diff --git a/packages/pi/src/convert.ts b/packages/pi/src/convert.ts
index d6c2bfec..6b361cc3 100644
--- a/packages/pi/src/convert.ts
+++ b/packages/pi/src/convert.ts
@@ -75,6 +75,9 @@ export type AnthropicRequestBody = {
 function sanitize(text: string): string {
   return text.replace(/[\uD800-\uDFFF]/gu, '\uFFFD')
 }
+function isNonEmptyText(value: unknown): value is string {
+  return typeof value === 'string' && value.trim().length > 0
+}
 
 /**
  * Detect lone (unpaired) UTF-16 surrogates. With the `u` flag the character
@@ -494,7 +497,18 @@ export async function buildAnthropicRequest(
     },
     { type: 'text', text: CLAUDE_CODE_IDENTITY },
   ]
-  if (context.systemPrompt?.trim()) {
+  // Pi's host type permits prompt segments even though pi-ai declares a string.
+  const systemPrompt = context.systemPrompt as unknown as
+    | string
+    | readonly string[]
+    | undefined
+  const normalizedSystemPrompt = Array.isArray(systemPrompt)
+    ? systemPrompt
+        .filter(isNonEmptyText)
+        .map((text) => text.trim())
+        .join('\n\n')
+    : systemPrompt
+  if (isNonEmptyText(normalizedSystemPrompt)) {
     // Pi's prompt cannot sit whole in the top-level system[] array: two lines of
     // its documentation paragraph (the docs/*.md enumeration and the "follow .md
     // cross-references" instruction) are each independently sufficient to make
@@ -519,7 +533,7 @@ export async function buildAnthropicRequest(
     // cache_control is set explicitly because addEphemeralCacheControl's
     // message-level breakpoint only fires for array content on the *last* user
     // message, which is not this one after the first turn.
-    const prompt = splitPiSystemPrompt(context.systemPrompt)
+    const prompt = splitPiSystemPrompt(normalizedSystemPrompt)
     if (prompt.systemText) {
       system.push({ type: 'text', text: prompt.systemText })
     }
diff --git a/packages/pi/src/tests/convert.test.ts b/packages/pi/src/tests/convert.test.ts
index 984b8331..3d76b64a 100644
--- a/packages/pi/src/tests/convert.test.ts
+++ b/packages/pi/src/tests/convert.test.ts
@@ -343,6 +343,124 @@ describe('convertMessages — empty base64 image guard', () => {
     expect(content[1]?.type).toBe('image')
   })
 })
+describe('buildAnthropicRequest — system prompt arrays', () => {
+  test('normalizes non-empty prompt strings before relocation', async () => {
+    const { body } = await buildAnthropicRequest(
+      'claude-sonnet-4-20250514',
+      {
+        messages: [userMsg('hello')],
+        systemPrompt: [' first prompt ', 'second prompt'],
+        tools: [],
+      } as unknown as Parameters<typeof buildAnthropicRequest>[1],
+      undefined,
+      defaultCache,
+    )
+
+    expect(body.system).toHaveLength(2)
+    expect(body.messages[0]?.content).toEqual([
+      {
+        type: 'text',
+        text: 'first prompt\n\nsecond prompt',
+        cache_control: { type: 'ephemeral' },
+      },
+      {
+        type: 'text',
+        text: 'hello',
+        cache_control: { type: 'ephemeral' },
+      },
+    ])
+  })
+
+  test('skips empty and whitespace-only prompt strings', async () => {
+    const { body } = await buildAnthropicRequest(
+      'claude-sonnet-4-20250514',
+      {
+        messages: [userMsg('hello')],
+        systemPrompt: ['', '  ', 'valid'],
+        tools: [],
+      } as unknown as Parameters<typeof buildAnthropicRequest>[1],
+      undefined,
+      defaultCache,
+    )
+
+    expect(body.system).toHaveLength(2)
+    expect(body.messages[0]?.content).toEqual([
+      {
+        type: 'text',
+        text: 'valid',
+        cache_control: { type: 'ephemeral' },
+      },
+      {
+        type: 'text',
+        text: 'hello',
+        cache_control: { type: 'ephemeral' },
+      },
+    ])
+  })
+
+  test('does not add prompt blocks for an empty prompt array', async () => {
+    const { body } = await buildAnthropicRequest(
+      'claude-sonnet-4-20250514',
+      {
+        messages: [userMsg('hello')],
+        systemPrompt: [],
+        tools: [],
+      } as unknown as Parameters<typeof buildAnthropicRequest>[1],
+      undefined,
+      defaultCache,
+    )
+
+    expect(body.system).toHaveLength(2)
+  })
+
+  test('routes array prompts through documentation relocation', async () => {
+    const segments = [
+      'KEEP ONE: you are an assistant.',
+      'KEEP TWO: available tools.',
+      'Pi documentation (read only when the user asks about pi itself):\n- MOVE THIS',
+    ]
+    const stringBody = (
+      await buildAnthropicRequest(
+        'claude-sonnet-4-20250514',
+        {
+          messages: [userMsg('hello')],
+          systemPrompt: segments.join('\n\n'),
+          tools: [],
+        } as any,
+        undefined,
+        defaultCache,
+      )
+    ).body
+    const arrayBody = (
+      await buildAnthropicRequest(
+        'claude-sonnet-4-20250514',
+        {
+          messages: [userMsg('hello')],
+          systemPrompt: segments,
+          tools: [],
+        } as unknown as Parameters<typeof buildAnthropicRequest>[1],
+        undefined,
+        defaultCache,
+      )
+    ).body
+    const arraySystemText = JSON.stringify(arrayBody.system)
+    const firstUserContent = arrayBody.messages[0]?.content as Array<
+      Record<string, unknown>
+    >
+
+    expect(arraySystemText).not.toContain('MOVE THIS')
+    expect(firstUserContent[0]).toMatchObject({
+      type: 'text',
+      text: segments[2],
+      cache_control: { type: 'ephemeral' },
+    })
+    expect(arrayBody.system?.slice(2).map((block) => block.text)).toEqual([
+      'KEEP ONE: you are an assistant.\n\nKEEP TWO: available tools.',
+    ])
+    expect(arrayBody.system).toEqual(stringBody.system)
+    expect(arrayBody.messages).toEqual(stringBody.messages)
+  })
+})
 
 describe('buildAnthropicRequest — Claude Code system[] shape', () => {
   // Anthropic rejects Pi's documentation paragraph inside the top-level