[MRQC-st]

Fixes #16328.

What this changes

Standalone OmniRoute (no OpenCode CLI) returned 403 FreeTierError on every free-tier model. Two independent causes, two files, no new shared module — matching the scope agreed with @opensource-elearning.

1. open-sse/utils/opencodeHeaders.ts — mint the session the way the client does

The upstream decodes x-opencode-session back through OpenCode CLI's Identifier.descending() layout — the 48-bit value ~(ts_ms << 12 | counter) in hex — and has refused any other encoding since 2026-10-13. The old synthesizer hashed the seed and emitted arbitrary hex, so its decoded counter landed out of the accepted window every time.

• canonicalId("ses_") now takes the top 36 bits of the seed hash and inverts with counter = 1 in the bit domain: ~((ts36 << 12) | 1n) & 0xFFFFFFFFFFFn. The low 12 hex therefore read ffe, and stay deterministic per conversation (prompt-cache affinity is preserved). This is byte-identical to opencode2api v1.3.8's descendingTimePart().
• New OPENCODE_SESSION_TIME_PART_PATTERN = /^[0-9a-f]{9}f[c-f][0-9a-f]$/ and isValidOpencodeSessionValue(), the value-level check: the 12 hex time part must decode to the client's counter window (0xFC0–0xFFF, i.e. counter 0–63). OPENCODE_SESSION_PATTERN alone accepts any 12 hex, so a shape-valid but foreign-encoded id (raw hash, ascending id, counter ≥ 64) slipped through to a 403.
• applyCliDefaults() now re-encodes on the value, not just the shape: a genuine descending session is kept; everything else is re-minted from the client's own value. This is applied only on the gated (zen) path — the ungated surfaces (opencode-go, fix(providers): OpenCode Go rewrites third-party User-Agent as OpenCode CLI #15311) keep the client's session unchanged.

2. open-sse/executors/opencodeFreeTierContract.ts — derive free models from the synced catalog

isPremiumOpencodeModel() no longer reads a static set (the old OPENCODE_FREE_MODELS had already rotted: 5 of its 6 ids were absent from the live list, 4 deprecated). It now reads the catalog OmniRoute already syncs:

signal	source
available	id ∈ the synced live catalog for the opencode family (catches delisted ids)
free	models.dev cost.input === 0 && cost.output === 0 (catches big-pickle, which has no -free suffix)
not deprecated	models.dev status !== "deprecated" (secondary guard)

Read synchronously from syncedAvailableModels + modelsDevSync, memoized on getModelCatalogCacheVersion() (bumped by every catalog write) — no per-request fetch, no table to rot. With no synced catalog it falls back to the legacy heuristic (-free suffix + the known ids) rather than treating everything as premium, so an unsynced deployment keeps its current behavior instead of 402-ing every free model.

Tests

• tests/unit/opencode-free-tier-request-contract.test.ts — tightened the loose OPENCODE_SESSION_PATTERN assertions to the value-level check; added cases for genuine-descending-preserved, out-of-window-re-minted, ungated-untouched, and catalog-driven classification.
• tests/unit/opencode-background-identity-headers.test.ts — same tightening.
• tests/unit/opencode-premium-keyless-gate-8681.test.ts — classification cases moved to the catalog seam.

tsc -p tsconfig.typecheck-core.json is clean. The full tests/unit/opencode-*.test.ts set shows no new failures versus the pre-change baseline (the ~12 remaining reds are pre-existing proxy/egress/rotation tests that fail in this local environment with or without the patch).

On the un-reproduced 403

We still cannot reproduce the maintainer's "free tier can only be used from within OpenCode" on all free models from their host — our 200s come from an APAC direct egress. Exact envelope that returned 200:

POST https://opencode.ai/zen/v1/chat/completions
authorization: Bearer public
content-type: application/json
accept: */*
user-agent: opencode/1.19.0 ai-sdk/provider-utils/4.0.23 runtime/bun/1.3.14
x-opencode-client: desktop
x-opencode-project: 374321b05e315c1177ca43fdc7e4bd79d702f37b
x-opencode-request: msg_<12hex + 14base62>   (fresh random per request)
x-opencode-session: ses_<12hex + 14base62>    (descending encoding, low 12 = 0xffe)

body: a captured client request (~138 KB) — model: "space-bunny-free", stream: true, ~82 KB system prompt, real tools array. Decisive A/B (same envelope, only the session varies): ~(now<<12|1) → 200, ~(1<<12|1) (epoch+1ms, counter=1) → 200, ~(now<<12|64) (counter=64) → 403. The gate is the counter bit-pattern, and the timestamp is effectively unvalidated — which is what makes a deterministic, hash-derived mint safe here.

[MRQC-st]

Implemented the two-file scope from @opensource-elearning's review and opened it as a PR:

→ #16598 — open-sse/utils/opencodeHeaders.ts + open-sse/executors/opencodeFreeTierContract.ts

What landed (all three Ask-1 corrections + both Ask-2 corrections, no new shared module):

• mint: canonicalId("ses_") takes the top 36 bits of the seed hash and inverts with counter 1 in the bit domain (~((ts36 << 12) | 1n) & 0xFFFFFFFFFFFn), so the low 12 hex read ffe — byte-identical to opencode2api v1.3.8's descendingTimePart(), and deterministic per conversation so prompt-cache affinity survives.
• value-level re-encode: isValidOpencodeSessionValue() checks shape and the 0xFC0–0xFFF window; applyCliDefaults() re-mints a shape-valid but out-of-window client session (opencode2api feat(docs,backup): export/import endpoints, documentation overhaul, screenshots #70's descendingTimePartPattern), and does so only on the gated zen path so opencode-go (fix(providers): OpenCode Go rewrites third-party User-Agent as OpenCode CLI #15311) keeps the client's session.
• catalog-driven free set: dropped the static OPENCODE_FREE_MODELS; isPremiumOpencodeModel now reads id ∈ live /zen/v1/models ∧ models.dev cost.input === 0 && cost.output === 0 ∧ status !== "deprecated", memoized on getModelCatalogCacheVersion(). With no synced catalog it falls back to the legacy heuristic so an unsynced deploy isn't 402-ed wholesale.
• tests tightened/added in opencode-free-tier-request-contract, opencode-background-identity-headers, opencode-premium-keyless-gate-8681.

On the un-reproduced host gate ("free tier can only be used from within OpenCode"): our 200s come from an APAC direct egress with Bearer public; the exact request envelope that passes is in the PR body. Decisive A/B on that envelope — ~(now<<12|1) → 200, epoch+1ms counter=1 → 200, counter=64 → 403 — is what makes the deterministic hash-derived mint safe.