Gasoline v5.5.0: Rock-Solid MCP Protocol Compliance

Feb 4, 2026

Brenn Hill

Gasoline v5.5.0 is a stability release focused on MCP protocol compliance. If you experienced “Unexpected end of JSON input” errors or connection issues with Claude Desktop or Cursor, this release fixes them all.

The Problem: Intermittent Connection Failures

Users reported sporadic errors when connecting to Gasoline via Claude Desktop:

[error] Unexpected end of JSON input

The MCP server appeared to be working — valid JSON responses were logged — but immediately after each response, a parse error occurred.

Root Cause: Three Protocol Violations

Our investigation uncovered three distinct MCP protocol violations:

1. Double Newlines in stdio Output

Go’s json.Encoder.Encode() adds a trailing newline to JSON output. Our stdio bridge then called fmt.Println(), adding a second newline. The empty line between messages was parsed as an empty JSON message, causing the parse error.

Fix: Changed fmt.Println(string(body)) to fmt.Print(string(body)) in the bridge — the HTTP response already includes the trailing newline.

2. Notification Responses

JSON-RPC 2.0 notifications (requests without an id field) must not receive responses. We were responding to notifications/initialized with an empty response, violating the spec.

Fix: Notifications now return nil from the handler and receive no response. HTTP transport returns 204 No Content.

3. Exit Race Condition

The stdio bridge could exit before the final response was written to stdout, truncating the last message.

Fix: Implemented an exit gate pattern — the process waits for any pending responses to flush before exiting.

Comprehensive Protocol Tests

v5.5.0 adds 10 new Go tests that verify MCP protocol compliance:

TestMCPProtocol_ResponseNewlines — exactly one trailing newline per response
TestMCPProtocol_NotificationNoResponse — notifications receive no response
TestMCPProtocol_JSONRPCStructure — valid JSON-RPC 2.0 structure
TestMCPProtocol_IDNeverNull — response ID is never null (Cursor requirement)
TestMCPProtocol_ErrorCodes — standard JSON-RPC error codes
TestMCPProtocol_InitializeResponse — MCP initialize handshake
TestMCPProtocol_ToolsListStructure — tools/list response format
TestMCPProtocol_HandlerUnit — handler method dispatch
TestMCPProtocol_HTTPHandler — HTTP transport compliance
TestMCPProtocol_BridgeCodeVerification — static analysis of bridge code

These tests are designed to be unalterable — they verify the MCP spec, not implementation details. Any future change that breaks MCP compliance will fail these tests.

Silenced Version Check Errors

The GitHub API version check now fails silently. Previously, rate limit errors (403) would log warnings even though version checking is non-critical.

Prior Versions Deprecated

All npm packages prior to v5.5.0 have been deprecated. Users installing old versions will see a warning directing them to upgrade.

Upgrade

npx gasoline-mcp@5.5.0

Or update your MCP configuration:

{
  "mcpServers": {
    "gasoline": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "gasoline-mcp@5.5.0", "--port", "7890", "--persist"]
    }
  }
}

Full Changelog

GitHub Release