I believe they have to reveal all of their evidence but not all of the intelligence they used to obtain the evidence. A quick search turns up the following Wikipedia page: https://en.wikipedia.org/wiki/Brady_disclosure So in summary they must reveal if they have evidence against the defendant that they plan to use in court, and they must disclose if they have information that could prove the innocence of the defendant. If law enforcement obtains information from a human intelligence agent that a suspect will be transporting drugs, and then the police monitor the suspect and pull them over due to a routine traffic violation, I do not believe they need to disclose that the original reason they pulled the defendant over was because of intelligence that they were transporting drugs. The defendant still broke a traffic law and was legally pulled over, if the officer then smells drugs or believes the suspect is 'acting suspiciously' or whatever, the car could then be searched and evidence found. If they don't believe they need the testimony of the undercover agent, I don't think they will even mention the existence of the undercover agent, as doing so would identify their intelligence asset needlessly, and it doesn't appear to be required as testimony from the undercover agent would not prove the innocence of the suspect (but rather the guilt). I also am not a lawyer though and if a lawyer corrects me I will stand corrected. I have seen several examples of cases where it appears that intelligence was used to obtain evidence, without the intelligence ever being disclosed.