# Agent Note: Read card — the read tool's structured line window reaches the client

Status: implemented

English | [中文](2026-07-30-web-read-card.zh.md)

## Problem

The `read` tool returns a canonical output object `{ path, offset, lines: [{ number, text }], totalLines }`, but its presentation collapsed that structure. `presentCall` declared a `GenericCallView` (`kind: 'read'`, a follow-along location) and `presentResult` returned a `GenericResultView` whose only content was the model-facing text with its `<path>…</path><type>file</type><content>…</content>` envelope stripped. A UI receiving that view saw one flattened text block: the line numbers were baked into the text as `N: ` prefixes, the file's language was unknown, and `totalLines` was gone. There was no way for a capable client to render a read the way it renders a diff — a line-numbered, syntax-highlighted code view with the line-number gutter separate from the content.

The structured data cannot be recovered downstream. A tool result on the wire carries only the model-facing `ContentBlock[]` (the rendered text) plus an opaque `meta`; the canonical output object stays in the tool and never reaches the client or the session log. So a client that wants the line array, the total, and a language hint cannot parse them back out of the `N: text` text — the tool has to project them onto a channel that persists.

## Decision

Add a fourth `card` tag, `read`, to the [render-intent union](../architecture/2026-07-02-tool-render-intent-union.md) — result-side only. `ToolResultView` gains `ReadResultView { card: 'read'; title?; path; lines: ReadFileLine[]; totalLines; lang?; content? }`; `ReadFileLine { number; text }` is the shared line unit. `ToolCallView` is untouched: the pending state stays a `GenericCallView` (`kind: 'read'`) because a call carries no file content until `execute` returns, so there is nothing structured to show at call time. This diverges from the bash terminal card, which tags both sides — a terminal call already carries its command and cwd at call time, a read call carries neither content nor total, so tagging the call side would add an empty variant.

The read tool projects the structured window through `output.presentationMeta`, the same persisted channel write/edit use for their applied-diff hunks ([canonical tool output contract](../architecture/2026-07-20-canonical-tool-output-contract.md)). `presentationMeta` runs once for a top-level surface call, returns `{ path, offset, lines, totalLines, lang? }` as JSON the session validates and stores on the result's `meta`, and `presentResult` narrows that meta back into the `ReadResultView` on both live and replay paths. `offset` (the 1-based first line the window requested) rides along because a byte cap below the first selected line yields an empty `lines` array with a positive `totalLines`; without the persisted `offset` a replayed card of such a window could not report where it starts or where a continuation resumes, and the last-line and re-parse fallbacks are both lossy. Without this channel the line array and total would be unreachable: the raw output object is not on the wire, and re-parsing the `N: text` text is lossy and fragile against the truncation footer.

`presentResult` returns `undefined` — the generic fallback — whenever the meta is absent or malformed (`readMetaFromMeta` narrows it defensively, so a replay of an older logged result never throws), whenever the result is an error, and whenever the single text block is not the read envelope. A pre-card logged result — a valid read envelope with no persisted `meta`, recorded before this card existed — takes that same `undefined` path deliberately: the client falls back to the raw `result.content`, so it shows the enveloped `<path>/<type>/<content>` text rather than the envelope-stripped generic card the old presenter returned. This is the accepted degradation under the [pre-release stance](../../../../AGENTS.md#pre-release-stance-foundation-over-blast-radius): reject the old on-disk format rather than add an envelope-stripping compatibility branch, since this change re-records every published fixture and the session format promises no backward compatibility. On the success path `presentResult` carries `content` (the envelope-stripped text) alongside the structured fields, so a UI without the read capability renders the file text through its generic/default card arm. The former TUI established the need for this fallback: its non-exhaustive result switch read `view.content`, while a separate dim-Markdown gate also had to admit `card: 'read'`. That frontend has since been removed, but the content fallback remains part of the view contract for any consumer without a structured read card.

### Language hint derivation

`langFromPath` (in `read-render.ts`) maps a file extension to a syntax-highlighting language id through a small fixed table (`LANG_BY_EXTENSION`) covering common source, config, and markup extensions. It reads the extension after the last path segment and last dot, is case-insensitive, and returns `undefined` for a dotfile (`.gitignore`), an extensionless name (`/etc/hosts`), a trailing dot, and any unknown extension — the card then omits `lang` and a UI renders plain text. The table is not a tunable: it is a display hint a UI may ignore, not a deployment-varying choice, and an unknown extension degrades to plain text rather than failing. It is deliberately small rather than an exhaustive language registry; extending it is a one-line table addition.

## Alternatives considered

**Re-parse the `N: text` model-facing text in `presentResult`.** Rejected: the structured line array would have to be reconstructed by splitting each line on the first `: `, which is ambiguous (a line whose own text contains `: `), loses the exact `totalLines` (the footer only states it in some branches), and breaks the moment the render format changes. `presentationMeta` carries the already-structured data with no re-parse.

**Tag the call side too (`ReadCallView`), mirroring the terminal card's both-sides symmetry.** Rejected: a read call has no content, no line array, and no total until it executes — a call-side read card would be an empty variant duplicating what `GenericCallView` (`kind: 'read'`, follow-along location) already expresses. The terminal card tags both sides because a terminal call genuinely carries call-time data (command, cwd); a read call does not.

**Put the structured window in a new service or a side channel instead of `meta`.** Rejected: `meta` is the established persisted presentation channel (write/edit's applied diffs ride it), it replays for free with the session log, and it needs no new plumbing. A service would reinvent persistence and replay that the event log already provides.

**A merge-extensible union instead of a closed tag.** Rejected for the same reason the [render-intent union](../architecture/2026-07-02-tool-render-intent-union.md) closed: a new card needs consuming code to render it, so a variant a consumer silently drops is worse than a compile error. Adding `read` to the closed union is the sanctioned way to extend it — each consumer that switches on `card` keeps compiling because the new member falls through its generic default, and a consumer that wants the rich view adds its own arm.

## Consequences

`ToolResultView` has a fourth member. A consumer may render the structured `lines`/`lang`/`totalLines` shape or route an unsupported card to its generic path; the read card carries `content` so the latter still shows the file text. This producer change is the backend that makes the structured data reachable without requiring every consumer to implement the richer view at once.

The read tool now computes `presentationMeta` for every top-level read, a small per-call projection (a `lines.map` and one `langFromPath` call) on data already in hand. The meta is persisted with the session log, so a read result is slightly larger on disk — the line array it already rendered as text, now also structured.

## Testing

`packages/fs/tool-fs/tests/read-render.spec.ts` unit-tests `langFromPath` (known extensions case-insensitively, extension read after the last segment and last dot, and the `undefined` cases: dotfile, extensionless, trailing dot, unknown) and `readMetaFromMeta` (a well-formed narrow with and without `lang`, and every rejection: non-object, array, missing or wrong-typed `path`/`totalLines`/`lines`, a malformed line entry, a non-string `lang`, and — because the function narrows the opaque persisted `meta` boundary — the semantically invalid paths a well-typed replayed JSON can still carry: an `offset` that is not a 1-based integer, a first line `number` below `offset`, a line `number` that is not a 1-based integer (`0`, `1.5`, `NaN`, `Infinity`), a `totalLines` that is not a non-negative integer (`-1`, `1.5`, `NaN`), and lines whose numbers duplicate, decrease, or exceed `totalLines`; it also narrows an empty window at a positive `offset` (a byte cap below the first selected line). `packages/fs/tool-fs/tests/tools.spec.ts` pins the tool wiring: `execute` attaches the structured window (with and without a `lang` hint) as `meta`, `presentResult` narrows it into a `card: 'read'` view carrying the envelope-stripped `content`, and the decline paths (error result, non-single-text content, malformed envelope with valid meta, and valid envelope with absent or malformed meta) all fall back to `undefined`. Both changed source files hold per-file 100% coverage. This change carries the snapshot evidence for the persisted meta and the extended union, not for a new rendered view: the re-recorded ACP session fixtures (`fs-read`, `fs-read-window`, `fs-edit`, `fs-policy-reject`, `fs-write-overwrite`, `parallel-tool-calls`, `agent-instructions`, `workspace-edit`) pin the persisted read `meta` (with `{{cwd}}`-tokenized paths), and `cordis-inspect-jsdoc` pins the four-member `ToolResultView` union. The then-current terminal snapshot also pinned that a consumer's generic dim-Markdown fallback stayed byte-identical; the structured card's own assembled-application transcript belonged to its consuming frontend change.

## Related

- [Tagged render-intent union for tool-call presentation](../architecture/2026-07-02-tool-render-intent-union.md) — the `card`-tagged vocabulary this extends with the `read` result arm.
- [Canonical tool output contract](../architecture/2026-07-20-canonical-tool-output-contract.md) — owns the `presentationMeta` persisted channel this projects the read window onto.
- [Web terminal card](2026-07-28-web-terminal-card.md) — the precedent for a client consuming a structured card; the read card follows the same producer pattern, result-side only.
