diff --git a/commands/santa-loop.md b/commands/santa-loop.md
index 111087966..fcc749bdd 100644
--- a/commands/santa-loop.md
+++ b/commands/santa-loop.md
@@ -70,38 +70,71 @@ Launch an Agent (subagent_type: `code-reviewer`, model: `opus`) with the full ru
 - "You are an independent quality reviewer. You have NOT seen any other review. Your job is to find problems, not to approve."
 - Return the structured JSON verdict above
 
-#### Reviewer B: External Model (Claude fallback only if no external CLI installed)
+#### Reviewer B: External Model (Claude fallback if no external reviewer is ready)
 
-First, detect which CLIs are available:
-```bash
-command -v codex >/dev/null 2>&1 && echo "codex" || true
-command -v gemini >/dev/null 2>&1 && echo "gemini" || true
-```
+Antigravity is optional and requires the external `ccg-workflow` runtime; it is not included in the base ECC install. An existing installation must provide both an executable `~/.claude/bin/codeagent-wrapper` and a readable regular reviewer role file at `~/.claude/.ccg/prompts/antigravity/reviewer.md`. Otherwise, retain the Claude fallback. Use the wrapper's maintained model-selection contract; do not add an Antigravity model pin.
+
+Run detection, prompt creation, and the selected external review together in this single Bash subshell. Replace the prompt placeholder with the same rubric, file contents, and review-only instructions given to Reviewer A. The order remains Codex, Gemini, Antigravity, then Claude. The Claude fallback creates no prompt file.
 
-Build the reviewer prompt (identical rubric + instructions as Reviewer A) and write it to a unique temp file:
 ```bash
-PROMPT_FILE=$(mktemp /tmp/santa-reviewer-b-XXXXXX.txt)
-cat > "$PROMPT_FILE" << 'EOF'
+(
+  set -e
+  set -o pipefail
+
+  REVIEWER_WRAPPER="$HOME/.claude/bin/codeagent-wrapper"
+  REVIEWER_ROLE="$HOME/.claude/.ccg/prompts/antigravity/reviewer.md"
+  if command -v codex >/dev/null 2>&1; then
+    REVIEWER_BACKEND=codex
+  elif command -v gemini >/dev/null 2>&1; then
+    REVIEWER_BACKEND=gemini
+  elif [ -x "$REVIEWER_WRAPPER" ] && [ -f "$REVIEWER_ROLE" ] && [ -r "$REVIEWER_ROLE" ]; then
+    REVIEWER_BACKEND=antigravity
+  else
+    printf '%s\n' 'CLAUDE_FALLBACK'
+    exit 0
+  fi
+
+  cleanup_reviewer_prompt() {
+    reviewer_status=$?
+    trap - EXIT
+    if ! rm -f -- "$PROMPT_FILE"; then
+      printf '%s\n' 'Could not remove reviewer prompt; remove the private temp file before continuing.' >&2
+      if [ "$reviewer_status" -eq 0 ]; then reviewer_status=1; fi
+    fi
+    exit "$reviewer_status"
+  }
+
+  umask 077
+  PROMPT_FILE=$(mktemp "${TMPDIR:-/tmp}/santa-reviewer-b.XXXXXX")
+  trap cleanup_reviewer_prompt EXIT
+  trap 'exit 129' HUP
+  trap 'exit 130' INT
+  trap 'exit 143' TERM
+  cat > "$PROMPT_FILE" << 'EOF'
 ... full rubric + file contents + reviewer instructions ...
 EOF
+
+  case "$REVIEWER_BACKEND" in
+    codex)
+      codex exec --sandbox read-only -m gpt-5.4 -C "$(pwd)" - < "$PROMPT_FILE"
+      ;;
+    gemini)
+      REVIEWER_PROMPT=$(cat "$PROMPT_FILE")
+      gemini -p "$REVIEWER_PROMPT" -m gemini-2.5-pro
+      ;;
+    antigravity)
+      {
+        printf 'ROLE_FILE: %s\n' "$REVIEWER_ROLE"
+        cat "$PROMPT_FILE"
+      } | "$REVIEWER_WRAPPER" --backend antigravity - "$PWD"
+      ;;
+  esac
+)
 ```
 
-Use the first available CLI:
+The subshell removes its prompt on success, backend or prompt-write failure, and handled HUP/INT/TERM signals, preserving the original failure status. A cleanup-only failure also returns nonzero. Forced termination such as SIGKILL cannot run cleanup; inspect private temp files after an interrupted process. A failed external invocation is not an approval and must not silently become a fallback or proceed to the verdict gate.
 
-**Codex CLI** (if installed)
-```bash
-codex exec --sandbox read-only -m gpt-5.4 -C "$(pwd)" - < "$PROMPT_FILE"
-rm -f "$PROMPT_FILE"
-```
-
-**Gemini CLI** (if installed and codex is not)
-```bash
-gemini -p "$(cat "$PROMPT_FILE")" -m gemini-2.5-pro
-rm -f "$PROMPT_FILE"
-```
-
-**Claude Agent fallback** (only if neither `codex` nor `gemini` is installed)
-Launch a second Claude Agent (subagent_type: `code-reviewer`, model: `opus`). Log a warning that both reviewers share the same model family — true model diversity was not achieved but context isolation is still enforced.
+**Claude Agent fallback:** If the subshell prints `CLAUDE_FALLBACK`, launch a second Claude Agent (subagent_type: `code-reviewer`, model: `opus`) with the rubric and file contents directly. The marker is a dispatch instruction, not a review verdict. Log that both reviewers share the same model family; keep their contexts separate.
 
 In all cases, the reviewer must return the same structured JSON verdict as Reviewer A.
 
@@ -166,9 +199,9 @@ Result:     [PUSHED / ESCALATED TO USER]
 ## Notes
 
 - Reviewer A (Claude Opus) always runs — guarantees at least one strong reviewer regardless of tooling.
-- Model diversity is the goal for Reviewer B. GPT-5.4 or Gemini 2.5 Pro gives true independence — different training data, different biases, different blind spots. The Claude-only fallback still provides value via context isolation but loses model diversity.
-- Strongest available models are used: Opus for Reviewer A, GPT-5.4 or Gemini 2.5 Pro for Reviewer B.
-- External reviewers run with `--sandbox read-only` (Codex) to prevent repo mutation during review.
+- Model diversity is the goal for Reviewer B. Record the actual model/provider reported by the selected backend; the Antigravity wrapper name alone does not establish a different model family. The Claude-only fallback provides context isolation but loses model diversity.
+- Use each backend's maintained model-selection contract. Do not pin a transient Antigravity model ID in this workflow.
+- Request review-only output from every reviewer. Codex uses `--sandbox read-only`; Gemini and Antigravity permissions depend on their installed runtime configuration. The CCG wrapper invocation alone does not guarantee a read-only sandbox.
 - Fresh reviewers each round prevents anchoring bias from prior findings.
 - The rubric is the most important input. Tighten it if reviewers rubber-stamp or flag subjective style issues.
 - Commits happen on NAUGHTY rounds so fixes are preserved even if the loop is interrupted.