fix: preserve first user message to guarantee API validity

[ranxianglei]

Summary

Fixes the month-long session-freeze bug (zhipuai-lb code 1214, "messages 参数非法") where compression leaves zero user-role messages in the request.

Root cause

The v1.13.2 preserve-last-user fix (lib/messages/prune.ts) searched the messages array for a pruned user message to restore when filtering would leave zero users. After OpenCode compaction removes pruned messages from the array, the search finds nothing — zero-user requests slip through and the provider hard-rejects.

Reproduced in ses_0b89319b1ffeK25eKU3GMfCK8U: pre-compress 128 msgs / 12 user → post-compress 11 msgs / 0 user → API 400 code 1214 → session frozen.

Fix

Replace preserve-last-user with preserve-first-user: the first user message (the session's original task, always present in the array) is unconditionally force-preserved (survive = true), regardless of prune state. This does not depend on the integrity of the pruned portion of the array.

const firstUserIdx = messages.findIndex((msg) => msg.info.role === "user")
if (firstUserIdx >= 0) {
    survive[firstUserIdx] = true
}

Trade-off

When the first user is compressed and a later user also survives, two adjacent user messages can appear (e.g., [u1, u2, a2]). This is an accepted trade-off:

• Zero user messages → hard API rejection (code 1214). MUST prevent.
• Adjacent user messages → accepted by virtually all providers (OpenAI, Anthropic, Google, zhipuai-lb).

The Bug 36 e2e test was updated: the force-preserved first user is exempted from the adjacent-user check. The no-adjacent-users invariant still holds for all non-first-user pairs.

Files

• lib/messages/prune.ts — preserve-first-user logic (replaces preserve-last-user)
• tests/prune.test.ts — 5 tests rewritten for new invariant
• tests/e2e-message-transform.test.ts — 3 e2e tests updated

Verification

• npm run typecheck — clean
• npm test — 846 pass, 0 fail
• npm run build — clean

Ultraworked with Sisyphus