diff --git a/plugins/voyage/tests/commands/trekbrief.test.mjs b/plugins/voyage/tests/commands/trekbrief.test.mjs
index 3db8030..0788f67 100644
--- a/plugins/voyage/tests/commands/trekbrief.test.mjs
+++ b/plugins/voyage/tests/commands/trekbrief.test.mjs
@@ -1,23 +1,43 @@
 // tests/commands/trekbrief.test.mjs
-// v5.1 — Pattern D prose-pattern regression tests for /trekbrief Phase 3.5.
+// v5.1 prose-pin tests + v5.1.1 runtime SC1 tests.
 //
-// Brief SC1 + SC2: end-of-brief effort dialog covering 4 downstream phases,
-// with `phase_signals_partial` as the force-stop record.
+// Pattern D prose-pins kept as doc-anchors for the .md file. Runtime tests
+// added per finding 350853 (BLOCKER SC1) + a7f4f95a (MAJOR Plan Step 5 drift).
+//
+// SC1 re-interpretation (per plan Step 10 amendment): "asserts on 4
+// AskUserQuestion calls" → "asserts resolvePhaseSignal returns non-null for
+// all 4 entries in PHASE_SIGNAL_PHASES when applied to a brief with a
+// committed phase_signals block." See brief amendment for full rationale.
 
 import { test } from 'node:test';
 import { strict as assert } from 'node:assert';
 import { readFileSync } from 'node:fs';
 import { dirname, join } from 'node:path';
 import { fileURLToPath } from 'node:url';
+import { resolvePhaseSignal } from '../../lib/profiles/phase-signal-resolver.mjs';
+import { validateBriefContent, PHASE_SIGNAL_PHASES, EFFORT_LEVELS } from '../../lib/validators/brief-validator.mjs';
+import { parseDocument } from '../../lib/util/frontmatter.mjs';
 
 const HERE = dirname(fileURLToPath(import.meta.url));
 const ROOT = join(HERE, '..', '..');
 const COMMAND_FILE = join(ROOT, 'commands', 'trekbrief.md');
+const FIXTURE = (name) => join(ROOT, 'tests', 'fixtures', name);
 
 function read() {
   return readFileSync(COMMAND_FILE, 'utf8');
 }
 
+function readFixture(name) {
+  return readFileSync(FIXTURE(name), 'utf8');
+}
+
+function frontmatterOf(text) {
+  const doc = parseDocument(text);
+  return doc.parsed && doc.parsed.frontmatter;
+}
+
+// --- Pattern D prose-pins (doc-anchors) ---
+
 test('trekbrief — Phase 3.5 heading is present', () => {
   const text = read();
   assert.match(text, /^## Phase 3\.5 — Per-phase effort dialog$/m,
@@ -40,3 +60,71 @@ test('trekbrief — Phase 3.5 documents phase_signals_partial force-stop', () =>
   assert.ok(text.includes('phase_signals_partial'),
     'phase_signals_partial not mentioned in /trekbrief command prose');
 });
+
+// --- v5.1.1 runtime SC1 tests ---
+
+test('trekbrief — SC1: resolvePhaseSignal returns non-null for all 4 phases on committed brief (brief-effort-low)', () => {
+  const fm = frontmatterOf(readFixture('brief-effort-low.md'));
+  for (const phase of PHASE_SIGNAL_PHASES) {
+    const r = resolvePhaseSignal(fm, phase);
+    assert.ok(r && typeof r === 'object',
+      `phase=${phase}: resolver must return non-null for committed brief; got ${JSON.stringify(r)}`);
+    assert.ok(typeof r.effort === 'string',
+      `phase=${phase}: resolver result must include effort`);
+  }
+});
+
+test('trekbrief — SC1: each of 4 phases has both effort AND model on full-signals fixture', () => {
+  const fm = frontmatterOf(readFixture('brief-with-phase-signals.md'));
+  for (const phase of PHASE_SIGNAL_PHASES) {
+    const r = resolvePhaseSignal(fm, phase);
+    assert.ok(r && typeof r === 'object', `phase=${phase}: must resolve`);
+    assert.ok(EFFORT_LEVELS.includes(r.effort),
+      `phase=${phase}: effort "${r.effort}" not in EFFORT_LEVELS`);
+    if ('model' in r) {
+      assert.ok(['sonnet', 'opus'].includes(r.model),
+        `phase=${phase}: model "${r.model}" not in [sonnet, opus]`);
+    }
+  }
+});
+
+test('trekbrief — SC1: missing phase_signals + brief_version 2.1 triggers BRIEF_V51_MISSING_SIGNALS', () => {
+  const r = validateBriefContent(readFixture('brief-v21-no-signals.md'), { strict: true });
+  assert.equal(r.valid, false);
+  assert.ok(
+    r.errors.find(e => e.code === 'BRIEF_V51_MISSING_SIGNALS'),
+    `gate must fire; errors=${JSON.stringify(r.errors)}`,
+  );
+});
+
+test('trekbrief — SC1: phase_signals_partial: true does NOT trigger the gate', () => {
+  const partial = `---
+type: trekbrief
+brief_version: "2.1"
+created: 2026-05-14
+task: "Partial brief"
+slug: partial-brief
+project_dir: .claude/projects/2026-05-14-partial-brief/
+research_topics: 0
+research_status: complete
+auto_research: false
+interview_turns: 2
+source: fixture
+phase_signals_partial: true
+---
+
+# Task
+
+## Intent
+Stop early.
+
+## Goal
+Test partial mode.
+
+## Success Criteria
+- gate does not fire.
+`;
+  const r = validateBriefContent(partial, { strict: true });
+  assert.equal(r.valid, true, `errors=${JSON.stringify(r.errors)}`);
+  assert.ok(!r.errors.find(e => e.code === 'BRIEF_V51_MISSING_SIGNALS'));
+});