diff --git a/.prettierignore b/.prettierignore
index 1aca0f3b..feb2b754 100644
--- a/.prettierignore
+++ b/.prettierignore
@@ -18,6 +18,15 @@ docs/lessons-index.md
 .claude/settings.json
 .claude/settings.local.json
 
+# Other Claude Code sessions' worktrees. They are git-excluded through `.git/info/exclude`, which prettier does not
+# read (it reads `.gitignore`), and they are not tracked, so this entry removes no repo file from format coverage.
+# Without it, a bare `format:check` in a main checkout reads every nested worktree and ignores that worktree's own
+# `.prettierignore`. Measured with one nested copy of HEAD: 62 committed files inside it were flagged, because this
+# file excludes them at the root only. The markdownlint twin is `.markdownlint-cli2.jsonc`'s `.claude/worktrees`
+# entry. Bound: it is tied to where Claude Code places worktrees today. If that location moves, the entry silently
+# matches nothing.
+.claude/worktrees/
+
 # Machine stage-reports that /pharn-dev-regress and /pharn-dev-verify must write VERBATIM — each is its
 # floor helper's stdout JSON, deliberately NOT formatted so the recorded verdict is the helper's own
 # bytes. Without these entries those two requirements conflict with whole-repo `format:check`, which the
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7978a055..1e2b38ee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,30 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
      `npm run check:changelog` holds this file's shape; the CI step "CHANGELOG per-PR entry check" holds
      each PR's diff. Details and known costs: CONTRIBUTING.md, "CHANGELOG entries". -->
 
+### Fixed
+
+- 2026-09-28: **A bare `npm run format:check` or `npm run lint` no longer reads other Claude Code sessions' worktrees
+  under `.claude/worktrees/`** ([`.prettierignore`](./.prettierignore), [`eslint.config.mjs`](./eslint.config.mjs)).
+  Repo-meta only, so `SKILLS_VERSION` does not move.
+  - **The failure.** On 2026-09-28, with 9 sibling worktrees present, `npm run check` from the main checkout failed
+    `format:check` with hundreds of `[warn]` lines, each one for a file inside `.claude/worktrees/<name>/`. Those
+    worktrees are excluded through `.git/info/exclude`, which prettier does not read. `prettier --check .` therefore
+    descended into each one and applied only the root `.prettierignore`, whose entries match at the root alone.
+    ESLint 10's flat config does not skip dot-directories either, so `eslint .` linted them too. 6.13.1 measured the
+    prettier half and deferred it; markdownlint has ignored `.claude/worktrees` since then.
+  - **Fix.** `.prettierignore` gains `.claude/worktrees/`, and `eslint.config.mjs`'s global `ignores` gains
+    `.claude/worktrees/**`. Each carries a comment giving the reason and the bound, as the `.markdownlint-cli2.jsonc`
+    entry does. The directory is untracked, so no repo file loses coverage.
+  - **Measured** in a worktree with no nested worktrees, with a probe at `.claude/worktrees/probe/` holding a copy of
+    HEAD plus a mis-formatted `.md` and `.mjs`. Before: `format:check` exited 1 on 64 files (the two probe files, and
+    62 committed files that the root `.prettierignore` excludes only at the root), and `lint` exited 1 with 2 errors
+    in the probe `.mjs`. After: both exit 0. Negative control: the same two files at the repo root still fail both
+    gates (2 prettier warnings, 2 ESLint errors). The probe was removed afterwards.
+  - **Not claimed.** No test pins either entry, unlike the markdownlint one (`command-hygiene.test.mjs`), so nothing
+    stops one being removed. Both entries are tied to where Claude Code places worktrees today. If that location
+    moves, they silently match nothing. `npm test`'s glob was already narrowed for the same reason (`package.json`,
+    `_test_glob_comment`).
+
 ## [6.32.1] - 2026-09-28
 
 ### Fixed
diff --git a/eslint.config.mjs b/eslint.config.mjs
index 64888308..c8922ee1 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -3,7 +3,19 @@ import globals from "globals";
 import prettier from "eslint-config-prettier";
 
 export default [
-  { ignores: ["node_modules/**", ".dev/floor/test-fixtures/**", "pharn/floor/test-fixtures/**"] },
+  {
+    ignores: [
+      "node_modules/**",
+      ".dev/floor/test-fixtures/**",
+      "pharn/floor/test-fixtures/**",
+      // Other Claude Code sessions' worktrees. They are git-excluded (.git/info/exclude), not tracked, so this removes
+      // no repo file from lint coverage. Flat config does not skip dot-directories, so without it a bare `lint` in a
+      // main checkout lints every nested worktree: measured, a .mjs with two lint errors in a nested copy of HEAD failed it.
+      // The markdownlint twin is `.markdownlint-cli2.jsonc`'s `.claude/worktrees` entry. Bound: it is tied to where
+      // Claude Code places worktrees today. If that location moves, the entry silently matches nothing.
+      ".claude/worktrees/**",
+    ],
+  },
   js.configs.recommended,
   {
     files: ["**/*.js", "**/*.cjs"],