diff --git a/workspace-server/internal/handlers/plugins_helpers_pure_test.go b/workspace-server/internal/handlers/plugins_helpers_pure_test.go
new file mode 100644
index 00000000..9ef499ea
--- /dev/null
+++ b/workspace-server/internal/handlers/plugins_helpers_pure_test.go
@@ -0,0 +1,80 @@
+package handlers
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// supportsRuntime tests — plugin runtime compatibility checking.
+
+func TestSupportsRuntime_EmptyRuntimes(t *testing.T) {
+	// Empty runtimes = unspecified, try it → always compatible.
+	info := pluginInfo{Name: "test", Runtimes: nil}
+	assert.True(t, info.supportsRuntime("claude_code"))
+	assert.True(t, info.supportsRuntime("any_runtime"))
+}
+
+func TestSupportsRuntime_ExactMatch(t *testing.T) {
+	info := pluginInfo{Name: "test", Runtimes: []string{"claude_code", "anthropic"}}
+	assert.True(t, info.supportsRuntime("claude_code"))
+	assert.True(t, info.supportsRuntime("anthropic"))
+}
+
+func TestSupportsRuntime_NoMatch(t *testing.T) {
+	info := pluginInfo{Name: "test", Runtimes: []string{"claude_code"}}
+	assert.False(t, info.supportsRuntime("openai"))
+}
+
+func TestSupportsRuntime_HyphenUnderscoreNormalized(t *testing.T) {
+	// "claude-code" and "claude_code" are considered equal.
+	info := pluginInfo{Name: "test", Runtimes: []string{"claude-code"}}
+	assert.True(t, info.supportsRuntime("claude_code"))
+	assert.True(t, info.supportsRuntime("anthropic_claude"))
+}
+
+func TestSupportsRuntime_HyphenVsUnderscoreReverse(t *testing.T) {
+	// Plugin declares underscore form; runtime uses hyphen.
+	info := pluginInfo{Name: "test", Runtimes: []string{"claude_code"}}
+	assert.True(t, info.supportsRuntime("claude-code"))
+}
+
+func TestSupportsRuntime_EmptyStringRuntime(t *testing.T) {
+	info := pluginInfo{Name: "test", Runtimes: []string{"claude_code"}}
+	// Empty runtime string: should not match any plugin.
+	assert.False(t, info.supportsRuntime(""))
+}
+
+func TestSupportsRuntime_SingleRuntimeMatch(t *testing.T) {
+	// Multiple declared runtimes: only matching one is sufficient.
+	info := pluginInfo{Name: "test", Runtimes: []string{"python", "nodejs", "claude_code"}}
+	assert.True(t, info.supportsRuntime("claude_code"))
+	assert.False(t, info.supportsRuntime("ruby"))
+}
+
+func TestSupportsRuntime_AllHyphenForms(t *testing.T) {
+	// Both plugin and runtime use hyphen form.
+	info := pluginInfo{Name: "test", Runtimes: []string{"claude-code"}}
+	assert.True(t, info.supportsRuntime("claude-code"))
+}
+
+func TestSupportsRuntime_MultipleHyphenNormalization(t *testing.T) {
+	// Mixed hyphen/underscore forms normalize to the same.
+	info := pluginInfo{Name: "test", Runtimes: []string{"some-runtime-name"}}
+	assert.True(t, info.supportsRuntime("some_runtime_name"))
+	assert.True(t, info.supportsRuntime("some-runtime-name"))
+}
+
+func TestSupportsRuntime_EmptyPluginRuntimesWithAnyInput(t *testing.T) {
+	// Empty Runtimes on plugin = try it regardless of runtime.
+	info := pluginInfo{Name: "test", Runtimes: []string{}}
+	assert.True(t, info.supportsRuntime(""))
+	assert.True(t, info.supportsRuntime("any"))
+	assert.True(t, info.supportsRuntime("unknown"))
+}
+
+func TestSupportsRuntime_ZeroLengthRuntimes(t *testing.T) {
+	// Empty slice vs nil: both should be treated as "unspecified".
+	info := pluginInfo{Name: "test"}
+	assert.True(t, info.supportsRuntime("anything"))
+}