fix(config): let a gateway ANTHROPIC_BASE_URL resolve as anthropic-compatible

Setting ANTHROPIC_BASE_URL to a proxy/gateway failed setup with "anthropic
provider anthropic requires official baseURL https://api.anthropic.com".
OPENAI_BASE_URL already auto-promotes a non-official env URL to the
openai-compatible kind, but Anthropic had no equivalent, so the universal
ANTHROPIC_BASE_URL proxy convention was rejected outright.

Mirror the OpenAI behaviour: a non-official ANTHROPIC_BASE_URL from the
environment promotes the profile to anthropic-compatible. This stays env-only
on purpose -- a custom baseURL in a project config.json is still rejected by
Resolve, so an untrusted project can't redirect a first-party key to another
host (TestResolveRejectsAnthropicOfficialProviderCustomBaseURL still passes).

Fixes #479

internal/config/resolver.go

@@ -590,9 +590,17 @@ func applyProviderEnv(cfg *FileConfig, providerKind ProviderKind, env envProfile
 	if profile.Name == "" {
 		profile.Name = string(providerKind)
 	}
+	// A non-official baseURL from the environment signals a proxy/gateway, so promote
+	// the first-party kind to its -compatible transport (which accepts a custom URL).
+	// Env-only by design: a custom baseURL in a project config.json is left as-is and
+	// rejected by Resolve, so an untrusted project can't redirect a first-party key to
+	// another host.
 	if providerKind == ProviderKindOpenAI && baseURL != "" && !isOfficialOpenAIBaseURL(baseURL) {
 		profile.ProviderKind = ProviderKindOpenAICompatible
 	}
+	if providerKind == ProviderKindAnthropic && baseURL != "" && !isOfficialAnthropicBaseURL(baseURL) {
+		profile.ProviderKind = ProviderKindAnthropicCompat
+	}
 	// When the env supplies only credentials (no baseURL) for a provider name that
 	// already exists, don't force the standard transport kind — a same-named
 	// openai-compatible/anthropic-compatible proxy or gateway must keep its kind.

internal/config/resolver_test.go

@@ -617,6 +617,35 @@ func TestResolveUsesOpenAIEnvFallback(t *testing.T) {
 	}
 }
 
+func TestResolveUsesAnthropicEnvBaseURLAsCompatible(t *testing.T) {
+	// ANTHROPIC_BASE_URL pointing at a proxy/gateway must resolve to an
+	// anthropic-compatible provider (mirroring OPENAI_BASE_URL), not the "requires
+	// official baseURL" rejection — issue #479. The env is user-controlled, so a
+	// custom URL there is a deliberate gateway choice, unlike a project config.json.
+	resolved, err := Resolve(ResolveOptions{
+		Env: map[string]string{
+			"ANTHROPIC_API_KEY":  "sk-ant-env",
+			"ANTHROPIC_BASE_URL": "https://gateway.example/anthropic",
+			"ANTHROPIC_MODEL":    "claude-custom",
+		},
+	})
+	if err != nil {
+		t.Fatalf("Resolve() error = %v", err)
+	}
+	if resolved.ActiveProvider != "anthropic" {
+		t.Fatalf("ActiveProvider = %q, want anthropic", resolved.ActiveProvider)
+	}
+	if resolved.Provider.ProviderKind != ProviderKindAnthropicCompat {
+		t.Fatalf("ProviderKind = %q, want anthropic-compatible", resolved.Provider.ProviderKind)
+	}
+	if resolved.Provider.BaseURL != "https://gateway.example/anthropic" {
+		t.Fatalf("BaseURL = %q, want gateway URL", resolved.Provider.BaseURL)
+	}
+	if resolved.Provider.APIKey != "sk-ant-env" || resolved.Provider.Model != "claude-custom" {
+		t.Fatalf("Provider = %#v, want env credentials/model", resolved.Provider)
+	}
+}
+
 func TestResolveUsesOpenAIAPIKeyOnlyWithDefaultModel(t *testing.T) {
 	resolved, err := Resolve(ResolveOptions{
 		Env: map[string]string{