diff --git a/docs/V1_16_PACKAGES_PROJECTS.md b/docs/V1_16_PACKAGES_PROJECTS.md
index 13fead2..63f078f 100644
--- a/docs/V1_16_PACKAGES_PROJECTS.md
+++ b/docs/V1_16_PACKAGES_PROJECTS.md
@@ -69,6 +69,16 @@ If `package doctor` reports `BAD package source: math-tools`, open `packages/mat
 
 If `package doctor` reports `BAD lock checksum: math-tools`, check the `math-tools` entry in `claro.lock`. Each package must have exactly one matching `checksum:` line. Duplicate checksums are rejected even when they agree, or when a correct checksum appears before or after an incorrect one. The doctor leaves the file unchanged. After reviewing your project and package manifests, run `claro package lock` to regenerate the lockfile, then run `claro package doctor` again.
 
+## Lock format safety
+
+`claro package doctor` requires exactly one `lock-version: 1` line in `claro.lock`. Missing, empty, unsupported (such as `10`), or duplicate format versions now fail with:
+
+```text
+BAD lock version: expected exactly one lock-version: 1 line
+```
+
+The doctor leaves your files unchanged. After reviewing your project and package manifests, run `claro package lock` to regenerate the lockfile, then run `claro package doctor` again. Spaces around the value and capitalization of the field name are accepted, as with other lockfile fields. Valid generated lockfiles continue to work unchanged; this check does not add registry downloads or verify package content hashes.
+
 ## Project-name safety
 
 `claro new NAME` checks the project name before creating folders. Project names may use only letters, numbers, dash, and underscore, and they must be 64 characters or fewer.
diff --git a/src/claro.c b/src/claro.c
index 4aacabe..cdc397c 100644
--- a/src/claro.c
+++ b/src/claro.c
@@ -644,7 +644,24 @@ static int package_manifest_name_matches(const char *text,const char *name){ con
 static int package_manifest_version_value_matches(const char *text){ const char *p=text; int fields=0; int matches=0; while(p&&*p){ const char *line=p; const char *end; while(*p&&*p!='\n'&&*p!='\r') p++; end=p; if((size_t)(end-line)>=8&&strncmp(line,"version:",8)==0){ const char *value=line+8; fields++; while(value<end&&isspace((unsigned char)*value)) value++; while(end>value&&isspace((unsigned char)end[-1])) end--; if((size_t)(end-value)==1&&value[0]=='1') matches++; } while(*p=='\n'||*p=='\r') p++; } return fields==1&&matches==1; }
 static int package_manifest_source_matches(const char *text){ const char *p=text; int fields=0; int matches=0; while(p&&*p){ const char *line=p; const char *end; while(*p&&*p!='\n'&&*p!='\r') p++; end=p; if((size_t)(end-line)>=7&&strncmp(line,"source:",7)==0){ const char *value=line+7; fields++; while(value<end&&isspace((unsigned char)*value)) value++; while(end>value&&isspace((unsigned char)end[-1])) end--; if((size_t)(end-value)==5&&strncmp(value,"local",5)==0) matches++; } while(*p=='\n'||*p=='\r') p++; } return fields==1&&matches==1; }
 static int package_manifest_checksum_matches(const char *text,const char *expected){ const char *p=text; size_t n=strlen(expected); int fields=0; int matches=0; while(p&&*p){ const char *line=p; const char *end; while(*p&&*p!='\n'&&*p!='\r') p++; end=p; if((size_t)(end-line)>=9&&strncmp(line,"checksum:",9)==0){ const char *value=line+9; fields++; while(value<end&&isspace((unsigned char)*value)) value++; while(end>value&&isspace((unsigned char)end[-1])) end--; if((size_t)(end-value)==n&&strncmp(value,expected,n)==0) matches++; } while(*p=='\n'||*p=='\r') p++; } return fields==1&&matches==1; }
-static int package_doctor(void){ int ok=1; char *mainfile; char *txt; printf("%s\n",CLARO_VERSION); printf("Package/project doctor:\n"); printf("  %s claro.project\n",file_exists_simple("claro.project")?"OK":"MISSING"); if(!file_exists_simple("claro.project")) ok=0; mainfile=project_value("main"); if(mainfile&&*mainfile){ printf("  %s main script: %s\n",file_exists_simple(mainfile)?"OK":"MISSING",mainfile); if(!file_exists_simple(mainfile)) ok=0; } else { printf("  MISSING main setting\n"); ok=0; } printf("  %s packages folder\n",path_exists_simple("packages")?"OK":"MISSING"); if(!path_exists_simple("packages")) ok=0; printf("  %s claro.lock\n",file_exists_simple("claro.lock")?"OK":"MISSING"); if(!file_exists_simple("claro.lock")) ok=0; txt=read_file_text("claro.project"); if(txt&&!project_package_names_safe()){ ok=0; } if(txt&& !package_manifest_version_matches(txt)){ printf("  BAD project manifest version: expected 1\n"); ok=0; } if(txt&& !package_project_name_valid(txt)){ printf("  BAD project manifest name: expected a non-empty name\n"); ok=0; } if(txt){ char *p=txt; while(*p){ char *line=p; char save; while(*p&&*p!='\n'&&*p!='\r') p++; save=*p; *p=0; { char *t=trim_inplace(line); if(strnicmp2(t,"package:",8)==0){ char *pkg=trim_inplace(t+8); if(*pkg){ char meta[512], *mt, sum[32]; if(!package_name_safe(pkg)){ printf("  BAD package name: %s\n",pkg); ok=0; } else { snprintf(meta,sizeof(meta),"packages/%s/claro.package",pkg); mt=read_file_text(meta); if(mt&&!package_manifest_version_value_matches(mt)){ printf("  BAD package version: %s\n",pkg); ok=0; } else if(mt&&!package_manifest_source_matches(mt)){ printf("  BAD package source: %s\n",pkg); ok=0; } else if(mt&&package_manifest_version_matches(mt)&&strstr(mt,"checksum:")){ if(!package_manifest_name_matches(mt,pkg)){ printf("  BAD package manifest name: %s\n",pkg); ok=0; } else { printf("  OK package manifest: %s\n",pkg); package_checksum(pkg,sum,sizeof(sum)); if(package_manifest_checksum_matches(mt,sum)) printf("  OK package checksum: %s\n",pkg); else { printf("  BAD package checksum: %s\n",pkg); ok=0; } if(package_lock_checksum_ok(pkg)) printf("  OK lock checksum: %s\n",pkg); else { printf("  BAD lock checksum: %s\n",pkg); ok=0; } } } else { printf("  MISSING package manifest: %s\n",pkg); ok=0; } free(mt); } } } } if(save) p++; while(*p=='\n'||*p=='\r') p++; } free(txt); } if(!lock_packages_match_project()) ok=0; free(mainfile); printf("%s\n",ok?"Package/project files look ready.":"Package/project files need attention."); return ok?0:1; }
+static int package_lock_version_matches(const char *text){
+ const char *p=text; int fields=0, matches=0;
+ while(p&&*p){
+  const char *line=p, *end;
+  while(*p&&*p!='\n'&&*p!='\r') p++;
+  end=p;
+  while(line<end&&isspace((unsigned char)*line)) line++;
+  if((size_t)(end-line)>=13&&strnicmp2(line,"lock-version:",13)==0){
+   const char *value=line+13; fields++;
+   while(value<end&&isspace((unsigned char)*value)) value++;
+   while(end>value&&isspace((unsigned char)end[-1])) end--;
+   if((size_t)(end-value)==1&&value[0]=='1') matches++;
+  }
+  while(*p=='\n'||*p=='\r') p++;
+ }
+ return fields==1&&matches==1;
+}
+static int package_doctor(void){ int ok=1; char *mainfile; char *txt; printf("%s\n",CLARO_VERSION); printf("Package/project doctor:\n"); printf("  %s claro.project\n",file_exists_simple("claro.project")?"OK":"MISSING"); if(!file_exists_simple("claro.project")) ok=0; mainfile=project_value("main"); if(mainfile&&*mainfile){ printf("  %s main script: %s\n",file_exists_simple(mainfile)?"OK":"MISSING",mainfile); if(!file_exists_simple(mainfile)) ok=0; } else { printf("  MISSING main setting\n"); ok=0; } printf("  %s packages folder\n",path_exists_simple("packages")?"OK":"MISSING"); if(!path_exists_simple("packages")) ok=0; printf("  %s claro.lock\n",file_exists_simple("claro.lock")?"OK":"MISSING"); if(!file_exists_simple("claro.lock")) ok=0; txt=read_file_text("claro.lock"); if(txt&&!package_lock_version_matches(txt)){ printf("  BAD lock version: expected exactly one lock-version: 1 line\n"); ok=0; } free(txt); txt=read_file_text("claro.project"); if(txt&&!project_package_names_safe()){ ok=0; } if(txt&& !package_manifest_version_matches(txt)){ printf("  BAD project manifest version: expected 1\n"); ok=0; } if(txt&& !package_project_name_valid(txt)){ printf("  BAD project manifest name: expected a non-empty name\n"); ok=0; } if(txt){ char *p=txt; while(*p){ char *line=p; char save; while(*p&&*p!='\n'&&*p!='\r') p++; save=*p; *p=0; { char *t=trim_inplace(line); if(strnicmp2(t,"package:",8)==0){ char *pkg=trim_inplace(t+8); if(*pkg){ char meta[512], *mt, sum[32]; if(!package_name_safe(pkg)){ printf("  BAD package name: %s\n",pkg); ok=0; } else { snprintf(meta,sizeof(meta),"packages/%s/claro.package",pkg); mt=read_file_text(meta); if(mt&&!package_manifest_version_value_matches(mt)){ printf("  BAD package version: %s\n",pkg); ok=0; } else if(mt&&!package_manifest_source_matches(mt)){ printf("  BAD package source: %s\n",pkg); ok=0; } else if(mt&&package_manifest_version_matches(mt)&&strstr(mt,"checksum:")){ if(!package_manifest_name_matches(mt,pkg)){ printf("  BAD package manifest name: %s\n",pkg); ok=0; } else { printf("  OK package manifest: %s\n",pkg); package_checksum(pkg,sum,sizeof(sum)); if(package_manifest_checksum_matches(mt,sum)) printf("  OK package checksum: %s\n",pkg); else { printf("  BAD package checksum: %s\n",pkg); ok=0; } if(package_lock_checksum_ok(pkg)) printf("  OK lock checksum: %s\n",pkg); else { printf("  BAD lock checksum: %s\n",pkg); ok=0; } } } else { printf("  MISSING package manifest: %s\n",pkg); ok=0; } free(mt); } } } } if(save) p++; while(*p=='\n'||*p=='\r') p++; } free(txt); } if(!lock_packages_match_project()) ok=0; free(mainfile); printf("%s\n",ok?"Package/project files look ready.":"Package/project files need attention."); return ok?0:1; }
 static int run_package_cmd(int argc,char **argv,int arg){ if(arg>=argc){ printf("Package commands:\n  claro package init\n  claro package add NAME\n  claro package remove NAME\n  claro package list\n  claro package doctor\n  claro package lock\n"); return 0; } if(strcmp(argv[arg],"init")==0){ ensure_project_file(); if(!project_package_names_safe()) return 1; make_folder("packages"); if(!write_package_lock()) return 1; printf("Package project ready: claro.project, claro.lock, packages/\n"); return 0; } if(strcmp(argv[arg],"add")==0&&arg+1<argc){ const char *name=argv[arg+1]; int already; if(package_name_too_long(name)){ fprintf(stderr,"Package names must be 64 characters or fewer.\n"); return 1; } if(!package_name_safe(name)){ fprintf(stderr,"Package names may use only letters, numbers, dash, and underscore.\n"); return 1; } ensure_project_file(); if(!project_package_names_safe()) return 1; make_folder("packages"); already=package_has_name(name); if(!append_package_to_project(name)) return 1; create_package_folder(name); if(!write_package_lock()) return 1; if(!already) printf("Added package: %s\n",name); return 0; } if(strcmp(argv[arg],"remove")==0&&arg+1<argc){ const char *name=argv[arg+1]; int removed; removed=remove_package_from_project(name); if(removed<0) return 1; if(removed) printf("Removed package from project: %s\n",name); else printf("Package was not listed: %s\n",name); return 0; } if(strcmp(argv[arg],"list")==0) return list_project_packages(); if(strcmp(argv[arg],"doctor")==0) return package_doctor(); if(strcmp(argv[arg],"lock")==0||strcmp(argv[arg],"update")==0){ ensure_project_file(); make_folder("packages"); if(write_package_lock()){ printf("Updated claro.lock\n"); return 0; } fprintf(stderr,"Could not write claro.lock\n"); return 1; } printf("Unknown package command. Try: claro package init\n"); return 1; }
 static int print_ide_info(void){
  printf("{\n");
diff --git a/tools/validate_package_security.py b/tools/validate_package_security.py
index b1477a6..c7f6ce3 100644
--- a/tools/validate_package_security.py
+++ b/tools/validate_package_security.py
@@ -104,6 +104,31 @@ def main():
             if phrase not in lock:
                 fail(f"claro.lock missing {phrase!r}:\n{lock}")
 
+        for version_line in ["lock-version: 1", "  LOCK-VERSION: \t1 \t"]:
+            valid_lock = lock.replace("lock-version: 1", version_line, 1)
+            (work / "claro.lock").write_text(valid_lock, encoding="utf-8")
+            rc, out = run([str(EXE), "package", "doctor"], work)
+            if rc != 0 or "Package/project files look ready." not in out:
+                fail(f"package doctor must accept a supported lock format version:\n{out}")
+            if read(work / "claro.lock") != valid_lock:
+                fail("package doctor must not rewrite a valid lockfile")
+        (work / "claro.lock").write_text(lock, encoding="utf-8")
+
+        for version_lines in ["", "lock-version: 10", "lock-version:",
+                              "lock-version: 1\nlock-version: 1",
+                              "lock-version: 1\nlock-version: 10",
+                              "lock-version: 10\nlock-version: 1"]:
+            invalid_lock = lock.replace("lock-version: 1", version_lines, 1)
+            (work / "claro.lock").write_text(invalid_lock, encoding="utf-8")
+            rc, out = run([str(EXE), "package", "doctor"], work)
+            if rc == 0:
+                fail(f"package doctor must reject invalid lock format versions: {version_lines!r}")
+            if "BAD lock version: expected exactly one lock-version: 1 line" not in out:
+                fail(f"Lock format-version diagnostic was unclear:\n{out}")
+            if read(work / "claro.lock") != invalid_lock:
+                fail("package doctor must not rewrite an invalid lock format version")
+        (work / "claro.lock").write_text(lock, encoding="utf-8")
+
         (work / "claro.lock").write_text(
             lock.replace("checksum:", "checksum: bad", 1),
             encoding="utf-8",