diff --git a/packages/paperclip-runner/src/protocol/replay-contract.test.ts b/packages/paperclip-runner/src/protocol/replay-contract.test.ts
index 2b98ffe4acb..f805104a92f 100644
--- a/packages/paperclip-runner/src/protocol/replay-contract.test.ts
+++ b/packages/paperclip-runner/src/protocol/replay-contract.test.ts
@@ -386,6 +386,19 @@ it("accepts marked whole-run usage through the runtime standalone event validato
   expect(result).toMatchObject({ ok: true });
 });
 
+it("rejects an event without normalizedSessionId, which the schemas leave optional", async () => {
+  const fixture = await readFixture();
+  const event = (fixture.events as Array<Record<string, unknown>>)[0]!;
+  expect(validatePrpEvent(event)).toMatchObject({ ok: true });
+
+  const { normalizedSessionId: _omitted, ...withoutSession } = event;
+  expect(validatePrpEvent(withoutSession)).toEqual({
+    ok: false,
+    event: null,
+    issues: [{ code: "schema_validation", path: "/normalizedSessionId", message: "event normalizedSessionId is required" }],
+  });
+});
+
 it("declares required incomplete reconciliation evidence in the strict usage contract", () => {
   const validate = new Ajv2020({ strict: true, allErrors: true }).compile(usageSchema);
   const measurement = { inputTokens: 120, outputTokens: 12, cacheReadTokens: 0, cacheWriteTokens: 0, activeSeconds: 0, requests: 0, providerCostUsd: 0 };
diff --git a/packages/paperclip-runner/src/protocol/replay-contract.ts b/packages/paperclip-runner/src/protocol/replay-contract.ts
index 18ee623889e..9b3a2f6d500 100644
--- a/packages/paperclip-runner/src/protocol/replay-contract.ts
+++ b/packages/paperclip-runner/src/protocol/replay-contract.ts
@@ -504,6 +504,23 @@ export function validatePrpEvent(value: unknown): EventValidationResult {
       issues: (eventValidator.errors ?? []).map(ajvIssue),
     };
   }
+  // The published schemas leave normalizedSessionId optional, but every
+  // producer sets it (the TypeScript PrpEvent type and the Rust event structs
+  // both require it), and consumers compare it to the expected session. The
+  // schema already limits its type and length when it is present.
+  if (record?.normalizedSessionId === undefined) {
+    return {
+      ok: false,
+      event: null,
+      issues: [
+        {
+          code: "schema_validation",
+          path: "/normalizedSessionId",
+          message: "event normalizedSessionId is required",
+        },
+      ],
+    };
+  }
   return { ok: true, event: value, issues: [] };
 }