Why Does Claude Use British Spelling?

Claude uses British spelling when it picks up signals from the conversation that British English is the right register, or when no language preference has been set and its training data tips toward British forms. A single phrase like colour or organise in your question can shift how Claude responds. Saving a clear language rule in a tool like fenbs means any connected assistant reads the preference before it writes a word.

The fix is straightforward: tell Claude which English variety you want. The more useful question is where to tell it so the instruction holds across every session.

Why Claude shifts to British English

Claude's training includes a large amount of British English text. When nothing in the conversation signals a preference, Claude draws on a mix of both varieties. If your question contains British spellings, a British place name, or a phrase more common in British usage, Claude takes that as a cue to match your register.

Pasted documents are a common cause. If you paste a contract, an article, or a document that uses British conventions, Claude may continue in the same style. The same applies to company names and proper nouns associated with British contexts.

• British spellings in your message or pasted content ("-ise" endings, colour, centre).
• British place names, legal bodies, or organizations in the context.
• A document with British formatting conventions pasted into the chat.
• No explicit language instruction set in your account or project settings.

Common British spellings Claude uses

The most frequent British forms that appear in Claude output are verb endings and a handful of nouns. Knowing them helps you spot when Claude has shifted registers.

• -ise verb endings: organise, recognise, analyse, prioritise, optimise.
• Colour, favour, behaviour, neighbour, honour.
• Centre, litre, metre (unit of measurement).
• Programme (non-technical usage), defence, licence (used as a verb).
• Travelling, modelling, cancelled (double-l forms).

How to ask for US spelling in a single chat

Add an explicit instruction to your message. "Use American English and US spelling throughout" is enough. Place it at the start of the chat or at the start of the message where you want the change to take effect.

If Claude has already produced several messages in British English, stating the preference at the start of a fresh conversation is more reliable than correcting mid-session. Long conversations can reduce how much weight Claude gives to early instructions.

You can also ask Claude to review existing text and replace British spellings. Paste the passage and give the instruction: "Rewrite this paragraph using American spelling. Change all -ise endings to -ize, and replace colour with color, centre with center."

How to set a permanent language preference

Claude.ai has a custom instructions field under Preferences. Add "Always use American English and US spelling" there. Claude applies it to every new conversation automatically.

In Claude Code, add the instruction to your CLAUDE.md or AGENTS.md file at the project root: "Use American English. American spelling only: -ize not -ise, color not colour, center not centre." Claude Code reads this file at the start of each session.

For a team using multiple AI apps, a shared record keeps the rule consistent. If one teammate uses ChatGPT and another uses Claude, a common writing preference saved in one place reaches both without each person maintaining their own copy.

What to do if the instruction does not hold

Claude follows language instructions reliably in most cases. It can still slip if the pasted content is heavily British or if the conversation is very long. When that happens, a brief reminder at the top of your next message usually corrects it.

Check the source material you are pasting. If you paste a British document and ask Claude to summarize it, Claude may write the summary in British English. Tell Claude: "Summarize this document in American English, even though the source uses British spelling."

If you are working on a document that mixes content from British and American sources, tell Claude at the start of each session which variety you want for the final output. Pasting mixed-variety content without that instruction is the most common cause of unexpected British forms.

Does it matter which English variety you use?

For most tasks it does not matter which variety Claude uses, as long as it is consistent. The problems arise when the output will be read by a specific audience, published under a style guide, or checked against a word list. A legal brief for a US court, a marketing document for American consumers, or an article for a US publication should use American spelling throughout.

Tell Claude which audience the output is for. "This is for a US audience" or "follow US style guide conventions" gives Claude enough context to make consistent choices without you listing every affected word.

How to do this with fenbs

1. Create a free account and open your personal board or a shared team board.
2. Go to Skills and prompts and add a new entry. Title it "Writing style" and write the rule: "Use American English throughout. American spelling only."
3. Connect your AI apps from Integrations. Claude, ChatGPT, Cursor and other supported apps can read your saved prompts.
4. At the start of a session, ask the assistant to read your project context and skills. It will find the writing rule and apply it without you retyping it.
5. If your team has a shared board, add the writing preference to Decisions and rules with the person who agreed on it noted. Connected assistants see that rule first.