diff --git a/src/server/responses/compact.ts b/src/server/responses/compact.ts
index b14800fc7b..696cf42944 100644
--- a/src/server/responses/compact.ts
+++ b/src/server/responses/compact.ts
@@ -178,8 +178,22 @@ function pruneCompactHandoffRoutes(now: number): void {
   }
 }
 
-function rememberCompactHandoffRoute(req: Request, model: string, now = Date.now()): void {
-  const key = sessionLaneIdFromRequest(req.headers);
+function compactHandoffRouteKey(req: Request, admission?: DataPlaneAdmission): string | null {
+  const lane = sessionLaneIdFromRequest(req.headers);
+  // A loopback request has no authenticated principal to bind this cross-request
+  // state to. Fail closed rather than treating a caller-controlled lane as identity.
+  if (!lane || !admission || admission.kind === "loopback") return null;
+  const principal = admission.kind === "configured" ? `configured:${admission.keyId}` : "environment";
+  return `${principal}\u0000${lane}`;
+}
+
+function rememberCompactHandoffRoute(
+  req: Request,
+  admission: DataPlaneAdmission | undefined,
+  model: string,
+  now = Date.now(),
+): void {
+  const key = compactHandoffRouteKey(req, admission);
   if (!key || model.length > COMPACT_HANDOFF_MODEL_MAX_LENGTH) return;
   pruneCompactHandoffRoutes(now);
   compactHandoffRoutes.delete(key);
@@ -187,13 +201,18 @@ function rememberCompactHandoffRoute(req: Request, model: string, now = Date.now
   pruneCompactHandoffRoutes(now);
 }
 
-function forgetCompactHandoffRoute(req: Request): void {
-  const key = sessionLaneIdFromRequest(req.headers);
+function forgetCompactHandoffRoute(req: Request, admission?: DataPlaneAdmission): void {
+  const key = compactHandoffRouteKey(req, admission);
   if (key) compactHandoffRoutes.delete(key);
 }
 
-function compactHandoffRoute(req: Request, previousModel: string, now = Date.now()): string | null {
-  const key = sessionLaneIdFromRequest(req.headers);
+function compactHandoffRoute(
+  req: Request,
+  admission: DataPlaneAdmission | undefined,
+  previousModel: string,
+  now = Date.now(),
+): string | null {
+  const key = compactHandoffRouteKey(req, admission);
   if (!key) return null;
   pruneCompactHandoffRoutes(now);
   const entry = compactHandoffRoutes.get(key);
@@ -953,9 +972,9 @@ export async function handleResponsesCompact(
     // synthetic buffer errors are not upstream bodies and stay uninspected.
     if (buffered.ok) {
       inspectResponseLogJson(logCtx, await buffered.clone().text());
-      forgetCompactHandoffRoute(req);
+      forgetCompactHandoffRoute(req, admission);
     } else if (quotaFailure && !storedPool401ReplayAttempted) {
-      const fallbackModel = compactHandoffRoute(req, raw.model);
+      const fallbackModel = compactHandoffRoute(req, admission, raw.model);
       if (fallbackModel && !req.signal.aborted) {
         const fallbackReq = new Request(req.url, {
           method: "POST",
@@ -1075,7 +1094,7 @@ export async function handleResponsesCompact(
     const result = new Response(JSON.stringify({ output: compactionItems }), {
       headers: { "Content-Type": "application/json" },
     });
-    rememberCompactHandoffRoute(req, raw.model);
+    rememberCompactHandoffRoute(req, admission, raw.model);
     return result;
   }
   const encrypted = compactionItems[0]!.encrypted_content;
@@ -1086,6 +1105,6 @@ export async function handleResponsesCompact(
   }
   const summary = decoded;
   const output = buildCompactV1Output(extractCompactUserMessages(inputItems), summary);
-  rememberCompactHandoffRoute(req, raw.model);
+  rememberCompactHandoffRoute(req, admission, raw.model);
   return new Response(JSON.stringify({ output }), { headers: { "Content-Type": "application/json" } });
 }
diff --git a/tests/responses-compaction-routing.test.ts b/tests/responses-compaction-routing.test.ts
index d97154c661..97f00aa7b7 100644
--- a/tests/responses-compaction-routing.test.ts
+++ b/tests/responses-compaction-routing.test.ts
@@ -1027,6 +1027,7 @@ describe("compact alternate-account attempt (#913)", () => {
         models: ["gpt-5.6-sol"],
       };
       const headers = { "x-codex-parent-thread-id": "compact-routed-handoff-thread" };
+      const admission = { kind: "configured", keyId: "compact-client", source: "dedicated" } as const;
       const calls: Array<{ model: string; nativeCompact: boolean }> = [];
       globalThis.fetch = (async (input: string | URL | Request, init?: RequestInit) => {
         const url = typeof input === "string"
@@ -1053,6 +1054,8 @@ describe("compact alternate-account attempt (#913)", () => {
         ),
         config,
         { model: "", provider: "" },
+        undefined,
+        admission,
       );
       expect(manual.status).toBe(200);
       expect(calls).toEqual([{ model: "deepseek-v4-flash", nativeCompact: false }]);
@@ -1066,6 +1069,8 @@ describe("compact alternate-account attempt (#913)", () => {
         ),
         config,
         { model: "", provider: "" },
+        undefined,
+        admission,
       );
       expect(unrelated.status).toBe(502);
       expect(calls.length).toBeGreaterThan(0);
@@ -1081,6 +1086,8 @@ describe("compact alternate-account attempt (#913)", () => {
         ),
         config,
         logCtx,
+        undefined,
+        admission,
       );
 
       expect(automatic.status).toBe(200);
@@ -1092,6 +1099,36 @@ describe("compact alternate-account attempt (#913)", () => {
       expect(calls.slice(0, -1).every(call => (
         call.model === "gpt-5.6-sol" && call.nativeCompact
       ))).toBe(true);
+
+      calls.length = 0;
+      const otherPrincipal = await handleResponsesCompact(
+        compactionRequest(
+          baseCompactionBody({ model: "openai-apikey/gpt-5.6-sol" }),
+          undefined,
+          headers,
+        ),
+        config,
+        { model: "", provider: "" },
+        undefined,
+        { kind: "configured", keyId: "different-client", source: "dedicated" },
+      );
+      expect(otherPrincipal.status).toBe(502);
+      expect(calls.every(call => call.model === "gpt-5.6-sol" && call.nativeCompact)).toBe(true);
+
+      calls.length = 0;
+      const unauthenticatedLoopback = await handleResponsesCompact(
+        compactionRequest(
+          baseCompactionBody({ model: "openai-apikey/gpt-5.6-sol" }),
+          undefined,
+          headers,
+        ),
+        config,
+        { model: "", provider: "" },
+        undefined,
+        { kind: "loopback", source: "loopback" },
+      );
+      expect(unauthenticatedLoopback.status).toBe(502);
+      expect(calls.every(call => call.model === "gpt-5.6-sol" && call.nativeCompact)).toBe(true);
     });
   });