {
+		if _, ok := stringMapFields[strings.ToLower(key)]; ok {
+			value = lambdaStringMapValueForViper(value)
+		}
 		effectiveConfig.Set(key, value)
 	}
 	return effectiveConfig
 }
 
+func lambdaStringMapFields(connectorSchema field.Configuration) map[string]struct{} {
+	out := make(map[string]struct{})
+	for _, schemaField := range connectorSchema.Fields {
+		if schemaField.Variant == field.StringMapVariant {
+			out[strings.ToLower(schemaField.FieldName)] = struct{}{}
+		}
+	}
+	for _, fieldGroup := range connectorSchema.FieldGroups {
+		for _, schemaField := range fieldGroup.Fields {
+			if schemaField.Variant == field.StringMapVariant {
+				out[strings.ToLower(schemaField.FieldName)] = struct{}{}
+			}
+		}
+	}
+	return out
+}
+
+func lambdaStringMapValueForViper(value any) any {
+	if _, ok := value.(string); ok {
+		return value
+	}
+	// Viper.GetStringMap and GetStringMapString parse JSON strings, while
+	// Viper.Set lowercases nested map[string]any keys.
+	encoded, err := json.Marshal(cast.ToStringMapString(value))
+	if err != nil {
+		return value
+	}
+	return string(encoded)
+}
+
 // Typed configs must decode the raw lambda payload directly. Decoding them from
 // effectiveConfig would send StringMap values through Viper.Set, which lowercases
 // nested map keys.
diff --git a/pkg/cli/lambda_server_test.go b/pkg/cli/lambda_server_test.go
index 7cf168a5e..3a4f2e81d 100644
--- a/pkg/cli/lambda_server_test.go
+++ b/pkg/cli/lambda_server_test.go
@@ -8,6 +8,7 @@ import (
 	"testing"
 
 	v2 "github.com/conductorone/baton-sdk/pb/c1/connector/v2"
+	"github.com/conductorone/baton-sdk/pkg/field"
 	"github.com/spf13/viper"
 )
 
@@ -107,7 +108,7 @@ func TestEffectiveLambdaConfigSyncResourceTypeIDs(t *testing.T) {
 		"sync-resource-types": []any{"user", "group"},
 	}
 
-	effectiveConfig := effectiveLambdaConfig(base, connectorConfig)
+	effectiveConfig := effectiveLambdaConfig(base, connectorConfig, field.Configuration{})
 
 	got := effectiveConfig.GetStringSlice("sync-resource-types")
 	want := []string{"user", "group"}
@@ -116,6 +117,44 @@ func TestEffectiveLambdaConfigSyncResourceTypeIDs(t *testing.T) {
 	}
 }
 
+func TestEffectiveLambdaConfigPreservesStringMapKeyCaseForViper(t *testing.T) {
+	t.Parallel()
+
+	base := viper.New()
+	connectorConfig := map[string]any{
+		"user-custom-fields": map[string]any{
+			"departmentNav/name": "Department",
+			"customString10":     "Custom String 10",
+			"isActive":           true,
+		},
+	}
+	schema := field.NewConfiguration([]field.SchemaField{
+		field.StringMapField("user-custom-fields"),
+	})
+
+	effectiveConfig := effectiveLambdaConfig(base, connectorConfig, schema)
+
+	gotStringMap := effectiveConfig.GetStringMap("user-custom-fields")
+	wantStringMap := map[string]any{
+		"departmentNav/name": "Department",
+		"customString10":     "Custom String 10",
+		"isActive":           "true",
+	}
+	if !reflect.DeepEqual(gotStringMap, wantStringMap) {
+		t.Fatalf("GetStringMap(user-custom-fields) = %#v, want %#v", gotStringMap, wantStringMap)
+	}
+
+	gotStringMapString := effectiveConfig.GetStringMapString("user-custom-fields")
+	wantStringMapString := map[string]string{
+		"departmentNav/name": "Department",
+		"customString10":     "Custom String 10",
+		"isActive":           "true",
+	}
+	if !reflect.DeepEqual(gotStringMapString, wantStringMapString) {
+		t.Fatalf("GetStringMapString(user-custom-fields) = %#v, want %#v", gotStringMapString, wantStringMapString)
+	}
+}
+
 func TestMakeLambdaConnectorConfigurationPreservesStringMapKeyCase(t *testing.T) {
 	t.Parallel()
 
@@ -127,7 +166,10 @@ func TestMakeLambdaConnectorConfigurationPreservesStringMapKeyCase(t *testing.T)
 		},
 	}
 
-	config, err := makeLambdaConnectorConfiguration[*testLambdaStringMapConfig](base, effectiveLambdaConfig(base, connectorConfig), connectorConfig)
+	schema := field.NewConfiguration([]field.SchemaField{
+		field.StringMapField("user-custom-fields"),
+	})
+	config, err := makeLambdaConnectorConfiguration[*testLambdaStringMapConfig](base, effectiveLambdaConfig(base, connectorConfig, schema), connectorConfig)
 	if err != nil {
 		t.Fatalf("makeLambdaConnectorConfiguration: %v", err)
 	}