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+# Tweet Detail and Conversation Design
+
+**Date:** 2026-07-13
+
+**Status:** Approved
+
+## Purpose
+
+Extend Twitter Lite with a read-only post detail page. A deliberate click on a
+post, or a manually entered internal status URL, shows that focal post and the
+visible conversation around it. Additional conversation pages load through the
+same intentional infinite-scroll behavior as the existing user and search
+feeds.
+
+This is an additive extension to
+`2026-07-13-twitter-lite-design.md`. Where that document lists only two routes
+and two server functions, this document adds one status route and one narrowly
+scoped read-only server function.
+
+## Decisions
+
+- A thread means the full visible conversation: ancestors and replies from all
+  participants sharing the conversation ID, not only the author's self-replies.
+- Bird remains unchanged. The existing `getTweet()` and `getThreadPaged()`
+  public methods are sufficient when composed through the conversation root.
+- No new relay operation is required. Both reads use the existing `TweetDetail`
+  operation.
+- Bird sorts the tweets within each fetched page chronologically.
+  Cursor pages are appended in retrieval order so already rendered content
+  does not jump. Preserving X's original ranked branch order is explicitly
+  deferred to a later Bird API investigation.
+- The focal post is shown once at the top of the page. If the conversation page
+  also contains it, the copy in the conversation list is removed.
+- The feature remains read-only and adds no history, recommendations, related
+  posts, author suggestions, or mutation controls.
+
+## Route and Navigation
+
+Add the typed route:
+
+```text
+/status/:tweetId
+```
+
+`tweetId` is a non-empty decimal X post ID. Invalid IDs render a non-retryable
+input error and make no relay request.
+
+Every non-quoted post card gains an internal `詳細・スレッド` link in its
+footer. The existing external `元の投稿を開く` link remains available. The
+whole card is not clickable because cards can contain text links and media
+controls. Quoted cards retain their current compact presentation.
+
+The status route uses the existing application shell. Neither `ユーザー` nor
+`検索` receives `aria-current` on this third route. The status page adds no
+third global navigation tab; it is reachable only from a deliberate post link
+or a manually entered URL.
+
+## Bird and Relay Boundary
+
+The application continues to construct one server-only `TwitterClient`.
+`TWITTER_RELAY_BASE_URL`, `BIRD_PROFILE_NAME`, Bird, and raw GraphQL responses
+must not enter the public browser bundle.
+
+Extend the application-local `BirdReader` interface with only the existing
+Bird methods needed here:
+
+```ts
+getTweet(tweetId: string): Promise<GetTweetResult>
+
+getThreadPaged(
+  tweetId: string,
+  options: {
+    cursor?: string
+    maxPages: number
+    pageDelayMs: number
+  },
+): Promise<SearchResult>
+```
+
+Add one TanStack Start GET server function, `loadThreadPosts`. This brings the
+intentional server-function boundary from two functions to exactly three. No
+generic Bird-method dispatcher or relay proxy is introduced.
+
+## Request and Result Contract
+
+The server input has two valid states:
+
+```ts
+type InitialThreadInput = {
+  tweetId: string
+}
+
+type ContinuedThreadInput = {
+  tweetId: string
+  conversationId: string
+  cursor: string
+}
+```
+
+All IDs must be decimal strings. Cursor values are opaque non-empty strings.
+`conversationId` and `cursor` must either both be absent or both be present.
+
+The successful page shape is:
+
+```ts
+type ThreadPage = {
+  tweets: Post[]
+  focalPost?: Post
+  conversationId: string
+  nextCursor?: string
+}
+```
+
+`focalPost` is present only on the initial page. All returned posts pass through
+the existing public-post sanitizer, including recursive removal of `_raw` from
+quoted posts.
+
+## Data Flow
+
+### Initial page
+
+1. Validate `tweetId` before creating a Bird request.
+2. Call `getTweet(tweetId)` once.
+3. If the post is missing or unavailable, return a non-retryable post error.
+4. Set `conversationId` to `focalPost.conversationId ?? focalPost.id`.
+5. Call `getThreadPaged(conversationId, { maxPages: 1, pageDelayMs: 0 })`.
+6. Return the sanitized focal post, the first conversation page, its
+   conversation ID, and its next cursor.
+
+Using the conversation root for `getThreadPaged()` is important. It avoids the
+existing Bird edge case where a stateless cursor request beginning from a reply
+cannot rediscover the conversation root if the cursor page omits that reply.
+
+### Continued page
+
+1. Validate the supplied `tweetId`, `conversationId`, and cursor.
+2. Call `getThreadPaged(conversationId, {
+   cursor, maxPages: 1, pageDelayMs: 0 })` once.
+3. Return the sanitized tweets with the same conversation ID and new cursor.
+
+The original `tweetId` remains part of the query identity and validation
+contract, but only the resolved conversation ID is used as Bird's focal ID for
+continuation requests.
+
+## Client Query and Infinite Scrolling
+
+Extend the existing feed request union with:
+
+```ts
+{ kind: 'thread'; tweetId: string }
+```
+
+The thread query key includes the focal tweet ID. Its initial page parameter is
+undefined. A successful page with a next cursor produces the next page
+parameter `{ cursor, conversationId }`, ensuring the resolved root survives the
+stateless server boundary.
+
+Keep the existing intentional query policy:
+
+- no request without a valid route ID;
+- no automatic retry;
+- no refetch on window focus or reconnect;
+- a 600px observer margin;
+- one in-flight next-page request;
+- explicit initial and later-page retry controls;
+- cross-page post de-duplication by post ID; and
+- continuation through an empty page when a new cursor is present.
+
+The focal post is taken from the first page and rendered above the conversation
+feed. The flattened conversation removes any post with the focal ID, preventing
+duplicate content.
+
+## Interface
+
+The page heading is `会話`. A short introduction states that only the selected
+post and its conversation are shown.
+
+The focal card uses the existing post renderer and media behavior, with a quiet
+`表示中の投稿` label and accent border. Its internal detail link is omitted to
+avoid a link to the current page. The external X link remains.
+
+Below it, a `会話` section renders the available conversation posts using the
+existing card presentation. Loading, empty, error, retry, and terminal states
+reuse the established Mist Instrument language. Reduced-motion behavior remains
+unchanged.
+
+## Error Handling
+
+- Invalid decimal tweet ID: non-retryable `invalid-input` error, no Bird call.
+- Missing or unavailable focal post: non-retryable `post-not-found` or
+  `post-unavailable` message.
+- Missing relay configuration: non-retryable `relay-config` message.
+- Timeout or unexpected upstream failure: retryable error.
+- Initial conversation failure: show the initial error state and refetch the
+  complete initial operation only after explicit Retry.
+- Later-page failure: retain the focal post and all loaded conversation posts;
+  Retry requests only the failed cursor page.
+- Empty conversation with no cursor: keep the focal post visible and show that
+  there are no additional conversation posts.
+
+Diagnostic details remain server-side. Browser-visible errors use the existing
+small error union and never contain relay URLs, response bodies, or raw content.
+
+## Testing
+
+### Unit and component coverage
+
+- decimal tweet ID validation and invalid-input no-call behavior;
+- initial `getTweet()` then root `getThreadPaged()` call order;
+- continuation calls the root conversation ID exactly once;
+- `maxPages: 1` and `pageDelayMs: 0` on every thread page;
+- public-post sanitization for focal, conversation, and quoted posts;
+- focal extraction and cross-page duplicate removal;
+- thread query key and `{ cursor, conversationId }` page parameters;
+- card internal link, external link, current-card label, and quoted-card
+  behavior;
+- status-route wiring and neutral global navigation state; and
+- existing user/search feed behavior remains unchanged.
+
+### Deterministic browser coverage
+
+Extend the local relay mock with GET `TweetDetail`, routed by operation suffix
+rather than query ID. It validates the `e2e` profile, method, Bird variable
+subset, feature object, and field-toggle object. Unsupported operations remain
+non-404 and the mock makes no outbound requests.
+
+Run the new flows on desktop and mobile:
+
+- open a post detail page from a card;
+- open a reply detail URL directly and resolve its conversation root;
+- show the focal post exactly once;
+- automatically load a second conversation page;
+- retain prior content across a later-page failure and explicit retry;
+- end without ambient discovery or mutation controls;
+- preserve keyboard focus and reduced-motion behavior;
+- report no browser console errors or uncaught page errors; and
+- capture reviewed detail-page snapshots.
+
+The existing live smoke test remains one Top search. This feature does not add
+an unconditional live `TweetDetail` read.
+
+### Final boundary checks
+
+- exactly three application server functions;
+- no Bird/client or relay configuration identifiers in `.output/public`;
+- no Bird mutation calls or generic relay proxy in application source;
+- route generation, lint, typecheck, unit tests, production build, and all
+  desktop/mobile E2E flows pass; and
+- both repositories remain clean except for their intentional local commits,
+  with no push or package publication.
+
+## Documentation Impact
+
+Update the original design's route and server-boundary lists, the implementation
+plan, README feature and omission lists, browser test documentation, and the
+expected server-function security count. `AGENTS.md` and `CLAUDE.md` remain
+absent unless repository-specific agent instructions change.
+
+## Deferred Work
+
+- preserving TweetDetail's original ranked branch order;
+- nested reply-tree rendering;
+- author-only self-thread filtering;
+- a dedicated status-ID input form;
+- related or quoted-post discovery feeds; and
+- any Bird API or relay-catalog change.