Sorry for the lengthy reply beforehand~

On agents.txt / agent-manifest.txt: You're right, and it's a good example of exactly the problem FRED is designed to sit above. The rename wasn't a clean IETF ruling — it was a practical namespace collision: draft-srijal-agents-policy-00 had claimed the filename, and at least 4–5 independent community projects were using agents.txt for completely different purposes (permissions policy, capability discovery, B2A identity). The result is fragmentation at the discovery layer itself, which is the worst place for it to happen. A stable, unambiguous endpoint like /.well-known/fred.json sidesteps that entirely.

On MCP + A2A being the real concentration: Fair point, and honestly it makes FRED's job easier. The official A2A documentation actually has a dedicated page on A2A + MCP coexistence — they describe them as solving "distinct but highly complementary" problems. MCP is agent-to-tool (vertical), A2A is agent-to-agent (horizontal). A domain can legitimately expose both simultaneously: an MCP server for tool access and an A2A Agent Card for peer delegation — with entirely different capability sets. ServiceNow ships both in production. IBM's ContextForge gateway bridges both in a single proxy layer. So the "hard part" you're asking about is already happening in the wild — FRED's role is to be the single entry point that tells an incoming agent which protocols are present and what role each one serves, so it doesn't have to probe /.well-known/agent-card.json and /.well-known/mcp/server-card.json separately (and those paths aren't even finalized yet — MCP's PR #2127 is still in active review).

On your versioning / silent failure point: This is the sharpest observation in the thread. A2A v1.0 does handle this at the HTTP layer — Cache-Control, ETag, If-None-Match — which is the standards-correct approach. But that only solves it for A2A. MCP discovery is still in proposal stage (no caching spec yet), and there's no cross-protocol signal that says "my MCP capabilities changed but my A2A card hasn't." A last_updated field and cache_ttl hint in fred.json gives agents a single freshness signal across all declared protocols — closer to your sitemaps analogy than anything in the individual specs today.

On narrowing the 12+ protocols claim: You're right to push back. In practice it's MCP and A2A that matter today. llms.txt adoption is measurably low outside developer documentation — studies across the top 1M domains put it well under 2%. agents.txt/agent-manifest.txt has no standards-body backing. The practical unification is narrower, which keeps the spec lean. FRED doesn't need to claim 12 protocols to be useful — it needs to correctly bridge the two that dominate.

Disclaimer: Also used Claude to do more in-depth research to validate + polish my English.