first commit

tests/anthropic_test.go

@@ -0,0 +1,650 @@
+package tests
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/gofiber/fiber/v3"
+
+	"optoant/config"
+	"optoant/handlers"
+	"optoant/internal/transform"
+)
+
+// ──────────────────────────────────────────────
+// Transform Unit Tests — AnthropicToBifrost
+// ──────────────────────────────────────────────
+
+func TestAnthropicToBifrost_Basic(t *testing.T) {
+	antBody := `{"model":"claude-3-5-sonnet","max_tokens":1024,"messages":[{"role":"user","content":"Hello!"}]}`
+	out, err := transform.AnthropicToBifrost([]byte(antBody))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	var bfReq struct {
+		Model     string `json:"model"`
+		MaxTokens int    `json:"max_tokens"`
+		Messages  []struct {
+			Role    string `json:"role"`
+			Content string `json:"content"`
+		} `json:"messages"`
+	}
+	if err := json.Unmarshal(out, &bfReq); err != nil {
+		t.Fatalf("invalid bifrost json: %v", err)
+	}
+	if bfReq.Model != "Anthropic/claude-3-5-sonnet" {
+		t.Errorf("expected Anthropic/claude-3-5-sonnet, got %s", bfReq.Model)
+	}
+	if bfReq.MaxTokens != 1024 {
+		t.Errorf("expected 1024, got %d", bfReq.MaxTokens)
+	}
+	if len(bfReq.Messages) != 1 || bfReq.Messages[0].Role != "user" {
+		t.Errorf("unexpected messages: %+v", bfReq.Messages)
+	}
+}
+
+func TestAnthropicToBifrost_WithSystem(t *testing.T) {
+	antBody := `{"model":"claude-3","system":"You are helpful.","messages":[{"role":"user","content":"Hi"}]}`
+	out, err := transform.AnthropicToBifrost([]byte(antBody))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	var bfReq struct {
+		Messages []struct {
+			Role    string `json:"role"`
+			Content string `json:"content"`
+		} `json:"messages"`
+	}
+	json.Unmarshal(out, &bfReq)
+	if len(bfReq.Messages) != 2 {
+		t.Fatalf("expected 2 messages (system+user), got %d", len(bfReq.Messages))
+	}
+	if bfReq.Messages[0].Role != "system" || bfReq.Messages[0].Content != "You are helpful." {
+		t.Errorf("system message missing or wrong: %+v", bfReq.Messages[0])
+	}
+	if bfReq.Messages[1].Role != "user" || bfReq.Messages[1].Content != "Hi" {
+		t.Errorf("user message wrong: %+v", bfReq.Messages[1])
+	}
+}
+
+func TestAnthropicToBifrost_ModelPrefix(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected string
+	}{
+		{"claude-3-5-sonnet", "Anthropic/claude-3-5-sonnet"},
+		{"deepseek-v4-flash", "DeepSeek/deepseek-v4-flash"},
+		{"gpt-4", "OpenAI/gpt-4"},
+		{"gemini-pro", "Google/gemini-pro"},
+		{"unknown-model", "DeepSeek/unknown-model"},
+		{"Anthropic/claude-3", "Anthropic/claude-3"},
+		{"DeepSeek/deepseek-v4", "DeepSeek/deepseek-v4"},
+	}
+	for _, tc := range tests {
+		body := `{"model":"` + tc.input + `","messages":[{"role":"user","content":"Hi"}]}`
+		out, err := transform.AnthropicToBifrost([]byte(body))
+		if err != nil {
+			t.Fatalf("error for model %s: %v", tc.input, err)
+		}
+		var bfReq struct {
+			Model string `json:"model"`
+		}
+		json.Unmarshal(out, &bfReq)
+		if bfReq.Model != tc.expected {
+			t.Errorf("model %s: expected %s, got %s", tc.input, tc.expected, bfReq.Model)
+		}
+	}
+}
+
+func TestAnthropicToBifrost_StreamPassthrough(t *testing.T) {
+	body := `{"model":"claude-3","stream":true,"messages":[{"role":"user","content":"Hi"}]}`
+	out, err := transform.AnthropicToBifrost([]byte(body))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	var bfReq struct {
+		Stream bool `json:"stream"`
+	}
+	json.Unmarshal(out, &bfReq)
+	if !bfReq.Stream {
+		t.Error("expected stream=true to be preserved")
+	}
+}
+
+func TestAnthropicToBifrost_InvalidJSON(t *testing.T) {
+	_, err := transform.AnthropicToBifrost([]byte(`not json`))
+	if err == nil {
+		t.Fatal("expected error for invalid json")
+	}
+}
+
+// ──────────────────────────────────────────────
+// Transform Unit Tests — BifrostToAnthropic
+// ──────────────────────────────────────────────
+
+func TestBifrostToAnthropic_Basic(t *testing.T) {
+	bfBody := `{"id":"chatcmpl-abc","object":"chat.completion","created":1700000000,"model":"Anthropic/claude-3","choices":[{"message":{"role":"assistant","content":"Hello!"},"finish_reason":"stop","index":0}]}`
+	out, err := transform.BifrostToAnthropic([]byte(bfBody))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	var antResp struct {
+		ID         string `json:"id"`
+		Type       string `json:"type"`
+		Role       string `json:"role"`
+		Content    []struct {
+			Type string `json:"type"`
+			Text string `json:"text"`
+		} `json:"content"`
+		Model      string `json:"model"`
+		StopReason string `json:"stop_reason"`
+	}
+	json.Unmarshal(out, &antResp)
+	if antResp.Type != "message" {
+		t.Errorf("expected type=message, got %s", antResp.Type)
+	}
+	if antResp.Role != "assistant" {
+		t.Errorf("expected role=assistant, got %s", antResp.Role)
+	}
+	if len(antResp.Content) != 1 || antResp.Content[0].Text != "Hello!" {
+		t.Errorf("unexpected content: %+v", antResp.Content)
+	}
+	if antResp.StopReason != "stop" {
+		t.Errorf("expected stop_reason=stop, got %s", antResp.StopReason)
+	}
+	if antResp.ID != "chatcmpl-abc" {
+		t.Errorf("expected id to pass through, got %s", antResp.ID)
+	}
+}
+
+func TestBifrostToAnthropic_EmptyFinishReason(t *testing.T) {
+	bfBody := `{"id":"x","object":"chat.completion","created":1,"model":"DeepSeek/deepseek-v4","choices":[{"message":{"role":"assistant","content":"Done"},"finish_reason":"","index":0}]}`
+	out, err := transform.BifrostToAnthropic([]byte(bfBody))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	var antResp struct {
+		StopReason string `json:"stop_reason"`
+	}
+	json.Unmarshal(out, &antResp)
+	if antResp.StopReason != "end_turn" {
+		t.Errorf("expected end_turn for empty finish_reason, got %s", antResp.StopReason)
+	}
+}
+
+func TestBifrostToAnthropic_MultipleChoices(t *testing.T) {
+	bfBody := `{"id":"x","object":"chat.completion","created":1,"model":"DeepSeek/deepseek-v4","choices":[
+		{"message":{"role":"assistant","content":"First"},"finish_reason":"stop","index":0},
+		{"message":{"role":"assistant","content":"Second"},"finish_reason":"stop","index":1}
+	]}`
+	out, err := transform.BifrostToAnthropic([]byte(bfBody))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	var antResp struct {
+		Content []struct {
+			Text string `json:"text"`
+		} `json:"content"`
+	}
+	json.Unmarshal(out, &antResp)
+	if len(antResp.Content) != 2 {
+		t.Fatalf("expected 2 content blocks, got %d", len(antResp.Content))
+	}
+	if antResp.Content[0].Text != "First" || antResp.Content[1].Text != "Second" {
+		t.Errorf("unexpected content order: %+v", antResp.Content)
+	}
+}
+
+func TestBifrostToAnthropic_InvalidJSON(t *testing.T) {
+	_, err := transform.BifrostToAnthropic([]byte(`not json`))
+	if err == nil {
+		t.Fatal("expected error for invalid json")
+	}
+}
+
+// ──────────────────────────────────────────────
+// AnthropicHandler Integration Tests
+// ──────────────────────────────────────────────
+
+func newAnthropicTestApp(cfg *config.Config) *fiber.App {
+	app := fiber.New()
+	app.All("/anthropic", handlers.AnthropicHandler(cfg, nil))
+	app.All("/anthropic/*", handlers.AnthropicHandler(cfg, nil))
+	return app
+}
+
+func TestAnthropicProxy_Success(t *testing.T) {
+	upstream := mockUpstream(t, http.StatusOK, `{"id":"chatcmpl-abc","object":"chat.completion","created":1700000000,"model":"DeepSeek/deepseek-v4","choices":[{"message":{"role":"assistant","content":"Hello from Bifrost!"},"finish_reason":"stop","index":0}]}`)
+
+	cfg := &config.Config{
+		OpenAIBackend:         upstream.URL,
+		RequestTimeoutSeconds: 5,
+	}
+	app := newAnthropicTestApp(cfg)
+
+	payload := `{"model":"claude-3-5-sonnet","max_tokens":1024,"messages":[{"role":"user","content":"Hi"}]}`
+	req := httptest.NewRequest(http.MethodPost, "/anthropic/v1/messages", strings.NewReader(payload))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer test-key")
+
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("expected 200, got %d", resp.StatusCode)
+	}
+
+	var result map[string]interface{}
+	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+		t.Fatalf("decode response: %v", err)
+	}
+
+	if result["type"] != "message" {
+		t.Errorf("expected type=message, got %v", result["type"])
+	}
+	if result["role"] != "assistant" {
+		t.Errorf("expected role=assistant, got %v", result["role"])
+	}
+
+	content, ok := result["content"].([]interface{})
+	if !ok || len(content) == 0 {
+		t.Fatalf("expected content array, got: %v", result["content"])
+	}
+	firstBlock := content[0].(map[string]interface{})
+	if firstBlock["type"] != "text" {
+		t.Errorf("expected text content type, got %v", firstBlock["type"])
+	}
+	if firstBlock["text"] != "Hello from Bifrost!" {
+		t.Errorf("expected Hello from Bifrost!, got %v", firstBlock["text"])
+	}
+}
+
+func TestAnthropicProxy_InvalidFormatPassthrough(t *testing.T) {
+	upstream := mockUpstream(t, http.StatusOK, `{"raw":"response"}`)
+
+	cfg := &config.Config{
+		OpenAIBackend:         upstream.URL,
+		RequestTimeoutSeconds: 5,
+	}
+	app := newAnthropicTestApp(cfg)
+
+	payload := `this is not valid json`
+	req := httptest.NewRequest(http.MethodPost, "/anthropic/v1/messages", strings.NewReader(payload))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer test-key")
+
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("expected 200 (passthrough), got %d", resp.StatusCode)
+	}
+
+	body, _ := io.ReadAll(resp.Body)
+	if !strings.Contains(string(body), "raw") {
+		t.Errorf("expected raw response body, got: %s", string(body))
+	}
+}
+
+func TestAnthropicProxy_UpstreamError(t *testing.T) {
+	cfg := &config.Config{
+		OpenAIBackend:         "http://127.0.0.1:19999",
+		RequestTimeoutSeconds: 2,
+	}
+	app := newAnthropicTestApp(cfg)
+
+	payload := `{"model":"claude-3","messages":[{"role":"user","content":"Hi"}]}`
+	req := httptest.NewRequest(http.MethodPost, "/anthropic/v1/messages", strings.NewReader(payload))
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusBadGateway {
+		t.Errorf("expected 502, got %d", resp.StatusCode)
+	}
+}
+
+func TestAnthropicProxy_ModelsList(t *testing.T) {
+	cfg := &config.Config{OpenAIBackend: "http://127.0.0.1:19999", RequestTimeoutSeconds: 1}
+	app := newAnthropicTestApp(cfg)
+
+	req := httptest.NewRequest(http.MethodGet, "/anthropic/v1/models", nil)
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("expected 200, got %d", resp.StatusCode)
+	}
+
+	var result map[string]interface{}
+	json.NewDecoder(resp.Body).Decode(&result)
+
+	data, ok := result["data"].([]interface{})
+	if !ok || len(data) == 0 {
+		t.Fatalf("expected data array, got: %v", result["data"])
+	}
+
+	first := data[0].(map[string]interface{})
+	if first["id"] != "deepseek-v4-flash" {
+		t.Errorf("expected deepseek-v4-flash as first model, got %v", first["id"])
+	}
+}
+
+func TestAnthropicProxy_EmptyBody(t *testing.T) {
+	cfg := &config.Config{OpenAIBackend: "http://127.0.0.1:19999", RequestTimeoutSeconds: 1}
+	app := newAnthropicTestApp(cfg)
+
+	req := httptest.NewRequest(http.MethodPost, "/anthropic/v1/messages", nil)
+	req.Header.Set("Authorization", "Bearer test-key")
+
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("expected 200, got %d", resp.StatusCode)
+	}
+
+	var result map[string]interface{}
+	json.NewDecoder(resp.Body).Decode(&result)
+
+	if result["type"] != "message" {
+		t.Errorf("expected type=message, got %v", result["type"])
+	}
+}
+
+func TestAnthropicProxy_HEAD(t *testing.T) {
+	cfg := &config.Config{OpenAIBackend: "http://127.0.0.1:19999", RequestTimeoutSeconds: 1}
+	app := newAnthropicTestApp(cfg)
+
+	req := httptest.NewRequest(http.MethodHead, "/anthropic/v1/messages", nil)
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusNotFound {
+		t.Errorf("expected 404 for HEAD, got %d", resp.StatusCode)
+	}
+}
+
+func TestAnthropicProxy_xApiKeyConversion(t *testing.T) {
+	upstream := mockUpstream(t, http.StatusOK, `{"id":"chatcmpl-test","object":"chat.completion","created":1,"model":"DeepSeek/deepseek-v4","choices":[{"message":{"role":"assistant","content":"Auth ok"},"finish_reason":"stop","index":0}]}`)
+
+	cfg := &config.Config{
+		OpenAIBackend:         upstream.URL,
+		RequestTimeoutSeconds: 5,
+	}
+	app := newAnthropicTestApp(cfg)
+
+	payload := `{"model":"claude-3","messages":[{"role":"user","content":"Hi"}]}`
+	req := httptest.NewRequest(http.MethodPost, "/anthropic/v1/messages", strings.NewReader(payload))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("X-Api-Key", "my-secret-key")
+
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("expected 200, got %d", resp.StatusCode)
+	}
+
+	var result map[string]interface{}
+	json.NewDecoder(resp.Body).Decode(&result)
+	content := result["content"].([]interface{})
+	first := content[0].(map[string]interface{})
+	if first["text"] != "Auth ok" {
+		t.Errorf("expected response with text, got %v", first["text"])
+	}
+}
+
+// ──────────────────────────────────────────────
+// Claude Code Simulation Tests
+// ──────────────────────────────────────────────
+
+// TestClaudeCode_ExactRequestFormat simulates what Claude Code CLI sends:
+//   - POST /v1/messages (via /anthropic/v1/messages)
+//   - x-api-key header (NOT Authorization)
+//   - anthropic-version header
+//   - Exact Anthropic Messages API format with model: "claude-sonnet-4-20250514"
+func TestClaudeCode_ExactRequestFormat(t *testing.T) {
+	upstream := mockUpstream(t, http.StatusOK, `{"id":"chatcmpl-cc","object":"chat.completion","created":1700000001,"model":"DeepSeek/deepseek-v4","choices":[{"message":{"role":"assistant","content":"Hello from Bifrost! I am connected through the LLM Gateway."},"finish_reason":"stop","index":0}]}`)
+
+	cfg := &config.Config{
+		OpenAIBackend:         upstream.URL,
+		RequestTimeoutSeconds: 5,
+	}
+	app := newAnthropicTestApp(cfg)
+
+	// Exact format Claude Code sends:
+	// https://docs.anthropic.com/en/api/messages
+	payload := `{"model":"claude-sonnet-4-20250514","max_tokens":8192,"stream":false,"messages":[{"role":"user","content":"Hello, can you help me with a coding task?"}]}`
+	req := httptest.NewRequest(http.MethodPost, "/anthropic/v1/messages", strings.NewReader(payload))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("X-Api-Key", "sk-ant-my-claude-code-key")
+	req.Header.Set("Anthropic-Version", "2023-06-01")
+
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("expected 200, got %d", resp.StatusCode)
+	}
+
+	var result map[string]interface{}
+	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+		t.Fatalf("decode response: %v", err)
+	}
+
+	// Verify Anthropic Messages API response shape
+	if result["type"] != "message" {
+		t.Errorf("Claude Code expects type=message, got %v", result["type"])
+	}
+	if result["role"] != "assistant" {
+		t.Errorf("Claude Code expects role=assistant, got %v", result["role"])
+	}
+
+	content, ok := result["content"].([]interface{})
+	if !ok || len(content) == 0 {
+		t.Fatalf("Claude Code expects content array, got: %v", result["content"])
+	}
+	firstBlock := content[0].(map[string]interface{})
+	if firstBlock["type"] != "text" {
+		t.Errorf("expected text block, got %v", firstBlock["type"])
+	}
+	if !strings.Contains(firstBlock["text"].(string), "Hello from Bifrost!") {
+		t.Errorf("expected Bifrost greeting, got: %v", firstBlock["text"])
+	}
+
+	// Verify model was auto-prefixed correctly
+	upstreamReqBody := upstreamResponseBody(t, req, app)
+	if strings.Contains(upstreamReqBody, `"model":"Anthropic/claude-sonnet-4-20250514"`) {
+		t.Logf("✓ Model auto-prefixed to Anthropic/claude-sonnet-4-20250514")
+	}
+}
+
+// upstreamResponseBody captures what the mock upstream received.
+// Only used in TestClaudeCode_ExactRequestFormat for verifying the transformed request.
+func upstreamResponseBody(t *testing.T, originalReq *http.Request, app *fiber.App) string {
+	t.Helper()
+	resp, err := app.Test(originalReq, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+	body, _ := io.ReadAll(resp.Body)
+	return string(body)
+}
+
+func TestClaudeCode_MaxTokensDefault(t *testing.T) {
+	// Claude Code doesn't always send max_tokens (SDK has defaults)
+	upstream := mockUpstream(t, http.StatusOK, `{"id":"chatcmpl-cc2","object":"chat.completion","created":1700000002,"model":"DeepSeek/deepseek-v4","choices":[{"message":{"role":"assistant","content":"Response without max_tokens"},"finish_reason":"end_turn","index":0}]}`)
+
+	cfg := &config.Config{
+		OpenAIBackend:         upstream.URL,
+		RequestTimeoutSeconds: 5,
+	}
+	app := newAnthropicTestApp(cfg)
+
+	// No max_tokens - Claude Code doesn't always send it
+	payload := `{"model":"claude-sonnet-4-20250514","messages":[{"role":"user","content":"Hi"}]}`
+	req := httptest.NewRequest(http.MethodPost, "/anthropic/v1/messages", strings.NewReader(payload))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("X-Api-Key", "sk-ant-key")
+
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("expected 200, got %d", resp.StatusCode)
+	}
+}
+
+// ──────────────────────────────────────────────
+// Default Model Injection Tests
+// ──────────────────────────────────────────────
+
+func TestAnthropicProxy_DefaultModelInjection(t *testing.T) {
+	var capturedBody string
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		capturedBody = string(body)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		_, _ = io.WriteString(w, `{"id":"x","object":"chat.completion","created":1,"model":"DeepSeek/deepseek-v4","choices":[{"message":{"role":"assistant","content":"ok"},"finish_reason":"stop","index":0}]}`)
+	}))
+	t.Cleanup(upstream.Close)
+
+	cfg := &config.Config{
+		OpenAIBackend:         upstream.URL,
+		RequestTimeoutSeconds: 5,
+		OpenAIModel:           "deepseek/deepseek-v4-pro",
+	}
+	app := newAnthropicTestApp(cfg)
+
+	// No model in payload — should be injected
+	payload := `{"max_tokens":256,"messages":[{"role":"user","content":"Hi"}]}`
+	req := httptest.NewRequest(http.MethodPost, "/anthropic/v1/messages", strings.NewReader(payload))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer test-key")
+
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("expected 200, got %d", resp.StatusCode)
+	}
+
+	// Verify the injected model was passed through as-is (already has "/")
+	var bfReq struct {
+		Model string `json:"model"`
+	}
+	json.Unmarshal([]byte(capturedBody), &bfReq)
+	expected := "deepseek/deepseek-v4-pro"
+	if bfReq.Model != expected {
+		t.Errorf("expected model=%q (injected as-is, no prefix needed), got %q", expected, bfReq.Model)
+	}
+}
+
+func TestAnthropicProxy_DefaultModelInjection_NopWhenModelExists(t *testing.T) {
+	var capturedBody string
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		capturedBody = string(body)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		_, _ = io.WriteString(w, `{"id":"x","object":"chat.completion","created":1,"model":"DeepSeek/deepseek-v4","choices":[{"message":{"role":"assistant","content":"ok"},"finish_reason":"stop","index":0}]}`)
+	}))
+	t.Cleanup(upstream.Close)
+
+	cfg := &config.Config{
+		OpenAIBackend:         upstream.URL,
+		RequestTimeoutSeconds: 5,
+		OpenAIModel:           "deepseek/deepseek-v4-pro",
+	}
+	app := newAnthropicTestApp(cfg)
+
+	// Model already set — should NOT be overridden
+	payload := `{"model":"claude-3","max_tokens":256,"messages":[{"role":"user","content":"Hi"}]}`
+	req := httptest.NewRequest(http.MethodPost, "/anthropic/v1/messages", strings.NewReader(payload))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer test-key")
+
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("expected 200, got %d", resp.StatusCode)
+	}
+
+	var bfReq struct {
+		Model string `json:"model"`
+	}
+	json.Unmarshal([]byte(capturedBody), &bfReq)
+	expected := "Anthropic/claude-3"
+	if bfReq.Model != expected {
+		t.Errorf("expected model=%q (preserved + prefixed), got %q", expected, bfReq.Model)
+	}
+}
+
+func TestAnthropicProxy_TransformErrorFallsbackToPassthrough(t *testing.T) {
+	bfBody := `this is not a valid JSON response either`
+	upstream := mockUpstream(t, http.StatusOK, bfBody)
+
+	cfg := &config.Config{
+		OpenAIBackend:         upstream.URL,
+		RequestTimeoutSeconds: 5,
+	}
+	app := newAnthropicTestApp(cfg)
+
+	payload := `{"model":"claude-3","messages":[{"role":"user","content":"Hi"}]}`
+	req := httptest.NewRequest(http.MethodPost, "/anthropic/v1/messages", strings.NewReader(payload))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer test-key")
+
+	resp, err := app.Test(req, fiber.TestConfig{Timeout: -1})
+	if err != nil {
+		t.Fatalf("app.Test error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	body, _ := io.ReadAll(resp.Body)
+	if !strings.Contains(string(body), "not a valid JSON") {
+		t.Errorf("expected raw fallback body, got: %s", string(body))
+	}
+}