Manual /compact with DeepSeek V4 Flash degenerates into thousands of repeated lines #41801

Description

A manual /compact request using opencode-go/deepseek-v4-flash entered an intra-response repetition loop. This differs from repeatedly triggering multiple Session compacted cycles: one compaction request itself generated a transcript-like prefix and then repeated the same short line 1,511 times.

The compaction never completed. I cancelled it after about 48 seconds, after which OpenCode recorded the message as Aborted. No HTTP, authentication, provider, or stream error was logged before cancellation.

Related, but not identical:

• Infinite "Session compacted" loop on multiple models (DeepSeek V4, MiMo V2.5, MiniMax M3) #30443
• bug(session): infinite compaction loop when compression fails to reduce context #27924

OpenCode version

1.18.16

Environment

• Ubuntu Linux
• TUI
• Provider: opencode-go
• Model: deepseek-v4-flash
• Manual /compact, not automatic overflow compaction
• No custom compaction agent or compaction prompt configured

Steps to reproduce

1. Use opencode-go/deepseek-v4-flash in a long-running session containing normal assistant messages and tool calls.
2. Wait for the active assistant turn to finish.
3. Run /compact manually.
4. Observe the compaction output.

Actual behavior

• The compaction response began by reproducing part of the preceding transcript, including serialized assistant/tool-call text.
• It then became stuck repeating one short line.
• The persisted compaction text was 29,389 characters when cancelled.
• The repeated line occurred 1,511 times in that single text part.
• The final text was cut off mid-token because the request was cancelled.
• OpenCode did not detect the high repetition or stop the compaction as failed.
• The provider/model still completed a minimal normal text-generation request successfully after the incident.

Sanitized log sequence:

stream providerID=opencode-go modelID=deepseek-v4-flash agent=compaction
llm runtime selected ... provider=opencode-go model=deepseek-v4-flash
[about 48 seconds of streaming]
cancel session.id=<redacted>
process ... error=Aborted

Expected behavior

The compaction agent should produce a concise handoff summary. If its output becomes highly repetitive or fails to reduce the conversation meaningfully, OpenCode should stop the request, mark compaction as failed, and preserve the previous usable session state.

A repetition/degeneration guard on compaction output would prevent a model or provider backend from streaming thousands of duplicate lines and consuming usage indefinitely.

Additional context

This may involve an interaction between the DeepSeek model/backend and the compaction prompt, but OpenCode currently has no client-side guard for degeneration inside a single compaction response. This report intentionally omits the full session export because it contains private project content.