diff --git a/internal/config/config_test.go b/internal/config/config_test.go
new file mode 100644
index 00000000..d8d66a31
--- /dev/null
+++ b/internal/config/config_test.go
@@ -0,0 +1,107 @@
+package config
+
+import (
+	"os"
+	"testing"
+)
+
+func TestFirstNonEmptyReturnsFirstNonBlank(t *testing.T) {
+	if got := firstNonEmpty("", "b", "c"); got != "b" {
+		t.Fatalf("got %q, want %q", got, "b")
+	}
+}
+
+func TestFirstNonEmptyReturnsEmptyWhenAllBlank(t *testing.T) {
+	if got := firstNonEmpty("", "", ""); got != "" {
+		t.Fatalf("got %q, want empty", got)
+	}
+}
+
+func TestFirstNonEmptySkipsWhitespaceOnly(t *testing.T) {
+	if got := firstNonEmpty("  ", "x"); got != "x" {
+		t.Fatalf("got %q, want %q", got, "x")
+	}
+}
+
+func TestTruthyRecognises1TrueYesYOn(t *testing.T) {
+	for _, v := range []string{"1", "true", "True", "TRUE", "yes", "Yes", "y", "Y", "on", "ON"} {
+		if !truthy(v) {
+			t.Fatalf("truthy(%q) = false, want true", v)
+		}
+	}
+}
+
+func TestTruthyReturnsFalseForEmptyAndGarbage(t *testing.T) {
+	for _, v := range []string{"", "0", "false", "no", "off", "maybe"} {
+		if truthy(v) {
+			t.Fatalf("truthy(%q) = true, want false", v)
+		}
+	}
+}
+
+func TestLoadDefaults(t *testing.T) {
+	cfg, err := Load()
+	if err != nil {
+		t.Fatalf("Load(): %v", err)
+	}
+	if cfg.Port != "3000" {
+		t.Fatalf("Port = %q, want %q", cfg.Port, "3000")
+	}
+	if cfg.DatabaseFile != "mal.db" {
+		t.Fatalf("DatabaseFile = %q, want %q", cfg.DatabaseFile, "mal.db")
+	}
+	if cfg.EpisodeAvailabilityMode != EpisodeAvailabilityModeAuto {
+		t.Fatalf("EpisodeAvailabilityMode = %q, want %q", cfg.EpisodeAvailabilityMode, EpisodeAvailabilityModeAuto)
+	}
+}
+
+func TestLoadReadsEnv(t *testing.T) {
+	os.Setenv("PORT", "8080")
+	os.Setenv("DATABASE_FILE", "/tmp/test.db")
+	os.Setenv("MAL_CORS_ALLOW_ALL", "1")
+	os.Setenv("MAL_JIKAN_TRACE", "true")
+	defer func() {
+		os.Unsetenv("PORT")
+		os.Unsetenv("DATABASE_FILE")
+		os.Unsetenv("MAL_CORS_ALLOW_ALL")
+		os.Unsetenv("MAL_JIKAN_TRACE")
+	}()
+
+	cfg, err := Load()
+	if err != nil {
+		t.Fatalf("Load(): %v", err)
+	}
+	if cfg.Port != "8080" {
+		t.Fatalf("Port = %q, want %q", cfg.Port, "8080")
+	}
+	if cfg.DatabaseFile != "/tmp/test.db" {
+		t.Fatalf("DatabaseFile = %q, want %q", cfg.DatabaseFile, "/tmp/test.db")
+	}
+	if !cfg.CORSAllowAll {
+		t.Fatal("CORSAllowAll = false, want true")
+	}
+	if !cfg.JikanTrace {
+		t.Fatal("JikanTrace = false, want true")
+	}
+}
+
+func TestLoadInvalidEpisodeAvailabilityMode(t *testing.T) {
+	os.Setenv("EPISODE_AVAILABILITY_MODE", "invalid")
+	defer os.Unsetenv("EPISODE_AVAILABILITY_MODE")
+
+	_, err := Load()
+	if err == nil {
+		t.Fatal("expected error for invalid EPISODE_AVAILABILITY_MODE")
+	}
+}
+
+func TestLoadEmptyPortIsError(t *testing.T) {
+	os.Setenv("PORT", "")
+	os.Setenv("DATABASE_FILE", "mal.db")
+	defer os.Unsetenv("PORT")
+
+	_, err := Load()
+	if err == nil {
+		t.Fatal("expected error for empty PORT")
+	}
+}