Foundry Agent Service: creating agent versions returns 404 “resource referenced… not found” in one project only; a new project in the same account works

Problem Since 27-Oct-2026, every agent create in one Foundry project fails, from the portal, the SDK and REST:

POST {project-endpoint}/agents/{name}/versions?api-version=v1

404 {"error":{"code":"not_found","message":"A resource referenced by this request was not found in the target project or workspace (for example, a connection or deployment). Verify that it exists and that the name is correct."}}

Creates in this project worked until 23-Oct. Existing agents in the project still run normally. Only creating new agents or versions fails.

Minimal repro (no tools, no connections):

{"definition":{"kind":"prompt","model":"gpt-5.4-nano","instructions":"hi"}}

What we’ve tested

• A new project in the same Foundry account accepts the identical request (200). So account settings, network, RBAC, model deployments and Entra agent identity setup all work.
• A second project in another account (same region and config) works.
• The failing project returns the same 404 for every model, including a model name that doesn’t exist, for every api-version (v1, 2025-11-15-preview, 2025-05-15-preview), and for both user and service principal callers.
• In the failing project, the classic /assistants create, the Responses API, conversations, deployments, connections and toolboxes all return 200.
• No resources, connections, deployments or role assignments were deleted or changed between the last success and the first failure.
• Capability hosts are empty, so we use the basic (Microsoft-managed) setup. No customer-managed keys. The account uses a system-assigned identity.

Request IDs (failing project): 4ae7af391abd4ce79533518371647b08 (portal), f43de4c5e64e1a364d541254572161b8, a6045a0730fc05b330c3b61ff4b37201, 81e6322506a53bd89c2530b5928318f8

Region: East US 2

Question What internal project-level resource could the agent service be failing to find, and how can it be restored without recreating the project? Agents already running there must keep working. Happy to share subscription and resource details privately.