ext-apps/examples/pdf-server

PDF Server

An interactive PDF viewer using PDF.js. Supports local files and remote URLs from academic sources (arxiv, biorxiv, zenodo, etc).

MCP Client Configuration

Add to your MCP client configuration (stdio transport):

{
  "mcpServers": {
    "pdf": {
      "command": "npx",
      "args": [
        "-y",
        "--silent",
        "--registry=https://registry.npmjs.org/",
        "@modelcontextprotocol/server-pdf",
        "--stdio"
      ]
    }
  }
}

Local Development

To test local modifications, use this configuration (replace ~/code/ext-apps with your clone path):

{
  "mcpServers": {
    "pdf": {
      "command": "bash",
      "args": [
        "-c",
        "cd ~/code/ext-apps/examples/pdf-server && npm run build >&2 && node dist/index.js --stdio"
      ]
    }
  }
}

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 streams PDFs in chunks using HTTP Range requests:

Server side (server.ts):

// Returns chunks with pagination metadata
{
  (bytes, offset, byteCount, totalBytes, hasMore);
}

Client side (mcp-app.ts):

// Load in chunks with progress
while (hasMore) {
  const chunk = await app.callServerTool({
    name: "read_pdf_bytes",
    arguments: { url, 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({
  content: [
    {
      type: "text",
      text: `PDF viewer | "${title}" | Current Page: ${page}/${total}\n\nPage content:\n${pageText}`,
    },
  ],
});

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);

5. View Persistence

Page position is saved per-view using viewUUID and localStorage.

6. Dark Mode / Theming

The viewer syncs with the host's theme using CSS light-dark() and the SDK's theming APIs:

app.onhostcontextchanged = (ctx) => {
  if (ctx.theme) applyDocumentTheme(ctx.theme);
  if (ctx.styles?.variables) applyHostStyleVariables(ctx.styles.variables);
};

Usage

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

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

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

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

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

Security: Client Roots

MCP clients may advertise roots — file:// URIs pointing to directories on the client's file system. The server uses these to allow access to local files under those directories.

• Stdio mode (--stdio): Client roots are always enabled — the client is typically on the same machine (e.g. Claude Desktop), so the roots are safe.
• HTTP mode (default): Client roots are ignored by default — the client may be remote, and its roots would be resolved against the server's filesystem. To opt in, pass --use-client-roots:

# Trust that the HTTP client is local and its roots are safe
bun examples/pdf-server/main.ts --use-client-roots

When roots are ignored the server logs:

[pdf-server] Client roots are ignored (default for remote transports). Pass --use-client-roots to allow the client to expose local directories.

Allowed Sources

• Local files: Must be passed as CLI arguments (or via client roots when enabled)
• Remote URLs: arxiv.org, biorxiv.org, medrxiv.org, chemrxiv.org, zenodo.org, osf.io, hal.science, ssrn.com, and more

Tools

Tool	Visibility	Purpose
list_pdfs	Model	List available local files and origins
display_pdf	Model + UI	Display interactive viewer
read_pdf_bytes	App only	Stream PDF data in chunks

Architecture

server.ts      # MCP server + tools
main.ts        # CLI entry point
src/
└── mcp-app.ts # Interactive viewer UI (PDF.js)

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()
View persistence	viewUUID + localStorage
Theming	applyDocumentTheme() + CSS light-dark()

Dependencies

• pdfjs-dist: PDF rendering (frontend only)
• @modelcontextprotocol/ext-apps: MCP Apps SDK