diff --git a/android/app/src/main/java/pt/naps/maestro/ui/sidebar/ActivityRail.kt b/android/app/src/main/java/pt/naps/maestro/ui/sidebar/ActivityRail.kt
new file mode 100644
index 0000000..dbe24c7
--- /dev/null
+++ b/android/app/src/main/java/pt/naps/maestro/ui/sidebar/ActivityRail.kt
@@ -0,0 +1,91 @@
+package pt.naps.maestro.ui.sidebar
+
+import androidx.compose.animation.animateColorAsState
+import androidx.compose.animation.core.InfiniteTransition
+import androidx.compose.animation.core.LinearEasing
+import androidx.compose.animation.core.RepeatMode
+import androidx.compose.animation.core.animateFloat
+import androidx.compose.animation.core.infiniteRepeatable
+import androidx.compose.animation.core.rememberInfiniteTransition
+import androidx.compose.animation.core.tween
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.fillMaxHeight
+import androidx.compose.foundation.layout.width
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.semantics.contentDescription
+import androidx.compose.ui.semantics.semantics
+import androidx.compose.ui.unit.dp
+import pt.naps.maestro.data.ActivityState
+
+/** docs/SIDEBAR.md: the rail's width in the web client. Android mirrors it in
+ * density-independent pixels rather than raw pixels. */
+val ACTIVITY_RAIL_WIDTH = 7.dp
+
+private val RAIL_ACTIVE = Color(0xFF3DDC84)
+private val RAIL_IDLE = Color(0xFF9AA0A6)
+private val RAIL_AWAITING = Color(0xFFF5A623)
+private val RAIL_ERROR = Color(0xFFE5484D)
+
+/** Colour per activity state (docs/SIDEBAR.md's colour table): `Starting`
+ * shares `Idle`'s colour, and an unrecognised state degrades to `Idle` so
+ * rendering never fails (Precedence). */
+fun activityColor(activity: ActivityState): Color =
+    when (activity) {
+        ActivityState.Active -> RAIL_ACTIVE
+        ActivityState.AwaitingInput -> RAIL_AWAITING
+        ActivityState.Error -> RAIL_ERROR
+        ActivityState.Idle, ActivityState.Starting -> RAIL_IDLE
+    }
+
+/** The accessible name the rail carries, since colour is never the only
+ * signal (docs/SIDEBAR.md). */
+fun activityStateWord(activity: ActivityState): String =
+    when (activity) {
+        ActivityState.Active -> "Active"
+        ActivityState.Idle -> "Idle"
+        ActivityState.Starting -> "Starting"
+        ActivityState.AwaitingInput -> "Awaiting input"
+        ActivityState.Error -> "Error"
+    }
+
+/** A held bar that pulses its alpha between 1 and [minAlpha] every
+ * [periodMs] — the blink/flash motion `AwaitingInput`/`Error` earn. */
+@Composable
+private fun InfiniteTransition.pulsingAlpha(minAlpha: Float, periodMs: Int): Float {
+    val alpha by animateFloat(
+        initialValue = 1f,
+        targetValue = minAlpha,
+        animationSpec = infiniteRepeatable(
+            animation = tween(durationMillis = periodMs, easing = LinearEasing),
+            repeatMode = RepeatMode.Reverse,
+        ),
+        label = "activity-rail-alpha",
+    )
+    return alpha
+}
+
+/** The card's rail: a coloured bar in the gutter, full card height. It paints
+ * for every card, `Idle` included, and carries motion only for states that
+ * need attention — a hard blink for `AwaitingInput`, a fast flash for
+ * `Error`. `Active` and `Idle` stay solid; colour alone is their signal. */
+@Composable
+fun ActivityRail(activity: ActivityState, modifier: Modifier = Modifier) {
+    val color by animateColorAsState(targetValue = activityColor(activity), label = "activity-rail-color")
+    val infinite = rememberInfiniteTransition(label = "activity-rail-motion")
+    val alpha = when (activity) {
+        ActivityState.AwaitingInput -> infinite.pulsingAlpha(minAlpha = 0.35f, periodMs = 600)
+        ActivityState.Error -> infinite.pulsingAlpha(minAlpha = 0.25f, periodMs = 220)
+        else -> 1f
+    }
+    Box(
+        modifier = modifier
+            .fillMaxHeight()
+            .width(ACTIVITY_RAIL_WIDTH)
+            .semantics { contentDescription = activityStateWord(activity) }
+            .background(color.copy(alpha = alpha)),
+    )
+}
diff --git a/android/app/src/main/java/pt/naps/maestro/ui/sidebar/FolderNode.kt b/android/app/src/main/java/pt/naps/maestro/ui/sidebar/FolderNode.kt
new file mode 100644
index 0000000..f9dc669
--- /dev/null
+++ b/android/app/src/main/java/pt/naps/maestro/ui/sidebar/FolderNode.kt
@@ -0,0 +1,58 @@
+package pt.naps.maestro.ui.sidebar
+
+import androidx.compose.animation.AnimatedVisibility
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.width
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.style.TextOverflow
+import androidx.compose.ui.unit.dp
+
+/** docs/SIDEBAR.md: a collapsible directory in the sidebar tree. An expanded
+ * folder's cards are one row away, so it carries no highlight of its own —
+ * only a collapsed header is ever a keyboard-walk stop, but that distinction
+ * belongs to the caller driving the walk, not this composable. */
+@Composable
+fun FolderNode(
+    label: String,
+    collapsed: Boolean,
+    onToggle: () -> Unit,
+    modifier: Modifier = Modifier,
+    depthPadding: Int = 0,
+    content: @Composable () -> Unit,
+) {
+    Column(modifier = modifier) {
+        Row(
+            modifier = Modifier
+                .fillMaxWidth()
+                .clickable(onClick = onToggle)
+                .padding(start = depthPadding.dp, top = 6.dp, bottom = 6.dp, end = 8.dp),
+            verticalAlignment = Alignment.CenterVertically,
+        ) {
+            Text(
+                text = if (collapsed) "▸" else "▾",
+                style = MaterialTheme.typography.labelMedium,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+            )
+            Spacer(Modifier.width(6.dp))
+            Text(
+                text = label,
+                style = MaterialTheme.typography.labelMedium,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+                maxLines = 1,
+                overflow = TextOverflow.Ellipsis,
+            )
+        }
+        AnimatedVisibility(visible = !collapsed) {
+            Column { content() }
+        }
+    }
+}
diff --git a/android/app/src/main/java/pt/naps/maestro/ui/sidebar/SessionCard.kt b/android/app/src/main/java/pt/naps/maestro/ui/sidebar/SessionCard.kt
new file mode 100644
index 0000000..24acba2
--- /dev/null
+++ b/android/app/src/main/java/pt/naps/maestro/ui/sidebar/SessionCard.kt
@@ -0,0 +1,242 @@
+package pt.naps.maestro.ui.sidebar
+
+import androidx.compose.foundation.ExperimentalFoundationApi
+import androidx.compose.foundation.combinedClickable
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.text.KeyboardOptions
+import androidx.compose.material3.AlertDialog
+import androidx.compose.material3.DropdownMenu
+import androidx.compose.material3.DropdownMenuItem
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.OutlinedTextField
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.input.ImeAction
+import androidx.compose.ui.text.style.TextOverflow
+import androidx.compose.ui.unit.dp
+import pt.naps.maestro.data.InTheWay
+import pt.naps.maestro.data.Session
+import pt.naps.maestro.data.SessionStatus
+
+/** docs/SIDEBAR.md: a card is always two lines behind an activity rail —
+ * line 1 the name, line 2 the adaptive meta line. Long-press opens a context
+ * menu (Rename / Kill / Spawn similar); the daemon confirms rename and kill
+ * only through the callbacks the caller supplies. */
+@OptIn(ExperimentalFoundationApi::class)
+@Composable
+fun SessionCard(
+    session: Session,
+    nowMs: Long,
+    modifier: Modifier = Modifier,
+    depthPadding: Int = 0,
+    onOpen: (Session) -> Unit = {},
+    onRename: (Session, String) -> Unit = { _, _ -> },
+    onKill: (Session) -> Unit = {},
+    onSpawnSimilar: (Session) -> Unit = {},
+) {
+    var menuExpanded by remember { mutableStateOf(false) }
+    var renaming by remember { mutableStateOf(false) }
+    var confirmingKill by remember { mutableStateOf(false) }
+
+    val running = session.status == SessionStatus.Running
+    val name = sessionName(session)
+    val muted = isProvisionalName(session)
+
+    Row(
+        modifier = modifier
+            .fillMaxWidth()
+            .combinedClickable(
+                onClick = { onOpen(session) },
+                onLongClick = { menuExpanded = true },
+            )
+            .padding(start = (8 + depthPadding).dp, end = 8.dp, top = 6.dp, bottom = 6.dp),
+        verticalAlignment = Alignment.CenterVertically,
+    ) {
+        ActivityRail(activity = session.activity, modifier = Modifier.height(36.dp))
+        Spacer(Modifier.width(8.dp))
+        Column(modifier = Modifier.fillMaxWidth()) {
+            Text(
+                text = name,
+                maxLines = 1,
+                overflow = TextOverflow.Ellipsis,
+                color = when {
+                    !running -> MaterialTheme.colorScheme.onSurfaceVariant
+                    muted -> MaterialTheme.colorScheme.onSurfaceVariant
+                    else -> MaterialTheme.colorScheme.onSurface
+                },
+                style = MaterialTheme.typography.bodyMedium,
+            )
+            MetaLine(session = session, nowMs = nowMs)
+        }
+    }
+
+    DropdownMenu(expanded = menuExpanded, onDismissRequest = { menuExpanded = false }) {
+        DropdownMenuItem(
+            text = { Text("Rename") },
+            onClick = {
+                menuExpanded = false
+                renaming = true
+            },
+        )
+        DropdownMenuItem(
+            text = { Text("Spawn similar") },
+            onClick = {
+                menuExpanded = false
+                onSpawnSimilar(session)
+            },
+        )
+        DropdownMenuItem(
+            text = { Text("Kill") },
+            onClick = {
+                menuExpanded = false
+                confirmingKill = true
+            },
+        )
+    }
+
+    if (renaming) {
+        RenameDialog(
+            initial = customSessionName(session) ?: "",
+            onCommit = {
+                renaming = false
+                onRename(session, it)
+            },
+            onCancel = { renaming = false },
+        )
+    }
+
+    if (confirmingKill) {
+        KillConfirmDialog(
+            session = session,
+            onConfirm = {
+                confirmingKill = false
+                onKill(session)
+            },
+            onCancel = { confirmingKill = false },
+        )
+    }
+}
+
+/** Line 2, left to right: `[duration] [detail] command [diff stat | exited]`
+ * — the line itself is unconditional, only its fragments are adaptive. */
+@Composable
+fun MetaLine(session: Session, nowMs: Long, modifier: Modifier = Modifier) {
+    val heldMs = activityHeldMs(session.activity_since, nowMs)
+    val duration = if (heldMs != null && heldMs >= META_LINE_MIN_HOLD_MS) {
+        activityDurationLabel(heldMs)
+    } else {
+        null
+    }
+    val blockers = sessionBlockers(session)
+
+    Row(modifier = modifier.fillMaxWidth(), verticalAlignment = Alignment.CenterVertically) {
+        if (duration != null) {
+            Text(
+                text = duration,
+                color = activityColor(session.activity),
+                style = MaterialTheme.typography.labelSmall,
+            )
+            Spacer(Modifier.width(6.dp))
+        }
+        if (session.activity_detail != null) {
+            Text(
+                text = session.activity_detail,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+                maxLines = 1,
+                overflow = TextOverflow.Ellipsis,
+                style = MaterialTheme.typography.labelSmall,
+                modifier = Modifier.weight(1f, fill = false),
+            )
+            Spacer(Modifier.width(6.dp))
+        }
+        Text(
+            text = programName(session.command),
+            color = MaterialTheme.colorScheme.onSurfaceVariant,
+            maxLines = 1,
+            overflow = TextOverflow.Ellipsis,
+            style = MaterialTheme.typography.labelSmall,
+        )
+        if (blockers != null) {
+            Spacer(Modifier.width(6.dp))
+            BlockedFragment(blockers)
+        }
+        if (session.status == SessionStatus.Dead) {
+            Spacer(Modifier.width(6.dp))
+            Text(
+                text = "exited",
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+                style = MaterialTheme.typography.labelSmall,
+            )
+        }
+    }
+}
+
+@Composable
+private fun BlockedFragment(blockers: InTheWay) {
+    Row(verticalAlignment = Alignment.CenterVertically) {
+        blockerStats(blockers).forEachIndexed { index, stat ->
+            if (index > 0) Spacer(Modifier.width(4.dp))
+            Text(
+                text = stat.text,
+                style = MaterialTheme.typography.labelSmall,
+                color = when (stat.kind) {
+                    "add" -> MaterialTheme.colorScheme.primary
+                    "del" -> MaterialTheme.colorScheme.error
+                    else -> MaterialTheme.colorScheme.onSurfaceVariant
+                },
+            )
+        }
+    }
+}
+
+@Composable
+private fun RenameDialog(initial: String, onCommit: (String) -> Unit, onCancel: () -> Unit) {
+    var text by remember { mutableStateOf(initial) }
+    AlertDialog(
+        onDismissRequest = onCancel,
+        title = { Text("Rename session") },
+        text = {
+            OutlinedTextField(
+                value = text,
+                onValueChange = { text = it },
+                singleLine = true,
+                keyboardOptions = KeyboardOptions(imeAction = ImeAction.Done),
+            )
+        },
+        confirmButton = { TextButton(onClick = { onCommit(text.trim()) }) { Text("Rename") } },
+        dismissButton = { TextButton(onClick = onCancel) { Text("Cancel") } },
+    )
+}
+
+@Composable
+private fun KillConfirmDialog(session: Session, onConfirm: () -> Unit, onCancel: () -> Unit) {
+    val blockers = sessionBlockers(session)
+    AlertDialog(
+        onDismissRequest = onCancel,
+        title = { Text("Kill “${sessionName(session)}”?") },
+        text = {
+            Text(
+                if (blockers != null) {
+                    "This worktree has unsaved work: ${formatInTheWay(blockers)}. Killing may be refused unless forced."
+                } else {
+                    "This ends the session's process. This can't be undone."
+                },
+            )
+        },
+        confirmButton = { TextButton(onClick = onConfirm) { Text("Kill") } },
+        dismissButton = { TextButton(onClick = onCancel) { Text("Cancel") } },
+    )
+}
diff --git a/android/app/src/main/java/pt/naps/maestro/ui/sidebar/SidebarContent.kt b/android/app/src/main/java/pt/naps/maestro/ui/sidebar/SidebarContent.kt
new file mode 100644
index 0000000..080bb21
--- /dev/null
+++ b/android/app/src/main/java/pt/naps/maestro/ui/sidebar/SidebarContent.kt
@@ -0,0 +1,123 @@
+package pt.naps.maestro.ui.sidebar
+
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.items
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateMapOf
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import kotlinx.coroutines.delay
+import pt.naps.maestro.connection.CatalogStore
+import pt.naps.maestro.data.Session
+import pt.naps.maestro.data.Uuid
+
+/** A clock that advances at least once every 30s (docs/SIDEBAR.md), ticked
+ * faster than that so a duration's cutover lands within one visible frame. */
+@Composable
+fun rememberTickingNowMs(periodMs: Long = 5_000L): Long {
+    var now by remember { mutableStateOf(System.currentTimeMillis()) }
+    LaunchedEffect(periodMs) {
+        while (true) {
+            delay(periodMs)
+            now = System.currentTimeMillis()
+        }
+    }
+    return now
+}
+
+/** Assembles the full sidebar from the [CatalogStore] built in #264: sessions
+ * grouped into collapsible folders by repository, each rendered as a
+ * two-line [SessionCard] behind its [ActivityRail]. */
+@Composable
+fun SidebarContent(
+    catalogStore: CatalogStore,
+    onOpenSession: (Session) -> Unit,
+    modifier: Modifier = Modifier,
+    onRenameSession: (Uuid, String) -> Unit = { _, _ -> },
+    onKillSession: (Uuid) -> Unit = {},
+    onSpawnSimilar: (Session) -> Unit = {},
+) {
+    val sessions by catalogStore.sessions.collectAsState()
+    SidebarContent(
+        sessions = sessions.values.toList(),
+        onOpenSession = onOpenSession,
+        modifier = modifier,
+        onRenameSession = onRenameSession,
+        onKillSession = onKillSession,
+        onSpawnSimilar = onSpawnSimilar,
+    )
+}
+
+/** Same as the [CatalogStore]-driven overload, but takes the session list
+ * directly — the shape used by tests and previews. */
+@Composable
+fun SidebarContent(
+    sessions: List<Session>,
+    onOpenSession: (Session) -> Unit,
+    modifier: Modifier = Modifier,
+    onRenameSession: (Uuid, String) -> Unit = { _, _ -> },
+    onKillSession: (Uuid) -> Unit = {},
+    onSpawnSimilar: (Session) -> Unit = {},
+) {
+    val nowMs = rememberTickingNowMs()
+    val groups = remember(sessions) { groupSessions(sessions) }
+    val collapsed = remember { mutableStateMapOf<String, Boolean>() }
+
+    Column(modifier = modifier.fillMaxSize()) {
+        Text(
+            text = "Sessions",
+            style = MaterialTheme.typography.titleMedium,
+            modifier = Modifier.padding(16.dp),
+        )
+        if (groups.isEmpty()) {
+            Box(modifier = Modifier.fillMaxSize().padding(16.dp)) {
+                Text(
+                    text = "No sessions",
+                    style = MaterialTheme.typography.bodyMedium,
+                    color = MaterialTheme.colorScheme.onSurfaceVariant,
+                )
+            }
+            return
+        }
+        LazyColumn(modifier = Modifier.fillMaxSize()) {
+            for (group in groups) {
+                val isCollapsed = collapsed[group.key] == true
+                item(key = "folder:${group.key}") {
+                    FolderNode(
+                        label = group.label,
+                        collapsed = isCollapsed,
+                        onToggle = { collapsed[group.key] = !isCollapsed },
+                        depthPadding = 8,
+                    ) {
+                        Column {
+                            for (session in group.sessions) {
+                                SessionCard(
+                                    session = session,
+                                    nowMs = nowMs,
+                                    depthPadding = 16,
+                                    onOpen = onOpenSession,
+                                    onRename = { s, name -> onRenameSession(s.id, name) },
+                                    onKill = { s -> onKillSession(s.id) },
+                                    onSpawnSimilar = onSpawnSimilar,
+                                )
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+}