diff --git a/src/app/api/openrouter/[...path]/route.ts b/src/app/api/openrouter/[...path]/route.ts
index 75d693b1d9..94faf0b9e2 100644
--- a/src/app/api/openrouter/[...path]/route.ts
+++ b/src/app/api/openrouter/[...path]/route.ts
@@ -173,9 +173,6 @@ export async function POST(request: NextRequest): Promise<NextResponseType<unkno
       requestBodyParsed = { kind: 'chat_completions', body };
     } else if (path === '/messages') {
       const body: GatewayMessagesRequest = JSON.parse(requestBodyText);
-      if (!body.cache_control && body.messages.length > 1) {
-        body.cache_control = { type: 'ephemeral' };
-      }
       requestBodyParsed = { kind: 'messages', body };
     } else {
       const body: GatewayResponsesRequest = JSON.parse(requestBodyText);
diff --git a/src/lib/providers/openrouter/request-helpers.ts b/src/lib/providers/openrouter/request-helpers.ts
index 7fcdd00bf1..7d10a9f331 100644
--- a/src/lib/providers/openrouter/request-helpers.ts
+++ b/src/lib/providers/openrouter/request-helpers.ts
@@ -80,11 +80,29 @@ function setCacheControlOnResponsesMessage(message: OpenAI.Responses.ResponseInp
   }
 }
 
+function isObjectRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === 'object' && value !== null;
+}
+
+function containsCacheControl(value: unknown): boolean {
+  if (Array.isArray(value)) {
+    return value.some(containsCacheControl);
+  }
+  if (!isObjectRecord(value)) {
+    return false;
+  }
+  if (Object.hasOwn(value, 'cache_control')) {
+    return true;
+  }
+  return Object.values(value).some(containsCacheControl);
+}
+
 export function addCacheBreakpoints(request: GatewayRequest) {
   if (
     request.kind === 'chat_completions' &&
     Array.isArray(request.body.messages) &&
-    request.body.messages.length > 1
+    request.body.messages.length > 1 &&
+    !containsCacheControl(request.body.messages)
   ) {
     const lastMessage = request.body.messages.findLast(
       msg => msg.role === 'user' || msg.role === 'tool'
@@ -98,7 +116,8 @@ export function addCacheBreakpoints(request: GatewayRequest) {
   } else if (
     request.kind === 'responses' &&
     Array.isArray(request.body.input) &&
-    request.body.input.length > 1
+    request.body.input.length > 1 &&
+    !containsCacheControl(request.body.input)
   ) {
     const lastMessage = request.body.input.findLast(
       msg => (msg.type === 'message' && msg.role === 'user') || msg.type === 'function_call_output'
@@ -109,6 +128,14 @@ export function addCacheBreakpoints(request: GatewayRequest) {
       );
       setCacheControlOnResponsesMessage(lastMessage);
     }
+  } else if (
+    request.kind === 'messages' &&
+    request.body.messages.length > 1 &&
+    !request.body.cache_control &&
+    !containsCacheControl(request.body.messages)
+  ) {
+    console.debug('[addCacheBreakpoints] setting cache breakpoint on messages request');
+    request.body.cache_control = { type: 'ephemeral' };
   }
 }
 
diff --git a/src/tests/openrouter-request-helpers.test.ts b/src/tests/openrouter-request-helpers.test.ts
new file mode 100644
index 0000000000..87bace7fa5
--- /dev/null
+++ b/src/tests/openrouter-request-helpers.test.ts
@@ -0,0 +1,166 @@
+import { describe, expect, test } from '@jest/globals';
+import { addCacheBreakpoints } from '@/lib/providers/openrouter/request-helpers';
+import type { GatewayRequest } from '@/lib/providers/openrouter/types';
+import type OpenAI from 'openai';
+
+describe('addCacheBreakpoints', () => {
+  test('adds a cache breakpoint to the last eligible chat completions message when none exist', () => {
+    const request: GatewayRequest = {
+      kind: 'chat_completions',
+      body: {
+        model: 'test-model',
+        messages: [
+          { role: 'system', content: 'You are helpful.' },
+          { role: 'user', content: 'First prompt' },
+          { role: 'assistant', content: 'First response' },
+          {
+            role: 'user',
+            content: [
+              { type: 'text', text: 'Latest prompt' },
+              { type: 'text', text: 'Latest detail' },
+            ],
+          },
+        ],
+      },
+    };
+
+    addCacheBreakpoints(request);
+
+    const lastContent = request.body.messages.at(-1)?.content;
+    expect(Array.isArray(lastContent)).toBe(true);
+    if (!Array.isArray(lastContent)) return;
+    expect(lastContent.at(-1)).toMatchObject({
+      type: 'text',
+      text: 'Latest detail',
+      cache_control: { type: 'ephemeral' },
+    });
+  });
+
+  test('does nothing for chat completions requests when any cache_control is already present', () => {
+    const request: GatewayRequest = {
+      kind: 'chat_completions',
+      body: {
+        model: 'test-model',
+        messages: [
+          { role: 'system', content: 'You are helpful.' },
+          {
+            role: 'user',
+            content: [
+              {
+                type: 'text',
+                text: 'First prompt',
+                cache_control: { type: 'ephemeral' },
+              } as OpenAI.ChatCompletionContentPartText,
+            ],
+          },
+          { role: 'assistant', content: 'First response' },
+          {
+            role: 'user',
+            content: [
+              { type: 'text', text: 'Latest prompt' },
+              { type: 'text', text: 'Latest detail' },
+            ],
+          },
+        ],
+      },
+    };
+
+    addCacheBreakpoints(request);
+
+    const lastContent =
+      request.kind === 'chat_completions' && request.body.messages.at(-1)?.content;
+    expect(lastContent).toEqual([
+      { type: 'text', text: 'Latest prompt' },
+      { type: 'text', text: 'Latest detail' },
+    ]);
+  });
+
+  test('does nothing for responses requests when any cache_control is already present', () => {
+    const request: GatewayRequest = {
+      kind: 'responses',
+      body: {
+        model: 'test-model',
+        input: [
+          {
+            type: 'message',
+            role: 'user',
+            content: [
+              {
+                type: 'input_text',
+                text: 'First prompt',
+                // @ts-expect-error non-standard cache_control extension
+                cache_control: { type: 'ephemeral' },
+              },
+            ],
+          },
+          {
+            type: 'function_call_output',
+            call_id: 'call_123',
+            output: [
+              { type: 'input_text', text: 'Tool output' },
+              { type: 'input_text', text: 'Tool detail' },
+            ],
+          },
+        ],
+      },
+    };
+
+    addCacheBreakpoints(request);
+
+    const lastItem = request.kind === 'responses' && request.body.input?.at(-1);
+    expect(lastItem).toMatchObject({
+      type: 'function_call_output',
+      output: [
+        { type: 'input_text', text: 'Tool output' },
+        { type: 'input_text', text: 'Tool detail' },
+      ],
+    });
+  });
+
+  test('adds top-level cache_control on messages request when none is present', () => {
+    const request: GatewayRequest = {
+      kind: 'messages',
+      body: {
+        model: 'anthropic/claude-sonnet-4-5',
+        max_tokens: 1024,
+        messages: [
+          { role: 'user', content: 'First prompt' },
+          { role: 'assistant', content: 'First response' },
+          { role: 'user', content: 'Latest prompt' },
+        ],
+      },
+    };
+
+    addCacheBreakpoints(request);
+
+    expect(request.body.cache_control).toEqual({ type: 'ephemeral' });
+  });
+
+  test('does nothing for messages request when any cache_control is already present', () => {
+    const request: GatewayRequest = {
+      kind: 'messages',
+      body: {
+        model: 'anthropic/claude-sonnet-4-5',
+        max_tokens: 1024,
+        messages: [
+          {
+            role: 'user',
+            content: [
+              {
+                type: 'text',
+                text: 'First prompt',
+                cache_control: { type: 'ephemeral' },
+              },
+            ],
+          },
+          { role: 'assistant', content: 'First response' },
+          { role: 'user', content: 'Latest prompt' },
+        ],
+      },
+    };
+
+    addCacheBreakpoints(request);
+
+    expect(request.body.cache_control).toBeUndefined();
+  });
+});