diff --git a/packages/pi/README.md b/packages/pi/README.md
index 81c7e3e9..3dd5ef4c 100644
--- a/packages/pi/README.md
+++ b/packages/pi/README.md
@@ -2,7 +2,7 @@
 
 Pi package for CortexKit Anthropic OAuth support. It overrides Pi's built-in `anthropic` provider with a CortexKit provider extension backed by the shared `@cortexkit/anthropic-auth-core` package.
 
-The Pi provider catalog includes Claude Fable 5 (`claude-fable-5`), limited-access Claude Mythos 5 (`claude-mythos-5`), Claude Opus 4.8, Claude Opus 4.5, Claude Sonnet 4.5, and Claude Sonnet 5 (`claude-sonnet-5`). Fable/Mythos reasoning uses Anthropic adaptive thinking with `thinking.display: "summarized"` and `output_config.effort`; the package does not send rejected manual `thinking.budget_tokens` for those models.
+The Pi provider catalog includes Claude Fable 5 (`claude-fable-5`), limited-access Claude Mythos 5 (`claude-mythos-5`), Claude Opus 5 (`claude-opus-5`), Claude Opus 4.8, Claude Opus 4.5, Claude Sonnet 4.5, and Claude Sonnet 5 (`claude-sonnet-5`). Fable/Mythos reasoning uses Anthropic adaptive thinking with `thinking.display: "summarized"` and `output_config.effort`; the package does not send rejected manual `thinking.budget_tokens` for those models.
 
 This package is part of the CortexKit Anthropic Auth monorepo, which supports both OpenCode (`@cortexkit/opencode-anthropic-auth`) and Pi (`@cortexkit/pi-anthropic-auth`) through the same shared core logic.
 
diff --git a/packages/pi/src/convert.ts b/packages/pi/src/convert.ts
index f7a9fdd1..3ab61dfe 100644
--- a/packages/pi/src/convert.ts
+++ b/packages/pi/src/convert.ts
@@ -27,6 +27,11 @@ import type {
   ToolResultMessage,
 } from '@earendil-works/pi-ai'
 
+// Anchor identifying Pi's documentation paragraph — the only part of the prompt
+// that Anthropic rejects in system[]. If pi upstream renames this heading the
+// split stops separating and the whole prompt returns to system[], which 400s.
+const PI_DOCS_ANCHOR = 'Pi documentation'
+
 const CLAUDE_CODE_TOOLS = new Map(
   [
     'Read',
@@ -391,7 +396,49 @@ export async function buildAnthropicRequest(
     { type: 'text', text: CLAUDE_CODE_IDENTITY },
   ]
   if (context.systemPrompt?.trim()) {
-    system.push({ type: 'text', text: sanitize(context.systemPrompt) })
+    // 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
+    // Anthropic reject the request with 400 "You're out of extra usage". Entry
+    // count and payload size are ruled out — 2697 bytes of neutral filler in the
+    // same position is accepted, and the same text is accepted inside messages[].
+    //
+    // Keep the identity, tool contract and guidelines in system[], where they
+    // carry system weight and survive context compaction, and carry only the
+    // documentation paragraph in messages[].
+    //
+    // It goes in as its own content block ahead of the user's text, not merged
+    // into it and not as a separate message. A cache prefix matches contiguously
+    // from the start of the request, so a block boundary here lets the prefix end
+    // before the user's words: a new conversation with a different first message
+    // still reads the paragraph from cache instead of re-writing ~1.1k tokens.
+    // A role: "system" message cannot be used at messages[0] — Anthropic rejects
+    // that — and placing one after the first user message puts it behind content
+    // that varies, which defeats the caching.
+    //
+    // 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 paras = sanitize(context.systemPrompt).split(/\n\n+/)
+    const keep = paras.filter((p) => !p.includes(PI_DOCS_ANCHOR))
+    const docs = paras.filter((p) => p.includes(PI_DOCS_ANCHOR))
+    if (keep.length) {
+      system.push({ type: 'text', text: keep.join('\n\n') })
+    }
+    if (docs.length) {
+      const firstUser = messages.find((m) => m.role === 'user')
+      const content = firstUser?.content
+      const docsBlock = {
+        type: 'text',
+        text: docs.join('\n\n'),
+        cache_control: { type: 'ephemeral' as const },
+      }
+      if (firstUser && typeof content === 'string') {
+        firstUser.content = [docsBlock, { type: 'text', text: content }]
+      } else if (firstUser && Array.isArray(content)) {
+        content.unshift(docsBlock)
+      }
+    }
   }
 
   const body: AnthropicRequestBody = {
diff --git a/packages/pi/src/index.ts b/packages/pi/src/index.ts
index 5023fc68..738846e3 100644
--- a/packages/pi/src/index.ts
+++ b/packages/pi/src/index.ts
@@ -80,6 +80,15 @@ export default function cortexKitPiAnthropicAuth(pi: ExtensionAPI) {
         contextWindow: CLAUDE_FABLE_MYTHOS_5_CONTEXT_WINDOW,
         maxTokens: CLAUDE_FABLE_MYTHOS_5_MAX_OUTPUT_TOKENS,
       })),
+      {
+        id: 'claude-opus-5',
+        name: 'Claude Opus 5',
+        reasoning: true,
+        input: textImageInput(),
+        cost: { input: 5, output: 25, cacheRead: 0.5, cacheWrite: 6.25 },
+        contextWindow: 1_000_000,
+        maxTokens: 128_000,
+      },
       {
         id: 'claude-opus-4-8',
         name: 'Claude Opus 4.8',
diff --git a/packages/pi/src/tests/convert.test.ts b/packages/pi/src/tests/convert.test.ts
index 148731c7..d0ed84ba 100644
--- a/packages/pi/src/tests/convert.test.ts
+++ b/packages/pi/src/tests/convert.test.ts
@@ -33,10 +33,22 @@ function toolResultMsg(toolCallId: string, text: string): Message {
 
 const defaultCache = { enabled: false, mode: 'hybrid' as const }
 
-async function buildMessages(messages: Message[]) {
+// Mirrors the shape of Pi's real prompt: instruction paragraphs followed by the
+// documentation paragraph, which is the only part Anthropic rejects in system[].
+const PI_PROMPT = [
+  'KEEP ONE: you are an assistant.',
+  'KEEP TWO: available tools.',
+  'Pi documentation (read only when the user asks about pi itself):\n- MOVE THIS',
+].join('\n\n')
+
+// systemPrompt is opt-in. buildAnthropicRequest splits a non-empty prompt between
+// system[] and the first user message, so tests that assert raw conversion output
+// pass no prompt and observe messages unchanged. The split itself is covered by
+// the "Claude Code system[] shape" block below.
+async function buildMessages(messages: Message[], systemPrompt?: string) {
   const context = {
     messages,
-    systemPrompt: 'test',
+    systemPrompt,
     tools: [],
   }
   const { body } = await buildAnthropicRequest(
@@ -326,6 +338,93 @@ describe('convertMessages — empty base64 image guard', () => {
   })
 })
 
+describe('buildAnthropicRequest — Claude Code system[] shape', () => {
+  // Anthropic rejects Pi's documentation paragraph inside the top-level
+  // system[] array — see the note in convert.ts. These cases pin the resulting
+  // shape: the remaining paragraphs stay in system[], and the documentation
+  // paragraph is carried as its own marked content block ahead of the user's
+  // text inside the first user message.
+  async function buildBody(messages: Message[], systemPrompt?: string) {
+    const { body } = await buildAnthropicRequest(
+      'claude-sonnet-4-20250514',
+      { messages, systemPrompt, tools: [] } as any,
+      undefined,
+      defaultCache,
+    )
+    return body
+  }
+
+  test('keeps the non-documentation paragraphs in system[]', async () => {
+    const body = await buildBody([userMsg('hello')], PI_PROMPT)
+    expect(body.system).toHaveLength(3)
+    const text = String(body.system?.[2]?.text)
+    expect(text).toContain('KEEP ONE')
+    expect(text).toContain('KEEP TWO')
+    expect(text).not.toContain('MOVE THIS')
+  })
+
+  test('carries the documentation paragraph as its own block ahead of the user text', async () => {
+    const body = await buildBody([userMsg('hello')], PI_PROMPT)
+    const content = body.messages[0]?.content as Array<Record<string, unknown>>
+    expect(content).toHaveLength(2)
+    expect(String(content[0]?.text)).toContain('MOVE THIS')
+    expect(content[1]).toMatchObject({ type: 'text', text: 'hello' })
+  })
+
+  test('marks the documentation block so the cached prefix ends before the user text', async () => {
+    // Without this marker the prefix would extend into the user's own words,
+    // and a new conversation with a different first message would have to
+    // re-cache the paragraph.
+    const body = await buildBody([userMsg('hello')], PI_PROMPT)
+    const content = body.messages[0]?.content as Array<Record<string, unknown>>
+    expect(content[0]?.cache_control).toEqual({ type: 'ephemeral' })
+  })
+
+  test('unshifts the documentation block onto a structured first user message', async () => {
+    const body = await buildBody(
+      [
+        {
+          role: 'user',
+          content: [
+            { type: 'text', text: 'see image' },
+            { type: 'image', mimeType: 'image/png', data: 'aGVsbG8=' },
+          ],
+          timestamp: 0,
+        } as Message,
+      ],
+      PI_PROMPT,
+    )
+    const content = body.messages[0]?.content as Array<Record<string, unknown>>
+    expect(content).toHaveLength(3)
+    expect(String(content[0]?.text)).toContain('MOVE THIS')
+    expect(content[1]).toMatchObject({ type: 'text', text: 'see image' })
+  })
+
+  test('drops the documentation paragraph when there is no user message to carry it', async () => {
+    // convertMessages emits only user/assistant and trailing assistants are
+    // stripped, so a conversation with no user message converts to empty. The
+    // remaining paragraphs still go to system[], which is a shape Anthropic
+    // accepts; only the documentation paragraph is dropped.
+    const body = await buildBody([assistantMsg('only assistant')], PI_PROMPT)
+    expect(body.messages).toHaveLength(0)
+    expect(body.system).toHaveLength(3)
+    expect(JSON.stringify(body.system)).not.toContain('MOVE THIS')
+  })
+
+  test('leaves the whole prompt in system[] when it has no documentation paragraph', async () => {
+    const body = await buildBody([userMsg('hello')], 'KEEP ONLY')
+    expect(body.system).toHaveLength(3)
+    expect(String(body.system?.[2]?.text)).toContain('KEEP ONLY')
+    expect(body.messages[0]).toEqual({ role: 'user', content: 'hello' })
+  })
+
+  test('leaves system[] and messages untouched when no prompt is set', async () => {
+    const body = await buildBody([userMsg('hello')])
+    expect(body.system).toHaveLength(2)
+    expect(body.messages[0]).toEqual({ role: 'user', content: 'hello' })
+  })
+})
+
 describe('buildAnthropicRequest — Fable/Mythos thinking', () => {
   test('maps Pi reasoning to output_config effort for Claude Fable 5', async () => {
     const { body } = await buildAnthropicRequest(
diff --git a/packages/pi/src/tests/index.test.ts b/packages/pi/src/tests/index.test.ts
index c6acf867..05c58f8c 100644
--- a/packages/pi/src/tests/index.test.ts
+++ b/packages/pi/src/tests/index.test.ts
@@ -44,4 +44,23 @@ describe('cortexKitPiAnthropicAuth provider registration', () => {
       maxTokens: 128_000,
     })
   })
+
+  test('exposes Claude Opus 5 in the Pi Anthropic catalog', () => {
+    const { pi, providers } = mockPi()
+
+    cortexKitPiAnthropicAuth(pi)
+
+    const opus5 = providers
+      .get('anthropic')
+      ?.models?.find((model) => model.id === 'claude-opus-5')
+    expect(opus5).toMatchObject({
+      id: 'claude-opus-5',
+      name: 'Claude Opus 5',
+      reasoning: true,
+      input: ['text', 'image'],
+      cost: { input: 5, output: 25, cacheRead: 0.5, cacheWrite: 6.25 },
+      contextWindow: 1_000_000,
+      maxTokens: 128_000,
+    })
+  })
 })