Kaal Sarpa yoga in the classical sense
Kaal Sarpa yoga, in the classical sense, is a combination described in some texts with the grahas hemmed between the nodes. Its name is more recent in common use, and the rule is not in every classical list.
What it is
The rule this site checks is that all seven grahas lie on one side of the Rahu–Ketu axis. Other traditions describe the yoga with different conditions, such as the grahas in particular nakshatras or houses.
How it is worked out or read
The site's chart check states the Rahu–Ketu rule and whether it holds. It does not add the other conditions or any remedy.
Where it appears on this site
See the chart checks and the cancellation article for the general idea of exceptions.
The definition
Kaal Sarpa yoga is said to form when all seven visible grahas, from the Sun to Saturn, lie on one side of the axis between Rahu and Ketu. Because Rahu and Ketu are always exactly opposite each other, the axis divides the zodiac into two halves, and the condition is that every planet falls within the same half. Some practitioners distinguish the case where the planets move from Rahu towards Ketu from the reverse, and some give twelve named types according to the house Rahu occupies.
Its place in the tradition
Kaal Sarpa is not described in the major classical texts, including the Brihat Parashara Hora Shastra, the Brihat Jataka, the Saravali and the Phaladeepika. It appears in later writing and became widely popular in the twentieth century. This does not by itself make it meaningless, since traditions develop, but it does mean that readers should not present it as a classical yoga, and that its definitions and results vary much more than those of yogas with a long textual history.
Partial forms and edge cases
Many practitioners speak of partial Kaal Sarpa when one planet lies just outside the axis, or when a planet is conjunct Rahu or Ketu. Others count the planets by degree, so that a planet in the same sign as Rahu but on the other side of its degree breaks the yoga. These variations mean that two readers can disagree about whether the yoga exists in the same chart. The chart checks tool on this site states the rule it applies, so that the result can be checked.
How common it is
Because Saturn and Jupiter move slowly, there are stretches of months when all the planets fall in one half of the zodiac, and everyone born during those months has the yoga in its strict form. Partial forms are more common still. A pattern shared by a large number of people born over several months cannot sensibly be read as a personal misfortune for each of them, which is one more reason to treat dramatic claims about it with caution.
Caution about remedies
Kaal Sarpa is one of the conditions most often used to sell expensive rituals, sometimes at particular temples with high fees. A reader who is told their chart has this yoga should ask which definition is used, whether it is complete or partial, and what the rest of the chart shows, before considering any remedy. If a remedy is chosen, a simple prayer or an affordable observance is fully within tradition, and no ritual should be undertaken under fear or pressure.