diff --git a/package.json b/package.json
index a55166b..752a9ef 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "codesys-mcp-sp21-plus",
-  "version": "0.6.0",
+  "version": "0.6.1",
   "description": "Codesys-MCP-SP21+ -- fork of luke-harriman/Codesys-MCP carrying CODESYS V3.5 SP22 Patch 1 fixes (and forward-compat with later SPs): script-engine API drift, online/runtime tool auto-login, dual-SHA release classifier, set_pou_code omitted-decl wipe fix, add_library managed-overload, etc. MCP server for CODESYS with persistent UI instance and file-based IPC.",
   "main": "dist/server.js",
   "bin": {
diff --git a/src/scripts/add_library.py b/src/scripts/add_library.py
index a38896b..c9d38d3 100644
--- a/src/scripts/add_library.py
+++ b/src/scripts/add_library.py
@@ -43,6 +43,7 @@ import sys, scriptengine as script_engine, os, traceback
 LIBRARY_NAME = "{LIBRARY_NAME}"
 USE_DIRECT = "{USE_DIRECT}" == "1"
 FORCE_DUP = "{FORCE_DUP}" == "1"
+ALLOW_UNRESOLVED = "{ALLOW_UNRESOLVED}" == "1"
 
 
 def _resolve_in_repo(name):
@@ -224,6 +225,27 @@ try:
         print("DEBUG: Pre-resolved '%s' to installed library '%s'." % (LIBRARY_NAME, disp))
     else:
         print("DEBUG: Pre-resolve via library_manager.find_library returned no hit for '%s'." % LIBRARY_NAME)
+        # HARD REFUSE: if the name does not resolve in the installed
+        # library repository, do NOT add anything. The post-add
+        # _is_resolved() guard alone is insufficient -- CODESYS lets
+        # add_placeholder(name_string) create a reference where
+        # is_placeholder=False but the library is still unresolvable on
+        # next project open, bricking compile with
+        # "The placeholder library '<name>' could not be resolved."
+        # Opt-in via ALLOW_UNRESOLVED=1 for the rare case where a
+        # placeholder for a not-yet-installed library is genuinely wanted.
+        if not ALLOW_UNRESOLVED:
+            msg = ("Refused: library '%s' is not installed in the CODESYS library "
+                   "repository (library_manager.find_library returned no hit). "
+                   "Install it via the Library Repository (Tools > Library Repository, "
+                   "or via CODESYS Installer for SL/add-on packages) before re-running, "
+                   "or pass allowUnresolved=true if you really want a placeholder for "
+                   "a not-yet-installed library."
+                   % LIBRARY_NAME)
+            print("ERROR: %s" % msg)
+            print("SCRIPT_ERROR: %s" % msg)
+            sys.exit(1)
+        print("DEBUG: ALLOW_UNRESOLVED=1 -- proceeding with placeholder add despite no repo hit.")
 
     # Step 2: add the reference. Default is add_placeholder() to match the
     # modern '<Name>, * (System)' convention (placeholder resolves at
diff --git a/src/server.ts b/src/server.ts
index 6fee077..cf70a3c 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -1871,14 +1871,15 @@ export async function startMcpServer(config: ServerConfig): Promise<void> {
 
   s.tool(
     'add_library',
-    "Adds a library reference to the CODESYS project. By default uses add_placeholder() to match the modern '<Name>, * (System)' convention so transitive deps resolve at compile time -- pass direct=true to opt into the legacy direct add_library() (specific version pin). Pre-checks lm.references for an existing reference with the same name and no-ops with a confirmation message unless force=true. The library must be installed in the CODESYS library repository.",
+    "Adds a library reference to the CODESYS project. **Refuses upfront** if the library name does not resolve in the installed library repository (Tools > Library Repository) -- this prevents the silent-broken-placeholder bug where add_placeholder() creates a hollow reference that bricks the next project open with 'placeholder library X could not be resolved'. Install the library first (Library Repository for stock libs, CODESYS Installer for SL/add-on packages), or pass allowUnresolved=true to opt into the dangerous behaviour. Default add path is add_placeholder() for the modern '<Name>, * (System)' convention; pass direct=true for the legacy specific-version pin. Pre-checks lm.references and no-ops with a confirmation message if a reference with the same name already exists, unless force=true.",
     {
       projectFilePath: z.string().describe("Path to the project file."),
-      libraryName: z.string().describe("Name of the library to add (e.g., 'Standard', 'Util', 'CAA Memory')."),
+      libraryName: z.string().describe("Name of the library to add (e.g., 'Standard', 'Util', 'CAA Memory'). Must match exactly an installed library in the CODESYS Library Repository unless allowUnresolved=true."),
       direct: z.boolean().optional().describe("If true, use direct add_library() (specific-version pin) instead of the default add_placeholder() (resolves at compile)."),
       force: z.boolean().optional().describe("If true, add even if a reference with the same name already exists (creates a duplicate). Default: dedup -- silently no-op with a confirmation message."),
+      allowUnresolved: z.boolean().optional().describe("DANGEROUS. If true, skip the pre-flight 'is this library installed?' check and add a placeholder anyway. Will brick the next project open if the library is genuinely not installed. Only use when you intentionally want a placeholder for a not-yet-installed library."),
     },
-    async (args: { projectFilePath: string; libraryName: string; direct?: boolean; force?: boolean }) => {
+    async (args: { projectFilePath: string; libraryName: string; direct?: boolean; force?: boolean; allowUnresolved?: boolean }) => {
       const escaped = resolvePath(args.projectFilePath, workspaceDir);
       const script = scriptManager.prepareScriptWithHelpers(
         'add_library',
@@ -1887,6 +1888,7 @@ export async function startMcpServer(config: ServerConfig): Promise<void> {
           LIBRARY_NAME: args.libraryName.trim(),
           USE_DIRECT: args.direct ? '1' : '0',
           FORCE_DUP: args.force ? '1' : '0',
+          ALLOW_UNRESOLVED: args.allowUnresolved ? '1' : '0',
         },
         ['ensure_project_open']
       );