---
title: "Designing Secure JSON-RPC 2.0 Tools for LLM Telephony Event Inspection"
---

Designing Secure JSON-RPC 2.0 Tools for LLM Telephony Event Inspection: SMSRiver

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Model Context Protocol & AI Agents

Published 2026-09-07

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8 min read

# Designing Secure JSON-RPC 2.0 Tools for LLM Telephony Event Inspection

Technical design patterns for JSON-RPC 2.0 telephony tools. Input validation, error masking, and token boundary optimization for LLM context windows.

Key Takeaway & Executive Summary

Optimize JSON-RPC tool schemas for LLM context windows: minimize token footprint, validate parameters strictly, and return compact, structured responses.

Large Language Models interact with external systems through structured JSON tool schemas. Inefficient tool definitions waste precious context tokens and trigger hallucinated parameters. This guide details how to build high-performance JSON-RPC 2.0 tools for telephony.

## Table of Contents

- [Context Window Optimization Best Practices](#section-1)

- [Frequently Asked Questions](#frequently-asked-questions)

## 1 Context Window Optimization Best Practices

1. Terse Descriptions: Keep schema descriptions clear and concise to conserve system prompt tokens.
2. Strict Parameter Types: Use explicit JSON Schema validation with regex patterns for phone numbers (e.g. ^\+[1-9]\d{1,14}$).
3. Truncated Message Bodies: When returning inbox lists, return only the most recent 5 messages with truncated bodies unless the agent explicitly requests full history.

## Frequently Asked Questions • Model Context Protocol & AI Agents

What JSON-RPC error codes should telephony tools return?

Use standard JSON-RPC 2.0 codes (-32602 for invalid parameters) alongside custom application error codes (e.g. -32001 for number not found, -32002 for quota exceeded).

## Related Technical Articles & Deep Dives

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Building Model Context Protocol (MCP) Servers for Telephony and SMS Testing](/articles/mcp-server-telephony-integration/)

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Regex vs LLM Extraction: Extracting OTP Codes and Magic Links from Raw SMS](/articles/prompt-engineering-for-otp-extraction/)

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Vaulting Telecom API Credentials in Edge Isolates and MCP Contexts](/articles/secure-secret-handling-for-ai-mcp-gateways/)

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