diff --git a/src/agents/cache-trace.ts b/src/agents/cache-trace.ts
index 0cc770dabe7..1edfd086f7a 100644
--- a/src/agents/cache-trace.ts
+++ b/src/agents/cache-trace.ts
@@ -129,12 +129,18 @@ function stableStringify(value: unknown): string {
     });
   }
   if (Array.isArray(value)) {
-    return `[${value.map((entry) => stableStringify(entry)).join(",")}]`;
+    const serializedEntries: string[] = [];
+    for (const entry of value) {
+      serializedEntries.push(stableStringify(entry));
+    }
+    return `[${serializedEntries.join(",")}]`;
   }
   const record = value as Record<string, unknown>;
-  const keys = Object.keys(record).toSorted();
-  const entries = keys.map((key) => `${JSON.stringify(key)}:${stableStringify(record[key])}`);
-  return `{${entries.join(",")}}`;
+  const serializedFields: string[] = [];
+  for (const key of Object.keys(record).toSorted()) {
+    serializedFields.push(`${JSON.stringify(key)}:${stableStringify(record[key])}`);
+  }
+  return `{${serializedFields.join(",")}}`;
 }
 
 function digest(value: unknown): string {
diff --git a/src/agents/openclaw-tools.subagents.sessions-spawn.lifecycle.e2e.test.ts b/src/agents/openclaw-tools.subagents.sessions-spawn.lifecycle.e2e.test.ts
index f45653d7bb8..c9ac6fee023 100644
--- a/src/agents/openclaw-tools.subagents.sessions-spawn.lifecycle.e2e.test.ts
+++ b/src/agents/openclaw-tools.subagents.sessions-spawn.lifecycle.e2e.test.ts
@@ -135,6 +135,21 @@ const waitFor = async (predicate: () => boolean, timeoutMs = 2000) => {
   }
 };
 
+function expectSingleCompletionSend(
+  calls: GatewayRequest[],
+  expected: { sessionKey: string; channel: string; to: string; message: string },
+) {
+  const sendCalls = calls.filter((call) => call.method === "send");
+  expect(sendCalls).toHaveLength(1);
+  const send = sendCalls[0]?.params as
+    | { sessionKey?: string; channel?: string; to?: string; message?: string }
+    | undefined;
+  expect(send?.sessionKey).toBe(expected.sessionKey);
+  expect(send?.channel).toBe(expected.channel);
+  expect(send?.to).toBe(expected.to);
+  expect(send?.message).toBe(expected.message);
+}
+
 describe("openclaw-tools: subagents (sessions_spawn lifecycle)", () => {
   beforeEach(() => {
     resetSessionsSpawnConfigOverride();
@@ -204,15 +219,12 @@ describe("openclaw-tools: subagents (sessions_spawn lifecycle)", () => {
     expect(first?.lane).toBe("subagent");
 
     // Direct send should route completion to the requester channel/session.
-    const sendCalls = ctx.calls.filter((c) => c.method === "send");
-    expect(sendCalls).toHaveLength(1);
-    const send = sendCalls[0]?.params as
-      | { sessionKey?: string; channel?: string; to?: string; message?: string }
-      | undefined;
-    expect(send?.sessionKey).toBe("agent:main:main");
-    expect(send?.channel).toBe("whatsapp");
-    expect(send?.to).toBe("+123");
-    expect(send?.message).toBe("✅ Subagent main finished\n\ndone");
+    expectSingleCompletionSend(ctx.calls, {
+      sessionKey: "agent:main:main",
+      channel: "whatsapp",
+      to: "+123",
+      message: "✅ Subagent main finished\n\ndone",
+    });
     expect(child.sessionKey?.startsWith("agent:main:subagent:")).toBe(true);
   });
 
@@ -289,15 +301,12 @@ describe("openclaw-tools: subagents (sessions_spawn lifecycle)", () => {
     expect(first?.sessionKey?.startsWith("agent:main:subagent:")).toBe(true);
     expect(child.sessionKey?.startsWith("agent:main:subagent:")).toBe(true);
 
-    const sendCalls = ctx.calls.filter((c) => c.method === "send");
-    expect(sendCalls).toHaveLength(1);
-    const send = sendCalls[0]?.params as
-      | { sessionKey?: string; channel?: string; to?: string; message?: string }
-      | undefined;
-    expect(send?.sessionKey).toBe("agent:main:discord:group:req");
-    expect(send?.channel).toBe("discord");
-    expect(send?.to).toBe("discord:dm:u123");
-    expect(send?.message).toBe("✅ Subagent main finished");
+    expectSingleCompletionSend(ctx.calls, {
+      sessionKey: "agent:main:discord:group:req",
+      channel: "discord",
+      to: "discord:dm:u123",
+      message: "✅ Subagent main finished",
+    });
 
     expect(deletedKey?.startsWith("agent:main:subagent:")).toBe(true);
   });
@@ -356,15 +365,12 @@ describe("openclaw-tools: subagents (sessions_spawn lifecycle)", () => {
     const first = agentCalls[0]?.params as { lane?: string } | undefined;
     expect(first?.lane).toBe("subagent");
 
-    const sendCalls = ctx.calls.filter((c) => c.method === "send");
-    expect(sendCalls).toHaveLength(1);
-    const send = sendCalls[0]?.params as
-      | { sessionKey?: string; channel?: string; to?: string; message?: string }
-      | undefined;
-    expect(send?.sessionKey).toBe("agent:main:discord:group:req");
-    expect(send?.channel).toBe("discord");
-    expect(send?.to).toBe("discord:dm:u123");
-    expect(send?.message).toBe("✅ Subagent main finished\n\ndone");
+    expectSingleCompletionSend(ctx.calls, {
+      sessionKey: "agent:main:discord:group:req",
+      channel: "discord",
+      to: "discord:dm:u123",
+      message: "✅ Subagent main finished\n\ndone",
+    });
 
     // Session should be deleted
     expect(deletedKey?.startsWith("agent:main:subagent:")).toBe(true);
diff --git a/src/agents/pi-embedded-helpers/errors.ts b/src/agents/pi-embedded-helpers/errors.ts
index 7c08ccef94c..253074ddd35 100644
--- a/src/agents/pi-embedded-helpers/errors.ts
+++ b/src/agents/pi-embedded-helpers/errors.ts
@@ -1,6 +1,7 @@
 import type { AssistantMessage } from "@mariozechner/pi-ai";
 import type { OpenClawConfig } from "../../config/config.js";
 import { formatSandboxToolPolicyBlockedMessage } from "../sandbox.js";
+import { stableStringify } from "../stable-stringify.js";
 import type { FailoverReason } from "./types.js";
 
 export function formatBillingErrorMessage(provider?: string): string {
@@ -309,19 +310,6 @@ function parseApiErrorPayload(raw: string): ErrorPayload | null {
   return null;
 }
 
-function stableStringify(value: unknown): string {
-  if (!value || typeof value !== "object") {
-    return JSON.stringify(value) ?? "null";
-  }
-  if (Array.isArray(value)) {
-    return `[${value.map((entry) => stableStringify(entry)).join(",")}]`;
-  }
-  const record = value as Record<string, unknown>;
-  const keys = Object.keys(record).toSorted();
-  const entries = keys.map((key) => `${JSON.stringify(key)}:${stableStringify(record[key])}`);
-  return `{${entries.join(",")}}`;
-}
-
 export function getApiErrorPayloadFingerprint(raw?: string): string | null {
   if (!raw) {
     return null;
diff --git a/src/agents/stable-stringify.ts b/src/agents/stable-stringify.ts
new file mode 100644
index 00000000000..d86b937b7f8
--- /dev/null
+++ b/src/agents/stable-stringify.ts
@@ -0,0 +1,12 @@
+export function stableStringify(value: unknown): string {
+  if (value === null || typeof value !== "object") {
+    return JSON.stringify(value) ?? "null";
+  }
+  if (Array.isArray(value)) {
+    return `[${value.map((entry) => stableStringify(entry)).join(",")}]`;
+  }
+  const record = value as Record<string, unknown>;
+  const keys = Object.keys(record).toSorted();
+  const entries = keys.map((key) => `${JSON.stringify(key)}:${stableStringify(record[key])}`);
+  return `{${entries.join(",")}}`;
+}