diff --git a/README.md b/README.md
index 2077d16..099ca1a 100644
--- a/README.md
+++ b/README.md
@@ -316,7 +316,7 @@ OpenAI Chat Completions. Required fields: `model`, `messages`. Optional: `stream
 OpenAI Responses API. Required fields: `model`, `input`. Optional: `instructions`, `stream`, `max_output_tokens`.
 
 ### POST /v1/messages
-Anthropic Messages API. Required fields: `model`, `messages`. Optional: `system`, `max_tokens`, `stream`.
+Anthropic Messages API. Required fields: `model`, `messages`. Optional: `system` (string or array of `{type: "text", text: string}` content blocks), `max_tokens`, `stream`.
 
 Errors are returned in Anthropic format: `{ "type": "error", "error": { "type": "...", "message": "..." } }`.
 
diff --git a/index.js b/index.js
index e3532e9..9701fef 100644
--- a/index.js
+++ b/index.js
@@ -592,6 +592,22 @@ export function normalizeAnthropicMessages(messages) {
     .filter((message) => message.content.length > 0)
 }
 
+export function normalizeAnthropicSystem(system) {
+  if (typeof system === "string") {
+    const trimmed = system.trim()
+    return trimmed || null
+  }
+  if (Array.isArray(system)) {
+    const text = system
+      .filter((block) => block && block.type === "text" && typeof block.text === "string")
+      .map((block) => block.text.trim())
+      .filter(Boolean)
+      .join("\n\n")
+    return text || null
+  }
+  return null
+}
+
 export function mapFinishReasonToAnthropic(finish) {
   if (!finish) return "end_turn"
   if (finish.includes("length")) return "max_tokens"
@@ -1040,11 +1056,13 @@ export function createProxyFetchHandler(client) {
         return anthropicBadRequest("No text content was found in the supplied messages.", 400, request)
       }
 
-      // Prepend Anthropic top-level system string as a system message so buildSystemPrompt picks it up.
-      const allMessages =
-        typeof body.system === "string" && body.system.trim()
-          ? [{ role: "system", content: body.system.trim() }, ...messages]
-          : messages
+      // Prepend Anthropic top-level `system` (string or array-of-content-blocks,
+      // per the Messages API spec) as a system message so buildSystemPrompt
+      // picks it up.
+      const systemText = normalizeAnthropicSystem(body.system)
+      const allMessages = systemText
+        ? [{ role: "system", content: systemText }, ...messages]
+        : messages
 
       const system = buildSystemPrompt(allMessages, {
         temperature: body.temperature,
diff --git a/index.test.js b/index.test.js
index f663ea0..f07c01d 100644
--- a/index.test.js
+++ b/index.test.js
@@ -14,6 +14,7 @@ import {
   resolveModel,
   normalizeAnthropicMessages,
   mapFinishReasonToAnthropic,
+  normalizeAnthropicSystem,
   normalizeGeminiContents,
   extractGeminiSystemInstruction,
   mapFinishReasonToGemini,
@@ -1388,6 +1389,114 @@ test("POST /v1/messages system string is included in prompt", async () => {
   assert.ok(capturedSystem?.includes("You are a pirate."))
 })
 
+test("POST /v1/messages system as content-block array is included in prompt", async () => {
+  let capturedSystem = null
+  const client = {
+    app: { log: async () => {} },
+    tool: { ids: async () => ({ data: [] }) },
+    config: {
+      providers: async () => ({
+        data: {
+          providers: [{ id: "anthropic", models: { "claude-3-5-sonnet": { id: "claude-3-5-sonnet" } } }],
+        },
+      }),
+    },
+    session: {
+      create: async () => ({ data: { id: "sess-ant-sys-arr" } }),
+      prompt: async ({ body }) => {
+        capturedSystem = body.system
+        return {
+          data: {
+            parts: [{ type: "text", text: "ok" }],
+            info: { tokens: { input: 1, output: 1, reasoning: 0, cache: { read: 0, write: 0 } }, finish: "end_turn" },
+          },
+        }
+      },
+    },
+  }
+
+  const handler = createProxyFetchHandler(client)
+  const request = new Request("http://127.0.0.1:4010/v1/messages", {
+    method: "POST",
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify({
+      model: "anthropic/claude-3-5-sonnet",
+      system: [{ type: "text", text: "You are a pirate." }],
+      messages: [{ role: "user", content: "Hello." }],
+    }),
+  })
+
+  await handler(request)
+  assert.ok(capturedSystem?.includes("You are a pirate."))
+})
+
+test("POST /v1/messages system as multi-block array concatenates text", async () => {
+  let capturedSystem = null
+  const client = {
+    app: { log: async () => {} },
+    tool: { ids: async () => ({ data: [] }) },
+    config: {
+      providers: async () => ({
+        data: {
+          providers: [{ id: "anthropic", models: { "claude-3-5-sonnet": { id: "claude-3-5-sonnet" } } }],
+        },
+      }),
+    },
+    session: {
+      create: async () => ({ data: { id: "sess-ant-sys-multi" } }),
+      prompt: async ({ body }) => {
+        capturedSystem = body.system
+        return {
+          data: {
+            parts: [{ type: "text", text: "ok" }],
+            info: { tokens: { input: 1, output: 1, reasoning: 0, cache: { read: 0, write: 0 } }, finish: "end_turn" },
+          },
+        }
+      },
+    },
+  }
+
+  const handler = createProxyFetchHandler(client)
+  const request = new Request("http://127.0.0.1:4010/v1/messages", {
+    method: "POST",
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify({
+      model: "anthropic/claude-3-5-sonnet",
+      system: [
+        { type: "text", text: "Line one." },
+        { type: "text", text: "Line two." },
+      ],
+      messages: [{ role: "user", content: "Hello." }],
+    }),
+  })
+
+  await handler(request)
+  assert.ok(capturedSystem?.includes("Line one."))
+  assert.ok(capturedSystem?.includes("Line two."))
+})
+
+test("normalizeAnthropicSystem handles string, array, and edge cases", () => {
+  assert.equal(normalizeAnthropicSystem("hello"), "hello")
+  assert.equal(normalizeAnthropicSystem("  hi  "), "hi")
+  assert.equal(normalizeAnthropicSystem(""), null)
+  assert.equal(normalizeAnthropicSystem("   "), null)
+  assert.equal(normalizeAnthropicSystem([{ type: "text", text: "a" }]), "a")
+  assert.equal(
+    normalizeAnthropicSystem([
+      { type: "text", text: "a" },
+      { type: "text", text: "b" },
+    ]),
+    "a\n\nb",
+  )
+  assert.equal(normalizeAnthropicSystem([{ type: "image", source: {} }]), null)
+  assert.equal(normalizeAnthropicSystem([]), null)
+  assert.equal(normalizeAnthropicSystem([{ type: "text", text: "" }]), null)
+  assert.equal(normalizeAnthropicSystem(undefined), null)
+  assert.equal(normalizeAnthropicSystem(null), null)
+  assert.equal(normalizeAnthropicSystem(42), null)
+  assert.equal(normalizeAnthropicSystem([null, { type: "text", text: "x" }]), "x")
+})
+
 test("POST /v1/messages missing model returns Anthropic error format", async () => {
   const handler = createProxyFetchHandler(createAnthropicClient())
   const request = new Request("http://127.0.0.1:4010/v1/messages", {