diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1177c6b133..4dc728dc3a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -129,6 +129,14 @@ aggregate instead: an italic *Catalog* line at the end of the version section an
 
 ### Fixed
 
+- **CI lint went red on every PR after a ruff minor bump** — `pyproject.toml` pins
+  `ruff>=0.15.21` without an upper bound, CI resolved **0.16.0**, and that version started
+  formatting Python code blocks inside Markdown: 20 tracked `.md` files suddenly "would be
+  reformatted", none of them touched by the PR that hit it. Ruff's exclude list now carries
+  `*.md`. Deliberately not fixed by reformatting those files — nine of them are
+  `prompts/library/*.md`, whose snippets steer code generation, so rewriting quote style there
+  is a pipeline change wearing a formatting costume. Verified against both 0.15.21 and 0.16.0
+  (`format --check` + `check` clean on 148 files) (#9899).
 - **A GitHub API brown-out could strand implementation PRs permanently — the hand-off calls
   are now retried** — on 2026-07-24 (~16:05–16:26 UTC) GitHub returned HTTP 502 on the
   workflow-dispatch endpoint and HTTP 504 on GraphQL, and four `radar-basic` PRs fell out of
diff --git a/pyproject.toml b/pyproject.toml
index ddf7ec08ae..0d6e725717 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -127,6 +127,12 @@ exclude = [
     "temp",
     "plots",
     "scripts",
+    # ruff 0.16 formats Python code blocks inside Markdown. Twenty tracked .md
+    # files change under it — including prompts/library/*.md, whose snippets
+    # steer code generation, so a quote-style rewrite there would be a pipeline
+    # change disguised as formatting. Ruff owns this repo's Python, not its
+    # prose; formatting the docs stays a deliberate, separate decision.
+    "*.md",
 ]
 
 [tool.ruff.lint]