Auto-detect DEFAULT_PROVIDER from whichever LLM API key is set

backend/app/core/config.py

@@ -229,9 +229,34 @@ def _empty_str_to_none_int(cls, v: object) -> object:
     FIRESTORE_DATABASE_NAME: str = "default"  # Firestore database name (default: "(default)")
 
     # LLM Provider Configuration
-    # Active LLM provider: "openrouter" (default) or "anthropic".
+    # Active LLM provider: "openrouter" (default) or "anthropic". Auto-detected from
+    # whichever API key is set when left unset — see _infer_default_provider_from_keys.
+    # Set explicitly to override the auto-detection (e.g. both keys present but you
+    # want Anthropic).
     DEFAULT_PROVIDER: LLMProvider = LLMProvider.OPENROUTER
 
+    @model_validator(mode="before")
+    @classmethod
+    def _infer_default_provider_from_keys(cls, data: object) -> object:
+        """Infer DEFAULT_PROVIDER from whichever API key is set, when not explicit.
+
+        Single source of truth for the auto-detection documented in
+        .env.quickstart.example ("set ONE of the two keys ... if both are set,
+        OpenRouter is used by default"). Runs on every Settings() construction —
+        not only at `specflow-init.sh` time — so switching keys by hand later
+        (without re-running init) still resolves to the right provider instead
+        of silently falling back to OpenRouter and failing startup validation.
+        """
+        if not isinstance(data, dict):
+            return data
+        if not data.get("DEFAULT_PROVIDER"):
+            data.pop("DEFAULT_PROVIDER", None)
+            anthropic_key = data.get("ANTHROPIC_API_KEY")
+            openrouter_key = data.get("OPENROUTER_API_KEY")
+            if anthropic_key and not openrouter_key:
+                data["DEFAULT_PROVIDER"] = LLMProvider.ANTHROPIC.value
+        return data
+
     # LLM Model Tier Configuration
     # Values follow OpenRouter naming convention: provider/model (e.g., anthropic/claude-opus-4.5)
     # Can be comma-separated for multiple models (used in multi-workspace generation for variance reduction)

backend/test/core/test_enums.py

@@ -68,7 +68,10 @@ class TestSettingsDefaultProvider:
     def test_default_is_openrouter(self):
         from app.core.config import Settings
 
-        s = Settings()
+        with pytest.MonkeyPatch.context() as mp:
+            mp.delenv("ANTHROPIC_API_KEY", raising=False)
+            mp.delenv("OPENROUTER_API_KEY", raising=False)
+            s = Settings()
         assert s.DEFAULT_PROVIDER == LLMProvider.OPENROUTER
         assert s.DEFAULT_PROVIDER == "openrouter"
 
@@ -87,3 +90,71 @@ def test_database_type_bogus_raises(self):
             mp.setenv("DATABASE_TYPE", "bogus")
             with pytest.raises(ValidationError, match="Invalid DATABASE_TYPE"):
                 Settings()
+
+
+class TestDefaultProviderInference:
+    """DEFAULT_PROVIDER auto-detection from whichever API key is set (unset case).
+
+    Regression coverage for the bug where hand-editing .env to switch from
+    OPENROUTER_API_KEY to ANTHROPIC_API_KEY (without re-running specflow-init.sh)
+    left DEFAULT_PROVIDER unset, and docker-compose's hardcoded
+    ``${DEFAULT_PROVIDER:-openrouter}`` fallback silently forced openrouter,
+    failing startup validation despite a valid Anthropic key being present.
+    """
+
+    def _settings(self, mp, *, anthropic: str | None, openrouter: str | None):
+        from app.core.config import Settings
+
+        mp.delenv("DEFAULT_PROVIDER", raising=False)
+        if anthropic is None:
+            mp.delenv("ANTHROPIC_API_KEY", raising=False)
+        else:
+            mp.setenv("ANTHROPIC_API_KEY", anthropic)
+        if openrouter is None:
+            mp.delenv("OPENROUTER_API_KEY", raising=False)
+        else:
+            mp.setenv("OPENROUTER_API_KEY", openrouter)
+        return Settings()
+
+    def test_anthropic_only_infers_anthropic(self):
+        with pytest.MonkeyPatch.context() as mp:
+            s = self._settings(mp, anthropic="sk-ant-test", openrouter=None)
+        assert s.DEFAULT_PROVIDER == LLMProvider.ANTHROPIC
+
+    def test_openrouter_only_infers_openrouter(self):
+        with pytest.MonkeyPatch.context() as mp:
+            s = self._settings(mp, anthropic=None, openrouter="or-test")
+        assert s.DEFAULT_PROVIDER == LLMProvider.OPENROUTER
+
+    def test_both_keys_set_defaults_to_openrouter(self):
+        with pytest.MonkeyPatch.context() as mp:
+            s = self._settings(mp, anthropic="sk-ant-test", openrouter="or-test")
+        assert s.DEFAULT_PROVIDER == LLMProvider.OPENROUTER
+
+    def test_neither_key_set_defaults_to_openrouter(self):
+        with pytest.MonkeyPatch.context() as mp:
+            s = self._settings(mp, anthropic=None, openrouter=None)
+        assert s.DEFAULT_PROVIDER == LLMProvider.OPENROUTER
+
+    def test_explicit_default_provider_wins_over_inference(self):
+        """An explicit DEFAULT_PROVIDER always overrides key-based inference."""
+        from app.core.config import Settings
+
+        with pytest.MonkeyPatch.context() as mp:
+            mp.setenv("ANTHROPIC_API_KEY", "sk-ant-test")
+            mp.delenv("OPENROUTER_API_KEY", raising=False)
+            mp.setenv("DEFAULT_PROVIDER", "openrouter")
+            s = Settings()
+        assert s.DEFAULT_PROVIDER == LLMProvider.OPENROUTER
+
+    def test_blank_default_provider_still_infers(self):
+        """Mirrors docker-compose's ${DEFAULT_PROVIDER:-} passing an empty string
+        rather than omitting the var entirely — must be treated as unset."""
+        from app.core.config import Settings
+
+        with pytest.MonkeyPatch.context() as mp:
+            mp.setenv("ANTHROPIC_API_KEY", "sk-ant-test")
+            mp.delenv("OPENROUTER_API_KEY", raising=False)
+            mp.setenv("DEFAULT_PROVIDER", "")
+            s = Settings()
+        assert s.DEFAULT_PROVIDER == LLMProvider.ANTHROPIC

backend/test/services/test_startup_validation.py

@@ -323,6 +323,29 @@ async def test_anthropic_only_passes(self, validator):
                 result = await validator._check_environment()
         assert result["passed"] is True
 
+    @pytest.mark.asyncio
+    async def test_anthropic_only_passes_without_explicit_default_provider(self, validator):
+        """Regression: switching to Anthropic by only setting ANTHROPIC_API_KEY
+        (leaving DEFAULT_PROVIDER unset, as .env.quickstart.example documents)
+        must pass without also requiring DEFAULT_PROVIDER=anthropic.
+
+        Previously this failed because DEFAULT_PROVIDER defaulted to openrouter
+        unless a one-time init script had persisted the override — so switching
+        keys by hand later silently broke startup validation.
+        """
+        from app.core.config import Settings
+        from unittest.mock import patch as mpatch
+
+        with patch.dict(os.environ, {
+            "ANTHROPIC_API_KEY": "sk-ant-key",
+            "DATABASE_TYPE": "memory",
+        }, clear=True):
+            anthropic_settings = Settings()
+            assert anthropic_settings.DEFAULT_PROVIDER == "anthropic"
+            with mpatch("app.services.startup_validation.settings", anthropic_settings):
+                result = await validator._check_environment()
+        assert result["passed"] is True
+
     @pytest.mark.asyncio
     async def test_both_provider_keys_missing_fails_with_provider_name(self, validator):
         """When no provider key set, message names both the variable and the active provider (FR-3)."""

docker-compose.yml

@@ -85,7 +85,10 @@ services:
       # Common environment variables
       - ANTHROPIC_API_KEY=${ANTHROPIC_API_KEY}
       - OPENROUTER_API_KEY=${OPENROUTER_API_KEY}
-      - DEFAULT_PROVIDER=${DEFAULT_PROVIDER:-openrouter}
+      # Blank (not "openrouter") when unset: the backend infers the provider from
+      # whichever API key is set (Settings._infer_default_provider_from_keys). A
+      # hardcoded default here would mask that inference and always force openrouter.
+      - DEFAULT_PROVIDER=${DEFAULT_PROVIDER:-}
       - LOG_LEVEL=${LOG_LEVEL:-DEBUG}
       - WORKSPACE_BASE_PATH=/workspaces
       - NOTIFY_EMAIL_USERNAME=${NOTIFY_EMAIL_USERNAME}