diff --git a/agentex/src/api/routes/agents.py b/agentex/src/api/routes/agents.py
index d82d5208..ac2cf13a 100644
--- a/agentex/src/api/routes/agents.py
+++ b/agentex/src/api/routes/agents.py
@@ -236,7 +236,7 @@ async def register_build(
     agents_use_case: DAgentsUseCase,
     authorization_service: DAuthorizationService,
 ) -> Agent:
-    """Create a build-only agent row and grant the caller access to it."""
+    """Create a build-only agent row and register its authz resource."""
     await authorization_service.check(
         AgentexResource.agent("*"),
         AuthorizedOperationType.create,
@@ -252,7 +252,7 @@ async def register_build(
         )
     except ValueError as e:
         raise HTTPException(status_code=400, detail=str(e)) from e
-    await authorization_service.grant(
+    await authorization_service.register_resource(
         AgentexResource.agent(agent_entity.id),
         principal_context=request.principal_context,
     )
diff --git a/agentex/tests/unit/api/test_agents_register_build_authz.py b/agentex/tests/unit/api/test_agents_register_build_authz.py
new file mode 100644
index 00000000..ab0b5807
--- /dev/null
+++ b/agentex/tests/unit/api/test_agents_register_build_authz.py
@@ -0,0 +1,117 @@
+"""Route-level authz tests for agent build registration."""
+
+from datetime import UTC, datetime
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+from src.api.routes.agents import register_build
+from src.api.schemas.agents import RegisterBuildRequest
+from src.api.schemas.authorization_types import AgentexResource, AuthorizedOperationType
+from src.domain.entities.agents import AgentEntity, AgentStatus
+
+
+def _agent() -> AgentEntity:
+    now = datetime.now(tz=UTC)
+    return AgentEntity(
+        id="agent-123",
+        name="build-agent",
+        description="Created from build",
+        status=AgentStatus.BUILD_ONLY,
+        status_reason="Agent build registered; not yet deployed.",
+        acp_url=None,
+        created_at=now,
+        updated_at=now,
+    )
+
+
+@pytest.mark.unit
+@pytest.mark.asyncio
+async def test_register_build_registers_agent_resource() -> None:
+    """Build-time agent creation must call register_resource, not grant-only."""
+    agents_use_case = MagicMock()
+    agents_use_case.register_build = AsyncMock(return_value=_agent())
+    authorization_service = MagicMock()
+    authorization_service.check = AsyncMock(return_value=True)
+    authorization_service.register_resource = AsyncMock(return_value=None)
+    authorization_service.grant = AsyncMock(return_value=None)
+    principal_context = {
+        "account_id": "account-123",
+        "user_id": "user-123",
+        "api_key": "test-key",
+    }
+
+    result = await register_build(
+        request=RegisterBuildRequest(
+            name="build-agent",
+            description="Created from build",
+            principal_context=principal_context,
+        ),
+        agents_use_case=agents_use_case,
+        authorization_service=authorization_service,
+    )
+
+    assert result.id == "agent-123"
+    authorization_service.check.assert_awaited_once_with(
+        AgentexResource.agent("*"),
+        AuthorizedOperationType.create,
+        principal_context=principal_context,
+    )
+    agents_use_case.register_build.assert_awaited_once_with(
+        name="build-agent",
+        description="Created from build",
+        registration_metadata=None,
+        agent_input_type=None,
+    )
+    authorization_service.register_resource.assert_awaited_once_with(
+        AgentexResource.agent("agent-123"),
+        principal_context=principal_context,
+    )
+    authorization_service.grant.assert_not_awaited()
+
+
+@pytest.mark.unit
+@pytest.mark.asyncio
+async def test_register_build_existing_agent_path_is_reentrant() -> None:
+    """Idempotent-by-name path returns an existing agent and re-registers safely.
+
+    The auth layer owns duplicate-resource idempotency; the route must keep
+    using the resource-registration path for the returned agent instead of
+    falling back to grant-only behavior.
+    """
+    existing_agent = _agent()
+    agents_use_case = MagicMock()
+    agents_use_case.register_build = AsyncMock(return_value=existing_agent)
+    authorization_service = MagicMock()
+    authorization_service.check = AsyncMock(return_value=True)
+    authorization_service.register_resource = AsyncMock(return_value=None)
+    authorization_service.grant = AsyncMock(return_value=None)
+    principal_context = {
+        "account_id": "account-123",
+        "user_id": "user-123",
+        "api_key": "test-key",
+    }
+    request = RegisterBuildRequest(
+        name="build-agent",
+        description="Created from build",
+        principal_context=principal_context,
+    )
+
+    first = await register_build(
+        request=request,
+        agents_use_case=agents_use_case,
+        authorization_service=authorization_service,
+    )
+    second = await register_build(
+        request=request,
+        agents_use_case=agents_use_case,
+        authorization_service=authorization_service,
+    )
+
+    assert first.id == existing_agent.id
+    assert second.id == existing_agent.id
+    assert agents_use_case.register_build.await_count == 2
+    assert authorization_service.register_resource.await_count == 2
+    for call in authorization_service.register_resource.await_args_list:
+        assert call.args == (AgentexResource.agent(existing_agent.id),)
+        assert call.kwargs == {"principal_context": principal_context}
+    authorization_service.grant.assert_not_awaited()