A new file is drawn as highlighted code; the agent page reopens a session by address

[danbarua]

What changes

• A tool result that is a diff with no old text is a new file. The card draws it as the file: line numbers, syntax highlighting chosen from the file extension, and only the first five non-blank lines until "Show N more lines" is pressed. A file in a language the highlighter does not know is plain text.
• A changed file is still drawn as a diff.
• When a diff is drawn, the card no longer repeats the same text in the Input section, in the "Wrote N bytes" line, or under Raw output. An argument the diff does not show (anything other than the text and the path) still appears under Input.
• The "Raw output" label is set like the "Input" and "Result" headings.
• The gallery has a new tile, "A new file".

Reopening a session in the web app

• /app/agent/{sessionId} loads that session from the agent. /app/agent starts a session and then takes that session's address, so reloading the page reopens it. The "agent" link in the bar is a full page load and starts a new session.
• Checked in the browser against the fake agent: a new session moved to /app/agent/fake-1 without reconnecting, a reload redrew the same transcript, and an unknown id shows the agent's error.
• Not checked against a real agent. The agent keeps sessions under .labkit-dev/agent-workspace of the checkout it was started from, so a session can only be reopened from that checkout.

How

• packages/ui/code.tsx: CodeView, using lowlight (highlight.js grammars: pure JavaScript, no WASM or eval, and colours are classes in ui.css). The cut is a React style prop, as the context meter's width already is. Not run in the VS Code webview. The whole file is highlighted once and the cut is a max-height in lines, so a string or comment that crosses the cut keeps its colour.
• packages/ui/tool.tsx: the rules are on the diff content, not on the tool's name, so any agent's new-file diff gets the same treatment.
• Colours come from the existing theme tokens, in light and dark.

Checked

• bun run check: all 28 passed, on both commits.
• In the browser, light and dark: the tile opens on 9 lines (5 non-blank), "Show 9 more lines" expands to all 18, "Show less" restores it.

Not in this change

• write_file still returns the whole written text (and, for an overwrite, the whole previous text) to the model as its tool result: packages/core-agent/host/commands/tools.ts:231 serialises the tool's entire output. The diff shown on reopening a session is rebuilt from that same output, so trimming it needs a separate place to keep what is shown.
• A path argument written differently from the diff's path (./app.py) still shows under Input.
• Rolling up a turn's diffs in a side pane.

🤖 Generated with Claude Code

https://claude.ai/code/session_01GVjyjkaADPAqLvtnf3xUNs