Findings · measured, dated, recomputable

3,386 observed MCP parameter changes — and 73.6% carried no version change.

When an MCP tool keeps its name but quietly changes the parameters it accepts, does anything signal it?

Cite as
73.6% of MCP tool-parameter changes carry no version change (Still OS Digital Holdings LLC, Surface History, n=3,386 parameter changes across 1,421 hosts, 2026-09-21 to 2026-10-30).

The numbers

Parameter changes observed3,386
Of those, no version change2,493 (73.6%)
Distinct hosts1,421
Tool-surface changes, same window6,384
Description-only changes, same window29,077
Observation window2026-09-21 → 2026-10-30

Why it matters

Adding or removing a tool is at least visible in the tool list. Changing a kept tool's parameters is not — a caller that pinned the version and hard-coded the call keeps sending the old shape to a tool that now expects a new one. These observations indicate it happens at scale and almost always without a version signal.

Method

Each server's live tool definitions are fetched on a repeating cadence; a change to a retained tool's input parameters with no change to its name is recorded as PARAMS_CHANGED, separately from tools added/removed (SURFACE_CHANGED) and description-only edits. Event-sourced and replayable to any past date.

Producer

The files these figures were derived from, on the machine that published this page

/home/marcus/state/mcp-census/surface-clock/surface-events.jsonl

Next

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