diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c0a81d86aea..97355e4e7ab 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -51,7 +51,6 @@ jobs:
               - 'scripts/check-file-sizes-core.mjs'
               - 'scripts/check-file-sizes-core.test.mjs'
               - 'desktop/**'
-              - '!desktop/src-tauri/**'
               - 'pnpm-lock.yaml'
             desktop-rust:
               - 'desktop/src-tauri/**'
@@ -86,6 +85,8 @@ jobs:
         run: scripts/test-mobile-worktree-overrides.sh
       - name: File size ratchet unit tests
         run: node --test scripts/check-file-sizes-core.test.mjs
+      - name: Changed-paths filter contract
+        run: scripts/test-ci-changed-paths-filter.sh
 
   rust-lint:
     name: Rust Lint
diff --git a/scripts/test-ci-changed-paths-filter.sh b/scripts/test-ci-changed-paths-filter.sh
new file mode 100755
index 00000000000..cdc8bd1346a
--- /dev/null
+++ b/scripts/test-ci-changed-paths-filter.sh
@@ -0,0 +1,69 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Guards against the bug behind launchpad-26/buzz#181: dorny/paths-filter
+# (via picomatch) matches each pattern in a filter's list as an independent
+# OR clause. A standalone negated pattern like '!desktop/src-tauri/**',
+# mixed into a filter that also has positive patterns, does NOT act as an
+# "AND NOT" exclusion the way a .gitignore-style reader would expect -- on
+# its own it matches every file outside that directory, which silently
+# makes the *entire* filter true for virtually any change in the repo.
+# Verified against picomatch directly: an array of
+#   ['desktop/**', '!desktop/src-tauri/**']
+# matches 'launchpad/plans/foo.md' as true, even though that path isn't
+# under desktop/ at all.
+#
+# The fix is not to mix quantifiers within one filter's pattern list. If a
+# filter genuinely needs "under A, but not under B", split B into its own
+# filter and OR the two outputs together at the job `if:` condition instead
+# (this repo already does exactly that for desktop vs. desktop-rust).
+
+repo_root=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)
+workflow="$repo_root/.github/workflows/ci.yml"
+
+# Pull just the `filters: |` block passed to dorny/paths-filter and walk it
+# filter-by-filter (a filter starts at 12-space indent, its patterns at 14).
+awk '
+  /filters: \|/ { in_block = 1; next }
+  in_block && /^ {12}[A-Za-z0-9_-]+:$/ {
+    if (name != "") { print name "\t" patterns }
+    name = $1
+    sub(/:$/, "", name)
+    patterns = ""
+    next
+  }
+  in_block && /^ {14}- /  {
+    pat = $0
+    sub(/^ *- /, "", pat)
+    patterns = patterns pat ","
+    next
+  }
+  in_block && name != "" && ! /^ {12,}/ { print name "\t" patterns; in_block = 0 }
+  END { if (in_block && name != "") print name "\t" patterns }
+' "$workflow" > /tmp/ci-changed-paths-filters.$$
+
+failed=0
+while IFS=$'\t' read -r filter_name patterns; do
+  has_positive=0
+  has_negative=0
+  IFS=',' read -ra items <<< "$patterns"
+  for item in "${items[@]}"; do
+    [ -z "$item" ] && continue
+    case "$item" in
+      \'!*) has_negative=1 ;;
+      *) has_positive=1 ;;
+    esac
+  done
+  if [ "$has_positive" -eq 1 ] && [ "$has_negative" -eq 1 ]; then
+    echo "::error::filter '$filter_name' in $workflow mixes a negated pattern with positive patterns -- picomatch treats each entry as an independent OR clause, so the negated pattern alone matches almost every file and the filter silently becomes true for nearly any change (this is exactly launchpad-26/buzz#181). Split the excluded path into its own filter and OR the outputs at the job if: condition instead." >&2
+    failed=1
+  fi
+done < /tmp/ci-changed-paths-filters.$$
+
+rm -f /tmp/ci-changed-paths-filters.$$
+
+if [ "$failed" -ne 0 ]; then
+  exit 1
+fi
+
+echo "changed-paths filter contract passed"