diff --git a/plugins/voyage/lib/profiles/resolver.mjs b/plugins/voyage/lib/profiles/resolver.mjs
new file mode 100644
index 0000000..45354e0
--- /dev/null
+++ b/plugins/voyage/lib/profiles/resolver.mjs
@@ -0,0 +1,203 @@
+// lib/profiles/resolver.mjs
+// Profile resolution layer (v4.1 SC #5-#9).
+//
+// Locked interface contract (per brief Preferences):
+//   loadProfile(name) → ProfileObject
+//     - Reads lib/profiles/<name>.yaml or custom voyage-profiles/<name>.yaml.
+//     - Throws Error (cause: PROFILE_NOT_FOUND) when not found.
+//     - Returns parsed object with phase_models flattened to {brief: 'sonnet', ...}
+//       (object form for downstream JSON-stats; conversion from YAML list-of-dicts).
+//
+//   resolveProfile(argv, env) → {profile, profile_source}
+//     - Resolution order: --profile flag > VOYAGE_PROFILE env > 'premium' default.
+//     - profile_source: 'flag' | 'env' | 'default'.
+//
+//   resolveTrekcontinueProfile(planPath, argv) → {profile, profile_source}
+//     - --profile flag in argv wins with 'flag'.
+//     - Otherwise reads plan.md frontmatter via parseDocument; returns
+//       plan-frontmatter `profile` field with 'inheritance'.
+//     - If flag overrides inheritance, console.error emits an advisory.
+//     - For v4.0-style plans without `profile:` field, returns 'default' premium.
+//
+//   validateProfileFile(path) → Result
+//     - Thin wrapper around validateProfile from profile-validator.mjs.
+//
+// Custom.yaml lookup order: <repo-root>/voyage-profiles/<name>.yaml > ~/.claude/voyage-profiles/<name>.yaml
+// Both attempted paths included in error message on miss (HIGH-risk-mitigering).
+
+import { readFileSync, existsSync } from 'node:fs';
+import { join, dirname, resolve } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { homedir } from 'node:os';
+import { parseDocument } from '../util/frontmatter.mjs';
+import { validateProfile } from '../validators/profile-validator.mjs';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const BUILTIN_PROFILES_DIR = __dirname; // lib/profiles/
+const BUILTIN_NAMES = new Set(['economy', 'balanced', 'premium']);
+
+/**
+ * Resolve the path to a profile file.
+ * Built-in profiles: lib/profiles/<name>.yaml
+ * Custom profiles: <repo-root>/voyage-profiles/<name>.yaml > ~/.claude/voyage-profiles/<name>.yaml
+ *
+ * @returns {{path: string|null, attempted: string[]}}
+ */
+export function findProfilePath(name, opts = {}) {
+  const cwd = opts.cwd || process.cwd();
+  const home = opts.home || homedir();
+  const attempted = [];
+
+  if (BUILTIN_NAMES.has(name)) {
+    const builtinPath = join(BUILTIN_PROFILES_DIR, `${name}.yaml`);
+    attempted.push(builtinPath);
+    if (existsSync(builtinPath)) return { path: builtinPath, attempted };
+  }
+
+  // Custom: repo-root first, then home
+  const repoCustom = join(cwd, 'voyage-profiles', `${name}.yaml`);
+  attempted.push(repoCustom);
+  if (existsSync(repoCustom)) return { path: repoCustom, attempted };
+
+  const homeCustom = join(home, '.claude', 'voyage-profiles', `${name}.yaml`);
+  attempted.push(homeCustom);
+  if (existsSync(homeCustom)) return { path: homeCustom, attempted };
+
+  return { path: null, attempted };
+}
+
+/**
+ * Flatten phase_models list-of-dicts to object form: {brief: 'sonnet', research: 'opus', ...}
+ */
+function flattenPhaseModels(list) {
+  const out = {};
+  if (!Array.isArray(list)) return out;
+  for (const entry of list) {
+    if (entry && typeof entry === 'object' && typeof entry.phase === 'string' && typeof entry.model === 'string') {
+      out[entry.phase] = entry.model;
+    }
+  }
+  return out;
+}
+
+/**
+ * Load and parse a profile file by name.
+ * @param {string} name
+ * @param {{cwd?: string, home?: string}} [opts]
+ * @returns {{name: string, profile_version: string, phase_models: object, parallel_agents_min: number, parallel_agents_max: number, external_research_enabled: boolean, brief_reviewer_iter_cap: number, _path: string}}
+ * @throws {Error} cause: PROFILE_NOT_FOUND | PROFILE_PARSE_ERROR
+ */
+export function loadProfile(name, opts = {}) {
+  const { path, attempted } = findProfilePath(name, opts);
+  if (!path) {
+    const err = new Error(
+      `Profile "${name}" not found. Attempted paths:\n  - ${attempted.join('\n  - ')}`,
+    );
+    err.cause = 'PROFILE_NOT_FOUND';
+    err.attempted = attempted;
+    throw err;
+  }
+
+  let text;
+  try { text = readFileSync(path, 'utf-8'); }
+  catch (e) {
+    const err = new Error(`Cannot read profile "${name}" at ${path}: ${e.message}`);
+    err.cause = 'PROFILE_READ_ERROR';
+    throw err;
+  }
+
+  const doc = parseDocument(text);
+  if (!doc.valid) {
+    const err = new Error(`Profile "${name}" parse error: ${doc.errors[0]?.message || 'unknown'}`);
+    err.cause = 'PROFILE_PARSE_ERROR';
+    throw err;
+  }
+
+  const fm = doc.parsed.frontmatter || {};
+  return {
+    name: fm.name,
+    profile_version: fm.profile_version,
+    phase_models: flattenPhaseModels(fm.phase_models),
+    parallel_agents_min: fm.parallel_agents_min,
+    parallel_agents_max: fm.parallel_agents_max,
+    external_research_enabled: fm.external_research_enabled,
+    brief_reviewer_iter_cap: fm.brief_reviewer_iter_cap,
+    _path: path,
+  };
+}
+
+/**
+ * Resolve profile name from argv + env + default.
+ * Order: --profile flag > VOYAGE_PROFILE env > 'premium'.
+ *
+ * @param {{flags?: object} | string[]} argv  Either parsed argv or a flags-object.
+ * @param {object} [env]
+ * @returns {{profile: string, profile_source: 'flag'|'env'|'default'}}
+ */
+export function resolveProfile(argv, env = process.env) {
+  const flags = (argv && typeof argv === 'object' && argv.flags) ? argv.flags : (argv || {});
+  if (typeof flags['--profile'] === 'string' && flags['--profile'].length > 0) {
+    return { profile: flags['--profile'], profile_source: 'flag' };
+  }
+  if (typeof env.VOYAGE_PROFILE === 'string' && env.VOYAGE_PROFILE.length > 0) {
+    return { profile: env.VOYAGE_PROFILE, profile_source: 'env' };
+  }
+  return { profile: 'premium', profile_source: 'default' };
+}
+
+/**
+ * Resolve profile for /trekcontinue: prefers explicit flag, falls back to plan
+ * frontmatter (inheritance), then 'premium' default if plan has no profile field.
+ *
+ * @param {string} planPath  Path to plan.md
+ * @param {{flags?: object} | string[]} argv
+ * @param {{env?: object, console?: Console}} [opts]
+ * @returns {{profile: string, profile_source: 'flag'|'inheritance'|'default'}}
+ */
+export function resolveTrekcontinueProfile(planPath, argv, opts = {}) {
+  const env = opts.env || process.env;
+  const con = opts.console || console;
+
+  const flags = (argv && typeof argv === 'object' && argv.flags) ? argv.flags : (argv || {});
+  const flagProfile = (typeof flags['--profile'] === 'string' && flags['--profile'].length > 0)
+    ? flags['--profile'] : null;
+
+  // Read plan-frontmatter to detect inheritance
+  let planProfile = null;
+  if (planPath && existsSync(planPath)) {
+    try {
+      const text = readFileSync(planPath, 'utf-8');
+      const doc = parseDocument(text);
+      if (doc.valid) {
+        const fm = doc.parsed.frontmatter || {};
+        if (typeof fm.profile === 'string' && fm.profile.length > 0) {
+          planProfile = fm.profile;
+        }
+      }
+    } catch {
+      // swallow — degrades gracefully to default
+    }
+  }
+
+  if (flagProfile) {
+    if (planProfile && planProfile !== flagProfile) {
+      con.error(`[voyage] profile inheritance overridden by --profile flag: ${planProfile} → ${flagProfile}`);
+    }
+    return { profile: flagProfile, profile_source: 'flag' };
+  }
+
+  if (planProfile) {
+    return { profile: planProfile, profile_source: 'inheritance' };
+  }
+
+  // v4.0-style plan without profile: default to premium
+  return { profile: 'premium', profile_source: 'default' };
+}
+
+/**
+ * Validate a profile YAML file.
+ * Thin wrapper for locked-interface compatibility.
+ */
+export function validateProfileFile(path, opts = {}) {
+  return validateProfile(path, opts);
+}
diff --git a/plugins/voyage/tests/fixtures/plan-with-profile.md b/plugins/voyage/tests/fixtures/plan-with-profile.md
new file mode 100644
index 0000000..c476ddb
--- /dev/null
+++ b/plugins/voyage/tests/fixtures/plan-with-profile.md
@@ -0,0 +1,26 @@
+---
+plan_version: "1.7"
+profile: balanced
+phase_models:
+  - phase: brief
+    model: sonnet
+  - phase: research
+    model: sonnet
+  - phase: plan
+    model: opus
+  - phase: execute
+    model: sonnet
+  - phase: review
+    model: opus
+  - phase: continue
+    model: sonnet
+---
+
+# Test plan (with profile)
+
+This fixture has explicit profile + phase_models in frontmatter.
+
+## Implementation Plan
+
+### Step 1: Stub
+- Files: src/stub.mjs
diff --git a/plugins/voyage/tests/fixtures/plan-without-profile.md b/plugins/voyage/tests/fixtures/plan-without-profile.md
new file mode 100644
index 0000000..1478e50
--- /dev/null
+++ b/plugins/voyage/tests/fixtures/plan-without-profile.md
@@ -0,0 +1,15 @@
+---
+plan_version: "1.6"
+---
+
+# Test plan (v4.0-style, no profile field)
+
+This fixture is a v4.0-style plan WITHOUT the v4.1 profile/phase_models fields.
+Used by tests/lib/profile-application.test.mjs to verify backward-compat
+edge-case: resolveTrekcontinueProfile returns {profile: 'premium', profile_source: 'default'}
+without throwing when the plan has no profile concept.
+
+## Implementation Plan
+
+### Step 1: Stub
+- Files: src/stub.mjs
diff --git a/plugins/voyage/tests/lib/profile-application.test.mjs b/plugins/voyage/tests/lib/profile-application.test.mjs
new file mode 100644
index 0000000..6a36513
--- /dev/null
+++ b/plugins/voyage/tests/lib/profile-application.test.mjs
@@ -0,0 +1,230 @@
+// tests/lib/profile-application.test.mjs
+// SC #5-#9 + backward-compat edge-case for lib/profiles/resolver.mjs.
+
+import { test } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { mkdtempSync, mkdirSync, writeFileSync, rmSync } from 'node:fs';
+import { join, dirname } from 'node:path';
+import { tmpdir } from 'node:os';
+import { fileURLToPath } from 'node:url';
+import {
+  loadProfile,
+  resolveProfile,
+  resolveTrekcontinueProfile,
+  validateProfileFile,
+  findProfilePath,
+} from '../../lib/profiles/resolver.mjs';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const REPO_ROOT = join(__dirname, '..', '..');
+
+// SC #5: loadProfile returns matrix-match for all 6 phase_models
+
+test('SC #5: loadProfile("economy") returns flattened phase_models with all 6 phases', () => {
+  const p = loadProfile('economy');
+  assert.equal(p.name, 'economy');
+  assert.equal(p.phase_models.brief, 'sonnet');
+  assert.equal(p.phase_models.research, 'sonnet');
+  assert.equal(p.phase_models.plan, 'sonnet');
+  assert.equal(p.phase_models.execute, 'sonnet');
+  assert.equal(p.phase_models.review, 'sonnet');
+  assert.equal(p.phase_models.continue, 'sonnet');
+  assert.equal(p.parallel_agents_min, 2);
+  assert.equal(p.parallel_agents_max, 3);
+  assert.equal(p.external_research_enabled, false);
+  assert.equal(p.brief_reviewer_iter_cap, 1);
+});
+
+test('SC #5: loadProfile("balanced") returns mixed phase_models', () => {
+  const p = loadProfile('balanced');
+  assert.equal(p.phase_models.plan, 'opus');
+  assert.equal(p.phase_models.review, 'opus');
+  assert.equal(p.phase_models.brief, 'sonnet');
+  assert.equal(p.phase_models.execute, 'sonnet');
+});
+
+test('SC #5: loadProfile("premium") returns all-opus', () => {
+  const p = loadProfile('premium');
+  for (const phase of ['brief', 'research', 'plan', 'execute', 'review', 'continue']) {
+    assert.equal(p.phase_models[phase], 'opus', `premium ${phase} should be opus`);
+  }
+});
+
+test('SC #5: loadProfile throws PROFILE_NOT_FOUND for unknown profile', () => {
+  try {
+    loadProfile('does-not-exist-xyz');
+    assert.fail('expected throw');
+  } catch (e) {
+    assert.equal(e.cause, 'PROFILE_NOT_FOUND');
+    assert.match(e.message, /not found/);
+    assert.ok(Array.isArray(e.attempted), 'should expose attempted paths');
+  }
+});
+
+// SC #6: env-var fallback flag > env > default
+
+test('SC #6: resolveProfile flag > env > default', () => {
+  // flag wins
+  const r1 = resolveProfile({ flags: { '--profile': 'balanced' } }, { VOYAGE_PROFILE: 'economy' });
+  assert.equal(r1.profile, 'balanced');
+  assert.equal(r1.profile_source, 'flag');
+
+  // env wins when no flag
+  const r2 = resolveProfile({ flags: {} }, { VOYAGE_PROFILE: 'economy' });
+  assert.equal(r2.profile, 'economy');
+  assert.equal(r2.profile_source, 'env');
+
+  // default when neither
+  const r3 = resolveProfile({ flags: {} }, {});
+  assert.equal(r3.profile, 'premium');
+  assert.equal(r3.profile_source, 'default');
+});
+
+// SC #7: performance — loadProfile 1000 iter < 50ms average (allowing some headroom)
+
+test('SC #7: loadProfile 1000-iter performance < 50ms average', () => {
+  const iterations = 1000;
+  const start = performance.now();
+  for (let i = 0; i < iterations; i++) {
+    loadProfile('economy');
+  }
+  const elapsed = performance.now() - start;
+  const avgMs = elapsed / iterations;
+  assert.ok(avgMs < 50, `loadProfile too slow: ${avgMs.toFixed(3)}ms average over ${iterations} iter`);
+});
+
+// SC #8: custom.yaml from repo-root trumps ~/.claude/
+
+test('SC #8: custom profile from <cwd>/voyage-profiles/<name>.yaml takes precedence over ~/.claude/', () => {
+  const tmpRepo = mkdtempSync(join(tmpdir(), 'voyage-resolver-repo-'));
+  const tmpHome = mkdtempSync(join(tmpdir(), 'voyage-resolver-home-'));
+  try {
+    // Place custom profile in repo and home — repo should win
+    mkdirSync(join(tmpRepo, 'voyage-profiles'), { recursive: true });
+    mkdirSync(join(tmpHome, '.claude', 'voyage-profiles'), { recursive: true });
+
+    writeFileSync(join(tmpRepo, 'voyage-profiles', 'mycustom.yaml'),
+      `---
+profile_version: "1.0"
+name: mycustom-repo
+phase_models:
+  - phase: brief
+    model: sonnet
+  - phase: research
+    model: sonnet
+  - phase: plan
+    model: sonnet
+  - phase: execute
+    model: sonnet
+  - phase: review
+    model: sonnet
+  - phase: continue
+    model: sonnet
+parallel_agents_min: 1
+parallel_agents_max: 2
+external_research_enabled: false
+brief_reviewer_iter_cap: 1
+---
+`);
+    writeFileSync(join(tmpHome, '.claude', 'voyage-profiles', 'mycustom.yaml'),
+      `---
+profile_version: "1.0"
+name: mycustom-home
+phase_models:
+  - phase: brief
+    model: opus
+  - phase: research
+    model: opus
+  - phase: plan
+    model: opus
+  - phase: execute
+    model: opus
+  - phase: review
+    model: opus
+  - phase: continue
+    model: opus
+parallel_agents_min: 1
+parallel_agents_max: 2
+external_research_enabled: true
+brief_reviewer_iter_cap: 3
+---
+`);
+
+    const found = findProfilePath('mycustom', { cwd: tmpRepo, home: tmpHome });
+    assert.ok(found.path, `expected to find mycustom; attempted: ${found.attempted.join(', ')}`);
+    assert.ok(found.path.startsWith(tmpRepo),
+      `expected repo-rot win (path under ${tmpRepo}), got: ${found.path}`);
+
+    const p = loadProfile('mycustom', { cwd: tmpRepo, home: tmpHome });
+    assert.equal(p.name, 'mycustom-repo', 'repo profile should win');
+    assert.equal(p.phase_models.brief, 'sonnet');
+  } finally {
+    rmSync(tmpRepo, { recursive: true, force: true });
+    rmSync(tmpHome, { recursive: true, force: true });
+  }
+});
+
+test('SC #8: missing profile error message includes both attempted paths', () => {
+  const tmpRepo = mkdtempSync(join(tmpdir(), 'voyage-resolver-empty-'));
+  const tmpHome = mkdtempSync(join(tmpdir(), 'voyage-resolver-emptyhome-'));
+  try {
+    try {
+      loadProfile('not-a-real-profile', { cwd: tmpRepo, home: tmpHome });
+      assert.fail('expected throw');
+    } catch (e) {
+      assert.equal(e.cause, 'PROFILE_NOT_FOUND');
+      // Both attempted paths should be in the error message for diagnostic clarity
+      const msg = e.message;
+      assert.match(msg, /voyage-profiles\/not-a-real-profile\.yaml/);
+      assert.match(msg, /\.claude\/voyage-profiles\/not-a-real-profile\.yaml/);
+    }
+  } finally {
+    rmSync(tmpRepo, { recursive: true, force: true });
+    rmSync(tmpHome, { recursive: true, force: true });
+  }
+});
+
+// SC #9: resolveTrekcontinueProfile inheritance from plan-frontmatter
+
+test('SC #9: resolveTrekcontinueProfile inherits from plan-frontmatter (profile: balanced)', () => {
+  const planPath = join(REPO_ROOT, 'tests', 'fixtures', 'plan-with-profile.md');
+  const r = resolveTrekcontinueProfile(planPath, { flags: {} });
+  assert.equal(r.profile, 'balanced');
+  assert.equal(r.profile_source, 'inheritance');
+});
+
+test('SC #9: resolveTrekcontinueProfile flag overrides plan-frontmatter (advisory)', () => {
+  const planPath = join(REPO_ROOT, 'tests', 'fixtures', 'plan-with-profile.md');
+  const advisories = [];
+  const fakeConsole = { error: (m) => advisories.push(m) };
+  const r = resolveTrekcontinueProfile(planPath,
+    { flags: { '--profile': 'economy' } },
+    { console: fakeConsole });
+  assert.equal(r.profile, 'economy');
+  assert.equal(r.profile_source, 'flag');
+  assert.equal(advisories.length, 1, 'expected one advisory message');
+  assert.match(advisories[0], /balanced.*economy/);
+  assert.match(advisories[0], /\[voyage\]/);
+});
+
+// Backward-compat edge-case: v4.0-style plan WITHOUT profile field
+
+test('Backward-compat: resolveTrekcontinueProfile on v4.0 plan without profile field returns default premium', () => {
+  const planPath = join(REPO_ROOT, 'tests', 'fixtures', 'plan-without-profile.md');
+  const r = resolveTrekcontinueProfile(planPath, { flags: {} });
+  assert.equal(r.profile, 'premium');
+  assert.equal(r.profile_source, 'default');
+});
+
+test('Backward-compat: resolveTrekcontinueProfile with non-existent plan path returns default premium', () => {
+  const r = resolveTrekcontinueProfile('/tmp/does-not-exist-plan-xyz.md', { flags: {} });
+  assert.equal(r.profile, 'premium');
+  assert.equal(r.profile_source, 'default');
+});
+
+// validateProfileFile re-export sanity
+
+test('validateProfileFile re-exports validateProfile (locked-interface compat)', () => {
+  const r = validateProfileFile(join(REPO_ROOT, 'lib', 'profiles', 'economy.yaml'));
+  assert.equal(r.valid, true);
+});