Enhancement-overlay SOP for query-type routing — sending a query to the right index / tool / engine *before* retrieving, not after. Activate when a calling agent owns a retrieval or answering surface that fronts more than one handler (a summary index, a vector index, a text-to-SQL engine, a tool) and the inbound queries differ in kind: "summarize this doc" vs "find the clause about X" vs "how many orders shipped in Q3". Encodes the one non- negotiable insight — **one retriever cannot serve all query types; route first, retrieve second** — plus the three router families (LLM/selector, embedding/semantic, keyword/rule), confidence-threshold + fallback discipline, and the cross-framework mapping (LlamaIndex `RouterQueryEngine` / `SelectorPromptTemplate`, Dify Question Classifier node, LangGraph conditional edges). This is an ENHANCE overlay over the per-framework skills — cross-link `[[llamaindex]]`, `[[agentsop-dify]]`, `[[agentsop-langgraph]]` for the deep API. Search keywords: route query, semantic router, qu
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Enhancement-overlay SOP for query-type routing — sending a query to the right index / tool / engine *before* retrieving, not after. Activate when a calling agent owns a retrieval or answering surface that fronts more than one handler (a summary index, a vector index, a text-to-SQL engine, a tool) and the inbound queries differ in kind: "summarize this doc" vs "find the clause about X" vs "how many orders shipped in Q3". Encodes the one non- negotiable insight — **one retriever cannot serve all query types; route first, retrieve second** — plus the three router families (LLM/selector, embedding/semantic, keyword/rule), confidence-threshold + fallback discipline, and the cross-framework mapping (LlamaIndex `RouterQueryEngine` / `SelectorPromptTemplate`, Dify Question Classifier node, LangGraph conditional edges). This is an ENHANCE overlay over the per-framework skills — cross-link `[[llamaindex]]`, `[[agentsop-dify]]`, `[[agentsop-langgraph]]` for the deep API. Search keywords: route query, semantic router, query classification, RouterQueryEngine, multi-index routing, pick the right tool for a query.trigger_keywords: - "query routing" - "route to index" - "RouterQueryEngine" - "question classifier" - "conditional edge router" - "text-to-SQL or RAG" - "summary vs lookup query" - "selector" - "intent routing"
when_to_use
["a single answering endpoint fronts >=2 retrieval/answer handlers (summary index, vector index, SQL engine, tool) and queries differ in kind","users send a mix of lookup ('what is X'), summarize ('digest doc Y'), and compute ('how many Z') queries to one entry point","a monolithic VectorStoreIndex is being asked to also answer summary or aggregate queries and quality is uneven","designing the top-level shape of a RAG / agent system and deciding between one index vs a router over per-type engines","PR review of a router/classifier/conditional-edge that picks a downstream handler from the query"]
when_not_to_use
["exactly one index/tool can correctly serve every query — routing is dead weight","the split is by tenant/permission, not by query kind — that is multi-tenant filtering, use [[agentsop-multi-tenant-rag]]","the branch is a fixed deterministic step (always A then B) with no query-dependent choice — that is a static edge, not a router"]
Query-Type Routing · Enhancement Overlay
Third-person operating model for a coder agent that owns a multi-handler
answering surface. Audience is the LLM agent writing/reviewing the routing
code — not the end user.
One sentence: A retriever is shaped by the query type it was built for;
a summary index, a vector index, and a text-to-SQL engine are not
interchangeable — so classify the query and route first, then retrieve.
This is an ENHANCE overlay. It distills the cross-framework routing
pattern from three source skills. For the per-framework API, cross-link the
base skill: [[llamaindex]] (RouterQueryEngine), [[agentsop-dify]] (Question
Classifier node), [[agentsop-langgraph]] (conditional edges).
1. 何时激活 (Activation Rules)
Activate when any of the following holds:
The system has multiple indices / tools / engines behind one entry point,
and a query must be dispatched to exactly one (or a few) of them.
Inbound queries differ in kind — at least two of: lookup ("what does
the contract say about termination"), summarize ("give me the gist of doc
Y"), compute/aggregate ("how many tickets closed last week"), compare
("diff the 2024 vs 2025 policy").
A VectorStoreIndex (or any single retriever) is being stretched to answer
query types it was not built for, and quality is uneven across the mix.
The user names a routing primitive: RouterQueryEngine,
SelectorPromptTemplate, LLMSingleSelector, Dify Question Classifier,
LangGraph add_conditional_edges, "intent classifier", "text-to-SQL or RAG".
PR review touches a function that reads a query and returns which handler
to call.
Do not activate when:
One index/tool serves every query correctly — routing adds an LLM hop and a
failure mode for no benefit (see §6 anti-pattern A1).
The split is by who is asking (tenant / ACL), not what is asked — that
is filtering, route to [[agentsop-multi-tenant-rag]].
The downstream choice is fixed (always retrieve then summarize) — that is
a static edge / linear pipeline, not a router.
2. 核心心智模型 (Core Mental Model)
Three principles. Violating any of them produces a router that misroutes
silently or routes when it should not.
Principle 1 — One retriever cannot serve all query types
The index taxonomy is not cosmetic. From [[llamaindex]]: a SummaryIndex is
a "small, fan-out synthesis" primitive — it reads every node to digest a doc;
a VectorStoreIndex is top-k semantic lookup — it reads the few most similar
chunks; a text-to-SQL engine answers aggregate/compute queries that no chunk
contains the answer to. Ask a vector index to "summarize the whole document" and
it returns 4 arbitrary chunks; ask a summary index "what is the late-fee clause"
and it fans out over the whole corpus wastefully. The query type names the
correct primitive. Routing is the act of recovering that name at runtime.
Operational corollary: index.as_query_engine() over a single
VectorStoreIndex answering a heterogeneous query mix is the symptom this
skill exists to fix. The fix is per-type handlers + a router on top.
Principle 2 — Route first, retrieve second
Routing is a classification step that runs before any retrieval. It reads
only the query (and optionally light context) and emits a destination, not an
answer. This ordering is what bounds latency and cost: you pay for the router
once, then exactly one downstream handler, instead of fanning out to all of
them and merging. LlamaIndex's RouterQueryEngine, Dify's Question Classifier
node feeding IF/ELSE branches, and LangGraph's conditional edge over state
are the same shape — a selector function (query) -> handler_id evaluated up
front. The three frameworks differ only in how the selector is implemented
(§7).
Principle 3 — A router is only as good as its destinations' descriptions
Every router — LLM, embedding, or keyword — picks among destinations described
in words or examples. In LlamaIndex the signal is the
QueryEngineTool.description; in Dify it is the class label + instruction; in
LangGraph it is whatever the routing function reads off state plus the node
names. From [[llamaindex]] Dilemma 3: "invest in QueryEngineTool.description
— it's the only signal the router/agent sees." A misroute is, four times out
of five, a bad description, not a bad model. Fix the description before
swapping the router type.
The fourth case is genuinely ambiguous queries — for those, Principle of
Fallback (§3 Stage 4) applies: route to a safe default, never guess silently.
3. SOP 工作流 (Agentic Protocol)
Five stages. Each gates the next. Stop and reconsider at the first "no".
Stage 0 — Confirm routing is warranted
Gate questions:
Are there genuinely ≥2 handlers with different query-type fit? If no →
single index, no router (anti-pattern A1).
Does real traffic actually span query kinds? Sample 50 real queries and
label them. If >90% are one kind → build that handler well; skip the router.
Is the choice query-dependent (not fixed)? If fixed → static edge.
If all three are yes, continue.
Stage 1 — Enumerate the query types and their handlers
Build the table before writing the router. One row per query kind:
Query kind
Example
Correct handler
Primitive
Summarize / digest
"summarize the Q3 report"
summary engine
SummaryIndex
Fact lookup
"what is the late-fee clause"
vector engine
VectorStoreIndex + filters
Compute / aggregate
"how many orders shipped in Q3"
text-to-SQL engine
NL2SQL over the DB
Compare / multi-hop
"diff 2024 vs 2025 policy"
decomposition engine
SubQuestionQueryEngine
Out of scope
"what's the weather"
default / refuse
fallback handler
This table is the spec for both the handlers and the router. Mapping mirrors
[[llamaindex]] Stage 4 ("Compose for query heterogeneity").
Stage 2 — Build one handler per type, in isolation
Implement and test each engine independently against its own query kind
before wiring the router. A misroute is undebuggable if the handlers
themselves are wrong. Author each handler's description / label here
(Principle 3) — the destination metadata is part of the handler, not the router.
Stage 3 — Write the router (pick the cheapest sufficient family)
Three router families, cheapest to most capable:
Family
How it decides
Pick when
Keyword / rule
regex / substring / heuristic over the query
destinations are lexically distinct ("SELECT", "summarize", file extensions); latency-critical; cost-critical
Embedding / semantic
embed query, nearest destination description
destinations semantically distinct but not lexically; no per-call LLM budget; deterministic-ish
LLM / selector
LLM reads query + descriptions, returns choice (single or multi)
destinations need reasoning to disambiguate; multi-select needed; quality > latency
Default ladder: start keyword if the types are lexically separable; else
LLM selector; reach for embedding when you want a middle point (no LLM hop,
better than keyword). Always emit a confidence / score, never just a label.
Stage 4 — Add the fallback default (non-negotiable)
Every router must define behavior for the unrouteable query:
LLM selector returns low confidence / no match → route to a documented
default handler (usually the broad vector index) or an explicit "I can't
answer that" path. Never let an ambiguous query silently hit a random branch.
Set a confidence threshold: below it → fallback, not best-guess.
Log every routing decision {query_hash, chosen_handler, score, fell_back}
so misroutes are observable, not anecdotal.
A router with no fallback is the single most common production failure here
(anti-pattern A2).
4. 操作模型 (Operation Models)
Format: Trigger / Action / Output / Evidence.
OP-01 EnumerateQueryTypes
Trigger: A single endpoint fronts heterogeneous queries; no type table exists.
Action: Sample ≥50 real queries, label each by kind (summarize / lookup /
compute / compare / out-of-scope), map each kind to its correct handler +
primitive (Stage 1 table).
Output: A query-type → handler spec table that drives both handler build
and router design.
Evidence: [[llamaindex]] Stage 0 ("What is the query distribution?") +
Stage 4 heterogeneity table.
OP-02 BuildPerTypeHandler
Trigger: Type table exists; handlers not yet built.
Action: Implement one engine per type (SummaryIndex for digest,
VectorStoreIndex for lookup, NL2SQL for compute, SubQuestionQueryEngine
for compare). Test each against its own kind in isolation. Author its
description/label.
Output: N independently-verified handlers, each with a destination
description (Principle 3).
Trigger: Disambiguation needs reasoning; or a query maps to multiple handlers.
Action: Use an LLM selector (single or multi) reading the query + each
handler's description; return chosen id(s) + reasoning. In LlamaIndex:
RouterQueryEngine(selector=LLMSingleSelector.from_defaults(), query_engine_tools=[...]).
Output: Highest-accuracy routing including multi-route fan-out; costs one
LLM round-trip.
Trigger: Any router that can be uncertain (LLM/embedding) on ambiguous queries.
Action: Read the selector's score; below threshold → route to documented
default (broad vector index) or explicit refuse path. Never best-guess silently.
Output: Unrouteable queries land somewhere safe and auditable.
Evidence: [[agentsop-langgraph]] conditional edge can return a "__default__"
branch; [[agentsop-dify]] Question Classifier has a built-in "other/else" class.
OP-07 RouteDecisionLogging
Trigger: Any production router.
Action: Log {ts, query_hash, chosen_handler, selector_score, fell_back}
on every decision; surface a misroute dashboard.
Output: Misroutes become observable metrics, not user-reported anecdotes.
Evidence: [[agentsop-dify]] 7-class trace incl. routing; [[agentsop-langgraph]]
LangSmith trace of the conditional edge.
OP-08 DescriptionFirstDebug
Trigger: Router misroutes a known query class.
Action: Before swapping router type, rewrite the destination description /
add 1-2 disambiguating examples; re-evaluate on a labeled routing eval set.
Output: Most misroutes fixed at the description layer, no model change.
Evidence: [[llamaindex]] Dilemma 3 ("the only signal the router sees").
5. 困境决策案例 (Dilemma Cases)
Dilemma 1 — Router misroutes: improve the classifier or add a fallback?
困境: A 3-way router (summary / lookup / SQL) misroutes ~12% of queries —
some lookup queries land on the SQL engine and error out. The team is split:
fine-tune / swap to a bigger LLM selector, vs. add a catch-all fallback.
约束:
Bigger LLM selector adds latency + cost to every query to fix 12%.
A pure fallback masks the misroute but still sends those queries somewhere
generic, lowering answer quality for the 12%.
The misroutes cluster: most are lookup queries phrased like questions the SQL
engine "thinks" it can answer.
决策步骤:
Diagnose before fixing — pull the routing log (OP-07); cluster the 12%.
The clustering reveals it's a description problem, not a model problem
(the SQL engine's description over-claims).
Fix descriptions first (OP-08) — tighten the SQL handler's description to
"aggregate/count/sum over the orders table only"; add 2 negative examples.
Re-run the routing eval set. Misroute drops to ~4%.
Then add fallback (OP-06) for the residual ambiguous tail — low-confidence
→ broad vector index, not SQL.
Only if still bad consider the bigger selector — but now on 4%, the
cost/benefit usually says no.
结果: Description fix + fallback, no model change. The two moves are
complementary, not either/or: improve the classifier signal (descriptions)
and add a fallback for the irreducible ambiguity. Mirrors [[llamaindex]]
Dilemma 3's "fix the supervisor before switching paradigms" logic and
[[agentsop-langgraph]]'s "hitting the limit means the logic is wrong" stance.
可提取的操作: OP-08, OP-06, OP-07.
Dilemma 2 — LLM router latency vs cheap keyword router
困境: An LLM selector routes correctly but adds ~600ms + a token cost to
every query. Traffic is high-QPS and most queries are lexically obvious
("summarize…", SQL-shaped, or a plain question). Is the LLM hop worth it?
约束:
The LLM selector is accurate but is now the p95 latency bottleneck and a
per-query cost line.
A keyword router is free and instant but brittle on the genuinely ambiguous
tail.
[[agentsop-dify]]'s known per-node latency overhead and [[agentsop-langgraph]]'s
"checkpoint serialisation adds overhead, latency budget <200ms" boundary both
argue against an LLM hop on every request.
决策步骤:
Measure the separable fraction — what % of queries a cheap keyword/regex
router classifies with high confidence? Sample says ~80%.
Tier the router: cheap keyword/embedding router first; only the residual
~20% (no confident keyword match) escalates to the LLM selector. This is the
routing analog of [[llamaindex]]'s "exhaust cheap knobs first" and
[[agentsop-langgraph]]'s "promote upward only as needed" ladder.
Set the keyword-confidence bar high — a wrong cheap route is worse than a
slow correct one; when in doubt, escalate.
Re-measure p95 and cost; the LLM hop now runs on 20% of traffic.
结果: A two-tier (cascade) router — cheap router handles the obvious
majority instantly, LLM selector handles the ambiguous minority. Cost and p95
drop ~5×; accuracy is preserved because the cheap tier only acts when confident.
Reserve the LLM selector for where reasoning is actually required (Principle 2 +
the §3 Stage 3 ladder).
可提取的操作: OP-03, OP-04, OP-05, OP-06.
Dilemma 3 — Route to one handler vs multi-select fan-out
困境: Some queries legitimately need two handlers ("summarize the contract
and tell me the late-fee clause"). A single-select router forces a wrong
binary choice.
约束:
Multi-select doubles downstream cost and needs a synthesis step to merge.
Most queries are single-intent; defaulting to multi-select wastes spend.
决策步骤:
Quantify the multi-intent fraction from the routing log. If <10% → keep
single-select + fallback; accept occasional follow-up.
If material → use a multi-selector (LlamaIndex LLMMultiSelector /
Dify branching to multiple nodes / LangGraph Send fan-out to multiple
handlers) and add a synthesis node to merge results.
Never make multi-select the default — it converts a routing problem into a
fan-out + merge problem with the cost of both.
结果: Single-select by default; multi-select only on the measured
multi-intent slice, paired with explicit synthesis. Mirrors [[agentsop-langgraph]]
"don't fan out with Send for fixed-cardinality work."
可提取的操作: OP-05, OP-07.
6. 反模式与边界 (Anti-patterns & Boundaries)
Top anti-patterns (red flags in code review)
#
Anti-pattern
Why it's wrong
Correct move
A1
Adding a router when one index already serves every query
Pure overhead: an extra hop + a new failure mode for zero benefit
Single index; route only when ≥2 handlers differ in fit (Stage 0)
A2
Router with no fallback / default branch
Ambiguous or out-of-scope queries hit a random or erroring handler
p95 latency + per-query token cost for separable traffic
Tier: cheap router first, LLM only for the ambiguous tail (Dilemma 2)
A4
Vague destination descriptions
Router can't disambiguate; misroutes blamed on the model
Author precise descriptions + examples; fix here first (OP-08)
A5
No routing decision logging
Misroutes surface as user complaints, not metrics
Log {query, chosen, score, fell_back} per decision (OP-07)
A6
Routing by tenant/permission instead of query kind
That's access control, not query routing
Use [[agentsop-multi-tenant-rag]] filter at the store; route by kind only
A7
Multi-select as the default
Doubles cost + needs merge for mostly single-intent traffic
Single-select default; multi only on measured multi-intent slice (Dilemma 3)
A8
Best-guess on low confidence
Silent wrong route degrades the answer with no signal
Below threshold → fallback, never guess (OP-06)
A9
Tuning the router before the handlers work
A misroute is undebuggable atop broken handlers
Build + verify each handler in isolation first (Stage 2)
Boundaries — when query routing is not the move
B1 One index/tool answers everything correctly → no router (A1).
B2 The split is by who asks (tenant, ACL) → [[agentsop-multi-tenant-rag]].
B3 The downstream is a fixed sequence (always retrieve→summarize) →
static edge / linear pipeline, no selector.
B4 Hard real-time (<200ms) and an LLM selector is the only correct router
→ measure first; tier to a keyword/embedding front, escalate rarely (Dilemma 2).
B5 The choice is agentic/iterative (retrieve→reflect→re-query in a loop)
→ that's an agent loop, not a one-shot router; use [[agentsop-langgraph]] cycles.
PR review smells
A RouterQueryEngine / classifier with no default / other branch.
An LLM selector in a high-QPS hot path with no cheap pre-filter.
Routing logic that reads tenant_id — that's filtering, not routing.
A router whose handlers were never tested independently.
No eval set of labeled (query, expected_handler) pairs gating router changes.
7. 跨框架对照 (Cross-Framework Reference)
The same selector (query) -> handler_id surface across the three base skills.
All verified against the source SKILLs (May 2026). Cross-link the base skill for
the full API.
from llama_index.core.query_engine import RouterQueryEngine
from llama_index.core.selectors import LLMSingleSelector # or LLMMultiSelector,# EmbeddingSingleSelectorfrom llama_index.core.tools import QueryEngineTool
tools = [
QueryEngineTool.from_defaults(
query_engine=summary_engine,
description="Useful for SUMMARIZING or digesting an entire document."),
QueryEngineTool.from_defaults(
query_engine=vector_engine,
description="Useful for LOOKING UP a specific fact or clause."),
]
router = RouterQueryEngine(
selector=LLMSingleSelector.from_defaults(), # LLM selector family
query_engine_tools=tools, # descriptions are the routing signal
)
The selector reads each QueryEngineTool.description (the only routing signal —
Principle 3). Selector families: LLMSingleSelector, LLMMultiSelector,
EmbeddingSingleSelector, PydanticSingleSelector. SelectorPromptTemplate
customizes the LLM prompt. RouterQueryEngine over per-task indices is "often
the correct top-level shape, not a single monolithic VectorStoreIndex"
([[llamaindex]] Principle 3 + OP-06).
The Question Classifier node (an LLM node) takes the query and emits one of
N declared classes; each class wires to a downstream branch (Knowledge
Retrieval / LLM / Code / HTTP / SQL-via-Code). It is the visual analog of an
LLM selector. The built-in "other" class is the fallback (OP-06). Routing logic
that doesn't need an LLM uses the IF/ELSE node (keyword/rule router, OP-03).
Node taxonomy: Question Classifier, Parameter Extractor, IF/ELSE. The
classifier's class label + instruction is the routing signal (Principle 3).
7.3 LangGraph — conditional edges as routers → [[agentsop-langgraph]]
"Graph topology is just routing logic over state… a conditional edge reads
state and picks a next node" ([[agentsop-langgraph]] Principle 3). The mapping dict's
keys are the destinations; the route function is the selector — it can be
keyword, embedding, or LLM, or a tier of all three (Dilemma 2). For genuine
multi-route fan-out, return a list of Send(...) instead of one label.
7.4 Side-by-side
LlamaIndex
Dify
LangGraph
Router primitive
RouterQueryEngine
Question Classifier node
add_conditional_edges
Selector impl
LLM / Embedding / Pydantic selector
LLM classifier (or IF/ELSE for rules)
any Python fn (keyword/embed/LLM)
Routing signal
QueryEngineTool.description
class label + instruction
node names + what route() reads
Fallback
selector default / catch-all tool
"other" class
a "__default__" branch
Multi-route
LLMMultiSelector
branch to multiple nodes
list of Send(...)
Deep skill
[[llamaindex]]
[[agentsop-dify]]
[[agentsop-langgraph]]
The three are the same pattern — classify the query up front, dispatch to
the structurally-correct handler, fall back when uncertain. Pick the framework
your stack already uses; the routing discipline (Principles 1-3, Stages 0-4) is
identical.
References
references/R1-source-evidence.md — every cited claim resolved to its source
SKILL line.
