diff --git a/standards/check-claude-ignore.sh b/standards/check-claude-ignore.sh
new file mode 100755
index 0000000..d29a50b
--- /dev/null
+++ b/standards/check-claude-ignore.sh
@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+# check-claude-ignore.sh <repo-dir>: warn-only (never fails); see dev-env#178.
+set -euo pipefail
+
+repo="${1:?usage: check-claude-ignore.sh <repo-dir>}"
+probe=".claude/__standards_probe__"
+ref="smartwatermelon/dev-env#178"
+fix="Add '.claude/' to .gitignore (or '.claude/*' plus '!.claude/<path>' for shared files),"
+fix+=" then 'git rm --cached' any tracked file listed. See ${ref}."
+
+# Blank global excludes. .git/info/exclude still applies.
+_git() { git -C "${repo}" -c core.excludesFile=/dev/null "$@"; }
+
+# Encode %, CR, LF for annotations.
+_esc() {
+  local s="$1"
+  s="${s//'%'/%25}"
+  s="${s//$'\r'/%0D}"
+  s="${s//$'\n'/%0A}"
+  printf '%s' "${s}"
+}
+
+if ! git -C "${repo}" rev-parse --git-dir >/dev/null 2>&1; then
+  echo "::error::${repo} is not a git repository"
+  exit 2
+fi
+
+warned=0
+
+# check-ignore: 0 ignored, 1 not, else error.
+rc=0
+_git check-ignore -q --no-index -- "${probe}" || rc=$?
+if ((rc == 1)); then
+  echo "::warning title=claude-ignore::$(_esc ".claude/ is not ignored by the committed .gitignore. ${fix}")"
+  warned=1
+elif ((rc != 0)); then
+  echo "::warning title=claude-ignore::git check-ignore failed (exit ${rc}); not checked"
+  exit 0
+fi
+
+# Tracked files matching an ignore rule.
+lsrc=0
+tracked="$(_git ls-files -ci --exclude-standard -- .claude/)" || lsrc=$?
+if ((lsrc != 0)); then
+  echo "::warning title=claude-ignore::git ls-files failed (exit ${lsrc}); tracked files not checked"
+  exit 0
+fi
+if [[ -n "${tracked}" ]]; then
+  echo "::warning title=claude-ignore::$(_esc "Tracked file(s) under .claude/ match an ignore rule:"$'\n'"${tracked}"$'\n'"${fix}")"
+  warned=1
+fi
+
+if ((warned == 0)); then echo ".claude/ ignore policy: OK"; fi
+exit 0
diff --git a/standards/run-standards.sh b/standards/run-standards.sh
index 8cfbf4e..c606931 100755
--- a/standards/run-standards.sh
+++ b/standards/run-standards.sh
@@ -16,6 +16,8 @@
 # opened fails their unrelated change. Measured across the fleet, that was the
 # single largest source of standards-check failures.
 #
+# claude-ignore: warn-only, whole-repo.
+#
 # Scope: the flag narrows the four linters that enumerate through _tracked
 # (shellcheck, yamllint, zizmor, markdownlint). actionlint and the Node-floor
 # check find their own inputs and stay whole-repo — both are cheap, and
@@ -293,6 +295,12 @@ if _skipped node-floor; then echo "== node-floor: skipped by input"; else
   _lint node-floor bash "${config_dir}/check-node-floor.sh" "${repo}" "${node_floor}"
 fi
 
+# claude-ignore: warning-only (dev-env#178); never adds to failed[].
+if _skipped claude-ignore; then echo "== claude-ignore: skipped by input"; else
+  _header claude-ignore
+  bash "${config_dir}/check-claude-ignore.sh" "${repo}"
+fi
+
 echo
 if ((${#failed[@]} > 0)); then
   _write_summary
diff --git a/tests/test-check-claude-ignore.sh b/tests/test-check-claude-ignore.sh
new file mode 100755
index 0000000..b32819b
--- /dev/null
+++ b/tests/test-check-claude-ignore.sh
@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+# Validation for standards/check-claude-ignore.sh on throwaway git repos.
+# It only warns, so cases assert on output and exit 0.
+set -euo pipefail
+here="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+checker="${here}/../standards/check-claude-ignore.sh"
+# Isolate from the developer's global git config (hooks, excludes).
+export GIT_CONFIG_GLOBAL=/dev/null GIT_CONFIG_SYSTEM=/dev/null
+tmp="$(mktemp -d)"
+trap 'rm -rf "${tmp}"' EXIT
+pass=0; fail=0
+_ok() { echo "  ok   $1"; pass=$((pass + 1)); }
+_bad() { echo "  FAIL $1"; fail=$((fail + 1)); }
+
+# _repo NAME GITIGNORE-CONTENT [TRACKED-FILE...]: build a fixture repo.
+_repo() {
+  local name="$1" ign="$2" d f
+  shift 2
+  d="${tmp}/${name}"
+  git init -q "${d}"
+  if [[ -n "${ign}" ]]; then printf '%b' "${ign}" >"${d}/.gitignore"; fi
+  mkdir -p "${d}/.claude"
+  for f in "$@"; do echo x >"${d}/${f}"; done
+  git -C "${d}" add -f -A
+}
+
+# _run NAME: run the checker; sets out and requires exit 0.
+_run() {
+  out="$("${checker}" "${tmp}/$1" 2>&1)" || { _bad "$1: exit status nonzero"; return 1; }
+}
+_warns() { grep -q '^::warning title=claude-ignore::' <<<"${out}"; }
+
+_repo nogi ""
+_run nogi && { if _warns; then _ok "no .gitignore warns"; else _bad "no .gitignore did not warn"; fi; }
+
+_repo clean '.claude/\n'
+_run clean && { if _warns; then _bad "'.claude/' ignored warned"; else _ok "'.claude/' ignored is clean"; fi; }
+
+_repo shared '.claude/*\n!.claude/pre-launch.sh\n' .claude/pre-launch.sh
+_run shared && { if _warns; then _bad "negated shared file warned"; else _ok "'.claude/*' + negation + tracked shared file is clean"; fi; }
+
+_repo tracked '.claude/\n' .claude/README.md
+_run tracked && { if _warns && grep -q '\.claude/README\.md' <<<"${out}"; then _ok "tracked file under ignored .claude/ warns and is named"; else _bad "tracked file case missing warning or name"; fi; }
+
+_repo nonegate '.claude/*\n' .claude/pre-launch.sh
+_run nonegate && { if _warns && grep -q '\.claude/pre-launch\.sh' <<<"${out}"; then _ok "'.claude/*' without negation warns and names file"; else _bad "no-negation case missing warning or name"; fi; }
+
+# A global excludes file must not mask a missing rule.
+_repo masked ""
+printf '.claude/\n' >"${tmp}/global-ignore"
+printf '[core]\n\texcludesFile = %s\n' "${tmp}/global-ignore" >"${tmp}/gitconfig"
+out="$(GIT_CONFIG_GLOBAL="${tmp}/gitconfig" "${checker}" "${tmp}/masked" 2>&1)"
+if _warns; then _ok "global excludes does not mask a missing rule"; else _bad "global excludes masked the check"; fi
+
+echo "${pass} passed, ${fail} failed"
+[[ "${fail}" -eq 0 ]]
diff --git a/tests/test-run-standards.sh b/tests/test-run-standards.sh
index 6e94c27..2735c28 100755
--- a/tests/test-run-standards.sh
+++ b/tests/test-run-standards.sh
@@ -113,6 +113,9 @@ printf 'on: push\npermissions:\n  contents: read\njobs:\n  a:\n    runs-on: ubun
 echo "lts/krypton" >"${tmp}/clean/.nvmrc"
 git -C "${tmp}/clean" add -A
 _expect_pass "clean repo passes every linter" clean
+# claude-ignore (dev-env#178) is warning-only: the clean fixture has no
+# .gitignore, so the run must warn AND still pass.
+if grep -q '^::warning title=claude-ignore::' "${tmp}/clean.log"; then _ok "claude-ignore warns without failing the run"; else _bad "claude-ignore warning missing from the clean run"; fi
 
 if bash "${runner}" --repo "${tmp}/bad-sh" --config-dir "${cfg}" --skip shellcheck >/dev/null 2>&1; then _ok "--skip shellcheck disables the linter"; else _bad "--skip shellcheck did not disable it"; fi