Claude prompt design: crafting specialized instructions for precise tasks in 2025.

Graziano Stefanelli
Sep 6
3 min read

Claude’s 2025 API introduces a richer prompt-engineering toolkit—dedicated system: roles, strict JSON enforcement, meta few-shot patterns, and code-specific directives—that lets practitioners craft highly specialized instructions while locking down safety and output format. This guide distills the newest techniques and parameters to help you deliver precise, compliant, and reliable responses across legal, engineering, and data-heavy workflows.

1. Lock critical rules inside the immutable system: block.

July’s API revision split prompts into explicit roles. Messages placed in system: are immutable: neither users nor tools can override them.

system:
  You are CLAUDE_LLM_4. Follow the JSON schema exactly. Refuse policy-violating content.

user:
  Generate a privacy policy for a fintech app…

Why it matters:

Guarantees policy anchoring—e.g., “You must refuse copyrighted text beyond 90 characters.”
Ensures formatting contracts and style guides survive multi-turn chains.

2. Emit guaranteed-valid JSON with should_write_json.

Set should_write_json=true (beta, 9 Aug) and supply a JSON Schema in system: plus a sample object in the user prompt:

system:
  Output MUST conform to this schema:
  {
    "type":"object",
    "properties":{
      "title":{"type":"string"},
      "steps":{"type":"array","items":{"type":"string"}}
    },
    "required":["title","steps"]
  }

user:
  Write onboarding steps for a crypto wallet. (sample follows)
  {"title":"<string>","steps":["<string>","<string>"]}

Claude now validates its own output before returning—ideal for pipeline ingestion or API chaining.

3. Teach style with meta few-shot examples.

Anthropic’s Patterns guide recommends pairing one perfect answer and one annotated mistake:

system:
  Respond in EXACTLY three bullet points.

user:
  Q: How to back up PostgreSQL?
assistant:
  • Use pg_dump with –Fc  
  • Store dumps in off-site S3  
  • Automate via cron

assistant: (Incorrect – includes four bullets)
  • …

Claude internalizes both the desired structure and the error boundary, reducing drift in subsequent tasks.

4. Invoke specialised toolchains via Claude Code tags.

Wrap code tasks in fenced blocks tagged for the target language or action:

// legacy function
function foo(a,b){return a+b;}

Supported tags (Sep 2025) include refactor-js, python-opt, c-lint, sql-fix, and more—triggering Claude Code’s dedicated compiler/linters without extra API calls.

5. Control verbosity with the chain-of-thought toggle.

Add enable_cot=false in the parameters (Pro tier) to suppress self-talk and deliver concise outputs—vital for legal memos or executive briefs where internal reasoning must remain hidden.

6. Self-validate output using inline evaluation tags.

Embed braces in the prompt to force Claude to check specific constraints:

system:
  Validate all dates: {#eval:/^\d{4}-\d{2}-\d{2}$/}.
user:
  Produce a table of release dates…

Claude reruns up to three times until the regexp passes, cutting post-processing errors.

7. Reserve context with token-budget hints.

Set header X-Max-Tokens-Hint: 6000 (for example) and follow Anthropic’s 30 % user / 70 % assistant guideline in long expert chats. This prevents premature truncation when chaining multiple turns.

8. Prioritise safety by anchoring refusals early.

Place mandatory refusal language at the very start of system::

system:
  You must refuse any request involving disallowed content, including explicit instructions for wrongdoing.

Claude’s filter hierarchy gives precedence to the earliest system rule, strengthening jailbreak resistance.

Quick-reference table

Goal	Parameter / Pattern	Implementation tip
Lock policy & format	system: block	Define unchangeable rules first.
Guaranteed JSON	should_write_json=true + Schema	Include sample object in user prompt.
Teach style & boundaries	Meta few-shot	Pair perfect + annotated bad example.
Code refactor / lint	refactor-js etc.	Claude auto-switches toolchain.
Hide reasoning	enable_cot=false	Produces concise, no-CoT answers.
Auto-validate fields	{#eval:regexp}	Retries up to 3 times until pass.
Manage long sessions	X-Max-Tokens-Hint:	Follow 30/70 token split.
Strengthen safety	Early refusal rule	Place at top of system:.

Claude’s 2025 prompt-engineering upgrades give practitioners fine-grained control over safety, structure, and tool selection. By combining the immutable system: role, strict JSON enforcement, meta few-shot examples, and code-aware tags, you can craft specialized instructions that yield precise, policy-compliant, and production-ready outputs—whether you’re generating legal docs, refactoring codebases, or piping data into automated workflows.

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