PM-301d · Module 1

Zero-Shot CoT vs. Few-Shot CoT

5 min read

Zero-shot CoT uses a trigger phrase to elicit reasoning without examples: "Think step by step before answering." Few-shot CoT provides worked examples that demonstrate the reasoning process. Both improve performance on complex tasks over direct answering, but their failure modes differ and their costs differ significantly.

Do This

  • Use zero-shot CoT ("Let's think step by step") for novel task types where examples are unavailable or hard to construct
  • Use few-shot CoT when you need the reasoning to follow a specific structure or methodology
  • Use zero-shot CoT first — it costs fewer tokens; upgrade to few-shot CoT only if quality is insufficient
  • Combine zero-shot CoT with format instructions: "Think step by step, then return your final answer in JSON"

Avoid This

  • Use few-shot CoT as the default without testing zero-shot first
  • Use zero-shot CoT for tasks where the reasoning structure is non-obvious and the model consistently gets it wrong
  • Apply CoT to tasks that require no multi-step reasoning — it adds tokens with no benefit
  • Omit the instruction to separate reasoning from final answer in production systems
# ZERO-SHOT CoT
Analyze this contract clause for risk. Think step by step, then provide your
risk assessment.

Clause: "Vendor shall deliver all milestones by the dates specified in Schedule A,
subject to delays caused by Client's failure to provide required materials."

Think step by step:

# FEW-SHOT CoT (with reasoning demonstration)
Analyze contract clauses for risk using this methodology:

Example:
Clause: "Payment due within 30 days of invoice."
Step 1: Identify the obligation — payment within 30 days.
Step 2: Identify the parties — which party bears the obligation (Client).
Step 3: Identify the risks — late payment fees? Interest accrual? Contract termination?
Step 4: Check for missing provisions — what happens if invoice is disputed?
Step 5: Risk rating — Medium. Missing: late payment consequences, dispute resolution.
Assessment: MEDIUM RISK — add late payment clause and dispute hold provision.

Now analyze:
Clause: "Vendor shall deliver all milestones by the dates specified in Schedule A,
subject to delays caused by Client's failure to provide required materials."
Step 1:
  1. Zero-Shot CoT Failure Mode The model chooses its own reasoning structure, which may be inappropriate for the task. For tasks with required methodologies (legal analysis, financial modeling, risk assessment), zero-shot CoT may produce reasoning that misses critical steps. When the reasoning structure matters, use few-shot CoT to demonstrate it.
  2. Few-Shot CoT Failure Mode The model overfits to the demonstrated reasoning structure and applies it mechanically to cases where a different approach is needed. The reasoning process becomes rigid. Ensure your few-shot examples cover variation in reasoning paths, not just variation in inputs.
  3. Production Choice Start with zero-shot CoT. If reasoning quality is insufficient or the model consistently misses critical steps, upgrade to few-shot CoT with 2-3 examples demonstrating the required methodology. Measure quality improvement per added example — the cost of few-shot CoT is real at production scale.