Empty reasoning step (no delta) in replayed history causes 400 on later Kimi K3 requests #37651

Description

Multi-turn sessions with opencode-go/kimi-k3 (max variant) start failing with Error from provider (Console Go): Upstream request failed (HTTP 400) once the history contains an assistant step where the provider opened and closed a reasoning stream without emitting any delta. Every later request in that session fails; a fresh session works until it hits the same history shape.

This is the second, independent failure path originally documented in #37552. That issue was closed when models.dev#3331 fixed the temperature path (thanks for the fast fix) — but this replay path does not involve temperature at all and still reproduces after the catalog fix.

Root cause

MessageV2.toModelMessages replays a reasoning part with empty text and no metadata (between duplicate step-start boundaries) as an assistant message with no usable content, which the upstream endpoint rejects.

Evidence

• Reproduced with a minimal config ("model": "opencode-go/kimi-k3" only — no agent files, no temperature anywhere).
• A/B: the exact affected session replayed through a build patched to skip empty metadata-free reasoning parts continued for 26 steps and finished normally; the same session on stock source fails immediately. Identical catalog and provider conditions in both runs.

Fix with regression test: #37624.

Environment

• opencode 1.18.3
• Windows 11 Pro (10.0.26200)
• Plugins: none active in the reproduction