sap-ai-core: Claude models fail with 400 "does not support assistant message prefill"

Bug Report

OpenCode Version: 1.18.23
Provider: sap-ai-core
Model: sap-ai-core/anthropic--claude-4.6-sonnet (affects all sap-ai-core/anthropic--* models)

Description

When using any Anthropic/Claude model through the SAP AI Core provider, every response is followed by a 400 error:

400 - LLM Module: This model does not support assistant message prefill.
The conversation must end with a user message.

Steps to Reproduce

1. Configure OpenCode with sap-ai-core provider and any anthropic--* model
2. Send any message (e.g. "Hi")
3. Receive a response
4. Observe 400 error immediately after

Root Cause

OpenCode appends a partial assistant message at the end of the conversation (assistant prefill) to steer the model's response. The SAP AI Core Orchestration API rejects this — it requires conversations to always end with a user message.

Relevant source file:
packages/opencode/src/provider/provider.ts — sap-ai-core custom loader

The fix requires stripping any trailing assistant message before sending to SAP AI Core, or disabling prefill specifically for this provider.

Expected Behavior

Messages to SAP AI Core Claude models should not include a trailing assistant message. The conversation should always end with a user message.

Workaround

None currently available via config. The only workaround is to not use Anthropic/Claude models through SAP AI Core.