Max-steps wrap-up appends MAX_STEPS_PROMPT as an assistant message → Anthropic 400 "does not support assistant message prefill" #37685

Description

When an agent reaches its configured steps limit, the final request fails with an Anthropic API 400 instead of producing the intended text-only wrap-up:

AI_APICallError: This model does not support assistant message prefill. The conversation must end with a user message.

Cause: packages/opencode/src/session/prompt.ts (~line 1281) appends the max-steps instruction as an assistant message when isLastStep is true:

messages: [
  ...modelMsgs,
  ...(isLastStep ? [{ role: "assistant" as const, content: MAX_STEPS_PROMPT }] : []),
],

Ending the request with an assistant message (a prefill) is rejected by the Anthropic API when thinking/effort is enabled (e.g. variant: max/xhigh on claude-sonnet-5 / claude-opus-4-8). So any agent that exhausts its steps budget on a thinking-enabled Anthropic model deterministically errors on its final step instead of returning the MAX_STEPS summary.

Reproduced twice on 1.18.3, each at exactly the agent's configured ceiling, with all prior steps succeeding:

• steps: 40 subagent (claude-sonnet-5, thinking variant): 39 successful tool-call steps, then the 400 on step 40.
• steps: 200 subagent (claude-opus-4-8, thinking variant): same 400 on step 200.

The line is unchanged on dev as of 2026-07-18.

Suggested fix: send MAX_STEPS_PROMPT as a user-role message, or only use the assistant prefill when the request does not have thinking enabled and the provider supports prefill.

Plugins

superpowers@git+https://github.com/obra/superpowers.git

OpenCode version

1.18.3

Steps to reproduce

1. Define a subagent on an Anthropic model with a thinking-enabling variant, e.g. model: anthropic/claude-sonnet-5, variant: max, and a low step cap, e.g. steps: 2.
2. Dispatch it (task tool) a task that needs more tool-call turns than the cap.
3. On the final permitted step the API call fails with This model does not support assistant message prefill. The conversation must end with a user message. instead of returning the max-steps text wrap-up.

Screenshot and/or share link

https://cln.sh/DfywsvTj

Operating System

macOS 26.5.2

Terminal

supacode