diff --git a/coding-agent/src/coding_agent/ui/http_server.py b/coding-agent/src/coding_agent/ui/http_server.py
index 68dfed855..3d25d0b0b 100644
--- a/coding-agent/src/coding_agent/ui/http_server.py
+++ b/coding-agent/src/coding_agent/ui/http_server.py
@@ -768,6 +768,11 @@ def _provisioned_execution_binding_from_request(
         raise ValueError(
             "cloud workspace provisioning requires cloud_workspace.enabled=true"
         )
+    if body.workspace_source.kind == "git":
+        raise HTTPException(
+            status_code=501,
+            detail="git workspace source is not implemented yet",
+        )
     _validate_workspace_source_phase_policy(
         body.workspace_source.model_dump(mode="python"),
         cloud_workspace_config,
@@ -1255,6 +1260,8 @@ async def create_session(
                 )
             except Exception:
                 logger.exception("Failed to roll back provisioned cloud workspace")
+        if isinstance(exc, HTTPException):
+            raise exc
         if isinstance(exc, RuntimeError):
             raise HTTPException(status_code=500, detail=str(exc)) from exc
         if isinstance(exc, (ValueError, TypeError)):
diff --git a/coding-agent/src/coding_agent/ui/schemas.py b/coding-agent/src/coding_agent/ui/schemas.py
index 280dcb891..78bcc0571 100644
--- a/coding-agent/src/coding_agent/ui/schemas.py
+++ b/coding-agent/src/coding_agent/ui/schemas.py
@@ -30,8 +30,16 @@ class DockerWorkspaceSourceRequest(BaseModel):
     ) = Field(None, min_length=1, max_length=20)
 
 
+class GitWorkspaceSourceRequest(BaseModel):
+    kind: Literal["git"]
+    remote_url: str = Field(..., min_length=1, max_length=1000)
+    base_ref: str = Field(..., min_length=1, max_length=200)
+    base_sha: str = Field(..., min_length=1, max_length=100)
+    runtime_profile: str | None = Field(None, min_length=1, max_length=100)
+
+
 ExecutionBindingRequest = LocalExecutionBindingRequest | CloudWorkspaceBindingRequest
-WorkspaceSourceRequest = DockerWorkspaceSourceRequest
+WorkspaceSourceRequest = DockerWorkspaceSourceRequest | GitWorkspaceSourceRequest
 
 
 class PromptRequest(BaseModel):
diff --git a/coding-agent/tests/ui/test_http_server.py b/coding-agent/tests/ui/test_http_server.py
index b8d8c7ef7..374bdd9bf 100644
--- a/coding-agent/tests/ui/test_http_server.py
+++ b/coding-agent/tests/ui/test_http_server.py
@@ -1013,6 +1013,44 @@ async def test_create_session_rejects_workspace_provisioning_when_disabled(
             "cloud workspace provisioning requires cloud_workspace.enabled=true"
         )
 
+    async def test_create_session_rejects_git_workspace_source_until_supported(
+        self, client, monkeypatch, tmp_path
+    ):
+        monkeypatch.setattr(
+            "coding_agent.ui.http_server._load_cloud_workspace_config",
+            lambda: {
+                "enabled": True,
+                "provider": "docker",
+                "workspace_root": str(tmp_path),
+                "container_name_prefix": "agent-",
+                **_test_runtime_profile_config(),
+            },
+        )
+        monkeypatch.setattr(
+            "coding_agent.ui.http_server.provision_cloud_binding_from_config",
+            lambda config, source: pytest.fail(
+                "git workspace source should fail before provider provisioning"
+            ),
+        )
+
+        response = await client.post(
+            "/sessions",
+            json={
+                "workspace_source": {
+                    "kind": "git",
+                    "remote_url": "https://github.com/org/repo.git",
+                    "base_ref": "main",
+                    "base_sha": "abc123",
+                    "runtime_profile": "universal",
+                }
+            },
+        )
+
+        assert response.status_code == 501
+        assert response.json()["detail"] == (
+            "git workspace source is not implemented yet"
+        )
+
     async def test_create_session_rolls_back_provisioned_workspace_on_failure(
         self, client, monkeypatch
     ):