Add GCF as an optional output format alongside TOON

[blackwell-systems]

Hi 👋 I'm Dayna, creator of GCF (Graph Compact Format). pb_mcp_server already supports MCP_OUTPUT_FORMAT=toon|json, so you clearly care about token cost on tool responses. I'd like to propose GCF as a third opt-in format, and I'm happy to send the PR.

It's a minimal, additive change in src/core/services/output-formatter.ts: extend OutputFormat to "json" | "toon" | "gcf" and add one case that calls encodeGeneric(data). Nothing else changes; JSON stays the default.

I pre-screened GCF against your actual encoders on canonical PocketBase list-records shapes (30 records), measured with cl100k_base, vs the real @toon-format/toon v2 encoder and your pretty-printed JSON default:

Payload	JSON default	TOON	GCF
Flat records	4078	2250	2291
Nested (?expand=category,supplier)	6678	5179	3049

• Flat lists: GCF and TOON are equivalent (~44% under JSON). No meaningful difference.
• Expanded relations: GCF is ~41% smaller than TOON and ~54% smaller than JSON. TOON only reaches ~22% under JSON on nested data because it re-declares the nested structure on every row; GCF flattens nested objects into path columns. Since agents expand relations constantly to get context in one call, this is the common case.

Both round-trip losslessly on this data.

For precedent, the official Chrome DevTools MCP server recently added the same default | toon | gcf opt-in (ChromeDevTools/chrome-devtools-mcp).

GCF is MIT, zero runtime dependencies: https://github.com/blackwell-systems/gcf. If you're open to it I'll open a PR against output-formatter.ts. Either way, thanks for building this.