diff --git a/prompts/implement-issue.md b/prompts/implement-issue.md
new file mode 100644
index 0000000..d86420d
--- /dev/null
+++ b/prompts/implement-issue.md
@@ -0,0 +1,160 @@
+## Purpose
+
+Implements a GitHub issue autonomously: validates the issue's claims against current code, makes the fix in a prepared git worktree, verifies the repo's checks and tests pass, then opens a draft PR.
+
+## Prerequisites
+
+- `gh` CLI authenticated (`gh auth status`)
+- `just` installed in the target repo
+- Worktree already created and branch already checked out by the caller
+
+## Context
+
+The task prompt provides:
+
+- `repo` — org/repo slug (e.g. `ooloth/hub`)
+- `issue` — GitHub issue number
+- `worktree` — absolute path to the prepared git worktree
+- `branch` — branch name
+
+## Workflow
+
+### 1. Fetch the issue
+
+```bash
+gh issue view <issue> --repo <repo> \
+  --json number,title,body,labels,comments,createdAt,updatedAt,state,author
+```
+
+Read the title, body, and all comments in full.
+
+### 2. Verify the worktree is clean
+
+```bash
+git -C <worktree> status --porcelain
+```
+
+If the output is non-empty, the worktree has uncommitted changes — something went wrong in a previous run. Stop without modifying any labels.
+
+### 3. Establish a baseline
+
+Run the repo's check and test commands before making any changes:
+
+```bash
+cd <worktree> && just check && just test
+```
+
+If they fail, the repo was already broken before you touched it. Stop without modifying any labels — this is not a problem you introduced and not yours to fix.
+
+### 4. Claim
+
+Relabel immediately to prevent a second agent from claiming the same issue:
+
+```bash
+# Create label if absent
+gh label list --repo <repo> --json name --jq '.[].name' | grep -q "status:agent-working" \
+  || gh label create "status:agent-working" --repo <repo> \
+       --color "0075ca" --description "An agent is currently implementing this"
+
+gh issue edit <issue> --repo <repo> \
+  --remove-label "status:ready-for-agent" \
+  --add-label "status:agent-working"
+```
+
+### 5. Validate the issue's claims
+
+Using `Read`, `rg`, and `fd` inside `<worktree>`, explore the areas the issue describes:
+
+- Find the files and symbols it references
+- Check whether the bug, gap, or violation it describes still exists in the current code
+- Check recent commits for evidence it was already addressed:
+
+```bash
+git -C <worktree> log --oneline -20
+```
+
+**If stale or already resolved** — comment explaining what you found, relabel, and stop:
+
+```bash
+gh issue comment <issue> --repo <repo> --body "..."
+
+gh label list --repo <repo> --json name --jq '.[].name' | grep -q "status:needs-human-review" \
+  || gh label create "status:needs-human-review" --repo <repo> \
+       --color "d93f0b" --description "Needs a human to review before proceeding"
+
+gh issue edit <issue> --repo <repo> \
+  --remove-label "status:agent-working" \
+  --add-label "status:needs-human-review"
+```
+
+### 6. Plan
+
+Read the relevant files and understand the module structure. Invoke
+`/uphold-invariants` to load the code quality invariants that apply to
+the affected areas — these constrain what changes are acceptable and
+should inform every decision you make. Identify exactly which files need
+to change and how. If the fix requires a design decision not already
+resolved by the issue body and comments, comment on the issue with the
+open question, relabel to `status:needs-human-review`, and stop — do
+not guess.
+
+### 7. Implement
+
+Make all changes inside `<worktree>`. Follow the repo's existing conventions (formatting, naming, error handling, style). Do not touch files unrelated to the issue.
+
+### 8. Write missing tests
+
+Before running checks, ask: what new decisions or behaviors did this
+change introduce — branches, filters, transformations, mappings? For
+each one: if the logic were wrong, would any existing test catch it? If
+not, and if the behavior can be exercised without standing up the full
+system, write a test for it.
+
+### 9. Fix until green
+
+Run the repo's check and test commands:
+
+```bash
+cd <worktree> && just check && just test
+```
+
+Your changes introduced any failures that appear now — the baseline
+passed in step 3. Read the errors, fix them in `<worktree>`, and re-run.
+Repeat until green. Do not bail here: this is your responsibility to
+resolve.
+
+If after multiple fix attempts the failures are intractable (e.g. the
+issue itself has a flaw that makes a correct implementation impossible),
+comment on the issue explaining the problem, relabel to
+`status:needs-human-review`, and stop.
+
+### 10. Verify manually
+
+Ask: how can I run this and confirm it actually works? Run the CLI, hit
+the endpoint, trigger the event, eyeball the output — whatever applies
+to what changed. Do not rely on tests alone. If end-to-end execution is
+impossible in this environment, say why explicitly.
+
+### 11. Commit and push
+
+Invoke `/commit` to stage, commit, and push your changes.
+
+### 12. Open draft PR
+
+Invoke `/write-pr-description` to draft and open the PR. The PR must be
+a draft and the body must include `Closes #<issue>`.
+
+### 13. Comment on the issue
+
+```bash
+gh issue comment <issue> --repo <repo> \
+  --body "Opened draft PR: <pr-url>"
+```
+
+## Output format
+
+**Done:** PR #N opened as draft — `<pr title>`
+
+Or if stopped early:
+
+**Stopped:** <one sentence reason> — issue relabeled `status:needs-human-review`
diff --git a/ui/tui/src/state/mod.rs b/ui/tui/src/state/mod.rs
index 1f520b8..eb39439 100644
--- a/ui/tui/src/state/mod.rs
+++ b/ui/tui/src/state/mod.rs
@@ -54,6 +54,10 @@ impl App {
                 *selected = selected.saturating_sub(1);
             }
             Screen::Detail { view, .. } => {
+                assert!(
+                    view.list_state.selected().is_some(),
+                    "list_state must be selected in Detail screen"
+                );
                 let sel = view.list_state.selected().unwrap_or(0);
                 if sel > 0 {
                     view.list_state.select(Some(sel - 1));
@@ -71,6 +75,10 @@ impl App {
                 }
             }
             Screen::Detail { view, .. } => {
+                assert!(
+                    view.list_state.selected().is_some(),
+                    "list_state must be selected in Detail screen"
+                );
                 let sel = view.list_state.selected().unwrap_or(0);
                 if len > 0 && sel < len - 1 {
                     view.list_state.select(Some(sel + 1));
@@ -104,6 +112,10 @@ impl App {
                 *selected = selected.saturating_sub(PAGE);
             }
             Screen::Detail { view, .. } => {
+                assert!(
+                    view.list_state.selected().is_some(),
+                    "list_state must be selected in Detail screen"
+                );
                 let sel = view.list_state.selected().unwrap_or(0);
                 view.list_state.select(Some(sel.saturating_sub(PAGE)));
             }
@@ -121,9 +133,105 @@ impl App {
                 *selected = (*selected + PAGE).min(len - 1);
             }
             Screen::Detail { view, .. } => {
+                assert!(
+                    view.list_state.selected().is_some(),
+                    "list_state must be selected in Detail screen"
+                );
                 let sel = view.list_state.selected().unwrap_or(0);
                 view.list_state.select(Some((sel + PAGE).min(len - 1)));
             }
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{App, DetailView, Screen, UiState};
+    use crate::display::{DisplayItem, Filter, ListSnapshot};
+    use rstest::rstest;
+    use workflows::status::StatusItem;
+
+    fn stub_item() -> StatusItem {
+        StatusItem::Ci(domain::CiFailure {
+            repo: domain::RepoSlug::new("ooloth", "hub"),
+            workflow_name: "CI".to_string(),
+            job_name: None,
+            step_name: None,
+            error: None,
+            age: chrono::Duration::zero(),
+            urgency: domain::Urgency::High,
+            url: "https://example.com".to_string(),
+        })
+    }
+
+    fn detail_app(item_count: usize, selected: usize) -> App {
+        let items: Vec<StatusItem> = (0..item_count).map(|_| stub_item()).collect();
+        App {
+            ui: UiState {
+                screen: Screen::Detail {
+                    parent: ListSnapshot {
+                        items: vec![DisplayItem::Group {
+                            label: "group".to_string(),
+                            items,
+                        }],
+                        selected: 0,
+                        filter: Filter::default(),
+                    },
+                    view: DetailView {
+                        group_index: 0,
+                        list_state: {
+                            let mut ls = ratatui::widgets::ListState::default();
+                            ls.select(Some(selected));
+                            ls
+                        },
+                    },
+                },
+                ..UiState::default()
+            },
+            ..App::default()
+        }
+    }
+
+    fn detail_selected(app: &App) -> usize {
+        match &app.ui.screen {
+            Screen::Detail { view, .. } => view.list_state.selected().unwrap(),
+            _ => panic!("not in detail screen"),
+        }
+    }
+
+    #[rstest]
+    #[case(3, 1, 0)]
+    #[case(3, 0, 0)]
+    fn detail_move_up(#[case] n: usize, #[case] start: usize, #[case] end: usize) {
+        let mut app = detail_app(n, start);
+        app.move_up();
+        assert_eq!(detail_selected(&app), end);
+    }
+
+    #[rstest]
+    #[case(3, 1, 2)]
+    #[case(3, 2, 2)]
+    fn detail_move_down(#[case] n: usize, #[case] start: usize, #[case] end: usize) {
+        let mut app = detail_app(n, start);
+        app.move_down();
+        assert_eq!(detail_selected(&app), end);
+    }
+
+    #[rstest]
+    #[case(20, 15, 5)]
+    #[case(20, 5, 0)]
+    fn detail_move_page_up(#[case] n: usize, #[case] start: usize, #[case] end: usize) {
+        let mut app = detail_app(n, start);
+        app.move_page_up();
+        assert_eq!(detail_selected(&app), end);
+    }
+
+    #[rstest]
+    #[case(25, 5, 15)]
+    #[case(25, 18, 24)]
+    fn detail_move_page_down(#[case] n: usize, #[case] start: usize, #[case] end: usize) {
+        let mut app = detail_app(n, start);
+        app.move_page_down();
+        assert_eq!(detail_selected(&app), end);
+    }
+}