fix(providers): route Anthropic API key to native ClaudeProvider instead of HTTPProvider

When using `provider: "anthropic"` with a plain `api_key` (no `auth_method`),
the factory incorrectly left `providerType` as `providerTypeHTTPCompat`,
causing requests to be sent via the OpenAI-compatible HTTP provider.
This resulted in 404 errors because the Anthropic API does not have a
`/v1/chat/completions` endpoint and requires different headers/body format.

Add a new `providerTypeClaudeAPIKey` that routes to `NewClaudeProviderWithBaseURL`,
which uses the native Anthropic SDK with the correct `/v1/messages` endpoint
and `x-api-key` header. Fix both the explicit provider path and the model-name
fallback detection path.

Closes #269

pkg/providers/factory.go

@@ -17,6 +17,7 @@ type providerType int
 const (
 	providerTypeHTTPCompat providerType = iota
 	providerTypeClaudeAuth
+	providerTypeClaudeAPIKey
 	providerTypeCodexAuth
 	providerTypeCodexCLIToken
 	providerTypeClaudeCLI
@@ -91,6 +92,8 @@ func resolveProviderSelection(cfg *config.Config) (providerSelection, error) {
 				if sel.apiBase == "" {
 					sel.apiBase = defaultAnthropicAPIBase
 				}
+				sel.providerType = providerTypeClaudeAPIKey
+				return sel, nil
 			}
 		case "openrouter":
 			if cfg.Providers.OpenRouter.APIKey != "" {
@@ -241,6 +244,8 @@ func resolveProviderSelection(cfg *config.Config) (providerSelection, error) {
 			if sel.apiBase == "" {
 				sel.apiBase = defaultAnthropicAPIBase
 			}
+			sel.providerType = providerTypeClaudeAPIKey
+			return sel, nil
 		case (strings.Contains(lowerModel, "gpt") || strings.HasPrefix(model, "openai/")) &&
 			(cfg.Providers.OpenAI.APIKey != "" || cfg.Providers.OpenAI.AuthMethod != ""):
 			sel.enableWebSearch = cfg.Providers.OpenAI.WebSearch

pkg/providers/factory_provider.go

@@ -120,7 +120,9 @@ func CreateProviderFromConfig(cfg *config.ModelConfig) (LLMProvider, string, err
 			}
 			return provider, modelID, nil
 		}
-		// Use API key with HTTP API
+		// Use API key with native Claude provider (not OpenAI-compatible HTTP).
+		// The Anthropic API uses /v1/messages (not /v1/chat/completions) and
+		// requires x-api-key header, so HTTPProvider would produce 404 errors.
 		apiBase := cfg.APIBase
 		if apiBase == "" {
 			apiBase = "https://api.anthropic.com/v1"
@@ -128,13 +130,7 @@ func CreateProviderFromConfig(cfg *config.ModelConfig) (LLMProvider, string, err
 		if cfg.APIKey == "" {
 			return nil, "", fmt.Errorf("api_key is required for anthropic protocol (model: %s)", cfg.Model)
 		}
-		return NewHTTPProviderWithMaxTokensFieldAndRequestTimeout(
-			cfg.APIKey,
-			apiBase,
-			cfg.Proxy,
-			cfg.MaxTokensField,
-			cfg.RequestTimeout,
-		), modelID, nil
+		return NewClaudeProviderWithBaseURL(cfg.APIKey, apiBase), modelID, nil
 
 	case "antigravity":
 		return NewAntigravityProvider(), modelID, nil

pkg/providers/factory_provider_test.go

@@ -178,6 +178,12 @@ func TestCreateProviderFromConfig_Anthropic(t *testing.T) {
 	if modelID != "claude-sonnet-4.6" {
 		t.Errorf("modelID = %q, want %q", modelID, "claude-sonnet-4.6")
 	}
+	// Anthropic API key must route to native ClaudeProvider (not HTTPProvider).
+	// The Anthropic API uses /v1/messages and x-api-key header, which are
+	// incompatible with the OpenAI-compatible HTTPProvider.
+	if _, ok := provider.(*ClaudeProvider); !ok {
+		t.Errorf("expected *ClaudeProvider for anthropic API key, got %T", provider)
+	}
 }
 
 func TestCreateProviderFromConfig_Antigravity(t *testing.T) {

pkg/providers/factory_test.go

@@ -105,6 +105,36 @@ func TestResolveProviderSelection(t *testing.T) {
 			},
 			wantType: providerTypeClaudeAuth,
 		},
+		{
+			name: "explicit anthropic provider with api key routes to claude api key provider",
+			setup: func(cfg *config.Config) {
+				cfg.Agents.Defaults.Provider = "anthropic"
+				cfg.Agents.Defaults.Model = "claude-opus-4-6"
+				cfg.Providers.Anthropic.APIKey = "sk-ant-api03-test"
+			},
+			wantType:    providerTypeClaudeAPIKey,
+			wantAPIBase: "https://api.anthropic.com/v1",
+		},
+		{
+			name: "explicit claude provider with api key and custom base routes to claude api key provider",
+			setup: func(cfg *config.Config) {
+				cfg.Agents.Defaults.Provider = "claude"
+				cfg.Agents.Defaults.Model = "claude-opus-4-6"
+				cfg.Providers.Anthropic.APIKey = "sk-ant-api03-test"
+				cfg.Providers.Anthropic.APIBase = "https://proxy.example.com/v1"
+			},
+			wantType:    providerTypeClaudeAPIKey,
+			wantAPIBase: "https://proxy.example.com/v1",
+		},
+		{
+			name: "claude model name with api key infers claude api key provider",
+			setup: func(cfg *config.Config) {
+				cfg.Agents.Defaults.Model = "claude-opus-4-6"
+				cfg.Providers.Anthropic.APIKey = "sk-ant-api03-test"
+			},
+			wantType:    providerTypeClaudeAPIKey,
+			wantAPIBase: "https://api.anthropic.com/v1",
+		},
 		{
 			name: "openai oauth routes to codex auth provider",
 			setup: func(cfg *config.Config) {
@@ -313,6 +343,22 @@ func TestCreateProviderReturnsClaudeProviderForAnthropicOAuth(t *testing.T) {
 	// TODO: Test custom APIBase when createClaudeAuthProvider supports it
 }
 
+func TestCreateProviderReturnsClaudeProviderForAnthropicAPIKey(t *testing.T) {
+	cfg := config.DefaultConfig()
+	cfg.Agents.Defaults.Provider = "anthropic"
+	cfg.Agents.Defaults.Model = "claude-opus-4-6"
+	cfg.Providers.Anthropic.APIKey = "sk-ant-api03-test"
+
+	provider, err := CreateProvider(cfg)
+	if err != nil {
+		t.Fatalf("CreateProvider() error = %v", err)
+	}
+
+	if _, ok := provider.(*ClaudeProvider); !ok {
+		t.Fatalf("provider type = %T, want *ClaudeProvider", provider)
+	}
+}
+
 func TestCreateProviderReturnsCodexProviderForOpenAIOAuth(t *testing.T) {
 	// TODO: This test requires openai protocol to support auth_method: "oauth"
 	// which is not yet implemented in the new factory_provider.go