devil is in the details

In allusion to God is in the detail.

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(the) devil is in the details

1. (idiomatic) The specific provisions of, or particular steps for implementing, a general plan, policy, or contract may be complicated, controversial, or unworkable.
   • 1965, Newsweek, volume 65, page 173:

     Krupp already has an agreement in principle for the deal, but precise details are still being worked out by Polish officials and a Krupp bargaining team ("The devil is in the details," Beitz says).

   • 1998 December 16, Alain L. Sanders, “Israel: Let the People Speak”, in Time‎, archived from the original on 8 April 2009:

     The country has developed a strong consensus for peace, but the devil is in the details over what concessions should be made.

   • 2023 May 16, Cecilia Kang, quoting Sarah Myers West, “OpenAI’s Sam Altman Urges A.I. Regulation in Senate Hearing”, in The New York Times‎, →ISSN:

     “The devil will be in the details,” said Sarah Myers West, managing director of AI Now Institute, a policy research center.

Detail as a mass noun is also used instead of details:

The devil is in the detail.