MCP Tool Errors Now Throw Typed Exceptions in Workflow Scripts

This is the most impactful behavioral change in this release for workflow authors.

Previously, MCP tool call failures inside workflow scripts silently resolved to {error: string} objects, making them indistinguishable from a successful tool call that returned an object with an error field. Now, MCP tool failures throw a proper McpToolError exception that crosses the VM sandbox boundary with structured fields.

The thrown error has:

• .name === "McpToolError"
• .toolName — the name of the MCP tool that failed
• .error — the error message string
• .detail — parsed JSON of the response body, if the error response was JSON

In scripts, use try/catch:

try {
  const result = await mcp__server__tool({ ... })
} catch (e) {
  if (e.name === "McpToolError") {
    console.log(e.toolName, e.error, e.detail)
  }
}

The behavior is gated by tengu_repl_mcp_error_throw (default: true). The system prompt documentation for workflow scripts has been updated to reflect that MCP tools THROW on failure while built-in tools return {error: string}.