diff --git a/internal/apierror/apierror.go b/internal/apierror/apierror.go
new file mode 100644
index 00000000..6ee53801
--- /dev/null
+++ b/internal/apierror/apierror.go
@@ -0,0 +1,65 @@
+package apierror
+
+import (
+	"errors"
+	"fmt"
+)
+
+type Error struct {
+	Status  int
+	Code    string
+	Message string
+	Cause   error
+}
+
+func (e *Error) Error() string {
+	if e.Cause != nil {
+		return fmt.Sprintf("%s: %v", e.Message, e.Cause)
+	}
+	return e.Message
+}
+
+func (e *Error) Unwrap() error { return e.Cause }
+
+// From returns err as *Error if it (or anything in its chain) is one;
+// otherwise wraps err in an Internal.
+func From(err error) *Error {
+	if err == nil {
+		return nil
+	}
+	var apiErr *Error
+	if errors.As(err, &apiErr) {
+		return apiErr
+	}
+	return Internal(err)
+}
+
+func NotFound(what string) *Error {
+	return &Error{Status: 404, Code: what + "_not_found", Message: what + " not found"}
+}
+
+func BadRequest(code, message string) *Error {
+	return &Error{Status: 400, Code: code, Message: message}
+}
+
+func Conflict(code, message string) *Error {
+	return &Error{Status: 409, Code: code, Message: message}
+}
+
+func Forbidden(code, message string) *Error {
+	return &Error{Status: 403, Code: code, Message: message}
+}
+
+func Unauthorized(code, message string) *Error {
+	return &Error{Status: 401, Code: code, Message: message}
+}
+
+func Internal(cause error) *Error {
+	return &Error{Status: 500, Code: "server_error", Message: "internal server error", Cause: cause}
+}
+
+var (
+	ErrNotFound     = &Error{Status: 404, Code: "not_found", Message: "not found"}
+	ErrForbidden    = &Error{Status: 403, Code: "forbidden", Message: "forbidden"}
+	ErrUnauthorized = &Error{Status: 401, Code: "unauthorized", Message: "authentication required"}
+)
diff --git a/internal/apierror/apierror_test.go b/internal/apierror/apierror_test.go
new file mode 100644
index 00000000..1a0b212f
--- /dev/null
+++ b/internal/apierror/apierror_test.go
@@ -0,0 +1,156 @@
+package apierror
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestError_ErrorString_NoCause(t *testing.T) {
+	e := &Error{Message: "x"}
+	if got := e.Error(); got != "x" {
+		t.Fatalf("Error() = %q, want %q", got, "x")
+	}
+}
+
+func TestError_ErrorString_WithCause(t *testing.T) {
+	e := &Error{Message: "x", Cause: errors.New("y")}
+	if got := e.Error(); got != "x: y" {
+		t.Fatalf("Error() = %q, want %q", got, "x: y")
+	}
+}
+
+func TestError_Unwrap(t *testing.T) {
+	cause := errors.New("boom")
+	e := &Error{Message: "x", Cause: cause}
+	if got := e.Unwrap(); got != cause {
+		t.Fatalf("Unwrap() = %v, want %v", got, cause)
+	}
+
+	e2 := &Error{Message: "x"}
+	if got := e2.Unwrap(); got != nil {
+		t.Fatalf("Unwrap() with nil Cause = %v, want nil", got)
+	}
+}
+
+func TestFrom_NilReturnsNil(t *testing.T) {
+	if got := From(nil); got != nil {
+		t.Fatalf("From(nil) = %v, want nil", got)
+	}
+}
+
+func TestFrom_PreservesAPIError(t *testing.T) {
+	orig := NotFound("track")
+	got := From(orig)
+	if got != orig {
+		t.Fatalf("From(orig) returned different pointer: got %p, want %p", got, orig)
+	}
+}
+
+func TestFrom_PreservesWrappedAPIError(t *testing.T) {
+	inner := NotFound("track")
+	wrapped := fmt.Errorf("ctx: %w", inner)
+	got := From(wrapped)
+	if got == nil {
+		t.Fatal("From(wrapped) = nil, want *Error")
+	}
+	if got.Code != "track_not_found" {
+		t.Fatalf("got.Code = %q, want %q", got.Code, "track_not_found")
+	}
+}
+
+func TestFrom_WrapsPlainError(t *testing.T) {
+	plain := errors.New("boom")
+	got := From(plain)
+	if got == nil {
+		t.Fatal("From(plain) = nil, want *Error")
+	}
+	if got.Status != 500 {
+		t.Errorf("Status = %d, want 500", got.Status)
+	}
+	if got.Code != "server_error" {
+		t.Errorf("Code = %q, want %q", got.Code, "server_error")
+	}
+	if got.Cause != plain {
+		t.Errorf("Cause = %v, want %v", got.Cause, plain)
+	}
+}
+
+func TestConstructors_FieldsCorrect(t *testing.T) {
+	tests := []struct {
+		name        string
+		err         *Error
+		wantStatus  int
+		wantCode    string
+		wantMessage string
+	}{
+		{
+			name:        "NotFound",
+			err:         NotFound("track"),
+			wantStatus:  404,
+			wantCode:    "track_not_found",
+			wantMessage: "track not found",
+		},
+		{
+			name:        "BadRequest",
+			err:         BadRequest("invalid_input", "invalid input"),
+			wantStatus:  400,
+			wantCode:    "invalid_input",
+			wantMessage: "invalid input",
+		},
+		{
+			name:        "Conflict",
+			err:         Conflict("already_exists", "already exists"),
+			wantStatus:  409,
+			wantCode:    "already_exists",
+			wantMessage: "already exists",
+		},
+		{
+			name:        "Forbidden",
+			err:         Forbidden("forbidden", "forbidden"),
+			wantStatus:  403,
+			wantCode:    "forbidden",
+			wantMessage: "forbidden",
+		},
+		{
+			name:        "Unauthorized",
+			err:         Unauthorized("unauthorized", "authentication required"),
+			wantStatus:  401,
+			wantCode:    "unauthorized",
+			wantMessage: "authentication required",
+		},
+		{
+			name:        "Internal",
+			err:         Internal(errors.New("boom")),
+			wantStatus:  500,
+			wantCode:    "server_error",
+			wantMessage: "internal server error",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.err.Status != tt.wantStatus {
+				t.Errorf("Status = %d, want %d", tt.err.Status, tt.wantStatus)
+			}
+			if tt.err.Code != tt.wantCode {
+				t.Errorf("Code = %q, want %q", tt.err.Code, tt.wantCode)
+			}
+			if tt.err.Message != tt.wantMessage {
+				t.Errorf("Message = %q, want %q", tt.err.Message, tt.wantMessage)
+			}
+		})
+	}
+}
+
+func TestSentinels_Identity(t *testing.T) {
+	wrapped := fmt.Errorf("ctx: %w", ErrNotFound)
+
+	var apiErr *Error
+	if !errors.As(wrapped, &apiErr) {
+		t.Fatal("errors.As(wrapped, &apiErr) = false, want true")
+	}
+	if apiErr != ErrNotFound {
+		t.Fatalf("recovered error pointer = %p, want %p", apiErr, ErrNotFound)
+	}
+}