diff --git a/src/server/retainedSummaryDelivery.ts b/src/server/retainedSummaryDelivery.ts
index 0bc703a7c..2ff7cee3d 100644
--- a/src/server/retainedSummaryDelivery.ts
+++ b/src/server/retainedSummaryDelivery.ts
@@ -1,3 +1,5 @@
+import { analysisDetailSchema } from "../contracts/analysisDetail.js";
+
 import type { EvidenceWriter } from "../application/investigation/InvestigationRecordPort.js";
 import type { EvidenceLookup } from "../application/EvidenceInputResolver.js";
 import {
@@ -57,6 +59,7 @@ export const withoutDeliveryDetail =
   (operations: ReadonlySet<string>) =>
   (operation: string, parameters: Parameters): Parameters => {
     if (!operations.has(operation)) return parameters;
+    analysisDetailSchema.unwrap().optional().parse(parameters.detail);
     const { detail: _detail, ...selected } = parameters;
     return selected;
   };
diff --git a/tests/boundary/mcp/interfaceBuilderSummaryMcp.test.ts b/tests/boundary/mcp/interfaceBuilderSummaryMcp.test.ts
index b62fcdf53..a33059c38 100644
--- a/tests/boundary/mcp/interfaceBuilderSummaryMcp.test.ts
+++ b/tests/boundary/mcp/interfaceBuilderSummaryMcp.test.ts
@@ -67,6 +67,14 @@ mcpTest(
     const app = await fixtureApp();
     const client = await openProductionTarget(mcp, onTestFinished, app);
 
+    const invalidDetail = await client.callTool({
+      name: "decode_interface_builder",
+      arguments: { detail: "brief" },
+    });
+    expect(parseMcpToolError(invalidDetail)).toMatchObject({
+      error: { code: "invalid_request" },
+    });
+
     // Summary first: its own retained parent must serve every later page.
     const summarized = await client.callTool({
       name: "decode_interface_builder",