diff --git a/src/agents/model-auth.ts b/src/agents/model-auth.ts
index 1f7882bb6fd..9cb23dbbf45 100644
--- a/src/agents/model-auth.ts
+++ b/src/agents/model-auth.ts
@@ -23,6 +23,7 @@ import {
   type AuthProfileStore,
   externalCliDiscoveryForProviderAuth,
   ensureAuthProfileStore,
+  ensureAuthProfileStoreWithoutExternalProfiles,
   listProfilesForProvider,
   resolveApiKeyForProfile,
   resolveAuthProfileOrder,
@@ -43,7 +44,11 @@ import {
 } from "./model-auth-runtime-shared.js";
 import { normalizeProviderId } from "./model-selection.js";
 
-export { ensureAuthProfileStore, resolveAuthProfileOrder } from "./auth-profiles.js";
+export {
+  ensureAuthProfileStore,
+  ensureAuthProfileStoreWithoutExternalProfiles,
+  resolveAuthProfileOrder,
+} from "./auth-profiles.js";
 export { requireApiKey, resolveAwsSdkEnvVarName } from "./model-auth-runtime-shared.js";
 export type { ResolvedProviderAuth } from "./model-auth-runtime-shared.js";
 export type ProviderCredentialPrecedence = "profile-first" | "env-first";
diff --git a/src/agents/pi-embedded-runner/compact.hooks.harness.ts b/src/agents/pi-embedded-runner/compact.hooks.harness.ts
index 5e634d1093b..340369fed3b 100644
--- a/src/agents/pi-embedded-runner/compact.hooks.harness.ts
+++ b/src/agents/pi-embedded-runner/compact.hooks.harness.ts
@@ -313,6 +313,7 @@ export async function loadCompactHooksHarness(): Promise<{
   vi.doMock("../model-auth.js", () => ({
     applyAuthHeaderOverride: vi.fn((model: unknown) => model),
     applyLocalNoAuthHeaderOverride: vi.fn((model: unknown) => model),
+    ensureAuthProfileStoreWithoutExternalProfiles: vi.fn(() => ({})),
     getApiKeyForModel: vi.fn(async () => ({ apiKey: "test", mode: "env" })),
     resolveModelAuthMode: vi.fn(() => "env"),
   }));
diff --git a/src/agents/pi-embedded-runner/run.overflow-compaction.harness.ts b/src/agents/pi-embedded-runner/run.overflow-compaction.harness.ts
index 33bb6a9030b..363cc57f991 100644
--- a/src/agents/pi-embedded-runner/run.overflow-compaction.harness.ts
+++ b/src/agents/pi-embedded-runner/run.overflow-compaction.harness.ts
@@ -218,6 +218,8 @@ export const mockedGetApiKeyForModel = vi.fn(
     mode: "api-key" as const,
   }),
 );
+export const mockedEnsureAuthProfileStore = vi.fn(() => ({}));
+export const mockedEnsureAuthProfileStoreWithoutExternalProfiles = vi.fn(() => ({}));
 export const mockedResolveAuthProfileOrder = vi.fn(() => [] as string[]);
 export const mockedShouldPreferExplicitConfigApiKeyAuth = vi.fn(() => false);
 
@@ -386,6 +388,10 @@ export function resetRunOverflowCompactionHarnessMocks(): void {
       mode: "api-key",
     }),
   );
+  mockedEnsureAuthProfileStore.mockReset();
+  mockedEnsureAuthProfileStore.mockReturnValue({});
+  mockedEnsureAuthProfileStoreWithoutExternalProfiles.mockReset();
+  mockedEnsureAuthProfileStoreWithoutExternalProfiles.mockReturnValue({});
   mockedResolveAuthProfileOrder.mockReset();
   mockedResolveAuthProfileOrder.mockReturnValue([]);
   mockedShouldPreferExplicitConfigApiKeyAuth.mockReset();
@@ -500,7 +506,9 @@ export async function loadRunOverflowCompactionHarness(): Promise<{
   vi.doMock("../model-auth.js", () => ({
     applyAuthHeaderOverride: vi.fn((model: unknown) => model),
     applyLocalNoAuthHeaderOverride: vi.fn((model: unknown) => model),
-    ensureAuthProfileStore: vi.fn(() => ({})),
+    ensureAuthProfileStore: mockedEnsureAuthProfileStore,
+    ensureAuthProfileStoreWithoutExternalProfiles:
+      mockedEnsureAuthProfileStoreWithoutExternalProfiles,
     getApiKeyForModel: mockedGetApiKeyForModel,
     resolveAuthProfileOrder: mockedResolveAuthProfileOrder,
     shouldPreferExplicitConfigApiKeyAuth: mockedShouldPreferExplicitConfigApiKeyAuth,
diff --git a/src/agents/pi-embedded-runner/run.overflow-compaction.test.ts b/src/agents/pi-embedded-runner/run.overflow-compaction.test.ts
index f35363941bb..a667f811d78 100644
--- a/src/agents/pi-embedded-runner/run.overflow-compaction.test.ts
+++ b/src/agents/pi-embedded-runner/run.overflow-compaction.test.ts
@@ -18,6 +18,8 @@ import {
   mockedContextEngine,
   mockedDescribeFailoverError,
   mockedEvaluateContextWindowGuard,
+  mockedEnsureAuthProfileStore,
+  mockedEnsureAuthProfileStoreWithoutExternalProfiles,
   mockedGlobalHookRunner,
   mockedGetApiKeyForModel,
   mockedPickFallbackThinkingLevel,
@@ -193,6 +195,21 @@ describe("runEmbeddedPiAgent overflow compaction trigger routing", () => {
     );
   });
 
+  it("uses the lightweight auth profile store during reply startup", async () => {
+    mockedRunEmbeddedAttempt.mockResolvedValueOnce(makeAttemptResult({ promptError: null }));
+
+    await runEmbeddedPiAgent({
+      ...overflowBaseRunParams,
+      runId: "run-lightweight-auth-store",
+    });
+
+    expect(mockedEnsureAuthProfileStore).not.toHaveBeenCalled();
+    expect(mockedEnsureAuthProfileStoreWithoutExternalProfiles).toHaveBeenCalledWith(
+      "/tmp/agent-dir",
+      { allowKeychainPrompt: false },
+    );
+  });
+
   it("forwards optional attempt params and the runtime plan into one attempt call", async () => {
     const internalEvents: AgentInternalEvent[] = [];
     const forwardingCase = makeForwardingCase(internalEvents);
diff --git a/src/agents/pi-embedded-runner/run.ts b/src/agents/pi-embedded-runner/run.ts
index b1c8a279c72..cffa69daebc 100644
--- a/src/agents/pi-embedded-runner/run.ts
+++ b/src/agents/pi-embedded-runner/run.ts
@@ -33,7 +33,6 @@ import {
   markAuthProfileGood,
   markAuthProfileUsed,
 } from "../auth-profiles.js";
-import { externalCliDiscoveryForProviderAuth } from "../auth-profiles/external-cli-discovery.js";
 import {
   resolveSessionKeyForRequest,
   resolveStoredSessionKeyForSessionId,
@@ -52,7 +51,7 @@ import { shouldSwitchToLiveModel, clearLiveModelSwitchPending } from "../live-mo
 import {
   applyAuthHeaderOverride,
   applyLocalNoAuthHeaderOverride,
-  ensureAuthProfileStore,
+  ensureAuthProfileStoreWithoutExternalProfiles,
   type ResolvedProviderAuth,
   resolveAuthProfileOrder,
   shouldPreferExplicitConfigApiKeyAuth,
@@ -514,12 +513,8 @@ export async function runEmbeddedPiAgent(
 
       const authStore = pluginHarnessOwnsTransport
         ? createEmptyAuthProfileStore()
-        : ensureAuthProfileStore(agentDir, {
-            externalCli: externalCliDiscoveryForProviderAuth({
-              cfg: params.config,
-              provider,
-              preferredProfile: params.authProfileId,
-            }),
+        : ensureAuthProfileStoreWithoutExternalProfiles(agentDir, {
+            allowKeychainPrompt: false,
           });
       const requestedProfileId = params.authProfileId?.trim();
       const resolvePluginHarnessPreferredProfileId = (): string | undefined => {
@@ -541,7 +536,7 @@ export async function runEmbeddedPiAgent(
         if (!harnessAuthProvider) {
           return undefined;
         }
-        const harnessAuthStore = ensureAuthProfileStore(agentDir, {
+        const harnessAuthStore = ensureAuthProfileStoreWithoutExternalProfiles(agentDir, {
           allowKeychainPrompt: false,
         });
         return resolveAuthProfileOrder({