[danbarua]

Problem

The composer's @ list was built but had nothing to list, and a mention sent nothing to the agent but its text.

Change

Every file's path at HEAD, in the files API

• GET /workspace/{slug}/paths answers the paths of the files in the tree at HEAD, in name order, as a paths array. Directories and submodules are not listed.
• At most 5,000 paths are listed. total counts them all, and truncated says when some are left out.
• The top directory at HEAD links it as paths; a directory below the top, and the tree at a commit, do not.

Mentions in the chat

• Once the chat has been shown in a workspace, it follows the workspace's file collection to paths and offers each path after @.
• A file a message mentions is sent with it as a resource_link: uri is file://<workspace folder>/<path>, each segment escaped, and name is the path. The text block is sent unchanged.
• The mention parser (mentionedLinks in src/ui/agent/mentions.ts):
  • It links each file once, in the order first mentioned.
  • It tries longer paths first, so @a.ts.bak is not @a.ts.
  • It finds paths with spaces.
  • A full stop that ends a sentence also ends the mention.
  • An @word that names no file, or an @ inside a word, stays text.

ACP client. promptBlocks and SessionClient.prompt take links. The blocks go text, links, then attached files, and a prompt of links alone has no text block. ACP requires every agent to take resource_link, so no capability is checked.

Controls

• Link, not embedded content. ACP prefers an embedded resource when the agent takes embedded context, because it saves a round trip and covers sources the agent cannot reach. Here the agent works in the same folder, and embedding would mean the browser downloading the file from the API and sending it back to the agent. The mention is a link the agent reads itself.
• How the agent treats the link. The pinned labkit-effect host turns a resource_link into a Markdown link line in the prompt's text (linesOf in host.ts). In a test on :8850 against bonsai-2026, the model saw [README.md](file:///…/bonsai-2026/README.md), called it "just markdown text", and offered to read it. Having the agent read linked files in its folder is a change for the agent runtime, not this repository.
• HEAD only. Mentions come from HEAD's tree, as the File Explorer does. An uncommitted or ignored file cannot be mentioned. The agent reads the file on disk, which may differ from HEAD.
• A cut-short list is used as it is, with a warning in the console naming how many files were listed and how many there are.
• The one address the page builds from data is the file: URI. It is an address for the agent, not an address of the API.
• The paperclip. It opens the file chooser in Chromium (checked with Playwright's filechooser event). On the device used for the test above, a click did nothing; this PR does not change it.

Verification

• bun run check: exit 0, 1557 pass, 0 fail.
• bun run e2e with E2E_PORT_API=9019 E2E_PORT_DEV=8970 E2E_PORT_BUILT=8971: 165 passed, 11 skipped (the dev-only tests, in the built bundle).
• Unit tests:
  • tests/git-api.test.ts: the top directory links paths. The listing has README.md and src/a.ts, no directories, and is in name order, with total and count equal and truncated false. A lower directory and the tree at a commit link no paths.
  • packages/acp-client/tests/prompt-blocks.test.ts: a link is a resource_link between the text and the attached files, whatever the agent takes. A prompt of links alone has no text block.
  • tests/mentions.test.ts: the file: URI escapes each segment. Each mentioned file is linked once, in order. The longer path wins. A path with a space and a sentence's full stop are handled. An unknown @word and an @ inside a word link nothing.
• e2e, dev server: typing @README.m offers README.md, and picking it writes @README.md . Sending it posts session/prompt with exactly the text block look at @README.md and one resource_link to file://<alpha's folder>/README.md, named README.md.

Linked issues

None. This follows #671.

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