Prompt Engineering Quick Reference#

Core principles#

  1. Be specific. Vague asks (“summarize this”) yield vague answers. State the audience, length, format, and success criteria.

  2. Show, don’t tell. A single well-chosen example is worth a paragraph of instructions.

  3. Separate concerns. Use XML-style tags or Markdown headings to isolate instructions, context, and examples.

  4. Constrain the output. Ask for JSON / a specific schema / a fixed structure whenever downstream code will parse the response.

  5. Let the model think. For any non-trivial reasoning task, allow intermediate reasoning before the final answer.

Role assignment#

Open with a system message describing the persona and scope. This is the single highest-leverage prompt technique.

You are a senior Python code reviewer at a fintech company.
Your job is to flag security issues (OWASP Top 10), then style problems.
Only comment on things that would block a merge. Be terse.

Few-shot examples#

Provide 2–5 input→output pairs that cover the edge cases you care about. The model will generalize from the pattern.

Convert each product description to a JSON tag list.

Example 1:
Input: "Wireless noise-cancelling over-ear headphones."
Output: ["wireless", "noise-cancelling", "over-ear", "headphones"]

Example 2:
Input: "Stainless steel insulated water bottle, 500ml."
Output: ["stainless-steel", "insulated", "water-bottle", "500ml"]

Now:
Input: "{user_input}"
Output:

Chain-of-thought (CoT)#

Ask the model to reason step-by-step before answering. On Claude 4.6 and other reasoning models, this is often implicit via extended/adaptive thinking, but the prompt still helps for older or cheaper models.

Think through the problem carefully, then give the final answer.

Problem: A train leaves station A at 3pm going 60 mph. Another leaves
station B at 4pm going 80 mph toward A. Stations are 280 miles apart.
When do they meet?

Reasoning:

For models that expose reasoning tokens natively (Claude Opus 4.6, Sonnet 4.6 with adaptive thinking; OpenAI o-series; Gemini 3 with ThinkingConfig), you usually do not need to add “think step by step” manually — the model handles it internally.

Structured output#

Always prefer schema-constrained output over regex-parsing free text.

With the Anthropic SDK#

import anthropic

client = anthropic.Anthropic()
response = client.messages.create(
    model="claude-sonnet-4-6",
    max_tokens=1024,
    tools=[{
        "name": "extract_contact",
        "description": "Extract structured contact info",
        "input_schema": {
            "type": "object",
            "properties": {
                "name": {"type": "string"},
                "email": {"type": "string"},
                "phone": {"type": "string"},
            },
            "required": ["name", "email"],
        },
        "strict": True,  # guarantees schema conformance
    }],
    tool_choice={"type": "tool", "name": "extract_contact"},
    messages=[{"role": "user", "content": "Jane Doe, jane@acme.co, 555-0100"}],
)

With LangChain create_agent#

from pydantic import BaseModel, Field
from langchain.agents import create_agent

class ContactInfo(BaseModel):
    name: str = Field(description="Full name")
    email: str = Field(description="Email address")
    phone: str | None = Field(default=None, description="Phone if present")

agent = create_agent(
    model="anthropic:claude-sonnet-4-6",
    tools=[],
    response_format=ContactInfo,
)
result = agent.invoke({"messages": [{"role": "user", "content": "..."}]})
print(result["structured_response"])  # ContactInfo instance

XML-style tagging (Anthropic-preferred)#

Claude is trained to respect XML-ish tags in prompts. Use them to isolate sections that the model should treat as data rather than instructions.

Review the pull request below and return a list of blocking issues.

<diff>
{pull_request_diff}
</diff>

<style_guide>
{internal_style_guide}
</style_guide>

Respond with a JSON array of {"line": int, "severity": str, "message": str}.

Negative instructions — use sparingly#

LLMs can interpret negative instructions (“don’t do X”) as emphasis on X. Rephrase as positive directives whenever possible.

  • ❌ “Don’t use passive voice.”

  • ✅ “Write every sentence in active voice.”

Grounding and anti-hallucination#

When the model must cite sources, be explicit:

Answer only using information contained in <context>. If the answer is
not in the context, reply exactly: "I don't know based on the provided
sources." Cite the source ID in square brackets after each claim.

<context>
[src-1] ...
[src-2] ...
</context>

Question: {question}

Temperature and sampling#

  • temperature=0.0 — deterministic, best for extraction, classification, code generation.

  • temperature=0.7 — balanced, good for most conversational tasks.

  • temperature=1.0 — Gemini 3 default and recommended; unusual to change.

For OpenAI Responses API and Anthropic messages with thinking enabled, temperature effects are muted — the model already has internal variance.

Prompt caching (long static context)#

If you send the same 50KB system prompt on every request, enable prompt caching so you pay the cache-read rate (typically 0.1× input) instead of the full input rate.

client.messages.create(
    model="claude-sonnet-4-6",
    system=[
        {"type": "text", "text": "You are..."},
        {
            "type": "text",
            "text": long_style_guide_text,
            "cache_control": {"type": "ephemeral"},
        },
    ],
    messages=[...],
)

See Observability: LangFuse & LangSmith for tracing these patterns in production.

Practice#

1. Rewrite a vague prompt#

Take the prompt: "summarize this article".

Rewrite it with:

  • An explicit audience (“a busy executive who has 30 seconds”)

  • A target length (“3 bullet points, each under 15 words”)

  • A format (“plain text, no Markdown”)

  • A success criterion (“must name the two parties and the dollar amount”)

Run both prompts through the same model and compare the outputs.

2. Few-shot classification#

Build a prompt that classifies customer support tickets into one of billing, bug, feature-request, other. Provide 4 examples in the prompt. Test with 10 held-out tickets and measure accuracy.

Target: ≥90% accuracy on a balanced test set using claude-sonnet-4-6 at temperature=0.

3. Structured extraction with strict tool use#

Use the Anthropic SDK with strict: true to extract the following from a job posting:

{
    "title": str,
    "company": str,
    "location": str,
    "remote": bool,
    "salary_min": int | None,
    "salary_max": int | None,
    "required_skills": list[str],
}

Verify the response parses as valid JSON on 20 real postings without any try/except fallbacks.

4. Prompt caching measurement#

Set up a long system prompt (~2,000 tokens). Issue 10 identical user queries. Compare total cost with and without cache_control using the usage.cache_creation_input_tokens and usage.cache_read_input_tokens fields from the response.

Expected: after the first request, subsequent requests should report most input tokens as cache reads, cutting cost by ~90% on the cached portion.

5. Grounding and refusal#

Build a RAG-style prompt that must answer only from a provided <context> block. Test with 5 questions that are answerable from the context and 5 that are not. The model should refuse exactly on the 5 unanswerable ones with the literal string "I don't know based on the provided sources.".

Measure refusal precision and recall.

Review Questions#

  1. Which prompt technique consistently produces the largest quality improvement with the smallest token budget?

    • A. Setting temperature to 1.0

    • B. Assigning a clear role in the system message

    • C. Using ALL CAPS for important instructions

    • D. Adding the word “please” to every instruction

  2. You need to extract a strict JSON schema from unstructured text. Which option is most reliable?

    • A. Ask the model to “return JSON” in the prompt and parse it yourself

    • B. Use tool use with strict: true (Anthropic) or response_format (OpenAI / LangChain)

    • C. Regex the output

    • D. Set temperature to 2.0 for variety

  3. For Claude Opus 4.6 with adaptive thinking enabled, do you still need to write “think step by step” in the prompt?

    • A. Yes, always

    • B. No — the model reasons internally; an explicit CoT instruction is usually redundant

    • C. Only on weekends

    • D. Only for math problems

  4. What is the recommended temperature for Gemini 3 models?

    • A. 0.0

    • B. 0.5

    • C. 1.0 (default)

    • D. 2.0

  5. Why is XML-style tagging particularly effective with Claude?

    • A. Claude was trained to respect XML-ish tags for isolating instructions from data

    • B. XML is faster to parse than JSON

    • C. It reduces token count

    • D. It’s required by the API

  6. Your system prompt is 50KB and never changes between requests. How do you cut cost?

    • A. Truncate it to 5KB

    • B. Enable prompt caching with cache_control: {"type": "ephemeral"}

    • C. Send it only on the first request

    • D. Use a smaller model

  7. What is the failure mode of negative instructions like “Don’t use passive voice”?

    • A. The model throws an error

    • B. LLMs sometimes interpret the negation as emphasis and do the opposite — prefer positive directives

    • C. They cost more tokens

    • D. They are case-sensitive

  8. A RAG system should refuse to answer when the context is insufficient. How do you enforce this?

    • A. Lower the temperature

    • B. Explicitly instruct the model to reply with a fixed refusal string when the answer is not in the context

    • C. Hope it works

    • D. Use a larger model

  9. When building a few-shot classifier, how many examples are typically enough?

    • A. Exactly 1

    • B. 2–5 examples that cover the edge cases

    • C. At least 100

    • D. As many as the context window allows

  10. Which parameter makes tool use schema-conforming on the Anthropic API?

    • A. temperature: 0

    • B. strict: true on the tool definition

    • C. json_mode: true

    • D. force_schema: true

View Answer Key

  1. B — Role assignment is the single highest-leverage technique.

  2. B — Schema-constrained tool use beats regex parsing every time.

  3. B — Reasoning models handle CoT internally; an explicit “think step by step” is redundant (and sometimes harmful).

  4. C — Google explicitly recommends keeping temperature=1.0 default for Gemini 3 models.

  5. A — Claude is trained to recognize XML-ish tags as structural boundaries.

  6. B — Prompt caching cuts cached reads to ~0.1× input cost.

  7. B — Rephrase as positive directives (“Write in active voice”).

  8. B — Explicit refusal instructions with a fixed string make refusals detectable and measurable.

  9. B — 2–5 carefully chosen examples typically get you most of the benefit.

  10. B — strict: true on the tool guarantees the model’s output matches the declared schema.