diff --git a/repl/repl_test.go b/repl/repl_test.go
new file mode 100644
index 0000000..1c902b1
--- /dev/null
+++ b/repl/repl_test.go
@@ -0,0 +1,150 @@
+package repl
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestRun_SingleExpression(t *testing.T) {
+	in := strings.NewReader("1 + 2\n")
+	var out bytes.Buffer
+
+	Run(in, &out)
+
+	got := out.String()
+	if !strings.Contains(got, "3") {
+		t.Errorf("expected output to contain '3', got %q", got)
+	}
+}
+
+func TestRun_MultipleExpressions(t *testing.T) {
+	in := strings.NewReader("1 + 2\n3 * 4\n")
+	var out bytes.Buffer
+
+	Run(in, &out)
+
+	got := out.String()
+	if !strings.Contains(got, "3") {
+		t.Errorf("expected output to contain '3', got %q", got)
+	}
+	if !strings.Contains(got, "12") {
+		t.Errorf("expected output to contain '12', got %q", got)
+	}
+}
+
+func TestRun_ErrorContinues(t *testing.T) {
+	// First line has error, second is valid.
+	in := strings.NewReader("1 +\n2 + 3\n")
+	var out bytes.Buffer
+
+	Run(in, &out)
+
+	got := out.String()
+	if !strings.Contains(got, "error:") {
+		t.Errorf("expected output to contain 'error:', got %q", got)
+	}
+	if !strings.Contains(got, "5") {
+		t.Errorf("expected output to contain '5' after error recovery, got %q", got)
+	}
+}
+
+func TestRun_DivisionByZero(t *testing.T) {
+	in := strings.NewReader("1 / 0\n")
+	var out bytes.Buffer
+
+	Run(in, &out)
+
+	got := out.String()
+	if !strings.Contains(got, "error:") {
+		t.Errorf("expected output to contain 'error:', got %q", got)
+	}
+	if !strings.Contains(got, "division by zero") {
+		t.Errorf("expected 'division by zero' in output, got %q", got)
+	}
+}
+
+func TestRun_EmptyLine(t *testing.T) {
+	// Empty lines should be skipped, not cause errors.
+	in := strings.NewReader("\n1 + 1\n")
+	var out bytes.Buffer
+
+	Run(in, &out)
+
+	got := out.String()
+	if !strings.Contains(got, "2") {
+		t.Errorf("expected output to contain '2', got %q", got)
+	}
+	// Should not contain any error.
+	if strings.Contains(got, "error:") {
+		t.Errorf("empty line should not produce error, got %q", got)
+	}
+}
+
+func TestRun_Prompt(t *testing.T) {
+	in := strings.NewReader("42\n")
+	var out bytes.Buffer
+
+	Run(in, &out)
+
+	got := out.String()
+	if !strings.Contains(got, ">> ") {
+		t.Errorf("expected prompt '>> ' in output, got %q", got)
+	}
+}
+
+func TestRun_FloatResult(t *testing.T) {
+	in := strings.NewReader("7 / 2\n")
+	var out bytes.Buffer
+
+	Run(in, &out)
+
+	got := out.String()
+	if !strings.Contains(got, "3.5") {
+		t.Errorf("expected output to contain '3.5', got %q", got)
+	}
+}
+
+func TestRun_InvalidCharacter(t *testing.T) {
+	in := strings.NewReader("1 @ 2\n")
+	var out bytes.Buffer
+
+	Run(in, &out)
+
+	got := out.String()
+	if !strings.Contains(got, "error:") {
+		t.Errorf("expected output to contain 'error:', got %q", got)
+	}
+}
+
+func TestRun_EmptyInput(t *testing.T) {
+	// No input at all — just EOF.
+	in := strings.NewReader("")
+	var out bytes.Buffer
+
+	Run(in, &out)
+
+	got := out.String()
+	// Should just show the prompt and exit gracefully.
+	if !strings.Contains(got, ">> ") {
+		t.Errorf("expected at least one prompt, got %q", got)
+	}
+}
+
+func TestRun_WholeIntegerNoTrailingZeros(t *testing.T) {
+	// 2 + 3 = 5, should print "5" not "5.000000".
+	in := strings.NewReader("2 + 3\n")
+	var out bytes.Buffer
+
+	Run(in, &out)
+
+	got := out.String()
+	// Result line is "5\n" (between prompts).
+	if !strings.Contains(got, "5\n") {
+		t.Errorf("expected '5\\n' in output, got %q", got)
+	}
+	// Should not contain "5.0"
+	if strings.Contains(got, "5.0") {
+		t.Errorf("expected no trailing zeros, got %q", got)
+	}
+}