Gemini-2.5-flash-lite returns 429 RESOURCE_EXHAUSTED on generate_content() despite all diagnostics passing — Project gen-lang-client-0949398102

Project ID: gen-lang-client-0949398102 (display name: gais-first)
Model: gemini-2.5-flash-lite
Region: us-central1
API: Vertex AI (vertexai=True), google-genai Python SDK

Every generate_content() call to gemini-2.5-flash-lite returns:

429 RESOURCE_EXHAUSTED
{‘error’: {‘code’: 429, ‘message’: ‘Resource exhausted. Please try
again later.’, ‘status’: ‘RESOURCE_EXHAUSTED’}}

This happens on every single attempt, including a minimal, isolated
test call (“Say hello in one word.”) with no other code involved,
and persists across multiple retries with long backoff (30s/60s/90s).

Diagnostics already completed, all passing:

  • client.models.list() succeeds and lists gemini-2.5-flash-lite as
    an available model for this project/region
  • IAM permissions confirmed: roles/owner on this project
  • Billing confirmed: active postpay (Google Cloud Services) account
    with substantial credit balance
  • Quota console checked: all Gemini/Vertex AI quota rows show 0%
    current usage (i.e. not at any visible limit)
  • Application Default Credentials freshly re-authenticated via
    gcloud auth application-default login
  • aiplatform.googleapis.com API confirmed enabled for this project
  • Network/IPv6 connectivity issue ruled out (forced IPv4 via
    socket.getaddrinfo, same result)
  • Switching only the model name to gemini-2.5-flash (same project,
    region, credentials, code) succeeds immediately and consistently

Since every other variable is held constant and only the model name
changes the outcome, this looks like an access/allocation restriction
specific to gemini-2.5-flash-lite on this project — possibly related
to its announced October 16, 2026 retirement. Is there a known issue
or an allowlist step needed to restore generate_content() access to
this specific model?

My last attempt to access the model was at 15:44 IST.
Happy to provide additional logs/request IDs if useful.