diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9da44db..df01f70 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -49,12 +49,14 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@v7
         with:
-          go-version: 'stable'
+          # Lint at the module's own Go. golangci-lint is a prebuilt binary
+          # that can only load stdlib export data from the Go it was built
+          # with; following 'stable' turns every Go release into a red lint
+          # job on every PR, triggered by nothing in the repo.
+          go-version-file: go.mod
 
       # golangci-lint v2 subsumes gofmt, gci, staticcheck, govet and the rest;
       # config in .golangci.yml. Generated files are auto-excluded by header.
-      # The official action ships a golangci-lint binary whose type-checker
-      # matches the Go toolchain, avoiding stdlib version skew.
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v9
         with:
diff --git a/flashduty_test.go b/flashduty_test.go
index 6a01336..fa1cda9 100644
--- a/flashduty_test.go
+++ b/flashduty_test.go
@@ -7,6 +7,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"reflect"
 	"strings"
 	"testing"
 	"time"
@@ -274,6 +275,42 @@ func TestIncidentNotificationOverrideOmitsUnsetPreference(t *testing.T) {
 	}
 }
 
+func TestCustomFieldValuesReachTheWire(t *testing.T) {
+	c, _ := NewClient("KEY", WithBaseURL("https://api.flashcat.cloud"), WithLogger(noopLogger{}))
+	values := CustomFieldValues{"region": "cn-beijing", "impact_users": float64(42), "tags": []any{"db"}}
+
+	tests := []struct {
+		name  string
+		path  string
+		body  any
+		field string
+	}{
+		{"create incident", "/incident/create", &CreateIncidentRequest{IncidentSeverity: "Critical", Fields: values}, "fields"},
+		{"ack incident", "/incident/ack", &AckIncidentRequest{CustomFields: values}, "custom_fields"},
+		{"resolve incident", "/incident/resolve", &ResolveIncidentRequest{CustomFields: values}, "custom_fields"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req, err := c.newRequest(context.Background(), http.MethodPost, tt.path, tt.body)
+			if err != nil {
+				t.Fatal(err)
+			}
+			body, err := io.ReadAll(req.Body)
+			if err != nil {
+				t.Fatal(err)
+			}
+			var payload map[string]any
+			if err := json.Unmarshal(body, &payload); err != nil {
+				t.Fatal(err)
+			}
+			got, _ := payload[tt.field].(map[string]any)
+			if !reflect.DeepEqual(got, map[string]any(values)) {
+				t.Fatalf("%s = %#v, want %#v (body = %s)", tt.field, payload[tt.field], values, body)
+			}
+		})
+	}
+}
+
 func TestNewRequestAppliesHookAndHeaders(t *testing.T) {
 	c, _ := NewClient("KEY",
 		WithRequestHeaders(map[string][]string{"X-Static": {"s"}}),
diff --git a/internal/cmd/gen/main.go b/internal/cmd/gen/main.go
index 60f97f1..13feae9 100644
--- a/internal/cmd/gen/main.go
+++ b/internal/cmd/gen/main.go
@@ -646,6 +646,10 @@ func (g *Gen) emitModels() string {
 			// a map payload (e.g. name -> count), not a struct.
 			v := g.goTypeOf(asMap(s["additionalProperties"]), goName(name)+"Value")
 			fmt.Fprintf(&enumsAndAliases, "// %s is a map response payload.\ntype %s map[string]%s\n\n", goName(name), goName(name), v)
+		case typeStr(s) == "object" && len(asMap(s["properties"])) == 0 && s["additionalProperties"] == true:
+			// A bare object with additionalProperties: true carries arbitrary
+			// keys (e.g. custom field values); a struct would drop them all.
+			fmt.Fprintf(&enumsAndAliases, "// %s is a free-form JSON object.\ntype %s map[string]any\n\n", goName(name), goName(name))
 		case typeStr(s) == "string", typeStr(s) == "integer", typeStr(s) == "number", typeStr(s) == "boolean":
 			gt := g.goTypeOf(s, goName(name))
 			fmt.Fprintf(&enumsAndAliases, "type %s %s\n\n", goName(name), gt)
diff --git a/models_gen.go b/models_gen.go
index 3a672e7..1d36a16 100644
--- a/models_gen.go
+++ b/models_gen.go
@@ -26,6 +26,9 @@ func (e AlertFeedType) String() string { return string(e) }
 // AlertRuleExportListResponse is a list response payload.
 type AlertRuleExportListResponse []AlertRuleExport
 
+// CustomFieldValues is a free-form JSON object.
+type CustomFieldValues map[string]any
+
 // DataSourceListResponse is a list response payload.
 type DataSourceListResponse []DataSourceItem
 
@@ -2420,9 +2423,6 @@ type CreateWorkItemRequest struct {
 	Title string `json:"title" toon:"title"`
 }
 
-// CustomFieldValues is generated from the Flashduty OpenAPI schema.
-type CustomFieldValues struct{}
-
 // DsClickHouseConfig is generated from the Flashduty OpenAPI schema.
 type DsClickHouseConfig struct {
 	// Default database for authentication.