331 MCP tool-list date-only changes invalidate caches

On 3 November 2026, DEV Community's AI channel published an article laying out a cache-invalidation problem caused by changes to MCP tool lists. The piece says a reader of their previous post, Paul Irolla, who builds a token optimizer for coding agents, pointed out something the authors had previously treated only as a correctness issue. In a model request, the tool list comes first. The authors maintain HISTOR, their public log of MCP tool definitions, which records every change it sees, so they went back and counted. Between 25 October and 1 November, HISTOR recorded 10,712 changes to tool lists across 4,491 MCP servers, and 331 of those changes altered only the date or the timestamp. Because the tool list sits at the front of the request, a one-byte change to a tool definition invalidates every prompt cache built on that request, even if the change carries no meaningful difference. According to the article, one cache miss costs roughly 12 times as much as a hit. The disclosure therefore rests on this week-long measurement plus an explanation of the caching mechanism; the material provides no information about responses from the parties involved or about any follow-up.