diff --git a/lib/lint.sh b/lib/lint.sh
index 367dda0..077f1e9 100644
--- a/lib/lint.sh
+++ b/lib/lint.sh
@@ -4,20 +4,28 @@
 # Validation logic for Knowledge Genome files.
 # =============================================================================
 
-# Validates YAML frontmatter and mandatory fields
+# Valid values for the 'type' frontmatter field.
+# Must stay in sync with the type list in templates/agents-genome.md.
+VALID_TYPES=("source" "entity" "concept" "query" "conflict" "private" "index" "log")
+
+# ---------------------------------------------------------------------------
+# lint_markdown_file <file> <genome_name>
+# Validates YAML frontmatter: delimiters, mandatory fields, domain, type value.
+# Returns the number of errors found.
+# ---------------------------------------------------------------------------
 lint_markdown_file() {
   local file="$1"
   local genome_name="$2"
   local errors=0
 
-  # 1. Check Frontmatter delimiters
+  # 1. Check frontmatter delimiters
   if [[ $(head -n 1 "$file") != "---" ]]; then
     warn "Missing frontmatter start (---) in: $file"
     errors=$((errors + 1))
   fi
 
   # 2. Check mandatory fields
-  local mandatory_fields=("title:" "type:" "domain:")
+  local mandatory_fields=("title:" "type:" "domain:" "maturity:" "last_updated:")
   for field in "${mandatory_fields[@]}"; do
     if ! grep -q "^${field}" "$file"; then
       warn "Missing mandatory field '${field}' in: $file"
@@ -25,52 +33,141 @@ lint_markdown_file() {
     fi
   done
 
-  # 3. Check if domain matches the genome name
+  # 3. Check domain matches genome name
   if grep -q "^domain:" "$file" && ! grep -q "^domain: ${genome_name}" "$file"; then
-    warn "Domain mismatch in $file (expected ${genome_name})"
+    warn "Domain mismatch in $file (expected '${genome_name}')"
     errors=$((errors + 1))
   fi
 
+  # 4. Validate type value against allowed list
+  local type_errors
+  check_valid_type "$file"
+  type_errors=$?
+  errors=$((errors + type_errors))
+
   return $errors
 }
 
-# Ensures files in private/ directories have the 'private: true' flag
+# ---------------------------------------------------------------------------
+# check_valid_type <file>
+# Verifies that the 'type' field contains a value from VALID_TYPES.
+# Returns 1 if invalid, 0 if valid or field absent (absence caught by lint_markdown_file).
+# ---------------------------------------------------------------------------
+check_valid_type() {
+  local file="$1"
+
+  local type_value
+  type_value=$(grep "^type:" "$file" | head -1 | sed 's/^type:[[:space:]]*//' | tr -d '"')
+
+  [[ -z "$type_value" ]] && return 0   # absence is caught upstream
+
+  local valid=0
+  for t in "${VALID_TYPES[@]}"; do
+    [[ "$type_value" == "$t" ]] && valid=1 && break
+  done
+
+  if [[ $valid -eq 0 ]]; then
+    warn "Invalid type value '${type_value}' in: $file"
+    warn "  Valid types: ${VALID_TYPES[*]}"
+    return 1
+  fi
+
+  return 0
+}
+
+# ---------------------------------------------------------------------------
+# check_privacy_consistency <file>
+# Ensures files in private/ directories carry 'private: true'.
+# Warns if a public file is incorrectly marked private.
+# ---------------------------------------------------------------------------
 check_privacy_consistency() {
   local file="$1"
   local errors=0
 
   if [[ "$file" == *"/private/"* ]]; then
     if ! grep -q "^private: true" "$file"; then
-      error "Privacy Leak: $file is in a private folder but lacks 'private: true' metadata."
+      error "Privacy leak: $file is in a private/ directory but lacks 'private: true'."
       errors=$((errors + 1))
     fi
   else
     if grep -q "^private: true" "$file"; then
-      warn "Metadata Mismatch: $file is marked private but located in a public directory."
-      # We count this as a warning unless you want to force strict isolation
+      warn "Metadata mismatch: $file is marked 'private: true' but is in a public directory."
     fi
   fi
 
   return $errors
 }
 
-# Basic check for internal wiki-links [[target]]
+# ---------------------------------------------------------------------------
+# check_knowledge_decay <file>
+# Reads 'maturity' and 'last_updated' from frontmatter and compares against
+# the staleness thresholds defined in agents-genome.md:
+#   maturity: stable  → flag if last_updated > 180 days ago
+#   maturity: draft   → flag if last_updated > 90 days ago
+#
+# Returns 1 if the file is stale, 0 otherwise.
+# Silently skips files with missing or unparseable date fields.
+# ---------------------------------------------------------------------------
+check_knowledge_decay() {
+  local file="$1"
+
+  local last_updated maturity
+  last_updated=$(grep "^last_updated:" "$file" | head -1 | sed 's/^last_updated:[[:space:]]*//' | tr -d '"')
+  maturity=$(grep "^maturity:" "$file" | head -1 | sed 's/^maturity:[[:space:]]*//' | tr -d '"')
+
+  # Skip if either field is absent or maturity is not decay-trackable
+  [[ -z "$last_updated" || -z "$maturity" ]] && return 0
+  [[ "$maturity" != "stable" && "$maturity" != "draft" ]] && return 0
+
+  # Parse date — handle both GNU date (Linux) and BSD date (macOS)
+  local updated_ts
+  if date --version &>/dev/null 2>&1; then
+    # GNU date
+    updated_ts=$(date -d "$last_updated" +%s 2>/dev/null)
+  else
+    # BSD date (macOS)
+    updated_ts=$(date -j -f "%Y-%m-%d" "$last_updated" +%s 2>/dev/null)
+  fi
+
+  [[ -z "$updated_ts" ]] && return 0   # unparseable date — skip silently
+
+  local now days_old threshold
+  now=$(date +%s)
+  days_old=$(( (now - updated_ts) / 86400 ))
+
+  case "$maturity" in
+    stable) threshold=180 ;;
+    draft)  threshold=90  ;;
+  esac
+
+  if [[ $days_old -gt $threshold ]]; then
+    warn "STALE: $file"
+    warn "  maturity: ${maturity} | last_updated: ${last_updated} | ${days_old} days ago (threshold: ${threshold})"
+    return 1
+  fi
+
+  return 0
+}
+
+# ---------------------------------------------------------------------------
+# check_broken_links <file>
+# Basic check for internal [[wikilinks]] that cannot be resolved locally.
+# Only emits warnings — cross-genome links may legitimately not resolve here.
+# ---------------------------------------------------------------------------
 check_broken_links() {
   local file="$1"
   local base_dir
   base_dir=$(dirname "$file")
 
-  # Extract links, stripping aliases: [[Link|Alias]] -> Link
+  # Extract link targets, stripping aliases: [[Link|Alias]] -> Link
   local links
-  links=$(grep -oP '\[\[\K[^\]]+' "$file" | cut -d'|' -f1)
+  links=$(grep -oP '\[\[\K[^\]]+' "$file" 2>/dev/null | cut -d'|' -f1)
 
   for link in $links; do
     local target="$link"
     [[ "$target" != *.md ]] && target="${target}.md"
 
-    # Simple relative check
     if [[ ! -f "${base_dir}/${target}" && ! -f "${base_dir}/../${target}" ]]; then
-      # Only a warning as links might point to other genomes or deep structures
       warn "Potential broken link: [[$link]] in $file"
     fi
   done