Native Backends

Semantic Router uses native Rust and CGo bindings for learned classifiers, embedding-backed signals, multimodal embedding, and MLP model selection. The router exposes the selected backend as a runtime capability contract through CurrentNativeBackendCapabilities; callers should use that contract instead of inferring support from build tags or package names.

Backend selection

Build shape	Backend name	How it is selected
Default CGo build	candle	Build without onnx and with CGo enabled.
ONNX build	onnx	Build with the onnx tag and CGo enabled.
Non-CGo or Windows build	stub	Build with CGO_ENABLED=0 or on Windows.

Examples:

# Default Candle-backed build from the Go router module.
cd src/semantic-router
go build ./cmd

# ONNX-backed build.
cd src/semantic-router
go build -tags=onnx ./cmd

# Non-CGo stub build for environments where native bindings are unavailable.
cd src/semantic-router
CGO_ENABLED=0 go build ./cmd

Runtime capabilities

Capability	candle	onnx	stub
Unified batch classification	Yes	No	No
LoRA batch classification	Yes	No	No
Batched embedding	Yes	Yes	No
Multimodal embedding	Yes	No	No
Modality routing	Yes	No	No
MLP selector	Yes	No	No
Explicit reset	No	No	No

Unsupported unified or LoRA batch classification fails early through the router classification layer when the selected backend does not advertise that capability. Do not assume an ONNX or non-CGo build has Candle-compatible classifier behavior just because the Go package compiles.

Lifecycle expectations

No current backend advertises explicit_reset. Treat native model state as process-owned for deployment and hot-reload planning:

• prefer process restart for backend swaps or model-family changes that need a clean native state
• use the runtime capability output when deciding whether to enable backend specific features in control planes
• keep ONNX deployments to features that advertise support, primarily batched embedding until ONNX classifier parity is implemented

Backend lifecycle reset support and deeper ONNX classifier parity are tracked as open architecture debt in the native binding workstream.