diff --git a/internal/test/checks/checks_test.go b/internal/test/checks/checks_test.go
new file mode 100644
index 000000000..0677054df
--- /dev/null
+++ b/internal/test/checks/checks_test.go
@@ -0,0 +1,19 @@
+package checks_test
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/sashabaranov/go-openai/internal/test/checks"
+)
+
+func TestChecksSuccessPaths(t *testing.T) {
+	checks.NoError(t, nil)
+	checks.NoErrorF(t, nil)
+	checks.HasError(t, errors.New("err"))
+	target := errors.New("x")
+	checks.ErrorIs(t, target, target)
+	checks.ErrorIsF(t, target, target, "msg")
+	checks.ErrorIsNot(t, errors.New("y"), target)
+	checks.ErrorIsNotf(t, errors.New("y"), target, "msg")
+}
diff --git a/internal/test/failer_test.go b/internal/test/failer_test.go
new file mode 100644
index 000000000..fb1f4bf06
--- /dev/null
+++ b/internal/test/failer_test.go
@@ -0,0 +1,24 @@
+//nolint:testpackage // need access to unexported fields and types for testing
+package test
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestFailingErrorBuffer(t *testing.T) {
+	buf := &FailingErrorBuffer{}
+	n, err := buf.Write([]byte("test"))
+	if !errors.Is(err, ErrTestErrorAccumulatorWriteFailed) {
+		t.Fatalf("expected %v, got %v", ErrTestErrorAccumulatorWriteFailed, err)
+	}
+	if n != 0 {
+		t.Fatalf("expected n=0, got %d", n)
+	}
+	if buf.Len() != 0 {
+		t.Fatalf("expected Len 0, got %d", buf.Len())
+	}
+	if len(buf.Bytes()) != 0 {
+		t.Fatalf("expected empty bytes")
+	}
+}
diff --git a/internal/test/helpers_test.go b/internal/test/helpers_test.go
new file mode 100644
index 000000000..aa177679b
--- /dev/null
+++ b/internal/test/helpers_test.go
@@ -0,0 +1,54 @@
+package test_test
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+
+	internaltest "github.com/sashabaranov/go-openai/internal/test"
+)
+
+func TestCreateTestFile(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "file.txt")
+	internaltest.CreateTestFile(t, path)
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("failed to read created file: %v", err)
+	}
+	if string(data) != "hello" {
+		t.Fatalf("unexpected file contents: %q", string(data))
+	}
+}
+
+func TestTokenRoundTripperAddsHeader(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Header.Get("Authorization") != "Bearer "+internaltest.GetTestToken() {
+			t.Fatalf("authorization header not set")
+		}
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer srv.Close()
+
+	client := srv.Client()
+	client.Transport = &internaltest.TokenRoundTripper{Token: internaltest.GetTestToken(), Fallback: client.Transport}
+
+	req, err := http.NewRequest(http.MethodGet, srv.URL, nil)
+	if err != nil {
+		t.Fatalf("request error: %v", err)
+	}
+	resp, err := client.Do(req)
+	if err != nil {
+		t.Fatalf("client request error: %v", err)
+	}
+	if _, err = io.Copy(io.Discard, resp.Body); err != nil {
+		t.Fatalf("read body: %v", err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("unexpected status: %d", resp.StatusCode)
+	}
+}
diff --git a/internal/test/server.go b/internal/test/server.go
index 127d4c16f..d32c3e4cb 100644
--- a/internal/test/server.go
+++ b/internal/test/server.go
@@ -23,6 +23,18 @@ func NewTestServer() *ServerTest {
 	return &ServerTest{handlers: make(map[string]handler)}
 }
 
+// HandlerCount returns the number of registered handlers.
+func (ts *ServerTest) HandlerCount() int {
+	return len(ts.handlers)
+}
+
+// HasHandler checks if a handler was registered for the given path.
+func (ts *ServerTest) HasHandler(path string) bool {
+	path = strings.ReplaceAll(path, "*", ".*")
+	_, ok := ts.handlers[path]
+	return ok
+}
+
 func (ts *ServerTest) RegisterHandler(path string, handler handler) {
 	// to make the registered paths friendlier to a regex match in the route handler
 	// in OpenAITestServer
diff --git a/internal/test/server_test.go b/internal/test/server_test.go
new file mode 100644
index 000000000..f8ce731d1
--- /dev/null
+++ b/internal/test/server_test.go
@@ -0,0 +1,80 @@
+package test_test
+
+import (
+	"io"
+	"net/http"
+	"testing"
+
+	internaltest "github.com/sashabaranov/go-openai/internal/test"
+)
+
+func TestGetTestToken(t *testing.T) {
+	if internaltest.GetTestToken() != "this-is-my-secure-token-do-not-steal!!" {
+		t.Fatalf("unexpected token")
+	}
+}
+
+func TestNewTestServer(t *testing.T) {
+	ts := internaltest.NewTestServer()
+	if ts == nil {
+		t.Fatalf("server not properly initialized")
+	}
+	if ts.HandlerCount() != 0 {
+		t.Fatalf("expected no handlers initially")
+	}
+}
+
+func TestRegisterHandlerTransformsPath(t *testing.T) {
+	ts := internaltest.NewTestServer()
+	h := func(_ http.ResponseWriter, _ *http.Request) {}
+	ts.RegisterHandler("/foo/*", h)
+	if !ts.HasHandler("/foo/*") {
+		t.Fatalf("handler not registered with transformed path")
+	}
+}
+
+func TestOpenAITestServer(t *testing.T) {
+	ts := internaltest.NewTestServer()
+	ts.RegisterHandler("/v1/test/*", func(w http.ResponseWriter, _ *http.Request) {
+		if _, err := io.WriteString(w, "ok"); err != nil {
+			t.Fatalf("write: %v", err)
+		}
+	})
+	srv := ts.OpenAITestServer()
+	srv.Start()
+	defer srv.Close()
+
+	base := srv.Client().Transport
+	client := &http.Client{Transport: &internaltest.TokenRoundTripper{Token: internaltest.GetTestToken(), Fallback: base}}
+	resp, err := client.Get(srv.URL + "/v1/test/123")
+	if err != nil {
+		t.Fatalf("request error: %v", err)
+	}
+	body, err := io.ReadAll(resp.Body)
+	resp.Body.Close()
+	if err != nil {
+		t.Fatalf("read response body: %v", err)
+	}
+	if resp.StatusCode != http.StatusOK || string(body) != "ok" {
+		t.Fatalf("unexpected response: %d %q", resp.StatusCode, string(body))
+	}
+
+	// unregistered path
+	resp, err = client.Get(srv.URL + "/unknown")
+	if err != nil {
+		t.Fatalf("request error: %v", err)
+	}
+	if resp.StatusCode != http.StatusNotFound {
+		t.Fatalf("expected 404, got %d", resp.StatusCode)
+	}
+
+	// missing token should return unauthorized
+	clientNoToken := srv.Client()
+	resp, err = clientNoToken.Get(srv.URL + "/v1/test/123")
+	if err != nil {
+		t.Fatalf("request error: %v", err)
+	}
+	if resp.StatusCode != http.StatusUnauthorized {
+		t.Fatalf("expected 401, got %d", resp.StatusCode)
+	}
+}