diff --git a/src/agents/pi-embedded-runner/compact.hooks.harness.ts b/src/agents/pi-embedded-runner/compact.hooks.harness.ts
index 8218e9cd363..663f9e90d65 100644
--- a/src/agents/pi-embedded-runner/compact.hooks.harness.ts
+++ b/src/agents/pi-embedded-runner/compact.hooks.harness.ts
@@ -1,6 +1,7 @@
 import { vi, type Mock } from "vitest";
 import type { PluginMetadataSnapshot } from "../../plugins/plugin-metadata-snapshot.js";
 import { clearAgentHarnesses } from "../harness/registry.js";
+import type { AgentRuntimePlan, BuildAgentRuntimePlanParams } from "../runtime-plan/types.js";
 import type { CompactionTranscriptRotation } from "./compaction-successor-transcript.js";
 
 type MockResolvedModel = {
@@ -110,6 +111,75 @@ export const rotateTranscriptAfterCompactionMock: Mock<
   rotated: false,
 }));
 
+function createCompactHooksRuntimePlan(params: BuildAgentRuntimePlanParams): AgentRuntimePlan {
+  const modelApi = params.modelApi ?? params.model?.api ?? undefined;
+  const transcriptPolicy = {
+    sanitizeMode: "full" as const,
+    sanitizeToolCallIds: false,
+    preserveNativeAnthropicToolUseIds: false,
+    repairToolUseResultPairing: false,
+    preserveSignatures: false,
+    sanitizeThinkingSignatures: false,
+    dropThinkingBlocks: false,
+    applyGoogleTurnOrdering: false,
+    validateGeminiTurns: false,
+    validateAnthropicTurns: false,
+    allowSyntheticToolResults: false,
+  };
+
+  return {
+    resolvedRef: {
+      provider: params.provider,
+      modelId: params.modelId,
+      ...(modelApi ? { modelApi } : {}),
+      ...(params.resolvedTransport ? { transport: params.resolvedTransport } : {}),
+    },
+    auth: {
+      providerForAuth: params.provider,
+      authProfileProviderForAuth: params.authProfileProvider ?? params.provider,
+      ...(params.sessionAuthProfileId
+        ? { forwardedAuthProfileId: params.sessionAuthProfileId }
+        : {}),
+    },
+    prompt: {
+      provider: params.provider,
+      modelId: params.modelId,
+      resolveSystemPromptContribution: vi.fn(() => undefined),
+      transformSystemPrompt: vi.fn((context: { systemPrompt: string }) => context.systemPrompt),
+    },
+    tools: {
+      preparedPlanning: {
+        loadMetadataSnapshot: () => ({}),
+      },
+      normalize: vi.fn((tools) => tools),
+      logDiagnostics: vi.fn(),
+    },
+    transcript: {
+      policy: transcriptPolicy,
+      resolvePolicy: vi.fn(() => transcriptPolicy),
+    },
+    delivery: {
+      isSilentPayload: vi.fn(() => false),
+      resolveFollowupRoute: vi.fn(() => undefined),
+    },
+    outcome: {
+      classifyRunResult: vi.fn(() => null),
+    },
+    transport: {
+      extraParams: {},
+      resolveExtraParams: vi.fn(() => ({})),
+    },
+    observability: {
+      resolvedRef: `${params.provider}/${params.modelId}`,
+      provider: params.provider,
+      modelId: params.modelId,
+      ...(modelApi ? { modelApi } : {}),
+      ...(params.sessionAuthProfileId ? { authProfileId: params.sessionAuthProfileId } : {}),
+      ...(params.resolvedTransport ? { transport: params.resolvedTransport } : {}),
+    },
+  };
+}
+
 const emptyPluginMetadataSnapshot: PluginMetadataSnapshot = {
   policyHash: "",
   index: {
@@ -569,6 +639,12 @@ export async function loadCompactHooksHarness(): Promise<{
     resolveMemorySearchConfig: resolveMemorySearchConfigMock,
   }));
 
+  vi.doMock("../runtime-plan/build.js", () => ({
+    buildAgentRuntimePlan: vi.fn((params: BuildAgentRuntimePlanParams) =>
+      createCompactHooksRuntimePlan(params),
+    ),
+  }));
+
   vi.doMock("../../plugins/memory-runtime.js", () => ({
     getActiveMemorySearchManager: getMemorySearchManagerMock,
   }));