Multi-turn RAG for Technical Documentation: Using Context-Aware Query Rewriting + Semantic Caching — Is This a Sound Approach?

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I’m building a conversational RAG system specifically for technical/engineering documentation (e.g., internal API guides, deployment manuals, configuration references). I’ve run into a common but frustrating issue:

In multi-turn conversations, follow-up questions often rely heavily on prior context (e.g., “How do I set permissions for that?”, “Does this parameter have a default value?”).
But since retrieval is based only on the raw user query—without resolving coreferences or leveraging dialogue history—the retrieved documents are frequently irrelevant, leading to poor or hallucinated answers.

To address this, I’m planning the following pipeline:

  1. Summarize or extract key entities/intent from the conversation history (e.g., current module, command, or config section being discussed).

  2. Rewrite the current user query into a self-contained, deictic-free statement using that context (e.g., transform “What about the timeout?” → “What is the default timeout value for the XYZ service?”).

  3. Use this rewritten query for vector retrieval, and cache the (rewritten_query → answer) pair in Redis with semantic similarity matching to improve latency and consistency for future similar requests.

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Seems good approach.

The approach is sound for conversational technical-doc QA. It is also incomplete as written. The missing pieces are the ones that usually determine whether it stays reliable under real engineering constraints: token preservation, hybrid retrieval, scope-aware caching, and cache validation.

Below is a “sound” version of your pipeline. It keeps your core idea (context-aware query rewriting + caching) but closes the common failure modes.

Why your current plan works in principle

Multi-turn follow-ups are underspecified

Follow-ups like “Does this parameter have a default?” omit the subject. Conversational search literature treats this as a primary retrieval problem, not a generation problem. QuReTeC frames it as “query resolution” because the current turn is often underspecified due to ellipsis, anaphora, and topic return. (arXiv)

Rewriting before retrieval is an established lever

The “Rewrite-Retrieve-Read” framing makes query adaptation explicit: if the query is misaligned, retrieval fails and downstream answers degrade. (arXiv)

Benchmarks bake this in

TREC CAsT’s baseline system includes rewriting and neural re-ranking. That is a strong signal that rewrite and rerank are not “nice-to-haves” for multi-turn retrieval. (TREC)

Where technical documentation changes the design

Technical corpora are identifier-heavy. Exact strings matter. Examples:

  • config keys and flags: --timeout, max_retries
  • API symbols: CreateFooRequest, FooClient::List
  • paths and resources: /v1/projects/{id}, arn:aws:...
  • environments and versions: prod, staging, v2.3

That changes two things:

  1. Rewriting must not paraphrase identifiers.
  2. Retrieval must not be vector-only because lexical match is often the highest-precision signal.

A sound end-to-end pipeline (your design, hardened)

Step 0: Intent gate

Only run retrieval when the turn is doc-QA. Do not rewrite-retrieve on casual chat or purely operational actions. This reduces cost and avoids “rewrite drift” from constant rewriting.

Practical heuristic signals:

  • contains a symbol, config key, error code, endpoint path
  • contains “default”, “permissions”, “flag”, “parameter”, “how do I”, “what does X mean”
  • references earlier answer: “that”, “it”, “this setting”

Step 1: Context extraction, but avoid “summary-only memory”

Why summarization is risky

Summaries drop tokens. That is catastrophic for config keys and API identifiers.

What to do instead

Maintain a small structured dialogue state (think “slots + anchors”), then optionally generate a short natural-language summary for readability, but never treat it as authoritative.

Minimum useful state:

  • Entity stack: most recent (component, symbol, config key, environment, version)
  • Constraints: tenant, role/permission scope, environment, version
  • Active anchors: last cited doc and chunk IDs (what the system actually used)

This is the practical cure for “What about the timeout?” because you can resolve “timeout” against the last active anchor and entity stack before you even involve an LLM.

Step 2: Context-aware query rewriting, but constrained

What “deictic-free” means

Deictic terms are “this, that, here, there, it, they”. A deictic-free query names the referent explicitly.

Example:

  • User: “How do I set permissions for that?”
  • Standalone query: “How do I set permissions for FooService deployment in prod on version v2.3?”

Constrained rewrite is the key

You want a rewrite that is:

  • explicit about the subject
  • explicit about constraints (env/version/role)
  • verbatim for identifiers

You can enforce this with two mechanisms:

  1. must_keep_tokens list extracted with rules (regex for snake_case, camelCase, flags, paths, error codes)
  2. rewrite prompt that says “copy these tokens exactly” and “do not invent versions/envs”

This keeps your “rewrite then retrieve” benefit while reducing the biggest technical-doc failure: identifier drift.

Add a conservative fallback for low confidence

When rewriting is ambiguous, do not force a single invented referent.

Use term-resolution fallback like QuReTeC: select terms from history to append to the current turn. It is less fluent than full rewriting but often safer in identifier-heavy domains. (arXiv)

Step 3: Retrieval should be hybrid, not vector-only

Vector-only retrieval is brittle when the “right answer” is gated by exact tokens.

Hybrid retrieval combines:

  • dense vectors for semantic similarity
  • sparse retrieval (BM25/BM25F) for exact keyword matching

This is widely documented and implemented:

  • Weaviate describes hybrid as fusing vector search and keyword (BM25F) search. (Weaviate)
  • Elastic describes hybrid as combining standard keyword queries with vector queries and merging results. (Elastic)
  • Qdrant’s guide explicitly pairs dense + sparse and then adds reranking. (Qdrant)
  • Weaviate’s explainer summarizes the intuition: dense is good for meaning, sparse is good for exact phrases. (Weaviate)

Add reranking for precision

After hybrid retrieval, rerank top N (say 50) with a cross-encoder or late-interaction model. That is the standard “precision stage” in conversational retrieval pipelines, and it is present in CAsT baselines. (TREC)

Step 4: Semantic caching is useful, but your cache key is unsafe as stated

Caching (rewritten_query → answer) with semantic similarity is a performance win. It is also a correctness trap in technical docs unless you scope and validate.

The good part

RedisVL’s SemanticCache is designed for semantic matching with tunable strictness and TTL. It exposes:

  • distance_threshold for semantic match strictness (Redis Vector Library)
  • filter_expression to restrict what can match (critical for scoping) (Redis Vector Library)
    LangChain’s Redis integration states the tradeoff directly: lower threshold increases precision but reduces cache hits. (LangChain)

The unsafe part

A “similar question” might require a different answer due to:

  • version differences
  • environment differences
  • permissions or tenant scope
  • doc updates

So you need scope tags and a validation step.

A safer caching design for technical docs

Tier 1: Exact cache

Key on a normalized structure, not just rewritten text:

  • normalized standalone query
  • must_keep_tokens
  • filters (version/env/component)
  • ACL scope tags (tenant, role)
  • corpus version (index build hash)
  • prompt version (answer format changes matter)

Exact cache is boring. It is also the highest-precision latency win.

Tier 2: Semantic cache, but scoped

Use RedisVL semantic search, but require:

  • filter_expression that enforces scope tags (tenant, role, env, version, corpus_version)
  • conservative distance_threshold

RedisVL documents both scoped filtering and thresholding as first-class. (Redis Vector Library)

Validate before serving a semantic hit

On a semantic hit, do a cheap retrieve (top 5 to 10) and compare to the cached “retrieval signature” (doc IDs, chunk IDs, fingerprints). If the evidence set is meaningfully different, regenerate and overwrite.

This prevents the worst-case failure: returning a confidently wrong cached answer because “the question looked similar.”

How to know it is working

Use component-level evaluation, not only “answer quality.”

Ragas provides retrieval and grounding metrics that map cleanly to your pipeline:

  • context precision and context recall for retrieval quality (docs.ragas.io)
  • faithfulness and answer relevancy for generation quality (docs.ragas.io)

A practical ablation plan:

  1. raw query only
    • rewrite / resolution
    • hybrid retrieve
    • rerank
    • exact cache
    • scoped semantic cache + validation

Track:

  • hallucination rate (faithfulness proxy)
  • retrieval relevance (context precision)
  • cache false-hit rate (semantic hits that fail validation)

Concrete “good default” architecture in one view

  1. Intent gate

  2. Dialogue state update (entities, constraints, anchors)

  3. Query contextualization

    • constrained rewrite producing a query bundle
    • fallback term-resolution if low confidence

  4. Retrieval

    • metadata filters
    • hybrid dense + sparse
    • rerank top N

  5. Answer with citations or refusal if evidence weak

  6. Caching

    • exact cache first
    • scoped semantic cache second
    • validate semantic hits against fresh retrieval

This is still your plan. It is your plan with the safety rails that technical documentation demands.

Curated references

Summary

  • Query rewriting for multi-turn retrieval is sound and well-supported. (arXiv)
  • For technical docs, enforce token preservation and use hybrid retrieval plus reranking. (Weaviate)
  • Semantic caching is safe only when scoped with filters and strict thresholds, and validated on hits. (Redis Vector Library)
  • Measure retrieval and grounding separately using context precision/recall and faithfulness. (docs.ragas.io)