Do not implement joins by asking an LLM to generate text over relational content. Text-to-text joining loses the structural constraints that make joins meaningful, and foundation models provide no rigorous query guarantees, so the reliable architecture is to use LLMs for natural language translation/planning and let a relational engine execute the resulting query.
Agentic systems that stitch data across documents/tables in prompt context are vulnerable to plausible but structurally invalid combinations; preserving a deterministic query path is necessary where correctness of the join matters.
If a subproblem can be solved by a deterministic engine with formal semantics, assign the deterministic engine the operation and use the LLM only for translation, generation, or summarization.
Treating data as raw text and performing joins via text-to-text generation throws away valuable structural information.
Foundation models are text-to-text only and lack rigorous structural query guarantees.