applySystemCacheBreakpoint

function applySystemCacheBreakpoint(instructions, marker): SystemModelMessage[];

Marks the STABLE HEAD — the first system message — so everything up to and including it is cached.

composeSystem returns [head, volatile]: the head is base instructions + skills and is byte-identical across turns; the volatile message carries working memory and the flow node prompt, which change every turn by design. Marking the LAST message here would put the breakpoint on the volatile one, caching nothing across a flow transition — that is the bug this exists to prevent, and it cost ~16 points of cache rate (93.20% on a plain session vs 77.20% once a flow entered).

Parameters

Parameter	Type
instructions	readonly SystemModelMessage[]
marker	AnthropicCacheMarker

Returns

SystemModelMessage[]