diff --git a/tools/brain-retro-opus-reviewer.mjs b/tools/brain-retro-opus-reviewer.mjs
new file mode 100644
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+++ b/tools/brain-retro-opus-reviewer.mjs
@@ -0,0 +1,127 @@
+#!/usr/bin/env node
+/**
+ * brain-retro reviewer — direct Opus API fallback handler (Phase 3 Task 18).
+ *
+ * Spec §4.6: the primary reviewer is a Claude Code subagent
+ * (`.claude/agents/reviewer-agent.md`) spawned via Task() from /brain-retro.
+ * THIS module is the FALLBACK handler invoked by the controller when the
+ * subagent crashes / times out: direct Opus API call with the same adaptive
+ * review prompt (but no cross-episode reading, no skill invocations).
+ *
+ * Pure layer: buildReviewPrompt + parseReview (this file's tests). Network
+ * layer: reviewViaDirectApi (zero-cost wrapper around router-classifier's
+ * callAnthropicAPI; the controller decides when to call it).
+ *
+ * G16 — file did not exist before Phase 3 Task 18; created here.
+ */
+
+import { REVIEWER_MODEL } from './router-config.mjs';
+
+const REQUIRED_REVIEW_FIELDS = [
+  'node_quality',
+  'chain_quality',
+  'gap_assessment',
+  'agent_self_assessment_accuracy',
+  'error_root_cause',
+  'outcome_reviewed',
+  'reasoning',
+];
+
+/**
+ * Build the adaptive review prompt for a given episode. Pure.
+ *
+ * Adaptive prompt template (spec §4.6):
+ *   - v4 → full prompt including alternatives_considered, self_assessment,
+ *     chain_gaps cues.
+ *   - v3 → omits alternatives_considered.
+ *   - v2 → omits both alternatives_considered and self_assessment.
+ *   - v1 → skipped upstream (caller filters them out).
+ */
+export function buildReviewPrompt(episode) {
+  const v = Number(episode?.schema_version) || 0;
+  const cues = [];
+
+  cues.push('node_quality: correct | wrong_node | overkill | underkill | disputable');
+  cues.push('chain_quality: correct | missing_step | extra_step | wrong_order | n/a');
+  cues.push('gap_assessment: acceptable | mistake_should_complete | mistake_should_not_start | n/a');
+  cues.push('agent_self_assessment_accuracy: accurate | over_confident | under_confident | no_self_assessment');
+  cues.push('error_root_cause: wrong_skill | wrong_tool | wrong_chain_order | external_failure | n/a');
+  cues.push('alternative_better: <node_id> | null');
+  cues.push('outcome_reviewed: success | soft_success | rework | blocked');
+  cues.push('reasoning: 1-3 sentences');
+
+  const adaptiveNotes = [];
+  if (v >= 3) {
+    adaptiveNotes.push('Episode is v3+: primary_rationale carries triggers/candidates/boundaries.');
+  }
+  if (v >= 4) {
+    adaptiveNotes.push('Episode is v4: classifier_output.alternatives_considered tells you what the classifier weighed.');
+    adaptiveNotes.push('self_assessment (if present and not pending) is the agent\'s post-hoc judgement — compare honesty.');
+    adaptiveNotes.push('execution_trace.chain_gaps shows whether the recommended chain ran in full.');
+  }
+
+  return [
+    'You are the independent reviewer of routing decisions for the Лидерра brain-governance experiment.',
+    'Return ONLY a JSON object with the 8 fields below. No prose, no code fences.',
+    '',
+    'Fields:',
+    ...cues.map((c) => '  - ' + c),
+    '',
+    adaptiveNotes.length ? 'Notes for this schema version:' : '',
+    ...adaptiveNotes.map((n) => '  - ' + n),
+    '',
+    'Episode (JSON):',
+    JSON.stringify(episode, null, 2),
+    '',
+    'Output JSON only.',
+  ].filter(Boolean).join('\n');
+}
+
+/**
+ * Parse the Opus reviewer response. Pure. Returns null on malformed JSON or
+ * when a required 8-dim field is missing. Passes through `reviewer_error`
+ * escalations from the subagent.
+ */
+export function parseReview(text) {
+  if (!text) return null;
+  const stripped = String(text).trim()
+    .replace(/^```(?:json)?\s*\n?/, '')
+    .replace(/\n?```$/, '')
+    .trim();
+  let parsed;
+  try { parsed = JSON.parse(stripped); }
+  catch { return null; }
+  if (!parsed || typeof parsed !== 'object') return null;
+
+  // Reviewer-agent escalation: pass through verbatim.
+  if (typeof parsed.reviewer_error === 'string') return parsed;
+
+  for (const f of REQUIRED_REVIEW_FIELDS) {
+    if (parsed[f] === undefined) return null;
+  }
+  return parsed;
+}
+
+/**
+ * Direct Opus API call. Wraps callAnthropicAPI from router-classifier with
+ * the reviewer model. Caller (controller inside /brain-retro) is responsible
+ * for decision (subagent first, this on failure).
+ *
+ * Returns the parsed review object or null on transport / parse failure.
+ */
+export async function reviewViaDirectApi(episode, options = {}) {
+  const { callAnthropicAPI } = await import('./router-classifier.mjs');
+  const apiKey = options.apiKey ?? process.env.ROUTER_LLM_KEY;
+  if (!apiKey) return null;
+  const prompt = buildReviewPrompt(episode);
+  try {
+    const text = await callAnthropicAPI(prompt, {
+      apiKey,
+      baseUrl: options.baseUrl ?? process.env.ROUTER_LLM_BASE_URL ?? undefined,
+      model: options.model ?? REVIEWER_MODEL,
+    });
+    return parseReview(text);
+  } catch {
+    return null;
+  }
+}
diff --git a/tools/brain-retro-opus-reviewer.test.mjs b/tools/brain-retro-opus-reviewer.test.mjs
new file mode 100644
index 00000000..bd927fe4
--- /dev/null
+++ b/tools/brain-retro-opus-reviewer.test.mjs
@@ -0,0 +1,65 @@
+// tools/brain-retro-opus-reviewer.test.mjs — TDD for Phase 3 Task 18 (G16, spec §4.6)
+import { describe, it, expect } from 'vitest';
+import { buildReviewPrompt, parseReview } from './brain-retro-opus-reviewer.mjs';
+
+describe('buildReviewPrompt — adaptive v2/v3/v4 (spec §4.6)', () => {
+  it('v4 includes alternatives_considered + self_assessment + chain_gaps cues', () => {
+    const ep = {
+      schema_version: 4,
+      schema_minor: 2,
+      task_id: 't',
+      primary_rationale: { task_classification: 'feature', node_chosen: 'direct' },
+      classifier_output: { recommended_node: '#19', alternatives_considered: [{ node: 'x', match_score: 0.5 }] },
+      self_assessment: { summary: 'ok', confidence_in_choice: 0.8 },
+      execution_trace: { chain_gaps: [] },
+    };
+    const p = buildReviewPrompt(ep);
+    expect(p).toContain('alternatives_considered');
+    expect(p).toContain('self_assessment');
+    expect(p).toContain('chain_gaps');
+  });
+
+  it('v3 omits alternatives_considered cue', () => {
+    expect(buildReviewPrompt({ schema_version: 3 })).not.toContain('alternatives_considered');
+  });
+
+  it('v2 omits alternatives + self_assessment cues', () => {
+    const p = buildReviewPrompt({ schema_version: 2 });
+    expect(p).not.toContain('alternatives_considered');
+    expect(p).not.toContain('self_assessment');
+  });
+
+  it('includes the episode JSON verbatim for the reviewer to read', () => {
+    const ep = { schema_version: 4, task_id: 'task-xyz-1' };
+    expect(buildReviewPrompt(ep)).toContain('task-xyz-1');
+  });
+});
+
+describe('parseReview — 8-dim review schema (spec §4.6)', () => {
+  it('parses a complete 8-dim review JSON', () => {
+    const r = parseReview('{"node_quality":"correct","chain_quality":"n/a","gap_assessment":"n/a","agent_self_assessment_accuracy":"accurate","error_root_cause":"n/a","alternative_better":null,"outcome_reviewed":"success","reasoning":"x"}');
+    expect(r.node_quality).toBe('correct');
+    expect(r.outcome_reviewed).toBe('success');
+    expect(r.alternative_better).toBeNull();
+    expect(r.reasoning).toBe('x');
+  });
+
+  it('strips ```json fence', () => {
+    const r = parseReview('```json\n{"node_quality":"wrong_node","chain_quality":"missing_step","gap_assessment":"acceptable","agent_self_assessment_accuracy":"over_confident","error_root_cause":"wrong_skill","alternative_better":"#19","outcome_reviewed":"rework","reasoning":"y"}\n```');
+    expect(r.node_quality).toBe('wrong_node');
+    expect(r.alternative_better).toBe('#19');
+  });
+
+  it('returns null on malformed JSON', () => {
+    expect(parseReview('not json')).toBeNull();
+  });
+
+  it('returns null when required field missing', () => {
+    expect(parseReview('{"node_quality":"correct"}')).toBeNull();
+  });
+
+  it('returns reviewer_error passthrough when reviewer escalates', () => {
+    const r = parseReview('{"reviewer_error":"malformed episode"}');
+    expect(r?.reviewer_error).toBe('malformed episode');
+  });
+});