diff --git a/foomo/docs/projects/gotsrpc/playground/03_todos.md b/foomo/docs/projects/gotsrpc/playground/03_todos.md
index 26877e8..3161ae1 100644
--- a/foomo/docs/projects/gotsrpc/playground/03_todos.md
+++ b/foomo/docs/projects/gotsrpc/playground/03_todos.md
@@ -16,3 +16,7 @@ https://github.com/foomo/gotsrpc-playground/blob/main/server/server/todos.go
 
 
 ## Next.js TypeScript client
+
+```typescript reference title="client/pages/wheel-of-fortune.tsx"
+https://github.com/foomo/gotsrpc-playground/blob/main/client/pages/todos.tsx
+```
diff --git a/foomo/docs/projects/gotsrpc/playground/04_union-errors.md b/foomo/docs/projects/gotsrpc/playground/04_union-errors.md
new file mode 100644
index 0000000..f943e8d
--- /dev/null
+++ b/foomo/docs/projects/gotsrpc/playground/04_union-errors.md
@@ -0,0 +1,22 @@
+# Union Errors
+
+## Go Service
+
+Service interface defintion
+
+```go reference title="server/services/wof/service.go" lines
+https://github.com/foomo/gotsrpc-playground/blob/main/server/services/ouch/service.go
+```
+
+Service implementation
+
+```go reference title="server/services/helloworld/service.go"
+https://github.com/foomo/gotsrpc-playground/blob/main/server/server/ouch.go
+```
+
+
+## Next.js TypeScript client
+
+```typescript reference title="client/pages/wheel-of-fortune.tsx"
+https://github.com/foomo/gotsrpc-playground/blob/main/client/pages/ouch.tsx
+```
diff --git a/foomo/docs/projects/gotsrpc/service-interfaces/errors.md b/foomo/docs/projects/gotsrpc/service-interfaces/errors.md
index 585f20e..4d8d6bd 100644
--- a/foomo/docs/projects/gotsrpc/service-interfaces/errors.md
+++ b/foomo/docs/projects/gotsrpc/service-interfaces/errors.md
@@ -1,2 +1,90 @@
 # Errors
 
+On the Go server side errors are types, that implement the error type:
+
+```go
+// from https://go.dev/blog/error-handling-and-go
+type error interface {
+    Error() string
+}
+```
+
+This does not transport well, since an interface implementation can not be marshalled.
+
+## String Error Types
+
+Scalar string error types provide a nice way to combine Go constants, that translate to TypeScript enums as errors:
+
+```go title="Go"
+type ScalarError string
+
+const (
+    ErrorFoo ScalarError = "foo"
+    ErrorBar ScalarError = "bar"
+)
+
+func (e *ScalarError) Error() string {
+    return string(*e)
+}
+
+type Service interface {
+    MightGoWrong() *ScalarError
+}
+```
+
+```typescript title="TypeScript"
+export enum ScalarError {
+	ErrorBar = "bar",
+	ErrorFoo = "foo",
+}
+
+export interface ServiceClient {
+    mightGoWrong(): Promise<ScalarError | null>;
+}
+```
+
+## Struct Error Types
+
+If an enumeration is not enough and you want to add information to your errors a struct is a good choice (be careful not to expose secrets 😉) :
+
+- it can still implement the Error type
+- it is still typed in contrast to other alternatives like maps
+
+```go title="Go"
+type ErrorCode int
+
+const (
+    ErrorCodeFoo ErrorCode = 1
+    ErrorCodeBar ErrorCode = 2
+)
+
+type StructError struct {
+    Message string `json:"message,omitempty"`
+    Code ErrorCode `json:"errorCode"`
+}
+
+func (e *StructError) Error() string {
+    return e.Message
+}
+
+type Service struct {
+    MightGoWrong() *StructError
+}
+```
+
+```typescript title="TypeScript"
+export enum ErrorCode {
+	Bar = 2,
+	Foo = 1,
+}
+
+export interface StructError {
+	message?:string;
+	errorCode:ErrorCode;
+}
+
+export interface ServiceClient {
+    mightGoWrong(): Promise<StructError | null>;
+}
+```
+