diff --git a/tests/unit/claude-cache-control-passthrough.test.mjs b/tests/unit/claude-cache-control-passthrough.test.mjs
new file mode 100644
index 0000000000..3b217175ac
--- /dev/null
+++ b/tests/unit/claude-cache-control-passthrough.test.mjs
@@ -0,0 +1,184 @@
+import { describe, test } from "node:test";
+import assert from "node:assert/strict";
+import { prepareClaudeRequest } from "../../open-sse/translator/helpers/claudeHelper.ts";
+
+describe("Claude cache_control passthrough", () => {
+  test("preserveCacheControl=true preserves cache_control in system blocks", () => {
+    const body = {
+      system: [
+        { type: "text", text: "System prompt 1" },
+        { type: "text", text: "System prompt 2", cache_control: { type: "ephemeral", ttl: "5m" } },
+      ],
+      messages: [],
+    };
+
+    const result = prepareClaudeRequest(body, "claude", true);
+
+    assert.equal(result.system.length, 2);
+    assert.equal(result.system[0].cache_control, undefined);
+    assert.deepEqual(result.system[1].cache_control, { type: "ephemeral", ttl: "5m" });
+  });
+
+  test("preserveCacheControl=false replaces cache_control in system blocks", () => {
+    const body = {
+      system: [
+        { type: "text", text: "System prompt 1" },
+        { type: "text", text: "System prompt 2", cache_control: { type: "ephemeral", ttl: "5m" } },
+      ],
+      messages: [],
+    };
+
+    const result = prepareClaudeRequest(body, "claude", false);
+
+    assert.equal(result.system.length, 2);
+    assert.equal(result.system[0].cache_control, undefined);
+    assert.deepEqual(result.system[1].cache_control, { type: "ephemeral", ttl: "1h" });
+  });
+
+  test("preserveCacheControl=true preserves cache_control in message content blocks", () => {
+    const body = {
+      messages: [
+        {
+          role: "user",
+          content: [
+            { type: "text", text: "User message 1" },
+            { type: "text", text: "User message 2", cache_control: { type: "ephemeral" } },
+          ],
+        },
+        {
+          role: "assistant",
+          content: [
+            {
+              type: "text",
+              text: "Assistant response",
+              cache_control: { type: "ephemeral", ttl: "10m" },
+            },
+          ],
+        },
+      ],
+    };
+
+    const result = prepareClaudeRequest(body, "claude", true);
+
+    assert.equal(result.messages.length, 2);
+    assert.equal(result.messages[0].content[0].cache_control, undefined);
+    assert.deepEqual(result.messages[0].content[1].cache_control, { type: "ephemeral" });
+    assert.deepEqual(result.messages[1].content[0].cache_control, {
+      type: "ephemeral",
+      ttl: "10m",
+    });
+  });
+
+  test("preserveCacheControl=false strips and re-adds cache_control in messages", () => {
+    const body = {
+      messages: [
+        {
+          role: "user",
+          content: [
+            { type: "text", text: "User message 1" },
+            { type: "text", text: "User message 2", cache_control: { type: "ephemeral" } },
+          ],
+        },
+        {
+          role: "assistant",
+          content: [
+            {
+              type: "text",
+              text: "Assistant response",
+              cache_control: { type: "ephemeral", ttl: "10m" },
+            },
+          ],
+        },
+      ],
+    };
+
+    const result = prepareClaudeRequest(body, "claude", false);
+
+    // Original cache_control should be stripped and OmniRoute's strategy applied
+    assert.equal(result.messages.length, 2);
+    // User message should not have cache_control (only second-to-last user gets it)
+    assert.equal(result.messages[0].content[0].cache_control, undefined);
+    assert.equal(result.messages[0].content[1].cache_control, undefined);
+    // Last assistant should have cache_control added by OmniRoute
+    assert.deepEqual(result.messages[1].content[0].cache_control, { type: "ephemeral" });
+  });
+
+  test("preserveCacheControl=true preserves cache_control in tools", () => {
+    const body = {
+      messages: [],
+      tools: [
+        { name: "tool1", description: "Tool 1", input_schema: { type: "object" } },
+        {
+          name: "tool2",
+          description: "Tool 2",
+          input_schema: { type: "object" },
+          cache_control: { type: "ephemeral", ttl: "30m" },
+        },
+      ],
+    };
+
+    const result = prepareClaudeRequest(body, "claude", true);
+
+    assert.equal(result.tools.length, 2);
+    assert.equal(result.tools[0].cache_control, undefined);
+    assert.deepEqual(result.tools[1].cache_control, { type: "ephemeral", ttl: "30m" });
+  });
+
+  test("preserveCacheControl=false replaces cache_control in tools", () => {
+    const body = {
+      messages: [],
+      tools: [
+        { name: "tool1", description: "Tool 1", input_schema: { type: "object" } },
+        {
+          name: "tool2",
+          description: "Tool 2",
+          input_schema: { type: "object" },
+          cache_control: { type: "ephemeral", ttl: "30m" },
+        },
+      ],
+    };
+
+    const result = prepareClaudeRequest(body, "claude", false);
+
+    assert.equal(result.tools.length, 2);
+    assert.equal(result.tools[0].cache_control, undefined);
+    assert.deepEqual(result.tools[1].cache_control, { type: "ephemeral", ttl: "1h" });
+  });
+
+  test("preserveCacheControl=true with Claude Code-style caching", () => {
+    const body = {
+      system: [{ type: "text", text: "System", cache_control: { type: "ephemeral", ttl: "5m" } }],
+      messages: [
+        {
+          role: "user",
+          content: [{ type: "text", text: "Turn 1", cache_control: { type: "ephemeral" } }],
+        },
+        {
+          role: "assistant",
+          content: [{ type: "text", text: "Response 1" }],
+        },
+        {
+          role: "user",
+          content: [{ type: "text", text: "Turn 2" }],
+        },
+      ],
+      tools: [
+        {
+          name: "bash",
+          description: "Execute bash",
+          input_schema: { type: "object" },
+          cache_control: { type: "ephemeral", ttl: "5m" },
+        },
+      ],
+    };
+
+    const result = prepareClaudeRequest(body, "claude", true);
+
+    // All original cache_control should be preserved
+    assert.deepEqual(result.system[0].cache_control, { type: "ephemeral", ttl: "5m" });
+    assert.deepEqual(result.messages[0].content[0].cache_control, { type: "ephemeral" });
+    assert.equal(result.messages[1].content[0].cache_control, undefined);
+    assert.equal(result.messages[2].content[0].cache_control, undefined);
+    assert.deepEqual(result.tools[0].cache_control, { type: "ephemeral", ttl: "5m" });
+  });
+});