Agent Loop

What Is an Agent Loop

The agent loop is the control flow underneath every AI agent: the model thinks, decides whether to call a tool, the tool runs, the result is fed back, and the model decides what to do next — repeated until the task is done or a stop condition is hit. It’s the engineering shape of ReAct in production code. Use the term when discussing how agents are implemented, not just what they do.

How It Works

A minimal agent loop in pseudocode:

while not done:
    response = model.complete(messages, tools=available_tools)
    if response.is_tool_call:
        result = run_tool(response.tool, response.args)
        messages.append(tool_result(result))
    else:
        done = True
        return response.text

Production-grade loops add:

• Step budget — hard limit on iterations to prevent runaway loops (typical: 25–100 steps)
• Cost budget — token or dollar cap that aborts before runaway spend
• Error handling — tool failures are caught and surfaced back as observations, not crashed
• Checkpointing — loop state is serializable so a workflow can resume after process restart
• Concurrency — many production loops dispatch parallel tool calls within a single iteration

Why It Matters

The agent loop is where every interesting failure mode lives: infinite loops, context-window overflow, runaway token cost, tool-call hallucinations, off-task drift. Every framework (LangGraph, CrewAI, Mastra, Pydantic AI, OpenAI Agents SDK) is fundamentally a wrapper around the agent loop with different opinions on state management, retries, and observability. Pick one based on which failure modes you’re most likely to hit.

Examples

• Claude Code — agent loop with file-system and bash tools, step budget enforced by the CLI
• Cursor Composer — multi-file agent loop with IDE-state tools
• LangGraph — graph-based loop with persistent state and conditional edges
• OpenAI Agents SDK — official OpenAI loop primitive, parallel tool calls supported
• Trinity — custom loop in the homelab agent referenced in the MCP production review