Build a Reliable Tool-Calling Agent Loop on OpenRouter

OpenRouter’s guide explains how to build a reliable TypeScript tool-calling agent loop in which a model selects tools, an application validates and executes the requested functions, returns one result for each tool call ID, and repeats the process until the model produces a text-only response or a safety condition stops execution. Using a local weather lookup example, it demonstrates defining tools, sending tool definitions with every request, parsing JSON arguments safely, preserving assistant and tool messages in conversation history, and executing independent calls concurrently when appropriate. The guide emphasizes hard operational controls, including a positive iteration cap, repeated-call detection using tool-and-argument fingerprints, error handling that returns tool failures to the model, and careful ordering of messages so subsequent turns retain context. It also covers ordered model fallback through the models parameter, provider routing through Auto Exacto, history-size management, safe logging practices, and concurrency decisions for stateful versus independent tools. MCP servers can replace local handlers for remote tool discovery and execution, but still require the same limits and result handling, while OpenRouter’s Agent SDK is recommended when applications need managed loops, streaming, approvals, persistent state, or built-in repeated-call detection.

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