Azure AI Claude rejects requests after tool abort: assistant message prefill not supported #19517

Bug Description

When using OpenCode with Azure AI Claude (via LiteLLM proxy), aborting a tool call (Ctrl+C or interrupt) causes the next LLM request to fail with:

litellm.BadRequestError: Azure_aiException - "This model does not support assistant message prefill. The conversation must end with a user message."

The raw XML of the attempted tool call leaks into the conversation as visible text:

<invoke name="task">
<parameter name="description">...</parameter>
<parameter name="prompt">...</parameter>
</invoke>
litellm.BadRequestError: Azure_aiException - ...

The user must send a new message to reset the conversation turn. The question tool also consistently triggers this error on Azure AI Claude.

Environment

• OpenCode 1.3.0 (Homebrew)
• Provider: Azure AI Claude (Opus 4.6) via LiteLLM proxy (Orion)
• Model group: azure_ai/claude-opus-4-6

Root Cause

Azure AI's Claude inference layer does not support "assistant message prefill" (trailing role: "assistant" in the messages array). The standard Anthropic API does support this, so OpenCode works fine with direct Anthropic, but fails through Azure AI / LiteLLM.

There are two code paths that produce a trailing assistant message:

1. Aborted assistant message with text but no tool calls

When a user aborts mid-stream after text has started streaming but before any tool call is registered, the aborted assistant message (with partial text) gets included in history.

The error filtering logic in toModelMessages() intentionally keeps aborted messages that have "substantive" parts (text qualifies):

packages/opencode/src/session/message-v2.ts lines 690-698:

if (
  msg.info.error &&
  !(
    MessageV2.AbortedError.isInstance(msg.info.error) &&
    msg.parts.some((part) => part.type !== "step-start" && part.type !== "reasoning")
  )
) {
  continue  // skip this assistant message
}

When an aborted message has tool calls, the tool results get appended as role: "user" (Anthropic convention), so the sequence is fine. But when it only has text, the sequence ends with role: "assistant".

2. isLastStep prefill injection

When the agentic loop hits the step limit, a hardcoded assistant message is appended unconditionally:

packages/opencode/src/session/prompt.ts lines 700-710:

messages: [
  ...(await MessageV2.toModelMessages(msgs, model)),
  ...(isLastStep
    ? [{ role: "assistant" as const, content: MAX_STEPS }]
    : []),
],

This is valid prefill for the direct Anthropic API but rejected by Azure AI Claude.

Suggested Fix

Option A (recommended): Strip trailing assistant messages in normalizeMessages()

packages/opencode/src/provider/transform.ts function normalizeMessages() (line 49) already performs provider-specific message normalization. Add logic to detect when the message array ends with role: "assistant" and either remove it or append a synthetic user message, for providers that don't support prefill.

The codebase already has precedents for this pattern:

• Synthetic user messages for Gemini reasoning models (prompt.ts lines 520-543)
• Message sequence fixes for Mistral (transform.ts lines 138-151)
• LiteLLM dummy tool injection (llm.ts lines 160-178)

Detection criteria for "no prefill support":

• Provider uses @ai-sdk/openai-compatible (LiteLLM setups)
• Provider ID contains azure_ai
• Or a provider-level capability flag

Option B: Guard isLastStep by provider capability

Only inject the MAX_STEPS assistant message for providers known to support prefill (direct Anthropic).

Option C: Proxy-side workaround

Configure LiteLLM to strip trailing assistant messages before forwarding to Azure AI. Less ideal since it papers over the issue.

Related Code Paths

Workaround

After the error occurs, send any new user message (even just "continue") to reset the conversation turn. This works because the new user message ensures the history no longer ends with an assistant message.