GLM-5.3-Flash thinking degenerates into '!!!!!' loop in agentic tool-calling scenarios (similar to #25129) #45533

Description

Using GLM-5.3-Flash (served via SGLang backend) with opencode, requests through the build agent trigger a thinking degeneration loop where the TUI shows a "Thought" block followed by endless ! characters — the same symptom as #25129.

Environment

• opencode 1.18.23
• Model: zai-org/GLM-5.3-Flash (FP8), served via SGLang lmsysorg/sglang:glm-5.3-flash on 8x NVIDIA H20
• Access: litellm proxy -> SGLang

Repro

1. Add GLM-5.3-Flash as an OpenAI-compatible model
2. Send any message through the build agent
3. TUI shows Thought: N.Ns followed by endless !!!!!!!!!!!!!!!!!!!

Root cause (confirmed server-side, not opencode rendering)

The model itself degenerates under complex agentic prompts. Direct curl tests against the backend confirm:

• Simple request (short prompt, no tools): correct answer returned.
• Complex request (long agentic system prompt + multiple tools + tool_choice=auto): the streaming response's reasoning_content field contains only ! characters (1800+ of them), and content is empty.

This is a server-side issue (SGLang's reasoning parsing for GLM-5.3-Flash under multi-tool prompts — tracked at sgl-project/sglang#36669), not an opencode rendering bug. However opencode surfaces it as the "Thought" + !!!! loop.

Suggestion

The proposed fix in #25129 — detect repetitive characters in thinking blocks (e.g. (.)\1{10,}) and abort/retry the request without the corrupted thinking context — would at least prevent the endless !!!! spam and make the failure recoverable. A more targeted mitigation would be to retry such requests with a reduced tool set or lower reasoning effort when a repetition loop is detected.

Why this matters

GLM-5.3-Flash is a current-gen 320B/18B MoE with always-on thinking. As more always-on-thinking models ship, opencode will hit this class of degeneration more often. A generic repetition guard would make opencode robust against model-side degeneration regardless of backend.