fix(provider): derive a price tier's threshold from its own minimum

[ANIRUDDHA ADAK (aniruddhaadak80)]

What

ManagedPricing.parse builds each tier's threshold from the previous tier's end:

const tiers = (prices) =>
  prices
    .map((tier, index) => ({
      ...tier,
      threshold: prices[index - 1]?.max_input_tokens ?? (tier.min_input_tokens ?? 1) - 1,
    }))

and finds the base tier by falsiness:

const first = model.pricing.tiers.find((tier) => !tier.min_input_tokens)
if (!first || (first.input === 0 && first.output === 0)) continue

Why it matters

A tier's start came from the tier above ending. The ?? prefers the predecessor's max_input_tokens, which is the same number only when the catalog is contiguous. Where it is not, the threshold lands in the direction that costs the user money:

tiers: [ { max_input_tokens: 100 }, { min_input_tokens: 201 } ]
-> threshold: 100        (the tier declares it starts at 201)

Selection in src/session/index.ts:827 is .filter((tier) => promptTokens > tier.threshold), so a 150-token prompt satisfies 150 > 100 and is billed at the 201+ rate — 51 tokens before the tier is meant to apply. An overlap errs the other way.

An explicit base tier of min_input_tokens: 1 made the model vanish. !tier.min_input_tokens is false for 1, so no base tier is found and the continue drops the entire row. Token counts start at 1, so min: 1 means the same as omitting it:

[{input:2,output:12,min_input_tokens:1}, {input:4,output:18,min_input_tokens:200001}]
-> parse(...)  ===  {}                                   the model is gone

[{input:2,output:12},              {input:4,output:18,min_input_tokens:200001}]
-> cost {input:2, output:12, tiers:[{... threshold:200000}]}

The same first lookup drives fast_mode_details pricing, so a fast-tier model can disappear the same way.

Verification

New test/provider/managed-pricing-tiers.test.ts. Three of four fail before:

Expected: 200
Received: 100
(fail) a tier starts where it declares, even when the previous one ends earlier

Expected: false
Received: true
(fail) a selection above the gap but below the tier's min does not pick that tier

Expected: 2
Received: undefined
(fail) an explicit base min of 1 still prices the model

After:

(pass) a tier starts where it declares, even when the previous one ends earlier
(pass) contiguous tiers are unaffected
(pass) a selection above the gap but below the tier's min does not pick that tier
(pass) an explicit base min of 1 still prices the model
 4 pass
 0 fail

Two of the four are guards against over-correcting: the contiguous case must be unchanged (it is, and the existing managed-pricing.test.ts suite still passes — 15/15 on three consecutive runs), and the third test states the selection rule directly so the threshold cannot drift from the > comparison that consumes it.

The change

-      const first = model.pricing.tiers.find((tier) => !tier.min_input_tokens)
+      // Token counts start at 1, so an explicit min of 1 is the base tier and
+      // means the same as omitting it. Looking it up by falsiness missed that
+      // spelling and dropped the whole model out of the catalog.
+      const first = model.pricing.tiers.find((tier) => !tier.min_input_tokens || tier.min_input_tokens <= 1)

-          .map((tier, index) => ({
+          .map((tier) => ({
             ...tier,
-            threshold: prices[index - 1]?.max_input_tokens ?? (tier.min_input_tokens ?? 1) - 1,
+            // A tier applies from its own min. The previous tier's max is the
+            // same number only when the catalog is contiguous, and preferring it
+            // charged a tier's rate from as early as the tier above ended.
+            threshold: (tier.min_input_tokens ?? 1) - 1,
           }))

The index parameter is no longer used.

Touched files are Prettier-clean (verified on LF-normalized copies; this Windows checkout's core.autocrlf=true makes Prettier flag every file repo-wide).

Fixes #838

[vercel]

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