Fit Extension for UCP

Name ai.vsize.shopping.fit
Schema https://vsize.ai/ucp/schemas/fit.json
Extends dev.ucp.shopping.catalog.search, dev.ucp.shopping.catalog.lookup
Discussion UCP #1027
Background The missing floor, drafted

Purpose

An agent buying shoes on someone's behalf cannot try the shoe on. Today the Universal Commerce Protocol carries size as a label (Size: EU 40) and fit as free text through ask. Neither is something an agent can compute with. This extension makes fit a measurable property: the variant carries the dimensions of the space inside the shoe; the buyer may supply consented foot measurements; when both are present, the variant also carries a zone-by-zone assessment with an explicit confidence.

Fit is a comparison between two measurements, not a summary of reviews.

Principles

MillimetresAll values in millimetres. Every dimension has a tolerance and a source.
UncertaintyEvery assessment has a confidence. unknown is a valid verdict; an assessor must not guess.
ConsentThe foot profile belongs to the buyer, is supplied per request, and is not stored by the business without separate consent.
VocabularyField names follow ISO 19409 and ISO 19410-1 for the inside of the shoe, ISO 7250 and Mondopoint (ISO 9407) for the foot.
ExclusionsNo reviews, no return statistics, no size conversion (ISO 19407 covers that).

1. The variant carries its internal dimensions

"fit": {
  "internal_dimensions": {
    "functional_length_mm": 257.0,
    "ball_width_mm": 92.5,
    "ball_girth_mm": 238.0,
    "instep_girth_mm": 244.0,
    "heel_width_mm": 63.0,
    "toe_height_mm": 27.5,
    "tolerance_mm": 2.0,
    "source": "last_derived",
    "vocabulary": "ISO 19409:2022 / ISO 19410-1:2022",
    "last_reference": "opaque-id"
  }
}

Field	Meaning
`functional_length_mm`	Usable internal length, heel to where the longest toe is meant to reach. Required.
`ball_width_mm`, `ball_girth_mm`	Width and girth at the ball.
`instep_girth_mm`	Girth at the instep.
`heel_width_mm`	Width at the heel seat.
`toe_height_mm`, `heel_height_mm`	Toe-box height; heel height of the shoe.
`tolerance_mm`	Measurement uncertainty. Required.
`source`	One of `last_measured`, `last_derived`, `in_shoe_measured`, `estimated`. Required.
`last_reference`	Opaque id of the last, so an agent can see that two variants share one.

2. The buyer supplies a foot profile, with consent

Sent in catalog context, which UCP defines as provisional, non-identifying buyer signals. One entry per foot; feet differ.

"context": {
  "foot_profile": {
    "feet": [
      { "side": "left",  "length_mm": 251.0, "ball_width_mm": 94.0, "ball_girth_mm": 236.0, "instep_girth_mm": 241.0, "heel_width_mm": 62.0 },
      { "side": "right", "length_mm": 254.5, "ball_width_mm": 95.0, "ball_girth_mm": 238.0, "instep_girth_mm": 243.0, "heel_width_mm": 62.5 }
    ],
    "measurement_method": "phone_scan",
    "measured_at": "2026-10-20",
    "tolerance_mm": 1.5,
    "consent": true
  }
}

3. The assessment carries its uncertainty

Returned on the variant only when a foot profile was supplied and the assessor could compare.

"fit": {
  "internal_dimensions": { "...": "..." },
  "fit_assessment": {
    "overall": "good",
    "confidence": 0.78,
    "foot_side": "right",
    "zones": [
      { "zone": "toe",    "delta_mm":  4.5, "verdict": "good" },
      { "zone": "ball",   "delta_mm": -1.5, "verdict": "snug" },
      { "zone": "instep", "delta_mm":  2.0, "verdict": "good" },
      { "zone": "heel",   "delta_mm":  0.5, "verdict": "good" }
    ],
    "limiting_zone": "ball",
    "assessed_by": "ai.vsize",
    "method": "geometric comparison, last-derived dimensions"
  }
}

overall and each zone verdict are one of good, snug, tight, loose, unknown. confidence and assessed_by are required, so an agent always knows how sure the assessment is and who made it.

Non-goals

No reviews, keep rates or return statistics.
No size-system conversion.
No storage of foot data on the business side.
Footwear only in this draft. The pattern may transfer to apparel; that is out of scope here.

Open questions

Do internal_dimensions belong on the catalog variant, or in a separate capability?
May foot_profile travel in catalog context, or does buyer measurement data need its own consent path, like identity linking?
May vocabulary reference an externally governed, paid standard, the question UCP #971 raises for print?

Status and path

This is a draft published under a vendor namespace, as the UCP contribution guide recommends for emerging use cases. If a second party, a merchant or a scanning provider, wants to expose or consume these fields, the intent is to propose dev.ucp.shopping.fit through an Enhancement Proposal to the Shopping Tech Council. Comments are welcome in UCP discussion #1027.

Draft 1, November 2026. Published by VSIZE, Novainvent AB, Sweden. Schema licensed under Apache 2.0, in line with UCP.

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