# Claude XML Anti-Patterns: What Not to Do When Prompting Claude > Common Claude XML prompting mistakes and how to fix them: over-tagging, misplaced documents, JSON fences, security pitfalls, more. > > Source: https://claudexml.com/anti-patterns/ · Last updated 2026-05-25 Home / Anti-patterns # Claude XML anti-patterns: what not to do Mistakes that look fine but quietly degrade reliability. Each entry: the trap, why it bites, and the fix. ## 1. Over-tagging trivial prompts The trap: Wrapping a one-line question in ``, ``, `` when none of them carry weight. Adds tokens, no quality gain. The fix: Tag when the structure is load-bearing. A simple zero-shot question doesn't need scaffolding. ## 2. Putting the user question inside `` The trap: Claude will follow the question as a directive rather than answer it. The fix: Keep `` for what the model should do; put the question in its own tag or unwrapped after the structured blocks. ## 3. Mismatched tags The trap: Opening `` and closing with `` or forgetting the close. The fix: XML is case-sensitive; Claude usually recovers but reliability drops. Lint your prompts. ## 4. Putting documents after the question on long contexts The trap: Anthropic's guidance is the opposite: long reference material first, then instructions, then question. The fix: Order: `` → `` → ``. Helps attention land on the instruction block. ## 5. Asking for JSON without saying "only JSON" The trap: Claude often wraps JSON in ````json` markdown fences by default. The fix: Add "Return only the JSON object, no prose, no markdown fences" — or wrap the output in `` and parse from there. ## 6. Reading `` output as ground truth The trap: Chain-of-thought scratchpads are reasoning aids, not verified logs. They can rationalize wrong answers. The fix: Use CoT to improve accuracy; don't show the scratchpad to end users as if it were proof. ## 7. Stuffing the system prompt and an inline `` with different personas The trap: The two will fight; Claude becomes inconsistent. The fix: One source of persona truth. Prefer the API `system` parameter. ## 8. Few-shot examples that don't span the real distribution The trap: If all your examples are happy-path, edge cases collapse to the happy-path label. The fix: Include hard cases, edge cases, and at least one example of each output class. ## 9. Forgetting `` in RAG prompts The trap: Then asking for citations. Claude invents source names. The fix: Always pair `` with `` when you want attribution. ## 10. Treating XML tags as a security boundary The trap: Users can inject `` into their input and break out of your scaffolding. The fix: Sanitize user input — escape angle brackets or strip XML-like sequences before splicing into the prompt. ## 11. Nesting tags that don't logically nest The trap: Putting `` inside ``, or `` inside ``. The fix: Keep major sections as siblings. Nest only when the semantics are containment (`` inside ``, `` inside ``). ## When to skip XML entirely XML is load-bearing when there's structural ambiguity in a single prompt the model has to resolve. No ambiguity → no scaffolding needed. Skip XML when: - Conversational chat. Multi-turn dialogue already establishes who is talking. Wrapping a question in `` doesn't help — the model knows. - Single-shot creative writing. "Write a haiku about Tuesday." No structure to scaffold. - One-line lookups. "What's the capital of France?" The tax is small but real. And skip specific tags when the API now does the job better: - Structured output: prefer the API's tool-use / JSON-mode over `` + "return only JSON". XML coercion is the second-best option in 2026. - Persona: the API `system` parameter is stronger than inline ``. - Reasoning: on Claude 3.7 and newer with extended thinking enabled, the `` tag is redundant and can fight the internal mechanism. Use one, not both. See also: when XML genuinely helps — the inverse list. Cite this page `Claude XML Anti-Patterns. claudexml.com. https://claudexml.com/anti-patterns/`