diff --git a/list-cnc-sim-core-sources.sh b/list-cnc-sim-core-sources.sh
index 7a38532..23a4f68 100755
--- a/list-cnc-sim-core-sources.sh
+++ b/list-cnc-sim-core-sources.sh
@@ -38,6 +38,7 @@ trap 'rm -f "$source_list"' EXIT
 } > "$source_list"
 
 ./list-filtered-existing-paths.sh \
+    --project-relative \
     "$filter" \
     "missing cnc sim core source" \
     "missing cnc sim core source in CMake list" < "$source_list"
diff --git a/list-filtered-existing-paths.sh b/list-filtered-existing-paths.sh
index 11d5839..38ff536 100755
--- a/list-filtered-existing-paths.sh
+++ b/list-filtered-existing-paths.sh
@@ -6,8 +6,14 @@ cd "$(dirname "$0")"
 # LinuxCNC source basis: source-list filters validate project bridge/shim
 # paths before native and wasm probes link them with LinuxCNC source objects,
 # keeping missing adapter files from silently dropping LinuxCNC-backed checks.
+project_relative=0
+if [[ "${1:-}" == "--project-relative" ]]; then
+    project_relative=1
+    shift
+fi
+
 if [[ "$#" -lt 3 ]]; then
-    echo "usage: $0 <filter> <missing-path-message> <missing-filter-message> [missing-path-hint]" >&2
+    echo "usage: $0 [--project-relative] <filter> <missing-path-message> <missing-filter-message> [missing-path-hint]" >&2
     exit 1
 fi
 
@@ -19,6 +25,11 @@ missing_path_hint=${4:-}
 found=0
 while IFS= read -r path; do
     [[ -z "$path" ]] && continue
+    if [[ "$project_relative" -eq 1 &&
+        ( "$path" = /* || "$path" == .. || "$path" == ../* || "$path" == */.. || "$path" == */../* ) ]]; then
+        echo "project source path must stay project-relative: $path" >&2
+        exit 1
+    fi
     if [[ ! -f "$path" ]]; then
         echo "$missing_path_message: $path" >&2
         if [[ -n "$missing_path_hint" ]]; then
diff --git a/list-linuxcnc-rs274-api-project-sources.sh b/list-linuxcnc-rs274-api-project-sources.sh
index 011f807..d3fe602 100755
--- a/list-linuxcnc-rs274-api-project-sources.sh
+++ b/list-linuxcnc-rs274-api-project-sources.sh
@@ -16,6 +16,7 @@ trap 'rm -f "$source_list"' EXIT
 } > "$source_list"
 
 ./list-filtered-existing-paths.sh \
+    --project-relative \
     "$filter" \
     "missing LinuxCNC rs274 API project source" \
     "missing LinuxCNC rs274 API project source in CMake list" < "$source_list"
diff --git a/list-linuxcnc-wasm-safe-project-sources.sh b/list-linuxcnc-wasm-safe-project-sources.sh
index 747b9d1..b3ac6ee 100755
--- a/list-linuxcnc-wasm-safe-project-sources.sh
+++ b/list-linuxcnc-wasm-safe-project-sources.sh
@@ -13,6 +13,7 @@ filter=${1:-}
     ./list-cmake-set-sources.sh cnc_sim_core_sources core/
     ./list-cmake-set-sources.sh cnc_sim_linuxcnc_rs274_backend_sources core/
 } | ./list-filtered-existing-paths.sh \
+    --project-relative \
     "$filter" \
     "missing wasm-safe project source" \
     "missing wasm-safe project source in CMake list"
diff --git a/list-linuxcnc-wasm-safe-shims.sh b/list-linuxcnc-wasm-safe-shims.sh
index aa11fc0..521f158 100755
--- a/list-linuxcnc-wasm-safe-shims.sh
+++ b/list-linuxcnc-wasm-safe-shims.sh
@@ -10,6 +10,7 @@ cd "$(dirname "$0")"
 filter=${1:-}
 
 ./list-cmake-set-sources.sh linuxcnc_wasm_safe_probe_shims | ./list-filtered-existing-paths.sh \
+    --project-relative \
     "$filter" \
     "missing wasm-safe shim source" \
     "missing wasm-safe shim in CMake list"
diff --git a/list-project-source-set.sh b/list-project-source-set.sh
index 6dd432c..dfddb05 100755
--- a/list-project-source-set.sh
+++ b/list-project-source-set.sh
@@ -120,6 +120,7 @@ case "$source_set" in
 esac
 
 printf '%s\n' "${sources[@]}" | ./list-filtered-existing-paths.sh \
+    --project-relative \
     "$filter" \
     "$missing_source_message" \
     "$missing_filter_message"
diff --git a/test-all-native.sh b/test-all-native.sh
index 7e9e123..ca6d449 100755
--- a/test-all-native.sh
+++ b/test-all-native.sh
@@ -28,6 +28,7 @@ missing_dump_source_log=$(mktemp "${TMPDIR:-/tmp}/cnc_sim_missing_rs274_dump_pro
 missing_api_source_log=$(mktemp "${TMPDIR:-/tmp}/cnc_sim_missing_rs274_api_project_source.XXXXXX.log")
 missing_core_source_log=$(mktemp "${TMPDIR:-/tmp}/cnc_sim_missing_core_source.XXXXXX.log")
 missing_bridge_source_log=$(mktemp "${TMPDIR:-/tmp}/cnc_sim_missing_bridge_smoke_project_source.XXXXXX.log")
+escaped_project_source_log=$(mktemp "${TMPDIR:-/tmp}/cnc_sim_escaped_project_source.XXXXXX.log")
 missing_support_object_log=$(mktemp "${TMPDIR:-/tmp}/cnc_sim_missing_linuxcnc_support_object.XXXXXX.log")
 missing_source_manifest=$(mktemp "${TMPDIR:-/tmp}/cnc_sim_native_syntax_missing_source_manifest.XXXXXX.txt")
 missing_source_log=$(mktemp "${TMPDIR:-/tmp}/cnc_sim_native_syntax_missing_source.XXXXXX.log")
@@ -57,6 +58,7 @@ cleanup_native_test_temps() {
         "$missing_api_source_log" \
         "$missing_core_source_log" \
         "$missing_bridge_source_log" \
+        "$escaped_project_source_log" \
         "$missing_support_object_log" \
         "$missing_source_manifest" \
         "$missing_source_log" \
@@ -171,6 +173,7 @@ for forbidden_manual_cleanup in \
     'rm -f "$missing_api_source_log"' \
     'rm -f "$missing_core_source_log"' \
     'rm -f "$missing_bridge_source_log"' \
+    'rm -f "$escaped_project_source_log"' \
     'rm -f "$missing_support_object_log"' \
     'rm -f "$missing_source_manifest" "$missing_source_log"' \
     'rm -f "$kinematics_unknown_filter_log"' \
@@ -212,7 +215,21 @@ grep -F 'exec 9>"${TMPDIR:-/tmp}/cnc_sim_rs274_source_link.lock"' test-linuxcnc-
 grep -F "flock 9" test-linuxcnc-source-link.sh >/dev/null
 grep -F "missing LinuxCNC support object" list-linuxcnc-source-support-objects.sh >/dev/null
 grep -F 'echo "$missing_path_message: $path"' list-filtered-existing-paths.sh >/dev/null
+grep -F 'project source path must stay project-relative: $path' list-filtered-existing-paths.sh >/dev/null
 grep -F "build LinuxCNC first or set LINUXCNC_ROOT to a built tree" list-linuxcnc-source-support-objects.sh >/dev/null
+if printf '../core/src/canon_event_sink.cpp\n' | ./list-filtered-existing-paths.sh \
+    --project-relative \
+    "" \
+    "missing project source" \
+    "missing project source in list" >"$escaped_project_source_log" 2>&1; then
+    echo "project source filter accepted an escaped project source path" >&2
+    exit 1
+fi
+if ! grep -F "project source path must stay project-relative: ../core/src/canon_event_sink.cpp" "$escaped_project_source_log" >/dev/null; then
+    echo "project source filter did not report escaped project paths clearly" >&2
+    sed -n '1,20p' "$escaped_project_source_log" >&2
+    exit 1
+fi
 native_link_flags=$(LINUXCNC_ROOT="$linuxcnc_root" ./list-linuxcnc-native-linkflags.sh)
 grep -Fx -- "-L" <<<"$native_link_flags" >/dev/null
 grep -Fx -- "$linuxcnc_root/lib" <<<"$native_link_flags" >/dev/null