Fantomas Issue

The input is a Fantomas GitHub issue URL (e.g. https://github.com/fsprojects/fantomas/issues/1234).

Follow these steps in order:

1. Fetch the issue

Use gh issue view <number> --repo fsprojects/fantomas --json title,body,labels to get the issue details. Extract the example code and expected behavior. Note the labels — bug (soundness) vs bug (stylistic) affects the changelog entry.

2. Reproduce the problem

Use the /format skill to format the example code and confirm the bug exists. If confirmed, try to trim the example down to the minimal reproduction case.

3. Add a failing unit test

Find a suitable test file in src/Fantomas.Core.Tests/. Look for existing tests related to the same AST node or concept using grep. If no good file exists, ask the user where to create a new file.

A new test file should follow this template:

module Fantomas.Core.Tests.MyNewConceptTests

open NUnit.Framework
open FsUnit
open Fantomas.Core.Tests.TestHelpers

// add tests here...

Test naming rules:

• Test names must start with a lowercase letter.
• When linked to a GitHub issue, add the issue number at the back with a comma: letmy test description, 1234() =
• You don't need to repeat the issue number for tests that are variations of the original report.

Verify signature files

Check if the fix should also apply to signature files (*.fsi). If so, add a test using formatSignatureString or the --signature flag.

Verify slight variations

• Check if additional tests are needed for different setting combinations.
• If the code involves #if/#else directives, use formatSourceStringWithDefines to test each define combination separately, plus a formatSourceString test for the merged result. Name suffixes: , no defines, , DEBUG, , 1234.

Run the test and assert it fails before proceeding to the fix.

4. Investigate the root cause

Use the /ast, /oak, and /writer-events skills to understand what's happening. Key files to inspect:

• src/Fantomas.Core/CodePrinter.fs — the main printer
• src/Fantomas.Core/Context.fs — writer context
• src/Fantomas.Core/ASTTransformer.fs — AST to Oak transformation
• src/Fantomas.Core/Trivia.fs — trivia (comments, blank lines, directives)

5. Implement the fix

Make the minimal change needed. Run the new test to confirm it passes.

6. Run all tests

Run dotnet test src/Fantomas.Core.Tests/Fantomas.Core.Tests.fsproj. If many tests fail, the fix is likely too broad — make it more targeted. If only a few tests fail and the new behavior is arguably better, update those tests and ask the user for their opinion (a git diff is easiest to review).

7. Update CHANGELOG.md

Add an entry under the ## [Unreleased] section. Never add to an already-published version section. If there is no Unreleased section, create one at the top above the most recent version.

• For bug (soundness) fixes, add under ### Fixed using the original issue title: - <Original GitHub issue title>. [#1234](https://github.com/fsprojects/fantomas/issues/1234)
• For bug (stylistic) fixes (not related to a style guide), also add under ### Fixed.
• For bug (stylistic) fixes related to a style guide, add under ### Changed: - Update style of xyz. [#1234](https://github.com/fsprojects/fantomas/issues/1234)

8. Post-task steps

Run dotnet fsi build.fsx -- -p FormatChanged, then dotnet fsi build.fsx -- -p AnalyzeChanged, and read analysis.sarif afterwards: the analyzer run exits 0 whatever it found. The Post-task Steps section of AGENTS.md says what each one covers.