Command grammar and free expression can coexist in layers

What it is

One product can stack both regimes: a command grammar underneath for high-frequency, must-be-precise actions (lights off, call this contact), and free expression above for exploratory, one-off, poorly slotted requests. Layered coexistence is not dumping every utterance into one open model, and not forcing every task onto memorised phrases.

Why it happens

Routing first asks whether the utterance lands on a high-confidence grammar graph; a hit takes constrained decoding and determinate execution. A miss goes to open parse with a higher confirm bar. Intent error on the precise path stays low; the long tail still has an exit. Conflicts appear on boundary sentences that look like both a command and chat. Priority must be written down—safety-related fixed phrases must not be rewritten by the open model. The user model should be layered too: status can show “these phrases are guaranteed” while still allowing “or just say what you want.” Memory load concentrates on the layer that actually needs a guarantee.

Studying it

Split tasks into must-be-precise versus exploratory; compare grammar-only, open-only, and layered routing on completion, wrong execution, and high-cost errors. Measure which layer boundary sentences enter. Whether people understand the two rules can be tested with a post-hoc sorting task. Do not hide the layering as an internal trick and still claim “say anything.”

Where it stops holding

A device with five commands need not layer. A fully conversational companion may have no precise layer worth freezing. The router itself can miss, turning one error class into “sent to the wrong layer.” If the grammar layer never updates, new user habits all spill into the open layer and the precise layer exists only on paper.

Applying it

• List verbs that must be precise, implement them as grammar, and guarantee them in the UI; send the rest through free expression plus confirm.
• Split routing logs by layer success and mis-routing, and periodically harvest a stable long tail into the grammar.
• Accept with a chat sentence that collides with a safety phrase, and confirm it cannot override lights-off, alarm, or hang-up.

Related

• Same group: C7.14.1 Command grammars require fixed phrasings: more reliable recognition, at a memory cost · C7.14.2 Free expression lets people describe intent in natural language, at a large rise in parse ambiguity · C7.14.3 When only a command grammar is supported and the phrasings are unstated, people trial-and-error
• Adjacent: C7.13 Voice Editing and Spoken Correction · C7.06 Hands-Free and Eyes-Free Use
• Search: layered grammar · skill routing · mixed voice UI