Automatic session titling can write a content-free title

[CancerTiN]

Summary

The built-in title provider (dsh-session-title-first-prompt-llm) can replace a usable fallback title with a generic phrase that identifies nothing. Observed on a real session: the provider overwrote the fallback 这里会有提示词文件吗 with the bare phrase 会话标题 — literally "session title", i.e. the instruction's own vocabulary echoed back.

There is a second, milder shape: the deterministic fallback can truncate mid-sentence, because the cap counts whitespace-delimited words (fallbackMaxWords: 5), which for Chinese input cuts inside a phrase.

Reproduction

1. Start a session whose first prompt is a normal Chinese question (observed: 这里会有提示词文件吗).
2. Let automatic titling run with the shipped dsh-base configuration:

- id: session-title
  config: { fallbackMaxWords: 5, fallbackMaxBytes: 40, maxTitleBytes: 80 }
- id: session-title-llm
  config: { targetWords: 5, targetCjkCharacters: 10, maxInputBytes: 4096, maxOutputTokens: 64, timeoutMs: 60000 }

3. Inspect the durable log.

Current behavior

{"type":"session/title","seq":12,"data":{"title":"这里会有提示词文件吗","source":{"kind":"fallback"}}}
{"type":"session/title-llm-request","seq":16,"data":{"titleProvider":"session-title-first-prompt-llm",
  "route":{"provider":"deepseek-official","model":"deepseek-v4-pro"}}}
{"type":"session/title","seq":17,"data":{"title":"会话标题","source":{"kind":"provider",
  "provider":"session-title-first-prompt-llm","model":{"provider":"deepseek-official","model":"deepseek-v4-pro"}}}}

A usable fallback is superseded by a title carrying no information about the session. In a second session the same pipeline produced a mid-sentence fragment as the durable title:

{"type":"session/title","seq":14,"data":{"title":"调用 set_session_title 工具，把本","source":{"kind":"fallback"}}}

Expected behavior

A provider revision should only become durable when it is at least as identifying as what it replaces. Concretely: a candidate that is generic vocabulary, or that merely echoes the titling instruction, should be rejected and the previous revision retained. The fallback path should not end on a dangling fragment when the input has no whitespace word boundaries (CJK).

Environment

Harness	DeepSeek Harness desktop app, @deepseek-ai/dsh-desktop-runtime 0.2.0-rc.2
Components	@deepseek-ai/dsh-session-title, @deepseek-ai/dsh-session-title-first-prompt-llm
Model route	deepseek-official / deepseek-v4-pro
OS	macOS 26 (darwin/arm64)
DSH_HOME	~/.dsh
Session format	v4

Notes toward a fix

• Reject a candidate that equals or consists only of generic title vocabulary: 会话标题, 未命名, 新会话, untitled, new session, session, chat, …
• Reject a candidate that is contained in, or equal to, the provider's own instruction vocabulary — 会话标题 is exactly the phrase the system prompt uses, which suggests echo rather than summarization.
• On rejection, keep the previous revision instead of writing the candidate (the service already has the machinery: it supersedes in-flight work and pins on user-sourced titles).
• In the fallback, treat the byte cap as the primary bound and prefer a boundary that does not split a CJK phrase; a trailing dangling function word (把本 above) is worse than a slightly shorter title.

Context

This is what motivated adding a rename path at all: the automatic result was bad enough to be worth fixing by hand, but 0.2.0-rc.2 exposes the rename capability (SessionTitleService.rename(session, title) via ctx.sessionTitle, plus the GUI rename dialog) to no model-facing surface — no tool, no session CLI subcommand (plugin / duplicate only), and /api behind the authority-bound signed cookie whose launch token is a per-process randomBytes() value that is never persisted. A model-facing set_session_title tool would make the bad automatic title self-correcting rather than permanent; see the companion discussion about the plugin that does this today.