docs(openai): document the PDF document-input support

Add a changeset (minor bump for `@tanstack/openai-base` and `@tanstack/ai-openai`)
and a multimodal-content docs example for the new `document` content part support
on the Responses adapter. The example uses `gpt-5.5`, one of the models that
declares the `document` input modality; the "Supported modalities by model" list
is expanded to include it and points readers at `model-meta.ts` for the full,
authoritative list.

Author: dangreenberggit

.changeset/openai-pdf-document-input.md

@@ -0,0 +1,26 @@
+---
+'@tanstack/openai-base': minor
+'@tanstack/ai-openai': minor
+---
+
+feat(ai-openai): support PDF `document` content parts in the Responses adapter.
+
+`openaiText`'s Responses adapter now accepts PDF `document` content parts, for any model whose `model-meta` entry declares the `document` input modality. Base64 data sources are sent as `input_file` with a `file_data` data URL and a `filename` (from `metadata.filename`, defaulting to `document.pdf`). URL sources are sent as `input_file` with `file_url`.
+
+```ts
+const adapter = openaiText('gpt-5.5')
+
+const message = {
+  role: 'user',
+  content: [
+    { type: 'text', content: 'Summarize this document' },
+    {
+      type: 'document',
+      source: { type: 'data', value: pdfBase64, mimeType: 'application/pdf' },
+      metadata: { filename: 'report.pdf' },
+    },
+  ],
+}
+```
+
+Non-PDF MIME types are rejected before the request is sent — including pre-wrapped `data:` URLs whose media type disagrees with `mimeType` — so callers get an actionable message instead of an opaque provider `400`. `OpenAIDocumentMetadata` gains `filename` and `detail`. The Chat Completions adapter throws a document-specific error pointing here, since documents are Responses-only.

docs/advanced/multimodal-content.md

@@ -94,11 +94,12 @@ const response = await chat({
 
 ### OpenAI
 
-OpenAI supports images and audio in their vision and audio models:
+OpenAI supports images, audio, and PDF documents in their vision, audio, and
+document-capable models:
 
 ```typescript
 import { openaiText } from '@tanstack/ai-openai'
-import { imageBase64 } from './data'
+import { imageBase64, pdfBase64 } from './data'
 
 const adapter = openaiText('gpt-5.5')
 
@@ -116,10 +117,30 @@ const message = {
 }
 ```
 
+```typescript
+import { pdfBase64 } from './data'
+
+// PDF document via base64 data (the API requires a filename alongside
+// inline data; defaults to "document.pdf" when omitted)
+const documentMessage = {
+  role: 'user',
+  content: [
+    { type: 'text', content: 'Summarize this document' },
+    {
+      type: 'document',
+      source: { type: 'data', value: pdfBase64, mimeType: 'application/pdf' },
+      metadata: { filename: 'report.pdf' }
+    }
+  ]
+}
+```
+
 **Supported modalities by model:**
-- `gpt-5.2`, `gpt-5-mini`: text, image
+- `gpt-5.5`, `gpt-5.2`, `gpt-5-mini` (among others): text, image, PDF document
 - `gpt-4o-audio`: text, audio
 
+Check each model's `supports.input` in `@tanstack/ai-openai`'s `model-meta.ts` for the authoritative per-model list.
+
 ### Anthropic
 
 Anthropic's Claude models support images and PDF documents:

docs/config.json

@@ -439,7 +439,8 @@
         {
           "label": "Multimodal Content",
           "to": "advanced/multimodal-content",
-          "addedAt": "2026-04-15"
+          "addedAt": "2026-04-15",
+          "updatedAt": "2026-07-21"
         },
         {
           "label": "Per-Model Type Safety",