diff --git a/plugins/voyage/lib/validators/brief-validator.mjs b/plugins/voyage/lib/validators/brief-validator.mjs
index 760ba9f..293ced9 100644
--- a/plugins/voyage/lib/validators/brief-validator.mjs
+++ b/plugins/voyage/lib/validators/brief-validator.mjs
@@ -9,12 +9,15 @@
 import { readFileSync, existsSync } from 'node:fs';
 import { parseDocument } from '../util/frontmatter.mjs';
 import { issue, ok, fail } from '../util/result.mjs';
+import { BASE_ALLOWED_MODELS } from './profile-validator.mjs';
 
 export const BRIEF_REQUIRED_FRONTMATTER = ['type', 'brief_version', 'task', 'slug', 'research_topics', 'research_status'];
 export const REVIEW_AS_BRIEF_REQUIRED_FRONTMATTER = ['type', 'task', 'slug', 'project_dir', 'findings'];
 export const BRIEF_TYPE_VALUES = Object.freeze(['trekbrief', 'trekreview']);
 export const BRIEF_RESEARCH_STATUS_VALUES = ['pending', 'in_progress', 'complete', 'skipped'];
 export const BRIEF_BODY_SECTIONS = ['Intent', 'Goal', 'Success Criteria'];
+export const PHASE_SIGNAL_PHASES = Object.freeze(['research', 'plan', 'execute', 'review']);
+export const EFFORT_LEVELS = Object.freeze(['low', 'standard', 'high']);
 
 function getRequiredFields(type) {
   return type === 'trekreview' ? REVIEW_AS_BRIEF_REQUIRED_FRONTMATTER : BRIEF_REQUIRED_FRONTMATTER;
@@ -36,6 +39,67 @@ export function validateBriefContent(text, opts = {}) {
     }
   }
 
+  // v5.1 — phase_signals (additive optional field) + version-conditional sequencing gate.
+  // Composition rule documented in each downstream command's "Composition rule (v5.1)" section.
+  const hasSignals = 'phase_signals' in fm;
+  const hasPartial = 'phase_signals_partial' in fm;
+  if (hasSignals && hasPartial) {
+    errors.push(issue(
+      'BRIEF_SIGNALS_MUTUALLY_EXCLUSIVE',
+      'phase_signals and phase_signals_partial are mutually exclusive — set exactly one',
+      'Either commit per-phase signals OR record phase_signals_partial: true (force-stop).',
+    ));
+  }
+  if (hasSignals) {
+    if (!Array.isArray(fm.phase_signals)) {
+      errors.push(issue(
+        'BRIEF_INVALID_PHASE_SIGNALS',
+        'phase_signals must be a list of {phase, effort?, model?} entries',
+      ));
+    } else {
+      for (const entry of fm.phase_signals) {
+        if (!entry || typeof entry !== 'object' || !('phase' in entry)) {
+          errors.push(issue('BRIEF_INVALID_PHASE_SIGNALS', `phase_signals entry must include a "phase" key`));
+          continue;
+        }
+        if (!PHASE_SIGNAL_PHASES.includes(entry.phase)) {
+          errors.push(issue(
+            'BRIEF_INVALID_PHASE_SIGNAL_PHASE',
+            `phase_signals.phase "${entry.phase}" not in [${PHASE_SIGNAL_PHASES.join(', ')}]`,
+          ));
+        }
+        if ('effort' in entry && !EFFORT_LEVELS.includes(entry.effort)) {
+          errors.push(issue(
+            'BRIEF_INVALID_EFFORT',
+            `phase_signals.effort "${entry.effort}" not in [${EFFORT_LEVELS.join(', ')}]`,
+          ));
+        }
+        if ('model' in entry && !BASE_ALLOWED_MODELS.includes(entry.model)) {
+          errors.push(issue(
+            'BRIEF_INVALID_MODEL',
+            `phase_signals.model "${entry.model}" not in [${BASE_ALLOWED_MODELS.join(', ')}]`,
+          ));
+        }
+      }
+    }
+  }
+  // Sequencing gate: brief_version ≥ 2.1 requires phase_signals OR phase_signals_partial.
+  if (typeof fm.brief_version === 'string') {
+    const vm = fm.brief_version.match(/^(\d+)\.(\d+)$/);
+    if (vm) {
+      const major = Number(vm[1]);
+      const minor = Number(vm[2]);
+      const atLeast21 = major > 2 || (major === 2 && minor >= 1);
+      if (atLeast21 && !hasSignals && !hasPartial && fm.type !== 'trekreview') {
+        errors.push(issue(
+          'BRIEF_V51_MISSING_SIGNALS',
+          'brief_version ≥ 2.1 requires phase_signals (or phase_signals_partial: true)',
+          'Re-run /trekbrief — Phase 3.5 collects per-phase effort + model signals.',
+        ));
+      }
+    }
+  }
+
   if (fm.type !== undefined && !BRIEF_TYPE_VALUES.includes(fm.type)) {
     errors.push(issue(
       'BRIEF_WRONG_TYPE',
diff --git a/plugins/voyage/lib/validators/profile-validator.mjs b/plugins/voyage/lib/validators/profile-validator.mjs
index 48e84d6..c7fec26 100644
--- a/plugins/voyage/lib/validators/profile-validator.mjs
+++ b/plugins/voyage/lib/validators/profile-validator.mjs
@@ -38,7 +38,7 @@ export const PROFILE_REQUIRED_PHASES = Object.freeze([
   'brief', 'research', 'plan', 'execute', 'review', 'continue',
 ]);
 
-const BASE_ALLOWED_MODELS = Object.freeze(['sonnet', 'opus']);
+export const BASE_ALLOWED_MODELS = Object.freeze(['sonnet', 'opus']);
 
 function getAllowedModels(env = process.env) {
   if (env.VOYAGE_ALLOW_HAIKU === '1') {
diff --git a/plugins/voyage/tests/validators/brief-validator.test.mjs b/plugins/voyage/tests/validators/brief-validator.test.mjs
index 6e501d2..a9fd185 100644
--- a/plugins/voyage/tests/validators/brief-validator.test.mjs
+++ b/plugins/voyage/tests/validators/brief-validator.test.mjs
@@ -152,3 +152,69 @@ test('validateBrief — wrong-type error message includes accepted set', () => {
   assert.ok(/trekbrief/.test(wrongType.message));
   assert.ok(/trekreview/.test(wrongType.message));
 });
+
+// --- v5.1 — phase_signals additive field + sequencing gate ---
+
+const SIGNALS_BLOCK = `phase_signals:
+  - phase: research
+    effort: standard
+  - phase: plan
+    effort: high
+    model: opus
+  - phase: execute
+    effort: low
+    model: sonnet
+  - phase: review
+    effort: standard
+`;
+
+test('validateBrief — v5.1 well-formed phase_signals accepted', () => {
+  const t = GOOD_BRIEF
+    .replace('brief_version: "2.0"', 'brief_version: "2.1"')
+    .replace('source: interview\n', `source: interview\n${SIGNALS_BLOCK}`);
+  const r = validateBriefContent(t, { strict: true });
+  assert.equal(r.valid, true, JSON.stringify(r.errors));
+});
+
+test('validateBrief — pre-v5.1 brief without phase_signals accepted (backward-compat)', () => {
+  const r = validateBriefContent(GOOD_BRIEF, { strict: true });
+  assert.equal(r.valid, true, JSON.stringify(r.errors));
+  assert.ok(!r.errors.find(e => e.code === 'BRIEF_V51_MISSING_SIGNALS'));
+});
+
+test('validateBrief — v5.1+ brief missing phase_signals + partial emits BRIEF_V51_MISSING_SIGNALS', () => {
+  const t = GOOD_BRIEF.replace('brief_version: "2.0"', 'brief_version: "2.1"');
+  const r = validateBriefContent(t, { strict: true });
+  assert.equal(r.valid, false);
+  assert.ok(r.errors.find(e => e.code === 'BRIEF_V51_MISSING_SIGNALS'));
+});
+
+test('validateBrief — v5.1+ brief with phase_signals_partial: true accepted', () => {
+  const t = GOOD_BRIEF
+    .replace('brief_version: "2.0"', 'brief_version: "2.1"')
+    .replace('source: interview\n', 'source: interview\nphase_signals_partial: true\n');
+  const r = validateBriefContent(t, { strict: true });
+  assert.equal(r.valid, true, JSON.stringify(r.errors));
+});
+
+test('validateBrief — phase_signals + phase_signals_partial both set rejected (mutually exclusive)', () => {
+  const t = GOOD_BRIEF
+    .replace('brief_version: "2.0"', 'brief_version: "2.1"')
+    .replace('source: interview\n', `source: interview\nphase_signals_partial: true\n${SIGNALS_BLOCK}`);
+  const r = validateBriefContent(t, { strict: true });
+  assert.equal(r.valid, false);
+  assert.ok(r.errors.find(e => e.code === 'BRIEF_SIGNALS_MUTUALLY_EXCLUSIVE'));
+});
+
+test('validateBrief — phase_signals with unknown phase rejected', () => {
+  const BAD_SIGNALS = `phase_signals:
+  - phase: nonsense
+    effort: standard
+`;
+  const t = GOOD_BRIEF
+    .replace('brief_version: "2.0"', 'brief_version: "2.1"')
+    .replace('source: interview\n', `source: interview\n${BAD_SIGNALS}`);
+  const r = validateBriefContent(t, { strict: true });
+  assert.equal(r.valid, false);
+  assert.ok(r.errors.find(e => e.code === 'BRIEF_INVALID_PHASE_SIGNAL_PHASE'));
+});