Size recommendations

Generate a recommendation

curl --fail-with-body -X POST "$METR_API_URL/v1/stores/$STORE_ID/fit-sessions/$SESSION_ID/recommendations" \
  -H "Authorization: Bearer $METR_API_KEY" \
  -H 'Content-Type: application/json' \
  --data '{
  "answers": {
    "usual_size": "M",
    "size_consistency": "consistent",
    "preferred_fit": "regular",
    "measurements": {
      "chest": 98,
      "shoulders": 45
    }
  }
}'

Response

{
  "id": "rec_example",
  "session_id": "fit_example",
  "recommended_size": "M",
  "probabilities": {
    "S": 0.048,
    "M": 0.904,
    "L": 0.048
  },
  "variant_ids": [
    "var_medium"
  ],
  "confidence": 0.9,
  "confidence_meaning": "uncalibrated_evidence_score",
  "fit": {
    "chest": "within_range",
    "shoulders": "within_range"
  },
  "evidence": {
    "chest": {
      "source": "measurement",
      "chart_min_cm": 98,
      "chart_max_cm": 98,
      "body_cm": 98.0
    },
    "shoulders": {
      "source": "measurement",
      "chart_min_cm": 45,
      "chart_max_cm": 45,
      "body_cm": 45.0
    }
  },
  "explanation": "M best matches measurement, usual size against this chart.",
  "warnings": [
    "Confidence is an uncalibrated evidence score, not a probability of fit."
  ],
  "algorithm_version": "metr-fit-2.0.0",
  "method": "rules",
  "chart_id": "cht_example",
  "chart_revision": 1,
  "created_at": "2026-10-01T12:00:00Z"
}

variant_ids contains currently available variants matching the recommended chart size and session color. Fit is unknown for body areas without a supplied measurement. Body charts describe below_range, within_range or above_range; garment charts describe the estimated ease or length relationship.

What confidence means

probabilities spreads belief over the available sizes. confidence is the chosen size's probability, clamped to 0.15–0.90 (0.60 without a tape measurement or reference brand). It is an uncalibrated evidence score, not the probability a size will fit. method is rules, or rules+jev when Jev chose among the plausible sizes. If Jev is unavailable the rules estimate is returned with a warning. Do not turn confidence into a ‘91% accurate’ claim.

Record the decision

curl --fail-with-body -X POST "$METR_API_URL/v1/stores/$STORE_ID/fit-sessions/$SESSION_ID/complete" \
  -H "Authorization: Bearer $METR_API_KEY" \
  -H 'Content-Type: application/json' \
  --data '{
  "decision": "accepted",
  "chosen_variant_id": "var_medium"
}'

• accepted: selected variant has the recommended size.
• overridden: selected variant has a different available size for the same product and session color.
• declined: no chosen_variant_id; the customer did not select a recommendation.

Retries and fallback

Identical recommendation retries return the same persisted response. Changed answers return 409: start another session. Identical completion retries are also safe. Recommendations do not reserve stock. Recheck availability before adding to cart. On no_suitable_size, missing approval, insufficient evidence, timeout or an unavailable service, show the merchant’s size chart and keep checkout available.