[Provider compatibility] Claude Code Task subagents receive false 429 on Antigravity Gemini 3.7 Flash #2899

Client or integration

Claude Code

Provider or upstream service

Google Antigravity / Cloud Code Assist

OpenCodex version

2.35.0

Endpoint or capability

/v1/messages; Claude Code Task subagents

Current behaviour

A Claude Code Task subagent routed to google-antigravity/gemini-3.7-flash fails with HTTP 429:

Resource has been exhausted (e.g. check quota).

Controlled comparisons indicate that this is not ordinary quota exhaustion:

• Main Claude Code requests using the same OpenCodex model succeed.
• Equivalent non-Task requests succeed.
• Removing one exact paragraph from the Task subagent system instruction changes the response from 429 to 200.
• Changing tool count, output-token limit, thinking configuration, session identity, or concurrency does not resolve the failure.

The triggering paragraph is:

You are a Claude agent, built on Anthropic's Claude Agent SDK.

A minimal Cloud Code Assist request containing that sentence reproduces the 429. Removing or generically rewriting it returns 200.

Google returns only:

{
  "error": {
    "code": 429,
    "message": "Resource has been exhausted (e.g. check quota).",
    "status": "RESOURCE_EXHAUSTED"
  }
}

OpenCodex therefore reports the failure as a rate limit even though it is prompt-content-specific.

Expected behaviour

Claude Code should be able to run a Task subagent through Antigravity Gemini 3.7 Flash when the upstream model is otherwise available.

For this compatibility case, OpenCodex should remove the exact rejected identity paragraph while preserving every other system instruction.

Unrelated Google 429 responses should retain their existing quota and rate-limit handling.

Minimal redacted request or reproduction

# Configure a Claude Code subagent to use:
# google-antigravity/gemini-3.7-flash

# Start Claude Code through OpenCodex and request delegation:
Use the Gemini 3.7 Flash subagent and have it reply exactly:
SUBAGENT_OK

# Observed:
# The Task subagent fails with HTTP 429.
#
# Control:
# A main Claude Code request through the same model succeeds.

Distinguishing redacted request fragment:

{
  "model": "gemini-3.7-flash-tiered",
  "request": {
    "systemInstruction": {
      "parts": [
        {
          "text": "You are a Claude agent, built on Anthropic's Claude Agent SDK."
        }
      ]
    },
    "contents": [
      {
        "role": "user",
        "parts": [{ "text": "Reply OK." }]
      }
    ]
  }
}

Controlled results:

Task system instruction containing the exact identity paragraph  429
No system instruction                                           200
No tools                                                        429
Exact identity paragraph alone                                  429
Generic agent identity instead                                  200
Exact identity paragraph removed                                200

Actual response or error

HTTP 429

{
  "error": {
    "code": 429,
    "message": "Resource has been exhausted (e.g. check quota).",
    "status": "RESOURCE_EXHAUSTED"
  }
}

Upstream documentation

No public specification was found for this internal Cloud Code Assist behaviour.

The expected behaviour is based on Claude Code's concrete Task-subagent request shape and controlled requests against the upstream service.

Suggested mapping or implementation notes

Apply the compatibility rewrite after the Gemini systemInstruction is constructed and before the Cloud Code Assist request is serialized:

• Apply only in Cloud Code Assist mode.
• Apply only to gemini-3.7-flash.
• Remove only a paragraph exactly equal to:

You are a Claude agent, built on Anthropic's Claude Agent SDK.

• Preserve all other system-instruction text and parts.
• Leave other Google transports and model IDs unchanged.
• Do not broadly reclassify Google 429 responses.

A candidate implementation was tested through an isolated OpenCodex instance using the real Claude Code Task flow. Before the rewrite, the subagent returned 429. After the rewrite, one real Task subagent completed and returned:

SUBAGENT_OK

The focused regression test, typecheck, full test suite, privacy scan, and diff validation passed.

The same patch was then applied to a normal running OpenCodex proxy and verified manually through the Claude Code CLI. A Gemini 3.7 Flash Task subagent completed 10 consecutive tool calls successfully:

Task subagent completed
Tool calls: 10/10 successful
Duration: approximately 43.3 seconds

This human verification exercised the normal Claude Code CLI → OpenCodex /v1/messages → Antigravity Gemini 3.7 Flash path rather than a synthetic adapter-only request.

Additional context and attachments

The full Task request continued to succeed after removing only the exact identity paragraph. Its tools, reasoning configuration, output limit, and remaining system instructions were unchanged.

Related but distinct issues:

• [Feature] Google Antigravity multi-account management, pool health and quota UX #1062 concerns Antigravity account-pool and quota UX.
• [Feature]: Generic OAuth account-pool failover with session affinity and quota-aware selection #695 concerns generic OAuth account failover.
• google-antigravity provider cannot use gemini-3.7-flash (adapter speaks /v1beta, gateway has it only on /v1internal Code Assist surface) #1906 concerned an older Gemini 3.7 routing 404.

No request bodies containing conversation history, credentials, account identifiers, local paths, session identifiers, timestamps, or user-specific configuration are attached.

Checks

• I searched existing provider and compatibility issues.
• The request and response were redacted.
• The expected behaviour is based on a concrete client requirement.