Select

JevSingleSelector and JevMultiSelector are LlamaIndex BaseSelectors: typed tool / query-engine choice with TypeSafe Jev, without an LLM-as-judge.

from llama_index.core.tools import ToolMetadata
from llama_index.selectors.jev import JevSingleSelector

selector = JevSingleSelector()
result = selector.select(
    [
        ToolMetadata(name="weather", description="Forecasts and current conditions"),
        ToolMetadata(name="docs", description="Product documentation lookup"),
    ],
    "Will it rain in Paris tomorrow?",
)
print(result.ind, result.reason)

JevSingleSelector plugs into RouterQueryEngine:

from llama_index.core.query_engine import RouterQueryEngine
from llama_index.selectors.jev import JevSingleSelector

engine = RouterQueryEngine(
    selector=JevSingleSelector(),
    query_engine_tools=[weather_tool, docs_tool],
)

See examples.

Fail closed

Wrong routing is worse than slightly-worse ranking. If the API errors, if Jev returns a choice that is not in the option set, or if confidence is below confidence_threshold, JevSingleSelector raises — unless you set default_index, in which case that one selection is returned with a reason that names both the fallback and the error.

This is TypeSafe's "don't act when uncertain." A low-confidence but in-schema choice is treated as failure, not as success.

Single vs multi

• JevSingleSelector asks one Choice question whose option names are the tool names (when those names are unique). The model sees names and descriptions, not opaque option_0 labels.
• JevMultiSelector asks one Noul per option in a single system_one call (the query is the whole state, so batching questions is correct). Options with noul > threshold are kept, in original order.

If there are more than 255 options, the multi-selector splits the question map across calls of at most 255 keys. That is the only 255-chunking in this repo; it is a safety cap on question-map size, not a rerank batch size.

Duplicate or empty tool names fall back to option_{i} so Choice keys stay unique.

Multi-select never returns an empty list

If zero Nouls clear threshold, JevMultiSelector returns exactly the highest-noul choice. An empty selection would leave a router with nothing to call. default_index is not used for this case — it is API-error fallback only.