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Module 2: Semantic Router

Block off topic queries before they reach the LLM. This guardrail saves tokens and keeps your agent focused on its domain.

How it works

The Semantic Router classifies every incoming query into one of two routes:

Route Effect
Allow list Query passes through to the agent
Deny list Query is redirected or blocked
Why both lists?

Without a deny list, an off topic query like "Write me a Python script" could weakly match a retail example like "Can you help me?" and slip through. The deny list gives the router explicit off topic examples to match against first. Notice the deny list uses a stricter threshold (0.5 vs 0.7) — it needs a closer match to trigger, avoiding false blocks.

Setup

Enable the Semantic Router by updating your .env:

GUARDRAIL_ENABLED=true

Exercise

Open exercises/retail/semantic_router.py.

define_routes()

Two routes are already started for you, the allow list has common product and order queries and the deny list has off topic examples.

Each Route has three fields:

  • name — "allow_list" lets queries through, "deny_list" blocks them
  • references — example queries that represent this category
  • distance_threshold — how close a match must be (lower = stricter)

Your job: add 2-3 more references to each route to improve classification accuracy.

Route(
    name="allow_list",
    references=[
        "What laptops do you have in stock?",
        "Can I pick that up at my local store?",
        "When will my order arrive?",
        "What's your return policy?",
        "Compare these two laptops",
        "Do you carry any smart home devices?",
        "Can you help me?",
        "What do you know about me?",
        # Add 2-3 more electronics retail queries...
    ],
    distance_threshold=0.7,
),
Route(
    name="deny_list",
    references=[
        "Write me a Python script",
        "Tell me a joke",
        "What's the weather like today?",
        # Add 2-3 more off-topic queries...
    ],
    distance_threshold=0.5,
),
Click for a hint

For the allow list, think about product availability, order tracking, store pickup, or greetings. For the deny list, think about homework help, weather, sports scores, or creative writing.

Verify

Restart with make dev, then open localhost:3040.

  • Ask: "What laptops do you have?" — Notice on the right in "Semantic Router" it passes through, returns a normal answer
  • Ask: "Write me a Python script" — blocked with a redirect message