तपस्तपःकर्मकस्यैव

Operational rule vidhi

The agent of the verb तप् 'to heat', becomes similar to the object, only when the object is the word तपस् itself.

Word by word (padaccheda)

Target तपः ṣaṣṭhī · ekavacanaTarget तपःकर्मकस्य ṣaṣṭhī · ekavacanaIndeclinable एव avyaya

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

तपः तपःकर्मकस्य एव

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The agent of the verb तप् 'to heat', becomes similar to the object, only when the object is the word तपस् itself.
Thus तप्यते तपस्तापसः 'the devotee performs austere devotion'; but not so in उत्तपति सुवर्णं सुवर्णकारः 'the goldsmith heats the gold'.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.