diff --git a/packages/agents/src/kb-shared/__tests__/fixtures/home-single-default/.agents/kb.yaml b/packages/agents/src/kb-shared/__tests__/fixtures/home-single-default/.agents/kb.yaml
new file mode 100644
index 00000000..7e614fe5
--- /dev/null
+++ b/packages/agents/src/kb-shared/__tests__/fixtures/home-single-default/.agents/kb.yaml
@@ -0,0 +1,7 @@
+# Single-entry user-global registry fixture (regression: #684).
+# Its sole entry is also the default, mirroring the issue's reported registry shape.
+kbs:
+  coding:
+    path: ../../vault-b
+    description: Fixture single-entry default vault
+    default: true
diff --git a/packages/agents/src/kb-shared/__tests__/resolve-writable-kb.test.ts b/packages/agents/src/kb-shared/__tests__/resolve-writable-kb.test.ts
index a950e92f..2920ed0d 100644
--- a/packages/agents/src/kb-shared/__tests__/resolve-writable-kb.test.ts
+++ b/packages/agents/src/kb-shared/__tests__/resolve-writable-kb.test.ts
@@ -15,6 +15,7 @@ const HOME_WITH_DEFAULT = join(FIXTURES, 'home-with-default');
 const HOME_MALFORMED = join(FIXTURES, 'malformed-registry');
 const HOME_READONLY_DEFAULT = join(FIXTURES, 'home-readonly-default');
 const HOME_READONLY_NAMED = join(FIXTURES, 'home-readonly-named');
+const HOME_SINGLE_DEFAULT = join(FIXTURES, 'home-single-default');
 // A home directory with no `.agents/kb.yaml`, so the user-global registry resolves empty.
 const HOME_EMPTY = FIXTURES;
 
@@ -68,6 +69,15 @@ describe(resolveWritableKb, () => {
     });
   });
 
+  it('resolves --kb for a single-entry registry whose sole entry is the default (regression: #684)', async () => {
+    const result = await resolveWritableKb({ startDir: '/', explicitKb: 'coding', home: HOME_SINGLE_DEFAULT });
+
+    expect(result).toEqual({
+      ok: true,
+      kb: { name: 'coding', path: VAULT_B, source: 'explicit' },
+    });
+  });
+
   it('returns no-kb-resolvable when --kb names an entry that does not exist', async () => {
     const result = await resolveWritableKb({ startDir: PROJECT_KB, explicitKb: 'nonexistent', home: HOME_EMPTY });