For investors deciding where to put new money in the U.S. this summer, the latest research from major asset managers points to a market that still offers opportunities — but requires more selectivity than during the earlier phase of the artificial-intelligence rally.

The broad message from Vanguard, J.P. Morgan, BlackRock, Fidelity and Schwab is not that investors should abandon equities. Instead, the firms increasingly emphasize diversification, high-quality fixed income, value-oriented stocks and the infrastructure supporting artificial intelligence, while warning that elevated valuations and persistent inflation make indiscriminate buying riskier.

The investment picture has also become more complicated in August. Vanguard…