diff --git a/src/celeste/credentials.py b/src/celeste/credentials.py
index 2feb118f..4e6cd29c 100644
--- a/src/celeste/credentials.py
+++ b/src/celeste/credentials.py
@@ -96,12 +96,18 @@ def get_credentials(
             SecretStr containing the API key for the provider.
 
         Raises:
-            MissingCredentialsError: If provider requires credentials but none are configured.
+            MissingCredentialsError: If provider requires credentials but none are configured,
+                or the configured value is empty/whitespace-only.
         """
-        if override_key:
-            if isinstance(override_key, str):
-                return SecretStr(override_key)
-            return override_key
+        if override_key is not None:
+            raw = (
+                override_key
+                if isinstance(override_key, str)
+                else override_key.get_secret_value()
+            )
+            if not raw.strip():
+                raise MissingCredentialsError(provider=provider)
+            return SecretStr(raw) if isinstance(override_key, str) else override_key
 
         registered = _auth_registry.get(provider)
         if registered is None:
@@ -116,7 +122,7 @@ def get_credentials(
         field_name = secret_name.lower()
 
         credential: SecretStr | None = getattr(self, field_name, None)
-        if credential is None:
+        if credential is None or not credential.get_secret_value().strip():
             raise MissingCredentialsError(provider=provider)
 
         return credential
@@ -130,7 +136,7 @@ def list_available_providers(self) -> list[Provider]:
         ]
 
     def has_credential(self, provider: Provider) -> bool:
-        """Check if a specific provider has credentials configured."""
+        """Check if a specific provider has non-empty credentials configured."""
         registered = _auth_registry.get(provider)
         if registered is None:
             raise UnsupportedProviderError(provider=provider)
@@ -142,7 +148,8 @@ def has_credential(self, provider: Provider) -> bool:
         secret_name, _, _ = registered
         field_name = secret_name.lower()
 
-        return getattr(self, field_name, None) is not None
+        credential = getattr(self, field_name, None)
+        return credential is not None and bool(credential.get_secret_value().strip())
 
     def get_auth(
         self,
diff --git a/tests/unit_tests/test_credentials.py b/tests/unit_tests/test_credentials.py
index 5a22df4c..91decfce 100644
--- a/tests/unit_tests/test_credentials.py
+++ b/tests/unit_tests/test_credentials.py
@@ -362,23 +362,48 @@ class TestEdgeCases:
     """Test edge cases and error conditions."""
 
     @pytest.mark.parametrize("value", ["", "   ", "\t\n"])
-    def test_whitespace_credentials(
+    def test_whitespace_credentials_are_treated_as_missing(
         self, monkeypatch: pytest.MonkeyPatch, value: str
     ) -> None:
-        """Test handling of whitespace-only credentials.
+        """Test that empty/whitespace-only credentials are treated as missing.
 
-        Note: Currently treats whitespace as 'present' credentials.
-        This is a deliberate choice - empty/whitespace values are still
-        considered as having credentials set (may be used for optional APIs).
+        When an env var is set to empty string or whitespace (e.g. OPENAI_API_KEY=""),
+        Pydantic BaseSettings produces SecretStr("") instead of None. The SDK must
+        detect this and raise MissingCredentialsError rather than sending invalid
+        Authorization headers.
         """
         # Arrange
         monkeypatch.setenv("OPENAI_API_KEY", value)
         creds = Credentials()  # type: ignore[call-arg]
 
         # Act & Assert
-        assert creds.has_credential(Provider.OPENAI) is True
-        credential = creds.get_credentials(Provider.OPENAI)
-        assert credential.get_secret_value() == value
+        assert creds.has_credential(Provider.OPENAI) is False
+        with pytest.raises(MissingCredentialsError):
+            creds.get_credentials(Provider.OPENAI)
+
+    @pytest.mark.parametrize(
+        "value",
+        ["", "   ", "\t\n", SecretStr(""), SecretStr("   "), SecretStr("\t\n")],
+    )
+    def test_whitespace_override_key_treated_as_missing(
+        self, value: str | SecretStr
+    ) -> None:
+        """Test that empty/whitespace override_key raises MissingCredentialsError."""
+        creds = Credentials()  # type: ignore[call-arg]
+
+        with pytest.raises(MissingCredentialsError):
+            creds.get_credentials(Provider.OPENAI, override_key=value)
+
+    @pytest.mark.parametrize("value", ["", "   ", SecretStr(""), SecretStr("   ")])
+    def test_whitespace_override_key_not_fallthrough(
+        self, monkeypatch: pytest.MonkeyPatch, value: str | SecretStr
+    ) -> None:
+        """Test that empty override_key raises even when a valid env var exists."""
+        monkeypatch.setenv("OPENAI_API_KEY", "valid-env-key")
+        creds = Credentials()  # type: ignore[call-arg]
+
+        with pytest.raises(MissingCredentialsError):
+            creds.get_credentials(Provider.OPENAI, override_key=value)
 
     def test_special_characters_in_credentials(
         self, monkeypatch: pytest.MonkeyPatch