diff --git a/src/agents/openai-transport-stream.test.ts b/src/agents/openai-transport-stream.test.ts
index 7479371d81f..47980905be6 100644
--- a/src/agents/openai-transport-stream.test.ts
+++ b/src/agents/openai-transport-stream.test.ts
@@ -2932,6 +2932,53 @@ describe("openai transport stream", () => {
     expect(params.tool_choice).toBe("required");
   });
 
+  it("sorts Responses tools by name for stable prompt-cache payloads", () => {
+    const model = {
+      id: "gpt-5.4",
+      name: "GPT-5.4",
+      api: "openai-responses",
+      provider: "openai",
+      baseUrl: "https://api.openai.com/v1",
+      reasoning: true,
+      input: ["text"],
+      cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+      contextWindow: 200000,
+      maxTokens: 8192,
+    } satisfies Model<"openai-responses">;
+    const zetaTool = {
+      name: "zeta",
+      description: "Z",
+      parameters: { type: "object", properties: {}, additionalProperties: false },
+    };
+    const alphaTool = {
+      name: "alpha",
+      description: "A",
+      parameters: { type: "object", properties: {}, additionalProperties: false },
+    };
+
+    const first = buildOpenAIResponsesParams(
+      model,
+      {
+        systemPrompt: "system",
+        messages: [],
+        tools: [zetaTool, alphaTool],
+      } as never,
+      { sessionId: "session-123" } as never,
+    ) as { tools?: Array<{ name?: string }> };
+    const second = buildOpenAIResponsesParams(
+      model,
+      {
+        systemPrompt: "system",
+        messages: [],
+        tools: [alphaTool, zetaTool],
+      } as never,
+      { sessionId: "session-123" } as never,
+    ) as { tools?: Array<{ name?: string }> };
+
+    expect(first.tools?.map((tool) => tool.name)).toEqual(["alpha", "zeta"]);
+    expect(first.tools).toEqual(second.tools);
+  });
+
   it("falls back to strict:false when a native OpenAI tool schema is not strict-compatible", () => {
     const params = buildOpenAIResponsesParams(
       {
@@ -3809,6 +3856,54 @@ describe("openai transport stream", () => {
     expect(notOptedIn.prompt_cache_key).toBeUndefined();
   });
 
+  it("sorts Chat Completions tools by function name for stable prompt-cache payloads", () => {
+    const model = {
+      id: "custom-model",
+      name: "Custom Model",
+      api: "openai-completions",
+      provider: "custom-cpa",
+      baseUrl: "https://proxy.example.com/v1",
+      compat: { supportsPromptCacheKey: true },
+      reasoning: false,
+      input: ["text"],
+      cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+      contextWindow: 32768,
+      maxTokens: 8192,
+    } as unknown as Model<"openai-completions">;
+    const zetaTool = {
+      name: "zeta",
+      description: "Z",
+      parameters: { type: "object", properties: {} },
+    };
+    const alphaTool = {
+      name: "alpha",
+      description: "A",
+      parameters: { type: "object", properties: {} },
+    };
+
+    const first = buildOpenAICompletionsParams(
+      model,
+      {
+        systemPrompt: "system",
+        messages: [],
+        tools: [zetaTool, alphaTool],
+      } as never,
+      { sessionId: "session-123" },
+    ) as { tools?: Array<{ function?: { name?: string } }> };
+    const second = buildOpenAICompletionsParams(
+      model,
+      {
+        systemPrompt: "system",
+        messages: [],
+        tools: [alphaTool, zetaTool],
+      } as never,
+      { sessionId: "session-123" },
+    ) as { tools?: Array<{ function?: { name?: string } }> };
+
+    expect(first.tools?.map((tool) => tool.function?.name)).toEqual(["alpha", "zeta"]);
+    expect(first.tools).toEqual(second.tools);
+  });
+
   it("disables developer-role-only compat defaults for configured custom proxy completions providers", () => {
     const params = buildOpenAICompletionsParams(
       {
diff --git a/src/agents/openai-transport-stream.ts b/src/agents/openai-transport-stream.ts
index 1bcb0e1cb53..7fa2baff74f 100644
--- a/src/agents/openai-transport-stream.ts
+++ b/src/agents/openai-transport-stream.ts
@@ -950,7 +950,7 @@ function convertResponsesTools(
     transport: "responses",
     model,
   });
-  return tools.map((tool): FunctionTool => {
+  return sortTransportToolsByName(tools).map((tool): FunctionTool => {
     const base = {
       type: "function" as const,
       name: tool.name,
@@ -2686,7 +2686,7 @@ function convertTools(
       model,
     },
   );
-  return tools.map((tool) => ({
+  return sortTransportToolsByName(tools).map((tool) => ({
     type: "function",
     function: {
       name: tool.name,
@@ -2701,6 +2701,28 @@ function convertTools(
   }));
 }
 
+function compareTransportToolText(left: string | undefined, right: string | undefined): number {
+  const leftText = left ?? "";
+  const rightText = right ?? "";
+  if (leftText < rightText) {
+    return -1;
+  }
+  if (leftText > rightText) {
+    return 1;
+  }
+  return 0;
+}
+
+function sortTransportToolsByName<T extends { name?: string; description?: string }>(
+  tools: readonly T[],
+): T[] {
+  return tools.toSorted(
+    (left, right) =>
+      compareTransportToolText(left.name, right.name) ||
+      compareTransportToolText(left.description, right.description),
+  );
+}
+
 function extractGoogleThoughtSignature(toolCall: unknown): string | undefined {
   const tc = toolCall as Record<string, unknown> | undefined;
   if (!tc) {