# AI agent

**Source:** https://promtable.com/glossary/agent

> An AI agent is a system where a language model autonomously plans and executes a sequence of tool calls to accomplish a goal.

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An AI agent is a system where a language model autonomously plans and executes a sequence of tool calls to accomplish a goal.

An agent loops: the LLM observes its state, decides on an action (usually a tool call), executes it via developer-supplied functions, observes the result, and decides the next step until the goal is reached or a stop condition fires. Frameworks like LangGraph, OpenAI Agents SDK, Claude Agent SDK, CrewAI, and AutoGen formalize this pattern. Reliability is the hard problem — agents fail on long horizons, drift off task, and burn tokens. Best practices in 2026: keep each step's context tight, use a planner-executor split, add deterministic guardrails (max steps, budget caps), and evaluate end-to-end success rates, not single-step quality.

## When to use

- Tasks that need real-time tool use (web search, code execution, browser).
- Workflows with branching that depend on intermediate results.

## Common mistakes

- Shipping a 50-step agent loop without budget caps or evals.
- Letting the agent's own context grow unbounded — it loses focus by step 15.
- Skipping a planner — pure ReAct loops drift.

## Related terms

- [function-calling](https://promtable.com/glossary/function-calling)
- [react-pattern](https://promtable.com/glossary/react-pattern)
- [reasoning-model](https://promtable.com/glossary/reasoning-model)

*Last updated: 2026-06-01*
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Original page: https://promtable.com/glossary/agent
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