docs(test): document protocol architecture and migration compatibility

README.md

@@ -662,6 +662,16 @@ The subagent has access to tools (message, web_search, etc.) and can communicate
 | `deepseek(To be tested)`   | LLM (DeepSeek direct)                   | [platform.deepseek.com](https://platform.deepseek.com) |
 | `groq`                     | LLM + **Voice transcription** (Whisper) | [console.groq.com](https://console.groq.com)           |
 
+### Provider Architecture
+
+PicoClaw routes providers by protocol family:
+
+- OpenAI-compatible protocol: OpenRouter, OpenAI-compatible gateways, Groq, Zhipu, and vLLM-style endpoints.
+- Anthropic protocol: Claude-native API behavior.
+- Codex/OAuth path: OpenAI OAuth/token authentication route.
+
+This keeps the runtime lightweight while making new OpenAI-compatible backends mostly a config operation (`api_base` + `api_key`).
+
 <details>
 <summary><b>Zhipu</b></summary>
 

pkg/migrate/migrate_test.go

@@ -299,6 +299,24 @@ func TestConvertConfig(t *testing.T) {
 	})
 }
 
+func TestSupportedProvidersCompatibility(t *testing.T) {
+	expected := []string{
+		"anthropic",
+		"openai",
+		"openrouter",
+		"groq",
+		"zhipu",
+		"vllm",
+		"gemini",
+	}
+
+	for _, provider := range expected {
+		if !supportedProviders[provider] {
+			t.Fatalf("supportedProviders missing expected key %q", provider)
+		}
+	}
+}
+
 func TestMergeConfig(t *testing.T) {
 	t.Run("fills empty fields", func(t *testing.T) {
 		existing := config.DefaultConfig()