diff --git a/src/engine/file_cache.cpp b/src/engine/file_cache.cpp
index 5a9cbc7..437acd2 100644
--- a/src/engine/file_cache.cpp
+++ b/src/engine/file_cache.cpp
@@ -271,13 +271,17 @@ FileCacheClassification classify_cached_file(
     const std::filesystem::path& path,
     std::optional<FileCacheEntry> previous,
     CacheValidation mode) {
-  if (!std::filesystem::exists(path)) {
-    return FileCacheClassification{.state = CacheState::Deleted};
+  // Non-throwing: Windows reports a file that an editor is replacing as
+  // "Access is denied". A check that fails is Unreadable, not Deleted, so a
+  // caller holding a verified entry keeps it (a rescan must not drop the
+  // file's nodes) and the daemon's enrichment walk sees no exception.
+  std::error_code error;
+  if (!std::filesystem::exists(path, error)) {
+    return FileCacheClassification{.state = error ? CacheState::Unreadable : CacheState::Deleted};
   }
 
   FileCacheEntry current;
   current.path = path;
-  std::error_code error;
   current.size = std::filesystem::file_size(path, error);
   if (error) {
     return FileCacheClassification{.state = CacheState::Deleted};
diff --git a/src/engine/incremental_update.cpp b/src/engine/incremental_update.cpp
index a59aedf..bbe7887 100644
--- a/src/engine/incremental_update.cpp
+++ b/src/engine/incremental_update.cpp
@@ -327,13 +327,17 @@ IncrementalUpdateResult apply_incremental_code_updates(
     }
 
     const auto language = detect_language(event.path);
-    if (language == DetectedLanguage::Unknown || !std::filesystem::exists(event.path)) {
+    // Non-throwing: a file mid-save reads as "Access is denied" on Windows. It
+    // counts as absent for this pass; the watcher reports it again once written.
+    std::error_code exists_error;
+    const bool present = std::filesystem::exists(event.path, exists_error);
+    if (language == DetectedLanguage::Unknown || !present) {
       if (language == DetectedLanguage::Unknown && index.files.contains(key)) {
         applied = true;
         changed_sources.push_back(key);
         continue;
       }
-      if (!std::filesystem::exists(event.path) && index.files.contains(key)) {
+      if (!present && index.files.contains(key)) {
         applied = true;
         changed_sources.push_back(key);
         continue;
diff --git a/src/engine/semantic_chunk_plan.cpp b/src/engine/semantic_chunk_plan.cpp
index 72cab79..b70e8e4 100644
--- a/src/engine/semantic_chunk_plan.cpp
+++ b/src/engine/semantic_chunk_plan.cpp
@@ -85,9 +85,10 @@ namespace {
 }
 
 [[nodiscard]] bool is_valid_cached_record(const SemanticCacheRecord& record, std::string_view current_hash) {
+  std::error_code error;  // unreadable counts as missing: the record is re-planned, never thrown past
   return record.state == SemanticCacheState::Valid &&
          record.content_hash == current_hash &&
-         std::filesystem::exists(record.fragment_path) &&
+         std::filesystem::exists(record.fragment_path, error) &&
          !record.fragment_hash.empty() &&
          sha256_file_hex(record.fragment_path) == record.fragment_hash;
 }
diff --git a/tests/smoke/daemon_server_test.cpp b/tests/smoke/daemon_server_test.cpp
index 4bf5588..b3a1029 100644
--- a/tests/smoke/daemon_server_test.cpp
+++ b/tests/smoke/daemon_server_test.cpp
@@ -47,6 +47,33 @@ void write_file(const std::filesystem::path& path, std::string contents) {
   std::ofstream(path, std::ios::binary) << contents;
 }
 
+// Reads a file the daemon may replace at any moment ("" if it cannot be
+// opened). On Windows std::ifstream opens without FILE_SHARE_DELETE, which
+// makes the daemon's MoveFileExW replace fail with access denied; a test that
+// polls graph.json every few milliseconds can starve the replace for longer
+// than its retry window. Sharing delete lets the replace through while the
+// read keeps the old file's bytes.
+std::string read_shared(const std::filesystem::path& path) {
+#ifdef _WIN32
+  const HANDLE file = ::CreateFileW(path.c_str(), GENERIC_READ, FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
+                                    nullptr, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
+  if (file == INVALID_HANDLE_VALUE) {
+    return {};
+  }
+  std::string contents;
+  std::array<char, 64 * 1024> buffer{};
+  DWORD read = 0;
+  while (::ReadFile(file, buffer.data(), static_cast<DWORD>(buffer.size()), &read, nullptr) && read > 0) {
+    contents.append(buffer.data(), read);
+  }
+  ::CloseHandle(file);
+  return contents;
+#else
+  std::ifstream input(path, std::ios::binary);
+  return {std::istreambuf_iterator<char>(input), std::istreambuf_iterator<char>()};
+#endif
+}
+
 // Records a failed expectation with a name, so a CI flake says WHICH step
 // failed instead of a bare non-zero exit.
 void expect(bool& ok, bool condition, const char* what) {
@@ -413,9 +440,7 @@ int main() {
   // once the persist interval elapses (no update op, no shutdown).
   bool persisted = false;
   for (int attempt = 0; attempt < 600 && !persisted; ++attempt) {
-    std::ifstream input(root / "cgraph-out" / "graph.json");
-    const std::string contents((std::istreambuf_iterator<char>(input)), std::istreambuf_iterator<char>());
-    persisted = contents.find("gamma") != std::string::npos;
+    persisted = read_shared(root / "cgraph-out" / "graph.json").find("gamma") != std::string::npos;
     if (!persisted) {
       std::this_thread::sleep_for(std::chrono::milliseconds(20));
     }
@@ -942,8 +967,7 @@ int main() {
       expect(ok, rc == 0, exited);
     };
     const auto persisted_graph_has_overlay = [&]() {
-      std::ifstream input(graph_path, std::ios::binary);
-      const auto graph = nlohmann::json::parse(input, nullptr, false);
+      const auto graph = nlohmann::json::parse(read_shared(graph_path), nullptr, false);
       if (!graph.is_object()) {
         return true;
       }
@@ -1062,8 +1086,7 @@ int main() {
            "overlay-events: live document deletion removed the invalid overlay");
     bool deleted_stat_evicted = false;
     for (int attempt = 0; attempt < 500 && !deleted_stat_evicted; ++attempt) {
-      std::ifstream input(stat_index_path, std::ios::binary);
-      const auto stat_json = nlohmann::json::parse(input, nullptr, false);
+      const auto stat_json = nlohmann::json::parse(read_shared(stat_index_path), nullptr, false);
       if (stat_json.is_object()) {
         deleted_stat_evicted = true;
         for (const auto& entry : stat_json.value("entries", nlohmann::json::array())) {
@@ -1174,8 +1197,7 @@ int main() {
     cov_server.join();
     expect(ok, cov_rc == 0, "coverage: daemon exited cleanly");
 
-    std::ifstream persisted(cov_root / "cgraph-out" / "graph.json", std::ios::binary);
-    const auto persisted_graph = nlohmann::json::parse(persisted, nullptr, false);
+    const auto persisted_graph = nlohmann::json::parse(read_shared(cov_root / "cgraph-out" / "graph.json"), nullptr, false);
     bool overlay_in_persisted = !persisted_graph.is_object();
     bool code_in_persisted = false;
     for (const auto& node : persisted_graph.value("nodes", nlohmann::json::array())) {
@@ -1190,7 +1212,6 @@ int main() {
     expect(ok, !overlay_in_persisted && code_in_persisted,
            "coverage: graph.json stayed deterministic-only across the refused update");
 
-    persisted.close();  // Windows cannot delete a file that is still open
     fs::remove_all(cov_root);
   }
 
diff --git a/tests/smoke/file_cache_test.cpp b/tests/smoke/file_cache_test.cpp
index 25b430f..dfacc74 100644
--- a/tests/smoke/file_cache_test.cpp
+++ b/tests/smoke/file_cache_test.cpp
@@ -192,6 +192,30 @@ int main() {
     return 1;
   }
 
+#if defined(__APPLE__) || defined(__unix__)
+  // A path whose existence cannot be checked classifies as Unreadable (so a
+  // verified entry is kept, not dropped as Deleted) instead of throwing out of
+  // the daemon's enrichment walk. Windows reports a file an
+  // editor is replacing as "Access is denied"; here an unsearchable parent
+  // makes the same check fail with EACCES. Root bypasses directory permissions.
+  if (::geteuid() != 0) {
+    const auto locked_dir = root / "locked";
+    write_file(locked_dir / "doc.md", "# doc\n");
+    std::filesystem::permissions(locked_dir, std::filesystem::perms::none, std::filesystem::perm_options::replace);
+    bool threw = false;
+    cgraph::FileCacheClassification locked;
+    try {
+      locked = cgraph::classify_cached_file(locked_dir / "doc.md", std::nullopt);
+    } catch (const std::exception&) {
+      threw = true;
+    }
+    std::filesystem::permissions(locked_dir, std::filesystem::perms::owner_all, std::filesystem::perm_options::replace);
+    if (threw || locked.state != cgraph::CacheState::Unreadable || locked.current.has_value()) {
+      return 1;
+    }
+  }
+#endif
+
   // === Content validation mode ===
 
   write_file(path, "print('hello')\n");
diff --git a/tests/smoke/incremental_update_test.cpp b/tests/smoke/incremental_update_test.cpp
index 8081ec2..68d86f5 100644
--- a/tests/smoke/incremental_update_test.cpp
+++ b/tests/smoke/incremental_update_test.cpp
@@ -14,6 +14,10 @@
 #include <unordered_set>
 #include <vector>
 
+#if defined(__APPLE__) || defined(__unix__)
+#include <unistd.h>
+#endif
+
 namespace {
 
 void write_file(const std::filesystem::path& path, std::string contents) {
@@ -107,6 +111,48 @@ int main() {
   }
   const auto created_root = graph->content_root;
 
+#if defined(__APPLE__) || defined(__unix__)
+  // A watcher event for an indexed file whose existence cannot be checked
+  // must not throw out of the serve loop, and must not drop the file's nodes:
+  // Windows reports a file mid-save as "Access is denied", and an unsearchable
+  // parent fails the same check with EACCES here. Root bypasses directory
+  // permissions. The file is removed again so the index matches created_root.
+  if (::geteuid() != 0) {
+    const auto locked_dir = root / "locked";
+    const auto delta = locked_dir / "delta.py";
+    write_file(delta, "class Delta:\n    pass\n");
+    cgraph::FileWatchEvent delta_created[] = {
+        {.path = delta, .change = cgraph::FileWatchChange::Created, .kind = cgraph::WatchedFileKind::Code},
+    };
+    (void)cgraph::apply_incremental_code_updates(state, index, delta_created);
+    const bool indexed = has_node_label(*cgraph::read_graph_snapshot(state), "Delta");
+    std::filesystem::permissions(locked_dir, std::filesystem::perms::none, std::filesystem::perm_options::replace);
+    cgraph::FileWatchEvent locked_event[] = {
+        {.path = delta, .change = cgraph::FileWatchChange::Modified, .kind = cgraph::WatchedFileKind::Code},
+    };
+    bool threw = false;
+    try {
+      (void)cgraph::apply_incremental_code_updates(state, index, locked_event);
+    } catch (const std::exception&) {
+      threw = true;
+    }
+    std::filesystem::permissions(locked_dir, std::filesystem::perms::owner_all, std::filesystem::perm_options::replace);
+    const bool kept = has_node_label(*cgraph::read_graph_snapshot(state), "Delta");
+    std::filesystem::remove_all(locked_dir);
+    cgraph::FileWatchEvent delta_deleted[] = {
+        {.path = delta, .change = cgraph::FileWatchChange::Deleted, .kind = cgraph::WatchedFileKind::Code},
+    };
+    (void)cgraph::apply_incremental_code_updates(state, index, delta_deleted);
+    graph = cgraph::read_graph_snapshot(state);
+    if (!indexed || threw || !kept || has_node_label(*graph, "Delta") ||
+        graph->content_root.sha256 != created_root.sha256) {
+      std::cerr << "FAIL: an unreadable indexed file threw or lost its nodes (indexed=" << indexed
+                << " threw=" << threw << " kept=" << kept << ")\n";
+      return 1;
+    }
+  }
+#endif
+
   // A source that becomes unreadable after a verified publish must not replace
   // either the live snapshot or its incremental index with unverified state.
   const auto verified_snapshot = graph;