diff --git a/plugins/voyage/lib/parsers/manifest-yaml.mjs b/plugins/voyage/lib/parsers/manifest-yaml.mjs
index d1cb24d..c5a3903 100644
--- a/plugins/voyage/lib/parsers/manifest-yaml.mjs
+++ b/plugins/voyage/lib/parsers/manifest-yaml.mjs
@@ -31,7 +31,20 @@ const OPTIONAL_KEYS = [
 
 const OPTIONAL_BOOLEAN_KEYS = new Set(OPTIONAL_KEYS);
 
-export { OPTIONAL_KEYS };
+// Optional string-typed manifest keys (v4.1 Step 3 — additive forward-compat).
+// `profile_used`: name of the model profile (economy|balanced|premium|<custom>) the
+//                 step was executed under. Absence is fine (v4.0 manifests have no
+//                 profile concept); presence MUST be a string.
+// Unlike OPTIONAL_BOOLEAN_KEYS, absence is NOT defaulted — the field is simply
+// missing from `parsed` so downstream consumers can distinguish "not set" from
+// "explicitly empty string".
+const OPTIONAL_STRING_KEYS = [
+  'profile_used',
+];
+
+const OPTIONAL_STRING_KEYS_SET = new Set(OPTIONAL_STRING_KEYS);
+
+export { OPTIONAL_KEYS, OPTIONAL_STRING_KEYS };
 
 /**
  * Extract the first fenced YAML block whose first non-blank line begins with
@@ -110,6 +123,19 @@ export function parseManifest(stepBody) {
     }
   }
 
+  // v4.1 Step 3 — string-typed optional keys. Absence == not-present (no default,
+  // unlike boolean keys above) so downstream can distinguish unset vs empty string.
+  for (const k of OPTIONAL_STRING_KEYS_SET) {
+    if (k in parsed) {
+      if (typeof parsed[k] !== 'string') {
+        errors.push(issue(
+          'MANIFEST_OPTIONAL_TYPE',
+          `${k} must be string if present (got ${typeof parsed[k]})`,
+        ));
+      }
+    }
+  }
+
   return { valid: errors.length === 0, errors, warnings, parsed };
 }
 
diff --git a/plugins/voyage/tests/lib/manifest-schema-extensions.test.mjs b/plugins/voyage/tests/lib/manifest-schema-extensions.test.mjs
index fa326f7..5f2fe00 100644
--- a/plugins/voyage/tests/lib/manifest-schema-extensions.test.mjs
+++ b/plugins/voyage/tests/lib/manifest-schema-extensions.test.mjs
@@ -8,7 +8,7 @@
 
 import { test } from 'node:test';
 import { strict as assert } from 'node:assert';
-import { parseManifest, OPTIONAL_KEYS } from '../../lib/parsers/manifest-yaml.mjs';
+import { parseManifest, OPTIONAL_KEYS, OPTIONAL_STRING_KEYS } from '../../lib/parsers/manifest-yaml.mjs';
 
 const BASE = `### Step 1: Cover
 - Manifest:
@@ -90,3 +90,44 @@ test('extension does NOT break REQUIRED_KEYS contract', () => {
     assert.ok(k in r.parsed, `required key ${k} missing after extension`);
   }
 });
+
+// v4.1 Step 3 — OPTIONAL_STRING_KEYS dispatch (profile_used)
+
+test('OPTIONAL_STRING_KEYS exports profile_used', () => {
+  assert.deepEqual(
+    [...OPTIONAL_STRING_KEYS].sort(),
+    ['profile_used'].sort(),
+    'OPTIONAL_STRING_KEYS export drift — pin contract',
+  );
+});
+
+test('profile_used: economy parses successfully (SC #10 forward-compat)', () => {
+  const r = parseManifest(bodyWithExtras('    profile_used: economy'));
+  assert.equal(r.valid, true, JSON.stringify(r.errors));
+  assert.equal(r.parsed.profile_used, 'economy');
+});
+
+test('profile_used: numeric rejected with MANIFEST_OPTIONAL_TYPE', () => {
+  const r = parseManifest(bodyWithExtras('    profile_used: 42'));
+  assert.equal(r.valid, false);
+  const found = r.errors.find(e => e.code === 'MANIFEST_OPTIONAL_TYPE');
+  assert.ok(found, `expected MANIFEST_OPTIONAL_TYPE, got: ${JSON.stringify(r.errors)}`);
+  assert.match(found.message, /profile_used/);
+  assert.match(found.message, /string/);
+});
+
+test('absence of profile_used: field is NOT in parsed (NOT defaulted, unlike boolean)', () => {
+  const r = parseManifest(bodyOnlyRequired());
+  assert.equal(r.valid, true, JSON.stringify(r.errors));
+  // Absence semantics differ from boolean: parsed should NOT contain the key
+  assert.equal('profile_used' in r.parsed, false,
+    'profile_used must NOT be auto-defaulted when absent — string-key semantics');
+});
+
+test('profile_used works alongside boolean optional keys (skip_commit_check + memory_write)', () => {
+  const r = parseManifest(bodyWithExtras('    skip_commit_check: true\n    memory_write: true\n    profile_used: balanced'));
+  assert.equal(r.valid, true, JSON.stringify(r.errors));
+  assert.equal(r.parsed.skip_commit_check, true);
+  assert.equal(r.parsed.memory_write, true);
+  assert.equal(r.parsed.profile_used, 'balanced');
+});