Add AI policy

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,3 +4,13 @@
 - [ ] Tests added
 - [ ] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
 - [ ] New functions/methods are listed in `api.rst`
+
+### AI Disclosure
+
+<!--- Please review our AI & contribution guidelines: https://docs.xarray.dev/en/stable/contribute/ai-policy.html. Remove this section if your PR does not contain AI-generated content. --->
+
+- [ ] This PR contains AI-generated content.
+  - [ ] I have tested any AI-generated content in my PR.
+  - [ ] I take responsibility for any AI-generated content in my PR.
+    <!--- If you used AI to generate code, please specify the tool used and the prompt below. --->
+    Tools: {e.g., Claude, Codex, GitHub Copilot, ChatGPT, etc.}

AI_POLICY.md

@@ -0,0 +1 @@
+doc/contribute/ai-policy.md

doc/contribute/ai-policy.md

@@ -0,0 +1,86 @@
+<!-- markdownlint-disable MD013 -->
+
+# AI Usage Policy
+
+**Note:** Some Xarray developers use AI tools as part of our development workflow.
+We assume this is now common. Tools, patterns, and norms are evolving fast — this
+policy aims to avoid restricting contributors' choice of tooling while ensuring that:
+
+- Reviewers are not overburdened
+- Contributions can be maintained
+- The submitter can vouch for and explain all changes
+- Developers can acquire new skills[^1]
+
+To that end this policy applies regardless of whether the code was written by hand, with
+AI assistance, or generated entirely by an AI tool.
+
+[^1]:
+    Over-reliance on AI tools has been shown to
+    [hinder skill formation amongst software developers](https://arxiv.org/abs/2601.20245).
+
+## Core Principle: Changes
+
+If you submit a pull request, you are responsible for understanding and having fully reviewed
+the changes. You must be able to explain why each change is correct[^2] and how it fits into
+the project. Strive to minimize changes to ease the burden on reviewers — avoid
+including unnecessary or loosely related changes.
+
+[^2]:
+    You may also open a draft PR with changes in order to discuss and receive feedback on the
+    best approach if you are not sure what the best way forward is.
+
+## Core Principle: Communication
+
+PR descriptions, issue comments, and review responses must be your own words. The
+substance and reasoning must come from you. Do not paste AI-generated text as
+comments or review responses. Please attempt to be concise.
+
+Using AI to improve the language of your writing (grammar, phrasing, spelling, etc.) is
+acceptable. Be careful that it does not introduce inaccurate details in the process.
+
+Maintainers reserve the right to delete or hide comments that violate our AI policy or code of conduct.
+
+## Code and Tests
+
+### Review Every Line
+
+You must have personally reviewed and understood all changes before submitting.
+
+If you used AI to generate code, you are expected to have read it critically and
+tested it. As with a hand-written PR, the description should explain the approach
+and reasoning behind the changes. Do not leave it to reviewers to figure out what
+the code does and why.
+
+#### Not Acceptable
+
+> I pointed an agent at the issue and here are the changes
+
+> This is what Claude came up with. 🤷
+
+#### Acceptable
+
+> I iterated multiple times with an agent to produce this. The agent wrote the code at my direction,
+> and I have fully read and validated the changes.
+
+> I pointed an agent at the issue and it generated a first draft. I reviewed the changes thoroughly and understand the implementation well.
+
+### Large AI-Assisted Contributions
+
+Generating code with agents is fast and easy. Reviewing it is not. Making a PR with a large diff
+shifts the burden from the contributor to the reviewer. To guard against this asymmetry:
+
+If you are planning a large AI-assisted contribution (e.g., a significant refactor, a
+framework migration, or a new subsystem), **open an issue first** to discuss the scope
+and approach with maintainers. This helps us decide if the change is worthwhile, how
+it should be structured, and any other important decisions.
+
+Maintainers reserve the right to close PRs where the scope makes meaningful review
+impractical, or when they suspect this policy has been violated. Similarly they may request
+that large changes be broken into smaller, reviewable pieces.
+
+## Documentation
+
+The same core principles apply to both code and documentation You must review the result
+for accuracy and are ultimately responsible for all changes made. Xarray has domain-specific
+semantics that AI tools frequently get wrong. Do not submit documentation that you
+haven't carefully read and verified.

doc/contribute/index.rst

@@ -13,6 +13,7 @@ In this section you will also find documentation on the internal organization of
    :hidden:
 
    contributing
+   ai-policy
    ../internals/index
    ../roadmap
    ../whats-new