I have heard a story that a sankalpa (solemn vow) to destroy the snake race was taken during Janmajeya's snake sacrifice (yajna). But as that sankalpa couldn't be fulfilled as the yajna was stopped by Astika. This made Agni, who was waiting for the sacrifice of all snakes, infuriated and hence he cursed the Vedic mantras to be ineffective just as poisonless snakes.
This story sounds logical. Where is it mentioned?
Mahabharata, in which the snake sacrifice was mentioned first, did not describe anything about Agni cursing Veda mantras to become ineffective.
The following is the extract from Mahabharata, where the final part of the snake sacrifice was described.
"Sauti said, 'Listen now to another very wonderful incident in connection with Astika. When king Janamejaya was about to gratify Astika by granting the boon, the snake (Takshaka), thrown off Indra's hands, remained in mid air without actually falling. King Janamejaya thereupon became curious, for Takshaka, afflicted with fear, did not at once fall into the fire although libations were poured in proper form into the blazing sacrificial Agni in his name.'
"Saunaka said, 'Was it, O Suta, that the mantras of those wise Brahmanas were not potent; since Takshaka did not fall into the fire?'
"Sauti replied, 'Unto the unconscious Takshaka, that best of snakes, after he had been cast off Indra's hands, Astika had thrice said, 'Stay,' 'Stay,' 'Stay.' And he succeeded in staying in the skies, with afflicted heart, like a person somehow staying between the welkin and the earth.
"The king then, on being repeatedly urged by his Sadasyas, said, 'Let it be done as Astika hath said. Let the sacrifice be ended, let the snakes be safe, let this Astika also be gratified, O Suta, thy words also be true.' When the boon was granted to Astika, plaudits expressive of joy rang through the air. Thus the sacrifice of the son of Parikshit--that king of the Pandava race--came to an end. The king Janamejaya of the Bharata race was himself pleased, and on the Ritwiks with the Sadasyas, and on all who had come there, the king, bestowed money by hundreds and thousands. And unto Suta Lohitaksha--conversant with the rules of building and foundations--who had at the commencement said that a Brahmana would be the cause of the interruption of the snake-sacrifice, the king gave much wealth.
The king, of uncommon kindness, also gave him various things, with food and wearing apparel, according to his desire, and became very much pleased. Then he concluded his sacrifice according to the prescribed rites, and after treating him with every respect, the king in joy sent home the wise Astika exceedingly gratified, for he had attained his object. And the king said unto him, 'Thou must come again to become a Sadasya in my great Horse-sacrifice.' And Astika said, 'yes' and then returned home in great joy, having achieved his great end after gratifying the monarch. And returning in joy to his uncle and mother and touching their feet, he recounted to them everything as it had happened.'
So this story must be from folklore or an interpolated story.
Further, Rig Veda I.164.46 says, Agni is the BRAHMAN.
They call him Indra, Mitra, Varuna, Agni, and he is heavenly nobly-winged Garutman. To what is One, sages give many a title they call it Agni, Yama, Matarisvan.
The BRAHMAN cannot and will not curse Vedic mantras to become ineffective.
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