diff --git a/lib/skills.sh b/lib/skills.sh
index 6e97f09..d1ce254 100644
--- a/lib/skills.sh
+++ b/lib/skills.sh
@@ -109,58 +109,135 @@ install_skills_to_persistent_source() {
   fi
 }
 
+# Resolve the skills dir for a given runtime without mutating the currently
+# sourced runtime functions permanently. We source the runtime file in a
+# subshell, call its runtime_skills_dir(), and echo the result.
+_resolve_skills_dir_for_runtime() {
+  local rt="$1"
+  local rt_file="$SCRIPT_DIR/runtimes/${rt}.sh"
+  [ -f "$rt_file" ] || { echo ""; return 1; }
+  (
+    # shellcheck disable=SC1090
+    source "$rt_file"
+    runtime_skills_dir
+  )
+}
+
 install_skills() {
+  # Primary skills dir — set from the currently sourced runtime (drives the
+  # summary output and the kimaki mirror source). Multi-runtime installs
+  # populate every detected runtime's skills dir below, but the primary
+  # stays the canonical one the rest of the script refers to.
   SKILLS_DIR="$(runtime_skills_dir)"
 
-  if [ "$INSTALL_SKILLS" = true ]; then
-    log "Phase 8.5: Installing agent skills..."
+  if [ "$INSTALL_SKILLS" != true ]; then
+    log "Phase 8.5: Skipping agent skills (--no-skills)"
+    return
+  fi
+
+  log "Phase 8.5: Installing agent skills..."
+
+  # Build the unique list of skills dirs to populate. claude-code and
+  # studio-code both resolve to $SITE_PATH/.claude/skills, so de-dupe.
+  local -a runtimes=("${DETECTED_RUNTIMES[@]:-$RUNTIME}")
+  local -a skills_dirs=()
+  local seen_dir rt dir
+  for rt in "${runtimes[@]}"; do
+    dir="$(_resolve_skills_dir_for_runtime "$rt")"
+    [ -n "$dir" ] || continue
+    local already=false
+    for seen_dir in "${skills_dirs[@]}"; do
+      [ "$seen_dir" = "$dir" ] && { already=true; break; }
+    done
+    [ "$already" = true ] || skills_dirs+=("$dir")
+  done
+
+  # Always guarantee the primary is in the list (for belt-and-braces when
+  # RUNTIME was set explicitly but somehow isn't in DETECTED_RUNTIMES).
+  local already=false
+  for seen_dir in "${skills_dirs[@]}"; do
+    [ "$seen_dir" = "$SKILLS_DIR" ] && { already=true; break; }
+  done
+  [ "$already" = true ] || skills_dirs+=("$SKILLS_DIR")
+
+  if [ ${#skills_dirs[@]} -gt 1 ]; then
+    log "  Detected ${#runtimes[@]} runtime(s): ${runtimes[*]}"
+    log "  Populating ${#skills_dirs[@]} unique skills dir(s)"
+  fi
+
+  # Install into every detected runtime's skills dir.
+  local target_dir
+  for target_dir in "${skills_dirs[@]}"; do
+    if [ ${#skills_dirs[@]} -gt 1 ]; then
+      log "→ Installing skills into $target_dir"
+    fi
+    SKILLS_DIR="$target_dir"
     run_cmd mkdir -p "$SKILLS_DIR"
 
     install_skills_from_local_repo
-
     install_skills_from_repo "https://github.com/WordPress/agent-skills.git" "WordPress agent skills"
-
     install_skills_from_repo "https://github.com/Extra-Chill/data-machine-skills.git" "Data Machine skills"
+  done
 
-    # Copy skills to Kimaki's directory if Kimaki is the chat bridge.
-    # Kimaki overrides OpenCode's skill discovery to only look in its
-    # own bundled skills dir, so the runtime skills dir alone isn't enough.
-    if [ "$CHAT_BRIDGE" = "kimaki" ]; then
-      if [ "$DRY_RUN" = true ]; then
-        KIMAKI_SKILLS_DIR="/usr/lib/node_modules/kimaki/skills"
-        echo -e "${BLUE}[dry-run]${NC} Would copy skills to $KIMAKI_SKILLS_DIR/ (if Kimaki installed)"
-      elif command -v kimaki &> /dev/null; then
-        KIMAKI_SKILLS_DIR="$(npm root -g 2>/dev/null)/kimaki/skills"
-        if [ -d "$KIMAKI_SKILLS_DIR" ]; then
-          for skill_dir in "$SKILLS_DIR"/*/; do
-            skill_name=$(basename "$skill_dir")
-            if [ -f "$skill_dir/SKILL.md" ]; then
-              cp -r "$skill_dir" "$KIMAKI_SKILLS_DIR/$skill_name"
-            fi
-          done
-          log "Skills also copied to Kimaki: $KIMAKI_SKILLS_DIR/"
-        fi
-      fi
+  # Reset SKILLS_DIR back to the primary for downstream consumers
+  # (kimaki mirror source, print_skills_summary, summary.sh).
+  SKILLS_DIR="$(runtime_skills_dir)"
 
-      # Mirror skills into the persistent kimaki-config/skills/ dir so
-      # post-upgrade.sh can restore them on every kimaki restart after
-      # `npm update -g kimaki` wipes $(npm root -g)/kimaki/skills/.
-      # Path mirrors the plugin-persistence pattern:
-      #   Local: $KIMAKI_DATA_DIR/kimaki-config/skills/ (defaults to ~/.kimaki/kimaki-config/skills/)
-      #   VPS:   /opt/kimaki-config/skills/
-      install_skills_to_persistent_source
+  # Copy skills to Kimaki's directory if Kimaki is the chat bridge.
+  # Kimaki overrides OpenCode's skill discovery to only look in its
+  # own bundled skills dir, so the runtime skills dir alone isn't enough.
+  if [ "$CHAT_BRIDGE" = "kimaki" ]; then
+    if [ "$DRY_RUN" = true ]; then
+      KIMAKI_SKILLS_DIR="/usr/lib/node_modules/kimaki/skills"
+      echo -e "${BLUE}[dry-run]${NC} Would copy skills to $KIMAKI_SKILLS_DIR/ (if Kimaki installed)"
+    elif command -v kimaki &> /dev/null; then
+      KIMAKI_SKILLS_DIR="$(npm root -g 2>/dev/null)/kimaki/skills"
+      if [ -d "$KIMAKI_SKILLS_DIR" ]; then
+        for skill_dir in "$SKILLS_DIR"/*/; do
+          skill_name=$(basename "$skill_dir")
+          if [ -f "$skill_dir/SKILL.md" ]; then
+            cp -r "$skill_dir" "$KIMAKI_SKILLS_DIR/$skill_name"
+          fi
+        done
+        log "Skills also copied to Kimaki: $KIMAKI_SKILLS_DIR/"
+      fi
     fi
-  else
-    log "Phase 8.5: Skipping agent skills (--no-skills)"
+
+    # Mirror skills into the persistent kimaki-config/skills/ dir so
+    # post-upgrade.sh can restore them on every kimaki restart after
+    # `npm update -g kimaki` wipes $(npm root -g)/kimaki/skills/.
+    # Path mirrors the plugin-persistence pattern:
+    #   Local: $KIMAKI_DATA_DIR/kimaki-config/skills/ (defaults to ~/.kimaki/kimaki-config/skills/)
+    #   VPS:   /opt/kimaki-config/skills/
+    install_skills_to_persistent_source
   fi
 }
 
 print_skills_summary() {
   echo ""
-  log "Skills installed to $SKILLS_DIR/"
-  if [ "$DRY_RUN" = false ]; then
-    ls -1 "$SKILLS_DIR" 2>/dev/null | while read -r skill; do
-      log "  - $skill"
+
+  # Collect unique skills dirs across detected runtimes, same logic as
+  # install_skills. Falls back to SKILLS_DIR if DETECTED_RUNTIMES is empty.
+  local -a runtimes=("${DETECTED_RUNTIMES[@]:-$RUNTIME}")
+  local -a skills_dirs=()
+  local seen_dir rt dir
+  for rt in "${runtimes[@]}"; do
+    dir="$(_resolve_skills_dir_for_runtime "$rt")"
+    [ -n "$dir" ] || continue
+    local already=false
+    for seen_dir in "${skills_dirs[@]}"; do
+      [ "$seen_dir" = "$dir" ] && { already=true; break; }
     done
-  fi
+    [ "$already" = true ] || skills_dirs+=("$dir")
+  done
+  [ ${#skills_dirs[@]} -gt 0 ] || skills_dirs=("$SKILLS_DIR")
+
+  for dir in "${skills_dirs[@]}"; do
+    log "Skills installed to $dir/"
+    if [ "$DRY_RUN" = false ]; then
+      ls -1 "$dir" 2>/dev/null | while read -r skill; do
+        log "  - $skill"
+      done
+    fi
+  done
 }
diff --git a/setup.sh b/setup.sh
index ab2876b..c2be80b 100755
--- a/setup.sh
+++ b/setup.sh
@@ -53,6 +53,7 @@ INSTALL_SKILLS=true
 SKILLS_ONLY=false
 RUNTIME_ONLY=false
 RUNTIME=""
+DETECTED_RUNTIMES=()
 IS_STUDIO=false
 
 while [[ $# -gt 0 ]]; do
@@ -223,18 +224,35 @@ fi
 # Runtime resolution
 # ============================================================================
 
-# Auto-detect runtime if not specified
-if [ -z "$RUNTIME" ]; then
+# Auto-detect runtime(s).
+#
+# RUNTIME is the "primary" runtime — the one that drives runtime_install,
+# runtime_generate_config, runtime_install_hooks, and the chat-bridge default.
+# First-match cascade: studio-code > claude-code > opencode.
+#
+# DETECTED_RUNTIMES is the list of ALL runtimes whose binary is present. On a
+# machine with both claude and opencode installed, skills get installed into
+# every detected runtime's skills dir (see install_skills in lib/skills.sh).
+# Explicit --runtime <name> narrows both lists to that single runtime.
+if [ -n "$RUNTIME" ]; then
+  # User passed --runtime explicitly — respect it, single-runtime mode.
+  DETECTED_RUNTIMES=("$RUNTIME")
+else
   if command -v studio &>/dev/null && [ "${IS_STUDIO:-false}" = true ]; then
-    RUNTIME="studio-code"
-  elif command -v claude &>/dev/null; then
-    RUNTIME="claude-code"
-  elif command -v opencode &>/dev/null; then
-    RUNTIME="opencode"
-  else
-    # Default to opencode (will be installed)
-    RUNTIME="opencode"
+    DETECTED_RUNTIMES+=("studio-code")
+  fi
+  if command -v claude &>/dev/null; then
+    DETECTED_RUNTIMES+=("claude-code")
+  fi
+  if command -v opencode &>/dev/null; then
+    DETECTED_RUNTIMES+=("opencode")
+  fi
+  if [ ${#DETECTED_RUNTIMES[@]} -eq 0 ]; then
+    # Nothing installed yet — default to opencode (will be installed).
+    DETECTED_RUNTIMES=("opencode")
   fi
+  # Primary = first match in the cascade above.
+  RUNTIME="${DETECTED_RUNTIMES[0]}"
 fi
 
 # Source the selected runtime
diff --git a/upgrade.sh b/upgrade.sh
index af6a563..e08019c 100755
--- a/upgrade.sh
+++ b/upgrade.sh
@@ -89,6 +89,7 @@ MULTISITE=false
 MULTISITE_TYPE="subdirectory"
 MODE="existing"
 RUNTIME=""
+DETECTED_RUNTIMES=()
 IS_STUDIO=false
 CHAT_BRIDGE=""
 
@@ -180,18 +181,26 @@ if [ -z "$EXISTING_WP" ]; then
   fi
 fi
 
-# Auto-detect runtime (same logic as setup.sh)
-if [ -z "$RUNTIME" ]; then
+# Auto-detect runtime(s). Same model as setup.sh: DETECTED_RUNTIMES is the
+# full list (drives multi-runtime skills install); RUNTIME is the primary
+# (first-match cascade). Explicit --runtime narrows to a single runtime.
+if [ -n "$RUNTIME" ]; then
+  DETECTED_RUNTIMES=("$RUNTIME")
+else
   if command -v studio &>/dev/null && [ -f "$EXISTING_WP/STUDIO.md" ]; then
-    RUNTIME="studio-code"
-  elif command -v opencode &>/dev/null; then
-    RUNTIME="opencode"
-  elif command -v claude &>/dev/null; then
-    RUNTIME="claude-code"
-  else
+    DETECTED_RUNTIMES+=("studio-code")
+  fi
+  if command -v claude &>/dev/null; then
+    DETECTED_RUNTIMES+=("claude-code")
+  fi
+  if command -v opencode &>/dev/null; then
+    DETECTED_RUNTIMES+=("opencode")
+  fi
+  if [ ${#DETECTED_RUNTIMES[@]} -eq 0 ]; then
     warn "No runtime binary found — defaulting to opencode"
-    RUNTIME="opencode"
+    DETECTED_RUNTIMES=("opencode")
   fi
+  RUNTIME="${DETECTED_RUNTIMES[0]}"
 fi
 
 RUNTIME_FILE="$SCRIPT_DIR/runtimes/${RUNTIME}.sh"