ext-apps/examples/pdf-server

PDF Server

A simple interactive PDF viewer that uses PDF.js. Launch it w/ a few PDF files and/or URLs as CLI args (+ support loading any additional pdf from arxiv.org).

What This Example Demonstrates

1. Chunked Data Through Size-Limited Tool Calls

On some host platforms, tool calls have size limits, so large PDFs cannot be sent in a single response. This example shows a possible workaround:

Server side (pdf-loader.ts):

// Returns chunks with pagination metadata
async function loadPdfBytesChunk(entry, offset, byteCount) {
  return {
    bytes: base64Chunk,
    offset,
    byteCount,
    totalBytes,
    hasMore: offset + byteCount < totalBytes,
  };
}

Client side (mcp-app.ts):

// Load in chunks with progress
while (hasMore) {
  const chunk = await app.callServerTool("read_pdf_bytes", { pdfId, offset });
  chunks.push(base64ToBytes(chunk.bytes));
  offset += chunk.byteCount;
  hasMore = chunk.hasMore;
  updateProgress(offset, chunk.totalBytes);
}

2. Model Context Updates

The viewer keeps the model informed about what the user is seeing:

app.updateModelContext({
  structuredContent: {
    title: pdfTitle,
    currentPage,
    totalPages,
    pageText: pageText.slice(0, 5000),
    selection: selectedText ? { text, start, end } : undefined,
  },
});

This enables the model to answer questions about the current page or selected text.

3. Display Modes: Fullscreen vs Inline

• Inline mode: App requests height changes to fit content
• Fullscreen mode: App fills the screen with internal scrolling

// Request fullscreen
app.requestDisplayMode({ mode: "fullscreen" });

// Listen for mode changes
app.ondisplaymodechange = (mode) => {
  if (mode === "fullscreen") enableScrolling();
  else disableScrolling();
};

4. External Links (openLink)

The viewer demonstrates opening external links (e.g., to the original arxiv page):

titleEl.onclick = () => app.openLink(sourceUrl);

Usage

# Default: loads a sample arxiv paper
bun examples/pdf-server/server.ts

# Load local files (converted to file:// URLs)
bun examples/pdf-server/server.ts ./docs/paper.pdf /path/to/thesis.pdf

# Load from URLs
bun examples/pdf-server/server.ts https://arxiv.org/pdf/2401.00001.pdf

# Mix local and remote
bun examples/pdf-server/server.ts ./local.pdf https://arxiv.org/pdf/2401.00001.pdf

# stdio mode for MCP clients
bun examples/pdf-server/server.ts --stdio ./papers/

Security: Dynamic URLs (via view_pdf tool) are restricted to arxiv.org. Local files must be in the initial list.

Tools

Tool	Visibility	Purpose
list_pdfs	Model	List indexed PDFs
display_pdf	Model + UI	Display interactive viewer in chat
read_pdf_bytes	App only	Chunked binary loading

Architecture

server.ts           # MCP server (233 lines)
├── src/
│   ├── types.ts        # Zod schemas (75 lines)
│   ├── pdf-indexer.ts  # URL-based indexing (44 lines)
│   ├── pdf-loader.ts   # Chunked loading (171 lines)
│   └── mcp-app.ts      # Interactive viewer UI

Key Patterns Shown

Pattern	Implementation
App-only tools	_meta: { ui: { visibility: ["app"] } }
Chunked responses	hasMore + offset pagination
Model context	app.updateModelContext()
Display modes	app.requestDisplayMode()
External links	app.openLink()
Size negotiation	app.sendSizeChanged()

Dependencies

• pdfjs-dist: PDF rendering
• @modelcontextprotocol/ext-apps: MCP Apps SDK