diff --git a/src/commands/sources.ts b/src/commands/sources.ts
index b08ebb2..4f8be68 100644
--- a/src/commands/sources.ts
+++ b/src/commands/sources.ts
@@ -47,17 +47,23 @@ function validateSourceId(id: string): void {
 //   - Single-level glob: 'wedding-planning/*'         (cosmetic; same as literal)
 //
 // Rules:
-//   - Allowed chars: lowercase a-z, 0-9, '-' (hyphen), '/' (slash)
+//   - Allowed chars: lowercase a-z, 0-9, '-' (hyphen), '_' (underscore), '/' (slash)
 //   - Optional single trailing '*' (no other position)
 //   - Length 1..64 (excluding the trailing '*')
-//   - Reject underscores ('_'), uppercase, whitespace, mid-string '*',
-//     multi-level globs ('**'), any other punctuation
+//   - Reject uppercase, whitespace, mid-string '*', multi-level globs
+//     ('**'), any other punctuation
 //
-// Why fail-fast at write time: a typo'd rule (`memory_dashboard/`) writes
-// successfully into config_jsonb but never matches anything at routing
-// time — the put_page silently falls to brain-default. Catching at
-// CLI-write moment surfaces "this is a typo" before bad data lands.
-const SLUG_PREFIX_RULE_RE = /^[a-z0-9](?:[a-z0-9-/]{0,62}[a-z0-9-/])?\*?$/;
+// Why fail-fast at write time: a typo'd rule writes successfully into
+// config_jsonb but never matches anything at routing time — the put_page
+// silently falls to brain-default. Catching at CLI-write moment surfaces
+// the typo before bad data lands.
+//
+// Underscore note (v0.18.2.fork.1 Phase 6 fix): chezmoi-managed prefixes
+// like `dot_claude/` are legitimate slugs — chezmoi conventionally maps
+// `~/.claude/` → `dot_claude/` in source. Earlier draft of this validator
+// rejected underscores, which broke Phase 4 source taxonomy. Underscore
+// is now first-class.
+const SLUG_PREFIX_RULE_RE = /^[a-z0-9_](?:[a-z0-9_\-/]{0,62}[a-z0-9_\-/])?\*?$/;
 function validateSlugPrefix(rule: string): void {
   if (!rule || rule.length === 0) {
     throw new Error('Empty slug-prefix rule. Each --slug-prefix entry must be a non-empty string.');
diff --git a/test/sources-update-slug-prefix.test.ts b/test/sources-update-slug-prefix.test.ts
index 68b386d..93156d6 100644
--- a/test/sources-update-slug-prefix.test.ts
+++ b/test/sources-update-slug-prefix.test.ts
@@ -106,8 +106,10 @@ describe('Prefix grammar validator (Issue #6 — fail-fast at write time)', () =
     if (hint) expect(msg).toContain(hint);
   };
 
-  test('reject underscore', async () => {
-    await expectReject('memory_dashboard/', 'Invalid slug-prefix');
+  test('accept underscore (chezmoi-style prefixes like dot_claude/)', async () => {
+    await runSources(engine, ['add', 'test-accept-under', '--slug-prefix', 'dot_claude/,foo_bar/']);
+    const cfg = await readConfig('test-accept-under');
+    expect(cfg.slug_prefix_rules).toEqual(['dot_claude/', 'foo_bar/']);
   });
   test('reject uppercase', async () => {
     await expectReject('MemoryDashboard/', 'Invalid slug-prefix');