fix: always use array content for system messages with block-level cache control

.changeset/fix-system-cache-control.md

@@ -0,0 +1,19 @@
+---
+"@openrouter/ai-sdk-provider": patch
+---
+
+Fix system message cache control to use block-level format
+
+When cache control is specified on a system message via `providerOptions`, the content is now converted to array format with `cache_control` on the text block, matching the existing behavior for user messages. This ensures consistent Anthropic prompt caching behavior across all message types.
+
+Before (message-level cache_control):
+```json
+{ "role": "system", "content": "...", "cache_control": { "type": "ephemeral" } }
+```
+
+After (block-level cache_control):
+```json
+{ "role": "system", "content": [{ "type": "text", "text": "...", "cache_control": { "type": "ephemeral" } }] }
+```
+
+Fixes #389

e2e/issues/issue-389-system-cache-control.test.ts

@@ -0,0 +1,107 @@
+/**
+ * Regression test for GitHub issue #389
+ * https://github.com/OpenRouterTeam/ai-sdk-provider/issues/389
+ *
+ * Issue: "Anthropic prompt caching not applied when `system` is a string
+ * in AI SDK (`ModelMessage[]`); only block content works"
+ *
+ * The user reported that prompt caching does not work when a system message
+ * is provided as a plain string with cache_control at the message level via
+ * providerOptions. Caching only worked when content was an array of text
+ * blocks with cache_control on each block.
+ *
+ * The fix converts system message content to array format with block-level
+ * cache_control when cache control is present, matching the existing behavior
+ * for user messages.
+ */
+import { generateText } from 'ai';
+import { describe, expect, it, vi } from 'vitest';
+import { createOpenRouter } from '@/src';
+
+vi.setConfig({
+  testTimeout: 120_000,
+});
+
+describe('Issue #389: System message cache control with string content', () => {
+  const openrouter = createOpenRouter({
+    apiKey: process.env.OPENROUTER_API_KEY,
+    baseUrl: `${process.env.OPENROUTER_API_BASE}/api/v1`,
+  });
+
+  const model = openrouter('anthropic/claude-sonnet-4');
+
+  const longSystemPrompt = `You are a helpful assistant. Here is some context that should be cached:
+
+${Array(50)
+  .fill(
+    'This is padding text to ensure the prompt meets the minimum token threshold for Anthropic prompt caching. ' +
+      'Prompt caching requires a minimum number of tokens in the prompt prefix. ' +
+      'This text is repeated multiple times to reach that threshold. ',
+  )
+  .join('\n')}
+
+Remember to be helpful and concise in your responses.`;
+
+  it('should trigger cache write on first request with system message cache control', async () => {
+    const response = await generateText({
+      model,
+      messages: [
+        {
+          role: 'system',
+          content: longSystemPrompt,
+          providerOptions: {
+            anthropic: {
+              cacheControl: { type: 'ephemeral' },
+            },
+          },
+        },
+        {
+          role: 'user',
+          content: 'What is 2+2? Answer with just the number.',
+        },
+      ],
+    });
+
+    expect(response.text).toBeDefined();
+    expect(response.text.length).toBeGreaterThan(0);
+    expect(response.finishReason).toBeDefined();
+  });
+
+  it('should trigger cache read on second request with system message cache control', async () => {
+    const makeRequest = () =>
+      generateText({
+        model,
+        messages: [
+          {
+            role: 'system',
+            content: longSystemPrompt,
+            providerOptions: {
+              anthropic: {
+                cacheControl: { type: 'ephemeral' },
+              },
+            },
+          },
+          {
+            role: 'user',
+            content: 'What is 2+2? Answer with just the number.',
+          },
+        ],
+      });
+
+    await makeRequest();
+
+    const response = await makeRequest();
+
+    const openrouterMetadata = response.providerMetadata?.openrouter as {
+      usage?: {
+        promptTokensDetails?: { cachedTokens?: number };
+      };
+    };
+
+    const cachedTokens =
+      openrouterMetadata?.usage?.promptTokensDetails?.cachedTokens;
+
+    expect(cachedTokens).toBeDefined();
+    expect(cachedTokens).toBeGreaterThan(0);
+  });
+});

src/chat/convert-to-openrouter-chat-messages.test.ts

@@ -232,7 +232,7 @@ describe('user messages', () => {
 });
 
 describe('cache control', () => {
-  it('should pass cache control from system message provider metadata', () => {
+  it('should convert system message to array content with block-level cache control', () => {
     const result = convertToOpenRouterChatMessages([
       {
         role: 'system',
@@ -246,10 +246,58 @@ describe('cache control', () => {
     ]);
 
     expect(result).toEqual([
+      {
+        role: 'system',
+        content: [
+          {
+            type: 'text',
+            text: 'System prompt',
+            cache_control: { type: 'ephemeral' },
+          },
+        ],
+      },
+    ]);
+  });
+
+  it('should keep system message as string when no cache control is present', () => {
+    const result = convertToOpenRouterChatMessages([
       {
         role: 'system',
         content: 'System prompt',
-        cache_control: { type: 'ephemeral' },
+      },
+    ]);
+
+    expect(result).toEqual([
+      {
+        role: 'system',
+        content: 'System prompt',
+      },
+    ]);
+  });
+
+  it('should convert system message to array content with openrouter namespace cache control', () => {
+    const result = convertToOpenRouterChatMessages([
+      {
+        role: 'system',
+        content: 'System prompt',
+        providerOptions: {
+          openrouter: {
+            cacheControl: { type: 'ephemeral' },
+          },
+        },
+      },
+    ]);
+
+    expect(result).toEqual([
+      {
+        role: 'system',
+        content: [
+          {
+            type: 'text',
+            text: 'System prompt',
+            cache_control: { type: 'ephemeral' },
+          },
+        ],
       },
     ]);
   });
@@ -677,8 +725,13 @@ describe('cache control', () => {
     expect(result).toEqual([
       {
         role: 'system',
-        content: 'System prompt',
-        cache_control: { type: 'ephemeral' },
+        content: [
+          {
+            type: 'text',
+            text: 'System prompt',
+            cache_control: { type: 'ephemeral' },
+          },
+        ],
       },
     ]);
   });

src/chat/convert-to-openrouter-chat-messages.ts

@@ -46,10 +46,18 @@ export function convertToOpenRouterChatMessages(
   for (const { role, content, providerOptions } of prompt) {
     switch (role) {
       case 'system': {
+        const cacheControl = getCacheControl(providerOptions);
         messages.push({
           role: 'system',
-          content,
-          cache_control: getCacheControl(providerOptions),
+          content: cacheControl
+            ? [
+                {
+                  type: 'text' as const,
+                  text: content,
+                  cache_control: cacheControl,
+                },
+              ]
+            : content,
         });
         break;
       }

src/types/openrouter-chat-completions-input.ts

@@ -14,7 +14,7 @@ export type ChatCompletionMessageParam =
 
 export interface ChatCompletionSystemMessageParam {
   role: 'system';
-  content: string;
+  content: string | Array<ChatCompletionContentPartText>;
   cache_control?: OpenRouterCacheControl;
 }