diff --git a/src/viewer/index.html b/src/viewer/index.html
index 551d016..2bdc687 100644
--- a/src/viewer/index.html
+++ b/src/viewer/index.html
@@ -1039,6 +1039,23 @@ <h1>agentmemory</h1>
       if (!s) return '';
       return s.length > n ? s.slice(0, n) + '...' : s;
     }
+    function sessionId(s) {
+      return s && s.id !== undefined && s.id !== null ? String(s.id) : '';
+    }
+    function shortSessionId(s, n) {
+      var id = sessionId(s);
+      return id ? id.slice(0, n || 8) : '';
+    }
+    function sessionDisplayName(s) {
+      var project = s && s.project ? String(s.project).split('/').pop() : '';
+      if (project) return project;
+      return shortSessionId(s, 8) || 'Unknown session';
+    }
+    function sessionLabel(s) {
+      var id = shortSessionId(s, 8);
+      var name = sessionDisplayName(s);
+      return id ? name + ' (' + id + ')' : name + ' (missing id)';
+    }
     function debounce(fn, ms) {
       var t;
       return function() {
@@ -1278,7 +1295,7 @@ <h1>agentmemory</h1>
         html += '<table><tr><th>Project</th><th>Status</th><th>Obs</th><th>Started</th></tr>';
         recent.forEach(function(s) {
           var statusBadge = s.status === 'active' ? 'badge-green' : s.status === 'completed' ? 'badge-blue' : 'badge-muted';
-          html += '<tr><td style="color:var(--ink);font-weight:500;">' + esc(s.project ? s.project.split('/').pop() : s.id.slice(0,8)) + '</td>';
+          html += '<tr><td style="color:var(--ink);font-weight:500;">' + esc(sessionDisplayName(s)) + '</td>';
           html += '<td><span class="badge ' + statusBadge + '">' + esc(s.status) + '</span></td>';
           html += '<td style="color:var(--ink-muted);font-family:var(--font-mono);font-size:12px;">' + (s.observationCount || 0) + '</td>';
           html += '<td style="font-family:var(--font-mono);font-size:11px;color:var(--ink-faint);">' + esc(shortTime(s.startedAt)) + '</td></tr>';
@@ -2192,7 +2209,8 @@ <h1>agentmemory</h1>
 
       if (sessions.length > 0 && !state.timeline.sessionId) {
         var sorted = sessions.slice().sort(function(a, b) { return (b.startedAt || '').localeCompare(a.startedAt || ''); });
-        state.timeline.sessionId = sorted[0].id;
+        var firstSelectable = sorted.find(function(s) { return sessionId(s); });
+        state.timeline.sessionId = firstSelectable ? sessionId(firstSelectable) : '';
       }
 
       renderTimelineToolbar(sessions);
@@ -2204,8 +2222,9 @@ <h1>agentmemory</h1>
       var html = '<div class="toolbar">';
       html += '<select id="tl-session"><option value="">Select session</option>';
       sessions.sort(function(a, b) { return (b.startedAt || '').localeCompare(a.startedAt || ''); }).forEach(function(s) {
-        var label = (s.project ? s.project.split('/').pop() : s.id.slice(0,8)) + ' (' + s.id.slice(0,8) + ')';
-        html += '<option value="' + esc(s.id) + '"' + (state.timeline.sessionId === s.id ? ' selected' : '') + '>' + esc(label) + '</option>';
+        var id = sessionId(s);
+        var disabled = id ? '' : ' disabled';
+        html += '<option value="' + esc(id) + '"' + (id && state.timeline.sessionId === id ? ' selected' : '') + disabled + '>' + esc(sessionLabel(s)) + '</option>';
       });
       html += '</select>';
       html += '<select id="tl-importance"><option value="0">All importance</option>';
@@ -2421,8 +2440,8 @@ <h1>agentmemory</h1>
       var sorted = sessions.slice().sort(function(a, b) { return (b.startedAt || '').localeCompare(a.startedAt || ''); });
       var recentSessions = sorted.slice(0, 5);
 
-      var obsResults = await Promise.all(recentSessions.map(function(s) {
-        return apiGet('observations?sessionId=' + encodeURIComponent(s.id));
+      var obsResults = await Promise.all(recentSessions.filter(function(s) { return sessionId(s); }).map(function(s) {
+        return apiGet('observations?sessionId=' + encodeURIComponent(sessionId(s)));
       }));
       obsResults.forEach(function(r) {
         if (r && r.observations) allObs = allObs.concat(r.observations);
@@ -2565,15 +2584,16 @@ <h1>agentmemory</h1>
       } else {
         items.forEach(function(s) {
           var statusBadge = s.status === 'active' ? 'badge-green' : s.status === 'completed' ? 'badge-blue' : 'badge-muted';
-          var selected = state.sessions.selectedId === s.id;
-          html += '<div class="session-item' + (selected ? ' selected' : '') + '" data-action="select-session" data-session-id="' + esc(s.id) + '">';
-          html += '<div class="session-top"><span class="session-project">' + esc(s.project ? s.project.split('/').pop() : 'Unknown') + '</span>';
+          var id = sessionId(s);
+          var selected = id && state.sessions.selectedId === id;
+          html += '<div class="session-item' + (selected ? ' selected' : '') + '"' + (id ? ' data-action="select-session" data-session-id="' + esc(id) + '"' : '') + '>';
+          html += '<div class="session-top"><span class="session-project">' + esc(sessionDisplayName(s)) + '</span>';
           html += '<span class="badge ' + statusBadge + '">' + esc(s.status) + '</span></div>';
           var preview = s.firstPrompt || s.summary || '';
           if (preview) {
             html += '<div class="session-preview" style="font-size:13px;color:var(--ink);margin:4px 0;line-height:1.4;">' + esc(truncate(preview, 140)) + '</div>';
           }
-          html += '<div class="session-meta">' + esc(s.id.slice(0, 12)) + ' &middot; ' + esc(formatTime(s.startedAt));
+          html += '<div class="session-meta">' + esc(shortSessionId(s, 12) || 'missing id') + ' &middot; ' + esc(formatTime(s.startedAt));
           html += ' &middot; ' + (s.observationCount || 0) + ' obs';
           if (s.model) html += ' &middot; ' + esc(s.model);
           html += '</div></div>';
@@ -2594,12 +2614,13 @@ <h1>agentmemory</h1>
     async function renderSessionDetail() {
       var panel = document.getElementById('session-detail');
       if (!panel) return;
-      var s = state.sessions.items.find(function(x) { return x.id === state.sessions.selectedId; });
-      if (!s) { panel.innerHTML = ''; return; }
+      var s = state.sessions.items.find(function(x) { return sessionId(x) === state.sessions.selectedId; });
+      var id = sessionId(s);
+      if (!s || !id) { panel.innerHTML = ''; return; }
 
       panel.innerHTML = '<div class="detail-panel"><h3>Loading session details…</h3></div>';
 
-      var obsRes = await apiGet('observations?sessionId=' + encodeURIComponent(s.id));
+      var obsRes = await apiGet('observations?sessionId=' + encodeURIComponent(id));
       var obs = (obsRes && obsRes.observations) || [];
 
       var typeCounts = {};
@@ -2672,7 +2693,8 @@ <h1>agentmemory</h1>
 
       html += '<div class="card" style="margin-bottom:12px;"><div class="card-title">Metadata</div>';
       html += '<div style="font-size:12px;font-family:var(--font-mono);margin-top:8px;line-height:1.7;">';
-      html += '<div><span style="color:var(--ink-muted);">id:</span> ' + esc(s.id) + '</div>';
+      var detailId = sessionId(s);
+      html += '<div><span style="color:var(--ink-muted);">id:</span> ' + esc(detailId || 'missing id') + '</div>';
       html += '<div><span style="color:var(--ink-muted);">cwd:</span> ' + esc(s.cwd || '-') + '</div>';
       html += '<div><span style="color:var(--ink-muted);">started:</span> ' + esc(formatTime(s.startedAt)) + '</div>';
       if (s.endedAt) html += '<div><span style="color:var(--ink-muted);">ended:</span> ' + esc(formatTime(s.endedAt)) + '</div>';
@@ -2681,10 +2703,14 @@ <h1>agentmemory</h1>
       html += '</div></div>';
 
       html += '<div style="display:flex;gap:8px;">';
-      if (s.status === 'active') {
-        html += '<button class="btn btn-danger" data-action="end-session" data-session-id="' + esc(s.id) + '">End Session</button>';
+      if (detailId && s.status === 'active') {
+        html += '<button class="btn btn-danger" data-action="end-session" data-session-id="' + esc(detailId) + '">End Session</button>';
+      }
+      if (detailId) {
+        html += '<button class="btn btn-primary" data-action="summarize-session" data-session-id="' + esc(detailId) + '">Summarize</button>';
+      } else {
+        html += '<button class="btn btn-primary" disabled>Summarize unavailable</button>';
       }
-      html += '<button class="btn btn-primary" data-action="summarize-session" data-session-id="' + esc(s.id) + '">Summarize</button>';
       html += '</div></div>';
       panel.innerHTML = html;
     }
@@ -3587,8 +3613,9 @@ <h1>agentmemory</h1>
       var el = document.getElementById('view-replay');
       var sessions = state.replay.sessions || [];
       var options = '<option value="">— pick a session —</option>' + sessions.map(function(s) {
-        var label = (s.project || 'unknown') + ' · ' + (s.id || '').slice(0, 8) + ' · ' + (s.observationCount || 0) + ' obs';
-        return '<option value="' + esc(s.id) + '"' + (s.id === state.replay.selectedId ? ' selected' : '') + '>' + esc(label) + '</option>';
+        var id = sessionId(s);
+        var label = sessionDisplayName(s) + ' · ' + (shortSessionId(s, 8) || 'missing id') + ' · ' + (s.observationCount || 0) + ' obs';
+        return '<option value="' + esc(id) + '"' + (id && id === state.replay.selectedId ? ' selected' : '') + (id ? '' : ' disabled') + '>' + esc(label) + '</option>';
       }).join('');
 
       var tl = state.replay.timeline;
diff --git a/test/viewer-session-id.test.ts b/test/viewer-session-id.test.ts
new file mode 100644
index 0000000..4450d0c
--- /dev/null
+++ b/test/viewer-session-id.test.ts
@@ -0,0 +1,189 @@
+import * as vm from "node:vm";
+import { describe, expect, it } from "vitest";
+import { renderViewerDocument } from "../src/viewer/document.js";
+
+function htmlEscape(value: string): string {
+  return value
+    .replace(/&/g, "&amp;")
+    .replace(/</g, "&lt;")
+    .replace(/>/g, "&gt;")
+    .replace(/"/g, "&quot;");
+}
+
+function loadViewerSandbox() {
+  const rendered = renderViewerDocument();
+  expect(rendered.found).toBe(true);
+  if (!rendered.found) throw new Error("viewer document not found");
+
+  const scriptMatch = rendered.html.match(/<script nonce="[^"]+">([\s\S]*?)<\/script>/);
+  expect(scriptMatch).not.toBeNull();
+  if (!scriptMatch) throw new Error("viewer script not found");
+
+  const elements = new Map<string, any>();
+  const createMockElement = (id = "") => {
+    const attributes = new Map<string, string>();
+    const classes = new Set<string>();
+    const listeners = new Map<string, Array<(event?: unknown) => void>>();
+    return {
+      id,
+      innerHTML: "",
+      textContent: "",
+      value: "",
+      checked: false,
+      dataset: {},
+      style: {},
+      listeners,
+      classList: {
+        add: (name: string) => classes.add(name),
+        remove: (name: string) => classes.delete(name),
+        contains: (name: string) => classes.has(name),
+        toggle: (name: string, force?: boolean) => {
+          const enabled = force ?? !classes.has(name);
+          if (enabled) classes.add(name);
+          else classes.delete(name);
+          return enabled;
+        },
+      },
+      addEventListener: (type: string, handler: (event?: unknown) => void) => {
+        const current = listeners.get(type) || [];
+        current.push(handler);
+        listeners.set(type, current);
+      },
+      getAttribute: (name: string) => attributes.get(name) ?? null,
+      setAttribute: (name: string, value: unknown) => {
+        attributes.set(name, String(value));
+      },
+      querySelectorAll: () => [],
+    };
+  };
+  const getElement = (id: string) => {
+    if (!elements.has(id)) elements.set(id, createMockElement(id));
+    return elements.get(id);
+  };
+
+  const tabs = [
+    "dashboard",
+    "graph",
+    "memories",
+    "timeline",
+    "sessions",
+    "lessons",
+    "actions",
+    "crystals",
+    "audit",
+    "activity",
+    "profile",
+    "replay",
+  ];
+  const tabButtons = tabs.map((tab) => ({ ...createMockElement(), dataset: { tab } }));
+  const views = tabs.map((tab) => ({ ...createMockElement(`view-${tab}`), id: `view-${tab}` }));
+  const checkboxes = [createMockElement(), createMockElement()].map((el) => ({ ...el, checked: false }));
+  const querySelectorAll = (selector: string) => {
+    if (selector === ".tab-bar button") return tabButtons;
+    if (selector === ".view") return views;
+    if (selector === 'input[type="checkbox"]') return checkboxes;
+    return [];
+  };
+
+  const document = {
+    documentElement: { dataset: {} },
+    createElement: () => {
+      let text = "";
+      return {
+        set textContent(value: unknown) {
+          text = String(value ?? "");
+        },
+        get innerHTML() {
+          return htmlEscape(text);
+        },
+      };
+    },
+    getElementById: getElement,
+    querySelectorAll,
+    addEventListener: () => {},
+  };
+
+  const sandbox: Record<string, any> = {
+    console: { log: () => {}, warn: () => {}, error: () => {} },
+    document,
+    window: {
+      location: {
+        search: "",
+        port: "3113",
+        protocol: "http:",
+        hostname: "localhost",
+        host: "localhost:3113",
+        origin: "http://localhost:3113",
+      },
+      matchMedia: () => ({ matches: false }),
+      addEventListener: () => {},
+    },
+    localStorage: { getItem: () => null, setItem: () => {} },
+    fetch: async () => ({ ok: true, json: async () => ({}) }),
+    WebSocket: function WebSocket() {},
+    navigator: { userAgent: "vitest" },
+    Element: function Element() {},
+    alert: () => {},
+    setInterval: () => 0,
+    clearInterval: () => {},
+    setTimeout: () => 0,
+    clearTimeout: () => {},
+    URLSearchParams,
+    Date,
+    Math,
+    Promise,
+    JSON,
+    Array,
+    Object,
+    String,
+    Number,
+    parseInt,
+    encodeURIComponent,
+  };
+
+  const scriptWithoutAutoStart = scriptMatch[1].replace(
+    /\n\s*loadTab\('dashboard'\);\n\s*connectWs\(\);\n\s*startDashboardAutoRefresh\(\);\s*$/,
+    "\n",
+  );
+
+  vm.createContext(sandbox);
+  vm.runInContext(scriptWithoutAutoStart, sandbox);
+
+  return { sandbox, getElement };
+}
+
+describe("viewer session rendering", () => {
+  it("does not throw when dashboard sessions are missing ids", () => {
+    const { sandbox, getElement } = loadViewerSandbox();
+    sandbox.state.dashboard = {
+      loaded: true,
+      health: { status: "healthy", health: {} },
+      sessions: [{ status: "active", observationCount: 3, startedAt: "2026-05-13T12:00:00Z" }],
+      memories: [],
+      graphStats: null,
+      recentAudit: [],
+      lessons: [],
+      crystals: [],
+    };
+
+    expect(() => sandbox.renderDashboard()).not.toThrow();
+    expect(getElement("view-dashboard").innerHTML).toContain("Unknown session");
+  });
+
+  it("does not throw when timeline and sessions tabs receive sessions missing ids", () => {
+    const { sandbox, getElement } = loadViewerSandbox();
+    const sessions = [{ status: "active", observationCount: 1, startedAt: "2026-05-13T12:00:00Z" }];
+
+    expect(() => sandbox.renderTimelineToolbar(sessions)).not.toThrow();
+    expect(getElement("view-timeline").innerHTML).toContain("Unknown session");
+
+    sandbox.state.sessions.items = sessions;
+    expect(() => sandbox.renderSessions()).not.toThrow();
+    expect(getElement("view-sessions").innerHTML).toContain("Unknown session");
+
+    const tabButtons = sandbox.document.querySelectorAll(".tab-bar button");
+    expect(tabButtons.length).toBeGreaterThan(0);
+    expect(() => sandbox.switchTab("sessions")).not.toThrow();
+    expect(tabButtons.some((button: any) => button.classList.contains("active"))).toBe(true);
+  });
+});