Google Antigravity mcp connect error invalid character ‘A’ looking for beginning of value

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Summary

The Google Antigravity tool consistently fails to connect to any MCP server, returning the error:
Error: ?? MCP Server v1.6.0 running on stdio: calling "initialize": invalid character 'A' looking for beginning of value.
This issue is specific to Antigravity, as other tools like Gemini-cli, Codex, and Claude-code function correctly.

Root Cause

The error stems from Antigravity sending malformed JSON data to the MCP server during the initialization handshake. The server expects a JSON object starting with {, but receives an unexpected character 'A', causing the parser to fail.

Why This Happens in Real Systems

  • Inconsistent Data Serialization: Antigravity’s client-side code incorrectly serializes the initialization payload, introducing invalid characters.
  • Mismatched Protocol Versions: The MCP server (v1.6.0) may enforce stricter JSON validation than older versions, exposing client-side bugs.
  • Tool-Specific Implementation: Antigravity’s unique implementation differs from other tools, leading to this isolated issue.

Real-World Impact

  • Service Downtime: Antigravity becomes unusable for MCP interactions.
  • Debugging Overhead: Engineers waste time isolating the issue due to other tools working correctly.
  • User Frustration: Blocks workflows reliant on Antigravity and MCP integration.

Example or Code (if necessary and relevant)

// Expected MCP Initialization Payload
{
  "command": "initialize",
  "version": "1.0"
}

// Actual Payload Sent by Antigravity (Hypothetical)
"Ainitialize command with version 1.0"

How Senior Engineers Fix It

  1. Inspect Payload: Log and compare Antigravity’s outgoing payload with the MCP server’s expected format.
  2. Patch Serialization: Fix the client-side code to generate valid JSON, ensuring proper escaping and structure.
  3. Version Pinning: Temporarily downgrade the MCP server to a version with lenient parsing if a quick fix is needed.
  4. Add Validation: Implement client-side checks to validate JSON before sending it to the server.

Why Juniors Miss It

  • Assumption of Tool Reliability: Juniors assume Antigravity is bug-free since it’s a widely used tool.
  • Overlooking Logs: They may ignore server-side error logs or misinterpret the 'A' character as irrelevant.
  • Lack of Protocol Knowledge: Insufficient understanding of MCP’s JSON requirements leads to incorrect debugging paths.

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