@@ -119,6 +119,77 @@ describe("applyProviderInstanceSettings", () => {
 
     expect(entry?.enabled).toBe(false);
   });
+
+  it.each(["constructor", "toString"])(
+    "treats a removed custom instance named %s as disabled",
+    (instanceId) => {
+      const entries = deriveProviderInstanceEntries([
+        provider({
+          provider: ProviderDriverKind.make("claudeAgent"),
+          instanceId,
+        }),
+      ]);
+      const [entry] = applyProviderInstanceSettings(entries, {
+        providerInstances: {},
+        providers: {} as never,
+      });
+
+      expect(entry?.enabled).toBe(false);
+    },
+  );
+
+  it("uses settings for a configured custom instance named constructor", () => {
+    const instanceId = ProviderInstanceId.make("constructor");
+    const entries = deriveProviderInstanceEntries([
+      provider({
+        provider: ProviderDriverKind.make("claudeAgent"),
+        instanceId,
+      }),
+    ]);
+    const [entry] = applyProviderInstanceSettings(entries, {
+      providerInstances: {
+        [instanceId]: {
+          driver: ProviderDriverKind.make("claudeAgent"),
+          enabled: false,
+        },
+      },
+      providers: {} as never,
+    });
+
+    expect(entry?.enabled).toBe(false);
+  });
+
+  it("treats a removed default instance for a fork driver as disabled", () => {
+    const driver = ProviderDriverKind.make("constructor");
+    const entries = deriveProviderInstanceEntries([
+      provider({
+        provider: driver,
+        instanceId: "constructor",
+      }),
+    ]);
+    const [entry] = applyProviderInstanceSettings(entries, {
+      providerInstances: {},
+      providers: {} as never,
+    });
+
+    expect(entry?.isDefault).toBe(true);
+    expect(entry?.enabled).toBe(false);
+  });
+
+  it("uses legacy settings for a built-in default instance", () => {
+    const entries = deriveProviderInstanceEntries([
+      provider({
+        provider: ProviderDriverKind.make("codex"),
+        instanceId: "codex",
+      }),
+    ]);
+    const [entry] = applyProviderInstanceSettings(entries, {
+      providerInstances: {},
+      providers: { codex: { enabled: false } } as never,
+    });
+
+    expect(entry?.enabled).toBe(false);
+  });
 });
 
 describe("deriveProviderInstanceEntries", () => {
 
@@ -233,9 +233,10 @@ export function deriveProviderEntriesByEnvironment(
  * settings write, so picker visibility must follow settings rather than waiting
  * for probe reconciliation.
  *
- * Non-default instances only exist through `providerInstances`; if one is
- * absent there, its streamed snapshot is stale (for example immediately after
- * deletion) and is treated as disabled.
+ * Only built-in default instances have a legacy `providers` entry. Every
+ * other instance exists through `providerInstances`; if it is absent there,
+ * its streamed snapshot is stale (for example immediately after deletion)
+ * and is treated as disabled.
  */
 export function applyProviderInstanceSettings(
   entries: ReadonlyArray<ProviderInstanceEntry>,
@@ -246,11 +247,16 @@ export function applyProviderInstanceSettings(
   >;
 
   return entries.map((entry) => {
-    const explicitInstance = settings.providerInstances?.[entry.instanceId];
+    const explicitInstance = Object.hasOwn(settings.providerInstances, entry.instanceId)
+      ? settings.providerInstances[entry.instanceId]
+      : undefined;
+    const legacyProvider = Object.hasOwn(legacyProviders, entry.driverKind)
+      ? legacyProviders[entry.driverKind]
+      : undefined;
     const enabled = explicitInstance
       ? resolveProviderInstanceEnabled(explicitInstance)
-      : entry.isDefault
-        ? (legacyProviders[entry.driverKind]?.enabled ?? entry.enabled)
+      : entry.isDefault && legacyProvider
+        ? (legacyProvider.enabled ?? entry.enabled)
         : false;
     return enabled === entry.enabled ? entry : { ...entry, enabled };
   });