IS-6 A budget on served tool schemas

A project that serves MCP tools to its own agent SHOULD hold the serialized size of their schemas under a ceiling that a gate enforces.

Field	Value
Keyword	SHOULD
Level	3
Checked	by judgement
Findings	F4, F406, F8
Harness facts	H8, H9
Threat	external: a harness release decides how much of the schema a session is sent
Status	active

Rationale: Shio and roadkeep measured their served tool schemas as the largest fixed cost of a session, held them under ceilings that fail the build, and met each later addition by trimming a description or raising the ceiling with a written reason; Shio's test failed on a real change three tokens over, answered by a trim (F4, R3/S4). The ceiling did not keep roadkeep's total from rising twelve times; what it did was make each rise a recorded decision. The other answer to the ceiling was to keep a capability off the list: Shio shipped three as CLI verbs because its agent had a shell (F8, R2/S2). The finding is contested by the harness documentation: with tool search on, the default read on 2026-09-24, Claude Code does not send every tool definition up front (H9). The measured sizes are not in doubt, but what a session pays for them is, and that disagreement is a property of the harness version rather than of the cases, so the rule stands one level below the finding's grade. The ceiling that projects put on surfaces they ship to other agents is a separate matter (F406), treated in the chapter on agent-facing product surfaces.