# Deliberate reasoning

**Source:** https://promtable.com/glossary/deliberate-reasoning

> Deliberate reasoning is the LLM mode where the model thinks step-by-step before answering — chain-of-thought, [[extended-thinking]], scratchpad — contrasted with fast pattern-match answers. The 'System 2' to plain chat's 'System 1'.

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Deliberate reasoning is the LLM mode where the model thinks step-by-step before answering — chain-of-thought, [[extended-thinking]], scratchpad — contrasted with fast pattern-match answers. The 'System 2' to plain chat's 'System 1'.

Dual-process theory in cognitive science distinguishes fast intuitive (System 1) from slow deliberate (System 2) thinking. 2024-2026 reasoning models brought this to LLMs: deliberate reasoning chains where the model thinks step-by-step before final answer — proven to improve hard-task performance. Implementations: chain-of-thought prompting (older), extended thinking (built into model — o-series, Opus 4, Gemini Ultra, DeepSeek R3), self-consistency (sample N reasoning chains + vote). Production trade-offs: 10-100× more cost + latency, but 2-5× quality gain on math, multi-step planning, and code. Routing decision: when speed + cost matter, use non-reasoning tier; when correctness on hard tasks matter, escalate to deliberate.

## When to use

- Hard tasks: math, code, planning, analysis.

## Common mistakes

- Forcing deliberate reasoning on every query — wastes compute.
- Skipping reasoning on hard tasks — answers degrade.

## Related terms

- [chain-of-thought](https://promtable.com/glossary/chain-of-thought)
- [extended-thinking](https://promtable.com/glossary/extended-thinking)
- [test-time-compute](https://promtable.com/glossary/test-time-compute)

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