As large language models rapidly evolve, developers often subscribe to different model ecosystems for distinct strengths—such as combining a Google AI Ultra subscription with the refined developer experience of Anthropic’s Claude Code CLI and Claude Desktop App.

However, integrating Claude-native clients with Google Gemini or Vertex AI backends presents two immediate challenges: protocol mismatch (Claude clients speak Anthropic Messages API, while Gemini speaks Google’s native API) and strict client-side validation (Claude Desktop blocks non-loopback HTTP and rejects non-claude-* model identifiers).

This guide walks through configuring Claude Code CLI and Claude Desktop App on Windows and macOS to seamlessly route through Antigravity Manager, an account-pooling reverse proxy running on a local server/NAS, backed by Gemini 3.8 Flash.

1. What is Antigravity Manager?

Antigravity Manager (community Docker image lbjlaq/antigravity-manager, powered by antigravity-tools) is an account-pooling and protocol-translation gateway tailored for Google Antigravity, Google AI Ultra, and Vertex AI backends.

Key Capabilities

1. Multi-Account & Quota Pooling: Centrally aggregates multiple Google accounts (including Google AI Ultra/One subscriptions, OAuth sessions, and Vertex AI service credentials). It handles automatic session keep-alive, quota distribution, and seamless failover.
2. Standardized Protocol Bridge: Translates Google backend responses into industry-standard Anthropic Messages API (/v1/messages) and OpenAI API (/v1/chat/completions) formats. This allows any client built exclusively for Claude or OpenAI to consume Gemini models without code modifications.
3. Dynamic Model Aliasing (custom_mapping): Allows wildcard routing rules at the gateway level. Incoming requests for claude* or claude-3-7-sonnet can be dynamically mapped to gemini-3.8-flash-high, cleanly bypassing client-side model validation and mitigating upstream rate limits (HTTP 429).
4. Web Administration Console: Provides a built-in web dashboard (default port 8045) to inspect health metrics, monitor query logs, configure model aliases, and set gateway access tokens.

[!NOTE]
Understanding the Gateway API Key
Google does not provide an “Anthropic API Key” for your Google account. In this setup, the API key entered into Claude Code or Claude Desktop is a self-configured proxy token (proxy.api_key in Antigravity Manager). It protects your gateway from unauthorized local network access.

2. Architecture & Core Constraints

Essential Constraints to Know

1. Network Constraint: Claude Desktop enforces a strict origin policy: baseUrl: must use https (or http on loopback). Supplying a plain LAN IP address (such as http://192.168.68.55:8045) will be rejected by client validation. Forwarding to the local loopback (http://127.0.0.1:8045) satisfies this requirement.
2. Model Identifier Constraint: Claude Desktop uses a hardcoded regex pattern requiring all model routes to begin with claude-* or anthropic/claude-*. Manually entering custom model names like gemini-3.8-flash-high triggers validation warnings and disables the “Apply Changes” button. Transparent routing must instead be handled by the proxy’s custom_mapping.

3. Step 1: Gateway & Network Configuration

3.1 Configure Model Mapping on the Gateway

To allow Claude clients requesting claude or claude-3-7-sonnet to be automatically served by Gemini 3.8 Flash without quota exhaustion, set up mapping rules on your server.

Configuration file path: /home/<user>/.antigravity_tools/gui_config.json

Add or update the custom_mapping section under proxy:

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{
  "proxy": {
    "custom_mapping": {
      "claude*": "gemini-3.8-flash-high",
      "claude": "gemini-3.8-flash-high",
      "claude-3-5-sonnet-*": "gemini-3.8-flash-high",
      "claude-3-7-sonnet-*": "gemini-3.8-flash-high",
      "claude-3-opus-*": "gemini-3.8-flash-high",
      "claude-opus-4-*": "gemini-3.8-flash-high",
      "claude-sonnet*": "gemini-3.8-flash-high",
      "claude-haiku-*": "gemini-3.8-flash-high"
    }
  }
}

Restart the container to apply the changes:

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sudo docker restart antigravity-manager

(Alternatively, navigate to http://<SERVER_IP>:8045 in your browser and update the mapping via Proxy Settings → Custom Model Mapping.)

3.2 Retrieve Your Gateway API Key

Your clients require an authorization key (<ANTIGRAVITY_MANAGER_API_KEY>) to authenticate against Antigravity Manager:

• Via Web UI: Open http://<SERVER_IP>:8045, go to Proxy Settings, and copy or set the API Key / Access Token.
• Via Configuration File: Inspect the "api_key" property inside gui_config.json:

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  { "proxy": { "api_key": "your_custom_proxy_key_here" } }

3.3 Set Up Local Loopback Port Forwarding

Forward port 8045 on your server to 127.0.0.1:8045 on your local workstation.

Windows (Native Portproxy, Persistent Across Reboots)

Open PowerShell as Administrator and execute:

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netsh interface portproxy add v4tov4 listenport=8045 listenaddress=127.0.0.1 connectport=8045 connectaddress=192.168.68.55

• Verify connection:

  1 2	netsh interface portproxy show all Test-NetConnection -ComputerName 127.0.0.1 -Port 8045

• Remove rule (if needed later):

  1	netsh interface portproxy delete v4tov4 listenport=8045 listenaddress=127.0.0.1

macOS (SSH Local Tunnel)

• Interactive Session:

  1	ssh -N -L 8045:127.0.0.1:8045 <server_ssh_host>

• Background Daemon (LaunchAgent):
  Create ~/Library/LaunchAgents/com.user.antigravity-tunnel.plist:

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  <?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> <plist version="1.0"> <dict> <key>Label</key> <string>com.user.antigravity-tunnel</string> <key>ProgramArguments</key> <array> <string>/usr/bin/ssh</string> <string>-N</string> <string>-L</string> <string>8045:127.0.0.1:8045</string> <string><server_ssh_host></string> </array> <key>RunAtLoad</key> <true/> <key>KeepAlive</key> <true/> </dict> </plist>

  Load and start the service:

  1	launchctl load ~/Library/LaunchAgents/com.user.antigravity-tunnel.plist

4. Configuring Claude Code CLI

[!WARNING]
Claude Code prioritizes its settings.json environment block over current shell environment variables. Setting temporary shell exports (export ANTHROPIC_BASE_URL=...) will be overridden by the config file. Modify the file directly.

4.1 Configuration File Setup

• Windows: C:\Users\<YourUsername>\.claude\settings.json
• macOS / Linux: ~/.claude/settings.json

Update the env section (replacing <ANTIGRAVITY_MANAGER_API_KEY> with your gateway key):

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{
  "env": {
    "ANTHROPIC_BASE_URL": "http://127.0.0.1:8045",
    "ANTHROPIC_AUTH_TOKEN": "<ANTIGRAVITY_MANAGER_API_KEY>",
    "ANTHROPIC_MODEL": "gemini-3.8-flash-high",
    "ANTHROPIC_DEFAULT_SONNET_MODEL": "gemini-3.8-flash-high",
    "ANTHROPIC_DEFAULT_OPUS_MODEL": "gemini-3.8-flash-high",
    "ANTHROPIC_DEFAULT_HAIKU_MODEL": "gemini-3.8-flash-high"
  },
  "permissions": {
    "defaultMode": "bypassPermissions"
  }
}

(Note: If a standalone "model": "sonnet[1m]" line exists at the root of settings.json, delete it so ANTHROPIC_MODEL within env handles routing.)

4.2 Verification Test

Run a quick probe command in any terminal:

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claude -p "Say 'Hello from Gemini 3.8 Flash'"

Expected output:

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Hello from Gemini 3.8 Flash

5. Configuring Claude Desktop App

5.1 Third-Party Inference Setup

1. Open Claude Desktop, open Settings → Configure third-party inference.
2. Select Gateway in the top dropdown.
3. Under GATEWAY CREDENTIALS:
   • Gateway base URL: http://127.0.0.1:8045 (do not append /v1)
   • Gateway API key: <ANTIGRAVITY_MANAGER_API_KEY>
   • Gateway auth scheme: bearer
   • Custom inference headers: Leave empty
   • Credential kind: Static API key
4. Under MODELS:
   • Model discovery: Enable (turn on). The proxy natively implements GET /v1/models. Enabling this lets the client automatically discover claude-* model aliases.
   • Model list: Keep completely empty (if you manually added custom entries previously, remove them using the × icon).

5.2 Bypassing UI Validation

Entering custom non-Anthropic model names manually causes Claude Desktop to display a validation error:

Doesn't look like an Anthropic model: expected a gateway model route referencing an Anthropic model...

This locks the Apply Changes button. Keeping the model list empty and relying on Model Discovery allows the gateway’s custom_mapping to handle transparent rewrites behind the scenes.

Click Test connection followed by Test model discovery, and then click Apply Changes.

5.3 Starting a Chat

Start a new chat in Claude Desktop. Keep the default model selected (or pick any Sonnet variant). All queries will now route seamlessly through Antigravity Manager to Gemini 3.8 Flash.

6. Troubleshooting & FAQ

Q1: Error: Model discovery — Invalid custom3p managed config: baseUrl: must use https (or http on loopback)

• Cause: The Base URL was configured with a LAN IP (e.g., http://192.168.68.55:8045). Claude Desktop enforces HTTPS for non-loopback addresses.
• Fix: Use local port forwarding and configure the client with http://127.0.0.1:8045.

Q2: Chat shows Model overloaded · Retrying (2/10) · 15s

• Cause: Requests to default Claude models hit rate limits (HTTP 429) on Anthropic accounts.
• Fix: Ensure custom_mapping in the server’s gui_config.json maps "claude*": "gemini-3.8-flash-high", and restart the Docker container.

Q3: Claude Code CLI returns Failed to authenticate. API Error: 401 Invalid API Key

• Cause: The value in ANTHROPIC_AUTH_TOKEN does not match the proxy.api_key configured in Antigravity Manager, or stale credentials remained in ~/.claude/settings.json.
• Fix: Verify proxy.api_key in gui_config.json and ensure settings.json matches exactly.

Q4: Connection drops after rebooting Windows

• Checks:
  1. Confirm the Windows iphlpsvc (IP Helper) service is running.
  2. Run netsh interface portproxy show all in an elevated terminal to verify the forwarding rule is intact.
  3. Ensure the server’s LAN IP address has not changed due to DHCP lease renewal.