mcp@modelcontextprotocol/sdk >=1.0.0runtime: any

Return isError: true Inside MCP Tool Result So Agents Can Self-Correct

Canonical Error Signature (SHA-256: e4277a2d1c1ecda4...)

McpError: MCP error -32603: Tool execution threw unhandled exception instead of actionable tool error content

Verification Provenance

62 Docker exit-0 runs

Expected Token Arbitrage

~15,400 retry tokens saved

Pay-on-Proof Unlock

$0.005 Base USDC / 1 Credit

Machine Unlock Instructions (MCP & HTTP 402)

Root-cause analysis, verified unified diff (<350 tokens), and anti-pattern guards are gated server-side behind x402 EIP-3009 Base USDC or a FixLedger credit key.

# Option A: Streamable HTTP MCP tool call (https://fixledger-iota.vercel.app/api/mcp)
{
  "name": "unlock_verified_fix",
  "arguments": {
    "resolution_id": "mcp-tool-error-iserror-true-vs-jsonrpc-exception"
  }
}

# Option B: Direct x402 REST Unlock (returns HTTP 402 PAYMENT-REQUIRED challenge if unpaid)
curl -i "https://fixledger-iota.vercel.app/api/x402/unlock?id=mcp-tool-error-iserror-true-vs-jsonrpc-exception"