diff --git a/tests-js/scripts/desktop-smoke.test.ts b/tests-js/scripts/desktop-smoke.test.ts
index 837de3e1a1..a06ff2c815 100644
--- a/tests-js/scripts/desktop-smoke.test.ts
+++ b/tests-js/scripts/desktop-smoke.test.ts
@@ -52,11 +52,11 @@ test('provider reconfiguration preserves feed, plugins, history and explicit fix
     writeEnvFile(home)
     expect(yaml.load(fs.readFileSync(path.join(home, 'config.yaml'), 'utf8'))).toMatchObject({
       updates: feed, plugins: { witness: true },
-      model: { provider: 'custom', temperature: 0.7, context_length: 12000 },
+      model: { provider: 'custom', base_url: 'http://127.0.0.1:9001/v1', temperature: 0.7, context_length: 12000 },
       providers: {},
-      // Exactly one entry, repointed at the second URL: a bare `custom` resolves
-      // from this list on every vintage, and reconfiguring must not stack
-      // duplicates.
+      // Exactly one entry, repointed at the second URL: reconfiguring must not
+      // stack duplicates. The readiness ladder reads model.base_url (above),
+      // not this list; the entry names the same endpoint for `custom:<name>`.
       custom_providers: [{ name: 'Mock', base_url: 'http://127.0.0.1:9001/v1', key_env: 'OPENAI_API_KEY' }],
       auxiliary: { title_generation: { enabled: false } }, approvals: { mode: 'smart' }, display: { interim_assistant_messages: true },
     })
diff --git a/tests-js/scripts/mock-provider-config.ts b/tests-js/scripts/mock-provider-config.ts
index 45dcd24fba..ad95cfa5a0 100644
--- a/tests-js/scripts/mock-provider-config.ts
+++ b/tests-js/scripts/mock-provider-config.ts
@@ -44,15 +44,19 @@ export function writeMockProviderConfig(
   const display = section.parse(extraDisplayConfig ? yaml.load(extraDisplayConfig) ?? {} : {})
   const merged = {
     ...config,
-    model: { ...config.model, default: 'mock-model', provider: 'custom', context_length: modelContextLength ?? 64000 },
-    // Two shapes, one endpoint. `providers:` stays untouched (a provider named
-    // 'mock' only resolved on trees that had registered such a profile, so
-    // older refs died with "Unknown provider 'mock'"). A bare `custom` resolves
-    // from `custom_providers:` on every vintage -- v2026.8.31's desktop
-    // readiness check reports "No usable credentials found for custom ...
-    // runtime resolution still failed" without an entry here -- while the .env
-    // OPENAI_BASE_URL/OPENAI_API_KEY pair (writeEnvFile) is how trees that
-    // resolve the endpoint from the environment reach the same URL.
+    // The endpoint must live in config.yaml: runtime_provider's bare-`custom`
+    // trust path reads the CONFIG base_url, and v2026.8.31 says so outright --
+    // "OPENAI_BASE_URL env var is no longer consulted -- config.yaml is the
+    // single source of truth for endpoint URLs". Without it `custom` resolves to
+    // no endpoint and the app boots on its onboarding overlay ("No usable
+    // credentials found for custom"), which is what every +desktop smoke leg
+    // hit. The `custom_providers` entry names the same endpoint for the
+    // named/`custom:<name>` form; the .env OPENAI_BASE_URL/OPENAI_API_KEY pair
+    // (writeEnvFile) stays for the trees that resolve the endpoint from the
+    // environment. `providers:` stays untouched (a provider named 'mock' only
+    // resolved on trees that had registered such a profile, so older refs died
+    // with "Unknown provider 'mock'").
+    model: { ...config.model, default: 'mock-model', provider: 'custom', base_url: `${url}/v1`, context_length: modelContextLength ?? 64000 },
     providers: { ...config.providers },
     custom_providers: [
       ...(config.custom_providers ?? []).filter(entry => entry?.name !== MOCK_PROVIDER_NAME),