pdf-server: display_pdf/interact rejected client-side by strict 2020-12 output-schema validators

[bonallure]

Bug

Calling display_pdf or interact in examples/pdf-server fails client-side, before the tool call ever reaches the server, on clients with a strict JSON Schema 2020-12-only output-schema validator (observed on Claude Desktop / Claude Code):

Tool 'display_pdf' has an invalid outputSchema: JSON Schema declares an unsupported dialect
("$schema": "http://json-schema.org/draft-07/schema#"). The default validator supports
JSON Schema 2020-12 only, pass a pre-configured Ajv instance to AjvJs

interact has no outputSchema of its own, but it fails too — the client rejects the entire tools/list response over the one bad tool (display_pdf), so every tool in the list breaks together.

Root cause

@modelcontextprotocol/sdk (pinned ^1.29.0 here, confirmed still present in latest 1.30.0) always emits "$schema": "http://json-schema.org/draft-07/schema#" on Zod-derived inputSchema/outputSchema, with no way to configure it via registerTool/registerAppTool:

• node_modules/@modelcontextprotocol/sdk/dist/esm/server/zod-json-schema-compat.js:9-17 — mapMiniTarget() falls back to 'draft-7' whenever no target is passed.
• node_modules/@modelcontextprotocol/sdk/dist/esm/server/mcp.js:88-96 — the tools/list handler never passes a target.

This is a known, already-filed, still-open upstream bug: modelcontextprotocol/typescript-sdk#2721. There is no released SDK version that fixes it.

display_pdf, read_pdf_bytes, and save_pdf in examples/pdf-server/server.ts all declare outputSchema: z.object({...}) via registerAppTool and trigger this.

Fix

Opening a PR that adds fixOutputSchemaDialect(server) to src/server/index.ts — a workaround that wraps Server.setRequestHandler to intercept the tools/list handler and rewrite the draft-07 $schema to https://json-schema.org/draft/2020-12/schema before the response goes out. Wired into examples/pdf-server/server.ts, called immediately after new McpServer(...).

Follow-up (not in the linked PR, scoped out to keep it focused)

The same unpatched bug affects other examples that declare outputSchema without calling the new helper:

• examples/budget-allocator-server/server.ts
• examples/cohort-heatmap-server/server.ts
• examples/basic-server-vanillajs/server.ts
• examples/integration-server/server.ts
• examples/customer-segmentation-server/server.ts
• examples/system-monitor-server/server.ts
• examples/debug-server/server.ts
• examples/wiki-explorer-server/server.ts
• examples/scenario-modeler-server/server.ts
• examples/video-resource-server/server.ts
• examples/threejs-server/server.ts

Happy to send a follow-up PR applying fixOutputSchemaDialect to these too, or maintainers may prefer to fold it into registerAppTool itself once there's agreement on the right call-site contract (it must run before any tool registration, since McpServer only installs its real tools/list handler once, lazily, on first registration).