Clean US phone numbers into one format in a CSV block

About this exercise

Your customer list has phone numbers typed many ways and your texting tool wants +1 followed by 10 digits. Write one CSV block (one code block) with the header original,clean and one row per number in the same order. Only US numbers with a 10-digit local part are valid; write INVALID for anything else, without guessing missing digits. At most 12 words outside the code.

Common mistakes when you ask AI for this

• Filling in the area code. The AI turns 555-0123 into +14155550123. Try: Add: "No area code means INVALID. Don't fill in digits."
• Keeping the UK number. The AI reformats the London number as valid. Try: Add: "Non-US numbers are INVALID."
• Wrong CSV shape. The AI drops the header or reorders rows. Try: Add: "Header original,clean, then 8 rows in the same order."
• Leaving the format to chance. The AI picks its own length and layout unless told the exact shape. Try: Add: "Put the code in exactly 1 code block. Keep the writing outside the code under 12 words."

What a strong prompt includes

• Gives the target format
• Says invalid
• Rules out guessing
• Names the header
• Keeps the order

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