ruff format rewrites JSON files into invalid JSON with exit 0

Agent diagnostic brief

Exact symptom

Illegal trailing comma before end of object

Where it has been observed

Python repository using uv run ruff format; Unfiltered changed-file list passed explicitly to ruff.

Likely distinct causes

Non-Python files were passed explicitly to ruff.
Another tool rewrote the JSON: check the formatter's reformatted-file count.
The JSON was already invalid: check the version from before formatting.

How to distinguish them

Look for 'N files reformatted' where the list included JSON paths.
Parse every changed JSON file with a JSON parser.
Run `ruff format --check` on the JSON file: it reports that it would reformat.

Current approaches

Pass only Python files to ruff format and re-validate JSON (solution-ruff-format-rewrites-json, revision 1).

Known obsolete approaches

Using unfiltered `git diff --name-only` output as ruff input.

Versions and freshness

Observed 2026-08-23; reproduced with ruff 0.16.7 on 2026-09-11
Editorial and primary-source review date: 2026-09-11.

What remains unknown

Whether later ruff versions will refuse unknown extensions.
Independent reproduction by a different operator has not been established.

Deeper evidence

Problem

ruff format rewrites JSON files into invalid JSON with exit 0

Observed symptom

`ruff format` is given a file list built from `git diff --name-only`, which includes JSON files; ruff formats them as Python, adds trailing commas, reports them as reformatted, and exits 0.
This historical observation is same-operator evidence and does not establish prevalence.

Operator goal

Format changed files as part of a verification step.

Current understanding

Ruff analyzes files passed directly on the command line regardless of include patterns; a JSON object is also valid Python syntax, so ruff formats it as Python and adds trailing commas.
With ruff 0.16.7 on 2026-09-11 the rewrite still happens with exit 0, while `ruff format --check` exits 1 without modifying the file.

Known limitations

Short JSON objects that fit on one line can remain valid, so damage can look intermittent.

Known negative results

Strip trailing commas with a simple regex. Result: Rejected before use. Why it misleads: A regex can alter commas inside string values.
No same-operator execution in this release counts as an independent reproduction.

Primary and recurrence sources

Ruff configuration and file discovery
Primary · Primary documentation · Date not supplied · Reviewed 2026-09-11 · Rights: allowed_to_summarize
Current primary material reviewed for claude-historical-corpus-1: Ruff configuration and file discovery.

Rights and provenance

Origin

Based on a real operator execution; identifying project details removed.

Rights

State: allowed_to_summarize
Review basis: Owner-authorized factual synthesis of the operator's own Claude-assisted execution, independently written from reviewed current primary sources; no transcript expression is published.

Editorial review date

2026-09-11

Known approaches

solution · Revision 1

Pass only Python files to ruff format and re-validate JSON

Production corpus importer · 2026-09-11T20:00:00.000Z
Operator Knowledge for Agents editorial · Historical operator record · Digital source: trainedAlgorithmicMedia · Rights: owned

## Candidate action - Pass only Python files to ruff format and re-validate JSON. Never pass unfiltered changed-file lists to ruff; filter to .py/.pyi (or rely on ruff's own discovery), and re-validate JSON after any bulk formatting. This is a candidate procedure supported by same-operator executions within the stated version boundary, not a universal fix. ## Applicability - Use when the observed signature is: Illegal trailing comma before end of object. - Observed scope: Python repository using uv run ruff format; Unfiltered changed-file list passed explicitly to ruff. - Stop if the first failing stage or product boundary differs. ## Procedure - Filter file lists to .py and .pyi, or let ruff discover files itself. - After any bulk formatting, parse the changed JSON files. - Repair damaged files with a quote-aware parser or restore them from version control. - Add a guard to scripts that build ruff file lists. ## Limitations - Short JSON objects that fit on one line can remain valid, so damage can look intermittent. ## Obsolete approaches - Using unfiltered `git diff --name-only` output as ruff input. ## Negative results - Strip trailing commas with a simple regex. Result: Rejected before use. Why it misleads: A regex can alter commas inside string values. - No external or same-operator report was promoted to independent reproduction credit. ## Evidence boundary - Grounded in current primary source records src-ruff-configuration-current. - Grade A same-operator observation (2026-08-23): After the quote-aware repair, all twelve affected JSON files parsed successfully; filtering ruff's inputs to Python files prevented recurrence. - Only immutable manifest executions count as Knowledge for Agents Attempts or Outcomes. ## What remains unknown - Whether later ruff versions will refuse unknown extensions. - Whether the approach works outside the stated environment remains unknown.

Candidate id

claude-ruff-format-rewrites-json

Historical period

2026-08-23

Historical date

2026-08-23

Verification grade

Operator relationship

same_operator

Independent reproduction

false

History source

same-operator Claude agent session history

Provenance disclosure

Based on a real operator execution; identifying project details removed.

Applicability

State

partial

Text

Applies when the failed stage matches: Illegal trailing comma before end of object.

Facts

Component: formatter file selection
Operation: format changed files during verification
Protocol: Python source formatting

Pack

Candidate action: Pass only Python files to ruff format and re-validate JSON. Never pass unfiltered changed-file lists to ruff; filter to .py/.pyi (or rely on ruff's own discovery), and re-validate JSON after any bulk formatting. This is a candidate procedure supported by same-operator executions within the stated version boundary, not a universal fix.
Applicability: Use when the observed signature is: Illegal trailing comma before end of object.
Observed scope: Python repository using uv run ruff format; Unfiltered changed-file list passed explicitly to ruff.
Stop if the first failing stage or product boundary differs.
Steps: Filter file lists to .py and .pyi, or let ruff discover files itself.
After any bulk formatting, parse the changed JSON files.
Repair damaged files with a quote-aware parser or restore them from version control.
Add a guard to scripts that build ruff file lists.
Limitations: Short JSON objects that fit on one line can remain valid, so damage can look intermittent.
Obsolete approaches: Using unfiltered `git diff --name-only` output as ruff input.
Negative results: Strip trailing commas with a simple regex. Result: Rejected before use. Why it misleads: A regex can alter commas inside string values.
No external or same-operator report was promoted to independent reproduction credit.
Evidence boundary: Grounded in current primary source records src-ruff-configuration-current.
Grade A same-operator observation (2026-08-23): After the quote-aware repair, all twelve affected JSON files parsed successfully; filtering ruff's inputs to Python files prevented recurrence.
Only immutable manifest executions count as Knowledge for Agents Attempts or Outcomes.
What remains unknown: Whether later ruff versions will refuse unknown extensions.
Whether the approach works outside the stated environment remains unknown.

Rights

State: allowed_to_summarize
Review basis: Original diagnostic procedure synthesized from owner-authorized execution facts and linked current primary sources; no private source code, logs, or transcript expression is published.

Source ids

src-ruff-configuration-current

Editorial review date

2026-09-11

Seo metadata

Meta title: Pass only Python files to ruff format and re-validate JSON | Knowledge for Agents
Meta description: Candidate procedure for ruff format rewrites json files into invalid json with exit 0: applicability, steps, current sources, limitations, negative results, and unknowns.

Historical execution

Disclosure: Based on a real operator execution; identifying project details removed.
Operator relationship: same_operator
Independent reproduction: false

Problem id

problem-ruff-format-rewrites-json

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Sources and related records

No source relations recorded.