diff --git a/pyproject.toml b/pyproject.toml
index 729d42b2..83d27ee4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -27,9 +27,11 @@ dependencies = [
     "websockets>=15.0",
     "asgiref>=3.11.0",
     "filetype>=1.2.0",
-    "google-auth[requests]>=2.0.0",
 ]
 
+[project.optional-dependencies]
+gcp = ["google-auth[requests]>=2.0.0"]
+
 [project.urls]
 Homepage = "https://withceleste.ai"
 Documentation = "https://withceleste.ai/docs"
diff --git a/src/celeste/exceptions.py b/src/celeste/exceptions.py
index 08f6cb5f..f2812745 100644
--- a/src/celeste/exceptions.py
+++ b/src/celeste/exceptions.py
@@ -170,6 +170,16 @@ def __init__(self) -> None:
         super().__init__("Stream completed but no chunks were produced")
 
 
+class MissingDependencyError(Error):
+    """Raised when a required optional dependency is not installed."""
+
+    def __init__(self, *, library: str, extra: str) -> None:
+        super().__init__(
+            f"Missing required dependency `{library}`. "
+            f'Install it with: pip install "celeste-ai[{extra}]"'
+        )
+
+
 class CredentialsError(Error):
     """Errors related to API credentials."""
 
@@ -217,6 +227,7 @@ def __init__(self, parameter: str, model_id: str) -> None:
     "ConstraintViolationError",
     "Error",
     "MissingCredentialsError",
+    "MissingDependencyError",
     "ModalityNotFoundError",
     "ModelNotFoundError",
     "StreamEmptyError",
diff --git a/src/celeste/providers/google/auth.py b/src/celeste/providers/google/auth.py
index 75d7253e..24eba8d8 100644
--- a/src/celeste/providers/google/auth.py
+++ b/src/celeste/providers/google/auth.py
@@ -5,6 +5,10 @@
 from pydantic import ConfigDict
 
 from celeste.auth import Authentication
+from celeste.exceptions import MissingDependencyError
+
+VERTEX_BASE_URL = "https://{location}-aiplatform.googleapis.com"
+VERTEX_GLOBAL_BASE_URL = "https://aiplatform.googleapis.com"
 
 
 class GoogleADC(Authentication):
@@ -18,6 +22,7 @@ class GoogleADC(Authentication):
 
     scopes: ClassVar[list[str]] = ["https://www.googleapis.com/auth/cloud-platform"]
     project_id: str | None = None
+    location: str = "global"
 
     model_config: ClassVar[ConfigDict] = ConfigDict(arbitrary_types_allowed=True)
 
@@ -35,8 +40,11 @@ def get_headers(self) -> dict[str, str]:
 
     def _get_access_token(self) -> tuple[str, str | None]:
         """Get OAuth access token using Application Default Credentials."""
-        import google.auth
-        import google.auth.transport.requests
+        try:
+            import google.auth
+            import google.auth.transport.requests
+        except ImportError as e:
+            raise MissingDependencyError(library="google-auth", extra="gcp") from e
 
         if self._credentials is None:
             self._credentials, self._project = google.auth.default(scopes=self.scopes)
@@ -47,5 +55,18 @@ def _get_access_token(self) -> tuple[str, str | None]:
 
         return self._credentials.token, self.project_id or self._project
 
+    @property
+    def resolved_project_id(self) -> str | None:
+        """Return effective project_id (explicit or from ADC credentials)."""
+        if self._credentials is None:
+            self._get_access_token()
+        return self.project_id or self._project
+
+    def get_vertex_base_url(self) -> str:
+        """Return the Vertex AI base URL for the configured location."""
+        if self.location == "global":
+            return VERTEX_GLOBAL_BASE_URL
+        return VERTEX_BASE_URL.format(location=self.location)
+
 
 __all__ = ["GoogleADC"]