Function Budget

internal/ctx/ctx.go:395–401

Budget subtracts the fixed reservations from the total context size, then leaves a headroom margin (see budgetHeadroomDivisor) so a char/4 undercount can't push the real prompt past the declared window.

(ctxSize int)

					break
				}
			}
			if dangling {
				continue
			}
		}
		out = append(out, m)
	}
	return out
}