TanStack
diff --git a/‎.changeset/feat-system-prompts-metadata.md‎
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diff --git a/‎packages/typescript/ai-anthropic/src/adapters/text.ts‎
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diff --git a/‎packages/typescript/ai-anthropic/src/index.ts‎
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diff --git a/‎packages/typescript/ai-anthropic/src/text/text-provider-options.ts‎
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diff --git a/‎packages/typescript/ai-anthropic/tests/anthropic-adapter.test.ts‎
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diff --git a/‎packages/typescript/ai-event-client/src/devtools-middleware.ts‎
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diff --git a/‎packages/typescript/ai-gemini/src/adapters/text.ts‎
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@@ -0,0 +1,86 @@
+---
+'@tanstack/ai': minor
+'@tanstack/ai-anthropic': minor
+'@tanstack/ai-event-client': patch
+'@tanstack/ai-gemini': patch
+'@tanstack/ai-ollama': patch
+'@tanstack/ai-openai': patch
+'@tanstack/ai-openrouter': patch
+'@tanstack/openai-base': patch
+---
+
+feat(ai): `systemPrompts` accept `{ content, metadata }` with adapter-inferred metadata typing
+
+`chat({ systemPrompts })` now accepts either a plain string (the existing
+shape — fully backward compatible) or `{ content, metadata }`. The `metadata`
+field's type is inferred from the adapter via a new
+`TSystemPromptMetadata` generic on `TextAdapter` / `BaseTextAdapter`:
+
+- `@tanstack/ai-anthropic` declares `AnthropicSystemPromptMetadata` →
+  users get `cache_control` autocomplete and type-checking on
+  `systemPrompts[i].metadata` for Anthropic chats.
+- Adapters with no per-prompt metadata (OpenAI, Gemini, Ollama,
+  OpenRouter, openai-base) inherit the default `never`, which means the
+  `metadata` field carries no meaningful value at the call site —
+  TypeScript only accepts `undefined` there. Provider-foreign metadata
+  that reaches an adapter via JS / `as any` is silently dropped, never
+  written to the wire.
+
+```ts
+import { chat } from '@tanstack/ai'
+import { anthropicText } from '@tanstack/ai-anthropic'
+
+// Anthropic — `cache_control` is autocompleted, no `satisfies` needed.
+chat({
+  adapter: anthropicText({ apiKey }, 'claude-sonnet-4-6'),
+  systemPrompts: [
+    {
+      content: 'Stable instructions — cache me.',
+      metadata: { cache_control: { type: 'ephemeral' } },
+    },
+    'Volatile per-request instruction.',
+  ],
+})
+
+// OpenAI — `metadata` is `never`; only `undefined` is assignable, so the
+// field is effectively unusable. The object form without `metadata` still
+// works for portability.
+chat({
+  adapter: openaiText({ apiKey }, 'gpt-4o-mini'),
+  systemPrompts: [
+    'Plain string.',
+    { content: 'Object form without metadata is allowed.' },
+  ],
+})
+```
+
+New exports:
+
+- `@tanstack/ai`: `SystemPrompt`, `NormalizedSystemPrompt` types and the
+  `normalizeSystemPrompts()` helper adapters use to normalize the wide
+  input shape to `{ content, metadata? }` before consumption.
+- `@tanstack/ai-anthropic`: `AnthropicSystemPromptMetadata` interface
+  (currently exposes `cache_control` for prompt caching).
+
+Internal:
+
+- New `TSystemPromptMetadata = never` generic on `TextAdapter` /
+  `BaseTextAdapter`, surfaced via `'~types'['systemPromptMetadata']`
+  for inference at the `chat()` call site.
+- Anthropic adapter reads `metadata.cache_control` and attaches it to
+  the corresponding `TextBlockParam`.
+- All other text adapters call `normalizeSystemPrompts()` and join
+  `.content` for their respective `instructions` / `system` /
+  `systemInstruction` fields. Foreign metadata that reaches them via JS
+  / `as any` is dropped (never written to the wire).
+- `normalizeSystemPrompts()` is the public API boundary and throws
+  `TypeError` (naming the offending index) for object-form entries whose
+  `content` isn't a string — preventing literal `"undefined"` from
+  reaching the model on stale call sites.
+- OpenTelemetry middleware attaches per-prompt metadata as the
+  `tanstack.ai.system_prompt.metadata` JSON span attribute when
+  `captureContent: true` and at least one entry carries metadata, so
+  observability backends can distinguish cache hit/miss for Anthropic.
+- `@tanstack/ai-event-client` mirrors the `SystemPrompt` shape locally
+  (avoids a circular import) and projects metadata away on the devtools
+  wire — devtools UI still receives `Array<string>`.
@@ -1,4 +1,4 @@
-import { EventType } from '@tanstack/ai'
+import { EventType, normalizeSystemPrompts } from '@tanstack/ai'
 import { BaseTextAdapter } from '@tanstack/ai/adapters'
 import { convertToolsToProviderFormat } from '../tools/tool-converter'
 import { validateTextProviderOptions } from '../text/text-provider-options'
@@ -38,6 +38,7 @@ import type {
   TextOptions,
 } from '@tanstack/ai'
 import type {
+  AnthropicSystemPromptMetadata,
   ExternalTextProviderOptions,
   InternalTextProviderOptions,
 } from '../text/text-provider-options'
@@ -115,7 +116,12 @@ export class AnthropicTextAdapter<
   TProviderOptions,
   TInputModalities,
   AnthropicMessageMetadataByModality,
-  TToolCapabilities
+  TToolCapabilities,
+  // TToolCallMetadata — anthropic has no tool-call metadata round-tripping
+  unknown,
+  // TSystemPromptMetadata — narrows `systemPrompts[i].metadata` at the
+  // chat() call site so users get `cache_control` autocomplete.
+  AnthropicSystemPromptMetadata
 > {
   readonly kind = 'text' as const
   readonly name = 'anthropic' as const
@@ -356,11 +362,22 @@ export class AnthropicTextAdapter<
       temperature: options.temperature,
       top_p: options.topP,
       messages: formattedMessages,
-      system: options.systemPrompts?.length
-        ? options.systemPrompts.map(
-            (text): TextBlockParam => ({ type: 'text', text }),
+      system: (() => {
+        const normalized =
+          normalizeSystemPrompts<AnthropicSystemPromptMetadata>(
+            options.systemPrompts,
           )
-        : undefined,
+        if (normalized.length === 0) return undefined
+        return normalized.map(
+          (p): TextBlockParam => ({
+            type: 'text',
+            text: p.content,
+            ...(p.metadata?.cache_control && {
+              cache_control: p.metadata.cache_control,
+            }),
+          }),
+        )
+      })(),
       tools: tools,
       ...validProviderOptions,
     }
 
@@ -10,6 +10,7 @@ export {
   type AnthropicTextConfig,
   type AnthropicTextProviderOptions,
 } from './adapters/text'
+export type { AnthropicSystemPromptMetadata } from './text/text-provider-options'
 
 // Summarize - thin factory functions over @tanstack/ai's ChatStreamSummarizeAdapter
 export {
 
@@ -4,12 +4,43 @@ import type {
   BetaToolChoiceAuto,
   BetaToolChoiceTool,
 } from '@anthropic-ai/sdk/resources/beta/messages/messages'
+import type { CacheControlEphemeral } from '@anthropic-ai/sdk/resources'
 import type { AnthropicTool } from '../tools'
 import type {
   MessageParam,
   TextBlockParam,
 } from '@anthropic-ai/sdk/resources/messages'
 
+/**
+ * Per-prompt metadata Anthropic understands on `systemPrompts` entries.
+ *
+ * Used via the structured form of `systemPrompts`:
+ *
+ * @example
+ *   import type { AnthropicSystemPromptMetadata } from '@tanstack/ai-anthropic'
+ *
+ *   chat({
+ *     adapter: anthropicText(),
+ *     model: 'claude-sonnet-4-6',
+ *     systemPrompts: [
+ *       {
+ *         content: 'Stable instructions — cache me.',
+ *         metadata: { cache_control: { type: 'ephemeral' } } satisfies AnthropicSystemPromptMetadata,
+ *       },
+ *       'Volatile per-request instruction.',
+ *     ],
+ *   })
+ */
+export interface AnthropicSystemPromptMetadata {
+  /**
+   * Anthropic prompt-caching control applied to this system prompt's
+   * `TextBlockParam`.
+   *
+   * @see https://docs.anthropic.com/en/docs/build-with-claude/prompt-caching
+   */
+  cache_control?: CacheControlEphemeral
+}
+
 export interface AnthropicContainerOptions {
   /**
    * Container identifier for reuse across requests.
 
@@ -117,6 +117,64 @@ describe('Anthropic adapter option mapping', () => {
     ])
   })
 
+  it('attaches cache_control to system TextBlockParams via systemPrompts metadata', async () => {
+    const mockStream = (async function* () {
+      yield {
+        type: 'content_block_start',
+        index: 0,
+        content_block: { type: 'text', text: '' },
+      }
+      yield {
+        type: 'content_block_delta',
+        index: 0,
+        delta: { type: 'text_delta', text: 'ok' },
+      }
+      yield {
+        type: 'message_delta',
+        delta: { stop_reason: 'end_turn' },
+        usage: { output_tokens: 1 },
+      }
+      yield { type: 'message_stop' }
+    })()
+
+    mocks.betaMessagesCreate.mockResolvedValueOnce(mockStream)
+
+    const adapter = createAdapter('claude-3-7-sonnet')
+
+    for await (const _ of chat({
+      adapter,
+      messages: [{ role: 'user', content: 'Hi' }],
+      systemPrompts: [
+        {
+          content: 'Stable instructions — cache me.',
+          // metadata is narrowed to AnthropicSystemPromptMetadata via the
+          // adapter's `~types['systemPromptMetadata']` declaration — no
+          // `satisfies` needed.
+          metadata: { cache_control: { type: 'ephemeral', ttl: '5m' } },
+        },
+        'Volatile per-request instruction.',
+      ],
+    })) {
+      // consume stream
+    }
+
+    const [payload] = mocks.betaMessagesCreate.mock.calls[0]!
+
+    // Object-form prompts attach their metadata cache_control; plain strings
+    // produce a TextBlockParam with no cache_control.
+    expect(payload.system).toEqual([
+      {
+        type: 'text',
+        text: 'Stable instructions — cache me.',
+        cache_control: { type: 'ephemeral', ttl: '5m' },
+      },
+      {
+        type: 'text',
+        text: 'Volatile per-request instruction.',
+      },
+    ])
+  })
+
   it('drops unknown modelOptions keys (e.g. `system`) and warns via logger.error', async () => {
     const mockStream = (async function* () {
       yield {
@@ -127,12 +185,12 @@ describe('Anthropic adapter option mapping', () => {
       yield {
         type: 'content_block_delta',
         index: 0,
-        delta: { type: 'text_delta', text: 'Hello' },
+        delta: { type: 'text_delta', text: 'ok' },
       }
       yield {
         type: 'message_delta',
         delta: { stop_reason: 'end_turn' },
-        usage: { output_tokens: 3 },
+        usage: { output_tokens: 1 },
       }
       yield { type: 'message_stop' }
     })()
@@ -148,8 +206,7 @@ describe('Anthropic adapter option mapping', () => {
       error: vi.fn(),
     }
 
-    const chunks: StreamChunk[] = []
-    for await (const chunk of chat({
+    for await (const _ of chat({
       adapter,
       messages: [{ role: 'user', content: 'Hi' }],
       systemPrompts: ['real system prompt'],
@@ -159,7 +216,7 @@ describe('Anthropic adapter option mapping', () => {
       } as unknown as AnthropicTextProviderOptions,
       debug: { logger, errors: true },
     })) {
-      chunks.push(chunk)
+      // consume stream
     }
 
     const [payload] = mocks.betaMessagesCreate.mock.calls[0]!
 
@@ -12,14 +12,21 @@ interface DevtoolsModelMessage {
   toolCalls?: unknown
 }
 
+/**
+ * Mirrors `SystemPrompt` from `@tanstack/ai` structurally so this package
+ * doesn't import from `@tanstack/ai` (which would introduce a circular dep,
+ * see file-top comment).
+ */
+type DevtoolsSystemPrompt = string | { content: string; metadata?: unknown }
+
 interface DevtoolsMiddlewareContext {
   requestId: string
   streamId: string
   conversationId?: string
   provider: string
   model: string
   source: 'client' | 'server'
-  systemPrompts: Array<string>
+  systemPrompts: ReadonlyArray<DevtoolsSystemPrompt>
   toolNames?: Array<string>
   options?: Record<string, unknown>
   modelOptions?: Record<string, unknown>
@@ -104,7 +111,13 @@ function buildEventContext(ctx: DevtoolsMiddlewareContext) {
     model: ctx.model,
     clientId: ctx.conversationId,
     source: ctx.source,
-    systemPrompts: ctx.systemPrompts.length > 0 ? ctx.systemPrompts : undefined,
+    // Devtools wire payload is plain strings; per-prompt metadata is
+    // irrelevant for observation and would require devtools-UI changes to
+    // render. Project metadata away here so the wire shape is unchanged.
+    systemPrompts:
+      ctx.systemPrompts.length > 0
+        ? ctx.systemPrompts.map((p) => (typeof p === 'string' ? p : p.content))
+        : undefined,
     toolNames: ctx.toolNames,
     options: ctx.options,
     modelOptions: ctx.modelOptions,
 
@@ -1,5 +1,5 @@
 import { FinishReason } from '@google/genai'
-import { EventType } from '@tanstack/ai'
+import { EventType, normalizeSystemPrompts } from '@tanstack/ai'
 import { BaseTextAdapter } from '@tanstack/ai/adapters'
 import { convertToolsToProviderFormat } from '../tools/tool-converter'
 import {
@@ -838,7 +838,12 @@ export class GeminiTextAdapter<
                 : undefined,
             }
           : undefined,
-        systemInstruction: options.systemPrompts?.join('\n'),
+        systemInstruction: (() => {
+          const prompts = normalizeSystemPrompts(options.systemPrompts)
+          return prompts.length > 0
+            ? prompts.map((p) => p.content).join('\n')
+            : undefined
+        })(),
         tools: convertToolsToProviderFormat(options.tools),
       },
     }