Thinking Message Preview Requirements

Overview

Thinking messages can contain several concise reasoning headings. Collapsed rows currently hide those headings behind the generic Thinking label whenever the source contains a newline. A compact preview helps users scan the agent's progress without expanding every reasoning block.

The UI system owns this presentation contract. Agent runtimes and the task system continue to own reasoning production, delivery, and persistence.

Terminology

Thinking message: An agent transcript message with the thinking type.
Meaningful line: The first source line that contains visible text after surrounding whitespace and inline Markdown decoration are removed.
Preview: The plain-text meaningful line shown beside the localized Thinking label in the collapsed row.

Requirements

REQ-UI-THINKING-MESSAGE-PREVIEW-001: Collapsed thinking message preview

Intent: Let users identify the subject of an expandable thinking message without opening the complete reasoning block.

User story: As a user reading an agent transcript, I want collapsed thinking rows to describe their subject, so that I can scan the agent's progress quickly.

Acceptance criteria

AC-UI-THINKING-MESSAGE-PREVIEW-001.1: When an expandable thinking message contains a meaningful line, Kandev shall show that line as plain text beside the localized Thinking label while the message is collapsed.
AC-UI-THINKING-MESSAGE-PREVIEW-001.2: When a thinking message starts with blank lines or Markdown decoration that produces no visible text, Kandev shall skip those lines. If no meaningful line exists, the row shall show only the localized Thinking label.
AC-UI-THINKING-MESSAGE-PREVIEW-001.3: When later reasoning content is appended to a thinking message after its first meaningful line, Kandev shall keep the original preview instead of replacing it with a later line.
AC-UI-THINKING-MESSAGE-PREVIEW-001.4: On desktop and mobile, an expandable thinking preview shall occupy one visual line, truncate within the available row width, and shall not widen the transcript or document.
AC-UI-THINKING-MESSAGE-PREVIEW-001.5: Expanding a thinking row shall continue to show the complete original reasoning content with its existing Markdown rendering. The preview shall not modify persisted or expanded content.
AC-UI-THINKING-MESSAGE-PREVIEW-001.6: Compact single-line thinking messages that already render their complete text inline shall keep their current inline and non-expandable behavior.
AC-UI-THINKING-MESSAGE-PREVIEW-001.7: Preview derivation shall apply to thinking messages from every agent provider without provider-specific labels or parsing rules.

Out of scope

Generating a new summary or choosing a later line because it appears more descriptive.
Changing ACP reasoning events, task-message persistence, WebSocket payloads, or streaming coalescing.
Changing the expanded Markdown renderer or the compact single-line threshold.
Redesigning the shared expandable-row interaction or its accessibility contract.

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