diff --git a/.changeset/honest-pots-drum.md b/.changeset/honest-pots-drum.md
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+---
+---
diff --git a/e2e/issues/issue-287-tool-calls-missing-arguments.test.ts b/e2e/issues/issue-287-tool-calls-missing-arguments.test.ts
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+/**
+ * Regression test for GitHub issue #287
+ * https://github.com/OpenRouterTeam/ai-sdk-provider/issues/287
+ *
+ * Reported error: AI_TypeValidationError when tool calls have missing arguments field
+ *
+ * Some upstream providers may omit the `arguments` field in tool calls when there
+ * are no arguments to pass. This caused validation errors because the schema
+ * required `arguments` to be a string.
+ *
+ * This test verifies that tool calls work correctly even when the tool has no
+ * parameters (and the upstream provider might omit the arguments field).
+ */
+import { generateText, tool } from 'ai';
+import { describe, expect, it, vi } from 'vitest';
+import { z } from 'zod/v4';
+import { createOpenRouter } from '@/src';
+
+vi.setConfig({
+  testTimeout: 60_000,
+});
+
+describe('Issue #287: Tool calls with missing arguments field', () => {
+  const openrouter = createOpenRouter({
+    apiKey: process.env.OPENROUTER_API_KEY,
+    baseUrl: `${process.env.OPENROUTER_API_BASE}/api/v1`,
+  });
+
+  // Use Anthropic Haiku model - mentioned in original issue context as potentially
+  // omitting the arguments field for tools with no parameters
+  const model = openrouter('anthropic/claude-3.5-haiku');
+
+  it('should handle tool with no parameters', async () => {
+    // Tool with no parameters - some providers may omit the arguments field entirely
+    const getCurrentTime = tool({
+      description: 'Gets the current time',
+      inputSchema: z.object({}),
+      execute: async () => {
+        return { time: new Date().toISOString() };
+      },
+    });
+
+    const response = await generateText({
+      model,
+      system:
+        'You are a helpful assistant. Always use the getCurrentTime tool when asked about time.',
+      prompt: 'What time is it right now? Use the getCurrentTime tool.',
+      tools: { getCurrentTime },
+      toolChoice: 'required',
+    });
+
+    // Should complete without AI_TypeValidationError
+    expect(response.text).toBeDefined();
+    expect(response.finishReason).toBeDefined();
+
+    // Verify tool was called
+    const toolCalls = response.steps.flatMap((step) => step.toolCalls || []);
+    expect(toolCalls.length).toBeGreaterThan(0);
+    expect(toolCalls[0]?.toolName).toBe('getCurrentTime');
+  });
+
+  it('should handle tool with optional parameters where none are provided', async () => {
+    // Tool with optional parameters - model might not provide any arguments
+    const greet = tool({
+      description: 'Greets the user',
+      inputSchema: z.object({
+        name: z.string().optional().describe('Optional name to greet'),
+      }),
+      execute: async ({ name }) => {
+        return { greeting: name ? `Hello, ${name}!` : 'Hello!' };
+      },
+    });
+
+    const response = await generateText({
+      model,
+      system:
+        'You are a helpful assistant. Use the greet tool when asked to say hello.',
+      prompt: 'Just say hello using the greet tool. No name needed.',
+      tools: { greet },
+      toolChoice: 'required',
+    });
+
+    // Should complete without AI_TypeValidationError
+    expect(response.text).toBeDefined();
+    expect(response.finishReason).toBeDefined();
+  });
+});
diff --git a/e2e/issues/issue-341-cache-control-last-text-part.test.ts b/e2e/issues/issue-341-cache-control-last-text-part.test.ts
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+++ b/e2e/issues/issue-341-cache-control-last-text-part.test.ts
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+/**
+ * Regression test for GitHub issue #341
+ * https://github.com/OpenRouterTeam/ai-sdk-provider/issues/341
+ *
+ * Reported error: Cache control bug exceeding Anthropic's 4-segment limit
+ *
+ * When message-level cache_control was set, it was being applied to ALL parts
+ * in a multi-part message, which could exceed provider cache segment limits.
+ *
+ * The fix ensures message-level cache_control only applies to the last text part,
+ * while part-specific cache_control still takes precedence for all part types.
+ *
+ * This test verifies that cache control works correctly with multi-part messages
+ * on Anthropic models.
+ */
+import { generateText } from 'ai';
+import { describe, expect, it, vi } from 'vitest';
+import { createOpenRouter } from '@/src';
+
+vi.setConfig({
+  testTimeout: 120_000,
+});
+
+describe('Issue #341: Cache control only applies to last text part', () => {
+  const openrouter = createOpenRouter({
+    apiKey: process.env.OPENROUTER_API_KEY,
+    baseUrl: `${process.env.OPENROUTER_API_BASE}/api/v1`,
+  });
+
+  // Use Anthropic model which supports cache control
+  const model = openrouter('anthropic/claude-3.5-haiku');
+
+  // Long system prompt to meet caching threshold
+  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 automatic caching. ' +
+      'Automatic 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 work with message-level cache control on multi-part user message', async () => {
+    // Multi-part message with message-level cache control
+    // The fix ensures only the last text part gets cache_control
+    const response = await generateText({
+      model,
+      messages: [
+        {
+          role: 'system',
+          content: longSystemPrompt,
+        },
+        {
+          role: 'user',
+          content: [
+            { type: 'text', text: 'First part of my question.' },
+            { type: 'text', text: 'Second part: What is 2+2?' },
+          ],
+          providerOptions: {
+            anthropic: {
+              cacheControl: { type: 'ephemeral' },
+            },
+          },
+        },
+      ],
+    });
+
+    // Should complete without errors about exceeding cache segment limits
+    expect(response.text).toBeDefined();
+    expect(response.text.length).toBeGreaterThan(0);
+    expect(response.finishReason).toBeDefined();
+  });
+
+  it('should work with explicit cache control on system message', async () => {
+    // Test cache control on system message
+    const response = await generateText({
+      model,
+      messages: [
+        {
+          role: 'system',
+          content: longSystemPrompt,
+          providerOptions: {
+            anthropic: {
+              cacheControl: { type: 'ephemeral' },
+            },
+          },
+        },
+        {
+          role: 'user',
+          content: 'What is the capital of France? Answer briefly.',
+        },
+      ],
+    });
+
+    expect(response.text).toBeDefined();
+    expect(response.text.length).toBeGreaterThan(0);
+    expect(response.finishReason).toBeDefined();
+  });
+
+  it('should handle multiple requests with caching enabled', async () => {
+    // Make multiple requests to verify caching works across requests
+    const responses = [];
+
+    for (let i = 0; i < 2; i++) {
+      const response = await generateText({
+        model,
+        messages: [
+          {
+            role: 'system',
+            content: longSystemPrompt,
+            providerOptions: {
+              anthropic: {
+                cacheControl: { type: 'ephemeral' },
+              },
+            },
+          },
+          {
+            role: 'user',
+            content: `Request ${i + 1}: What is ${i + 1} + ${i + 1}? Answer with just the number.`,
+          },
+        ],
+      });
+
+      expect(response.text).toBeDefined();
+      expect(response.finishReason).toBeDefined();
+      responses.push(response);
+    }
+
+    // Both requests should complete successfully
+    expect(responses.length).toBe(2);
+    responses.forEach((r) => {
+      expect(r.text.length).toBeGreaterThan(0);
+    });
+  });
+});