Understand the mechanism

Documents are parsed, normalized, split into addressable chunks, enriched with metadata, embedded, and indexed. At query time, the system may rewrite the question, apply access filters, run vector or hybrid search, rerank candidates, fit the best evidence into a context budget, and generate an answer whose claims point back to sources.

Trace causality rather than memorizing vocabulary. First identify the state that exists before the operation. Next identify the computation and anything it persists. Finally identify what reaches the caller and what remains uncertain. That separation prevents a common category error: treating a convenient interface as proof that the underlying system learned, retrieved, secured, or validated something.

Here is the compact calculation to anchor the mechanism: if 18 of 20 test questions retrieve at least one relevant chunk in top 5, recall@5 = 18/20 = 0.90. The equation is useful only with its assumptions. Ask which quantities were measured, which were estimated, and whether an average hides a tail or subgroup. If the mechanism cannot explain a surprising metric, inspect the boundary conditions before tuning randomly.

For “Can I carry over vacation?”, hybrid search retrieves policy chunks. A reranker promotes the region-specific 2026 policy over a popular 2024 FAQ. The answer quotes the two-day limit and cites the exact chunk; without supporting evidence it abstains.

The worked number is if 18 of 20 test questions retrieve at least one relevant chunk in top 5, recall@5 = 18/20 = 0.90. State the unit and denominator whenever you report it. A percentage without a denominator can conceal a tiny sample; a latency without a percentile can conceal slow users; a similarity score without a labeled task can conceal irrelevant neighbors. Compare the observed value with a threshold chosen before seeing the final test result.

Now test the tempting shortcut. Suppose the team optimizes only the most visible metric. The result may look better while the system becomes less trustworthy. The reason is concrete: Small chunks retrieve precisely but lose surrounding context; large chunks preserve context but dilute matches. Higher top-k improves recall while adding distractors and tokens. Reranking improves ordering at extra latency. Fresh indexing costs compute but stale evidence damages trust. This is why the decision record must include both the intended gain and the tolerated regression. If the tolerated regression is unknown, the change is not ready for a consequential workflow.

• Prefer a measured baseline over a persuasive demo.
• Keep versions, inputs, and thresholds reproducible.
• Separate syntactic success from semantic correctness and authorization.
• Escalate or abstain when evidence falls outside the contract.
• Re-evaluate when data, traffic, models, providers, or user goals change.

These rules turn the topic into an engineering decision rather than a slogan. They also make disagreement productive: another person can challenge the assumptions, rerun the evaluation, and reach a documented conclusion.

At each mechanism step, annotate three things: the shape or type of data entering, the state read or written, and the possible error returned. Then ask whether rerunning that step is deterministic, probabilistic, or dependent on external state. This exposes bugs hidden by a successful final response. A useful trace includes versions and units—for example, tokens rather than characters, milliseconds at a named percentile, or vectors produced by a named embedding version. When an intermediate value cannot be observed directly, record the proxy and explain why it is informative.