Which ancient Tamil work is the oldest extant Tamil grammar text?

The ancient Tamil work that is the oldest extant Tamil grammar text is the Tolkappiyam.

The Tolkappiyam is a comprehensive treatise covering Tamil letters and sounds, word formation, sentence structure, poetic metre and the relationship between language and literary context. Its surviving text is arranged in three books: Ezhuttatikaram, Sollatikaram and Porulatikaram, traditionally divided into nine chapters each.

The work contains 1,610 aphoristic sutras, although its date and composition are debated. Scholars commonly see it as a layered text whose sections may have been composed or expanded over several centuries, broadly from the late centuries BCE into the early centuries CE. Traditional accounts place it much earlier.

It is not the same thing as a Sangam anthology. Works such as Purananuru and Akananuru are collections of poetry, while Silappatikaram is an epic narrative. Later Tamil grammar, especially the Nannul in the medieval period, restated and clarified many grammatical principles, but it did not displace the Tolkappiyam’s status as the oldest surviving Tamil grammar.