diff --git a/app/main.py b/app/main.py
index 5bd191b..d75d1fb 100644
--- a/app/main.py
+++ b/app/main.py
@@ -1,5 +1,9 @@
+from pathlib import Path
+
 from fastapi import FastAPI, Depends, HTTPException, status
+from fastapi.responses import FileResponse
 from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials
+from fastapi.staticfiles import StaticFiles
 from sqlalchemy.orm import Session
 from typing import List
 import boto3
@@ -14,6 +18,12 @@
 Base.metadata.create_all(bind=engine)
 
 app = FastAPI(title="Agent Suite", version="0.1.0")
+
+# Static files directory
+STATIC_DIR = Path(__file__).resolve().parent / "static"
+
+# Mount static assets (CSS, JS)
+app.mount("/static", StaticFiles(directory=str(STATIC_DIR)), name="static")
 security = HTTPBearer()
 settings = get_settings()
 
@@ -35,6 +45,35 @@ def verify_api_key(credentials: HTTPAuthorizationCredentials = Depends(security)
     return inbox
 
 
+# ─── Web UI Routes ────────────────────────────────────────
+
+
+def _serve_ui():
+    """Serve the single-page application HTML."""
+    return FileResponse(str(STATIC_DIR / "index.html"), media_type="text/html")
+
+
+@app.get("/inbox", include_in_schema=False)
+def inbox_page():
+    """Inbox page - lists all received messages."""
+    return _serve_ui()
+
+
+@app.get("/inbox/{message_id}", include_in_schema=False)
+def message_page(message_id: str):
+    """Message detail page."""
+    return _serve_ui()
+
+
+@app.get("/compose", include_in_schema=False)
+def compose_page():
+    """Compose page - send a new email."""
+    return _serve_ui()
+
+
+# ─── API Routes ───────────────────────────────────────────
+
+
 @app.get("/health")
 def health_check():
     return {"status": "ok", "service": "agent-suite"}
diff --git a/app/static/app.js b/app/static/app.js
new file mode 100644
index 0000000..a474f32
--- /dev/null
+++ b/app/static/app.js
@@ -0,0 +1,549 @@
+/* Agent Suite - Email Inbox Web UI */
+
+(function () {
+    "use strict";
+
+    // ─── State ───────────────────────────────────────────────
+
+    var state = {
+        apiKey: localStorage.getItem("agent_suite_api_key") || "",
+        messages: [],
+        total: 0,
+        currentPage: 0,
+        pageSize: 50,
+    };
+
+    // ─── DOM References ──────────────────────────────────────
+
+    var appEl = document.getElementById("app");
+    var modalOverlay = document.getElementById("modal-overlay");
+    var apiKeyInput = document.getElementById("api-key-input");
+    var showKeyCheckbox = document.getElementById("show-key");
+    var settingsBtn = document.getElementById("settings-btn");
+    var saveKeyBtn = document.getElementById("save-key-btn");
+
+    // ─── API Client ──────────────────────────────────────────
+
+    var api = {
+        request: function (path, options) {
+            options = options || {};
+            if (!state.apiKey) {
+                return Promise.reject(
+                    new Error("No API key configured. Click the gear icon to set your API key.")
+                );
+            }
+            var headers = {
+                Authorization: "Bearer " + state.apiKey,
+                "Content-Type": "application/json",
+            };
+            if (options.headers) {
+                Object.keys(options.headers).forEach(function (k) {
+                    headers[k] = options.headers[k];
+                });
+            }
+            return fetch(window.location.origin + path, {
+                method: options.method || "GET",
+                headers: headers,
+                body: options.body || undefined,
+            }).then(function (res) {
+                if (!res.ok) {
+                    return res.json().catch(function () {
+                        return {};
+                    }).then(function (data) {
+                        throw new Error(data.detail || "Request failed (" + res.status + ")");
+                    });
+                }
+                return res.json();
+            });
+        },
+
+        getMessages: function (skip, limit) {
+            skip = skip || 0;
+            limit = limit || 50;
+            return api.request(
+                "/v1/inboxes/me/messages?skip=" + skip + "&limit=" + limit
+            );
+        },
+
+        getInbox: function () {
+            return api.request("/v1/inboxes/me");
+        },
+
+        sendEmail: function (to, subject, body) {
+            return api.request("/v1/inboxes/me/send", {
+                method: "POST",
+                body: JSON.stringify({ to: to, subject: subject, body: body }),
+            });
+        },
+    };
+
+    // ─── Toast Notifications ─────────────────────────────────
+
+    function showToast(message, type) {
+        type = type || "success";
+        var container = document.getElementById("toast-container");
+        var toast = document.createElement("div");
+        toast.className = "toast " + type;
+        toast.textContent = message;
+        container.appendChild(toast);
+        setTimeout(function () {
+            toast.style.opacity = "0";
+            toast.style.transition = "opacity 200ms";
+            setTimeout(function () {
+                if (toast.parentNode) {
+                    toast.parentNode.removeChild(toast);
+                }
+            }, 200);
+        }, 4000);
+    }
+
+    // ─── Utility ─────────────────────────────────────────────
+
+    function formatDate(dateStr) {
+        var d = new Date(dateStr);
+        var now = new Date();
+        var diff = now - d;
+        if (diff < 60000) return "just now";
+        if (diff < 3600000) return Math.floor(diff / 60000) + "m ago";
+        if (diff < 86400000) return Math.floor(diff / 3600000) + "h ago";
+        if (diff < 604800000) return Math.floor(diff / 86400000) + "d ago";
+        return d.toLocaleDateString();
+    }
+
+    function escapeHtml(str) {
+        if (!str) return "";
+        var div = document.createElement("div");
+        div.appendChild(document.createTextNode(str));
+        return div.innerHTML;
+    }
+
+    function truncate(str, len) {
+        if (!str) return "";
+        return str.length > len ? str.substring(0, len) + "..." : str;
+    }
+
+    // ─── Views ───────────────────────────────────────────────
+
+    function renderLoading() {
+        return '<div class="loading"><div class="spinner"></div> Loading...</div>';
+    }
+
+    function renderError(message) {
+        return '<div class="error-banner">' + escapeHtml(message) + "</div>";
+    }
+
+    function renderNoApiKey() {
+        return (
+            '<div class="empty-state">' +
+            '<div class="icon">&#128273;</div>' +
+            "<h3>API Key Required</h3>" +
+            '<p>Set your API key using the gear icon in the navigation bar to access your inbox.</p>' +
+            "</div>"
+        );
+    }
+
+    function renderEmptyInbox() {
+        return (
+            '<div class="empty-state">' +
+            '<div class="icon">&#128235;</div>' +
+            "<h3>No messages yet</h3>" +
+            "<p>Your inbox is empty. Messages sent to your agent email address will appear here.</p>" +
+            "</div>"
+        );
+    }
+
+    // ─── Inbox View ──────────────────────────────────────────
+
+    function renderInboxView() {
+        updateActiveNav("/inbox");
+
+        if (!state.apiKey) {
+            appEl.innerHTML = renderNoApiKey();
+            return;
+        }
+
+        appEl.innerHTML =
+            '<div class="page-header">' +
+            "<h1>Inbox</h1>" +
+            '<button class="btn" id="refresh-btn">Refresh</button>' +
+            "</div>" +
+            renderLoading();
+
+        var refreshBtn = document.getElementById("refresh-btn");
+        if (refreshBtn) {
+            refreshBtn.addEventListener("click", function () {
+                loadMessages();
+            });
+        }
+
+        loadMessages();
+    }
+
+    function loadMessages() {
+        var skip = state.currentPage * state.pageSize;
+
+        api.getMessages(skip, state.pageSize)
+            .then(function (data) {
+                state.messages = data.messages;
+                state.total = data.total;
+                renderMessageList();
+            })
+            .catch(function (err) {
+                // Keep the page header, replace only the content area
+                var existing = appEl.querySelector(".message-list, .loading, .error-banner, .empty-state");
+                var html = renderError(err.message);
+                if (existing) {
+                    existing.outerHTML = html;
+                } else {
+                    appEl.innerHTML =
+                        '<div class="page-header">' +
+                        "<h1>Inbox</h1>" +
+                        '<button class="btn" id="refresh-btn">Refresh</button>' +
+                        "</div>" +
+                        html;
+                }
+            });
+    }
+
+    function renderMessageList() {
+        var header =
+            '<div class="page-header">' +
+            "<h1>Inbox<span class=\"badge\">" + state.total + "</span></h1>" +
+            '<button class="btn" id="refresh-btn">Refresh</button>' +
+            "</div>";
+
+        if (state.messages.length === 0) {
+            appEl.innerHTML = header + renderEmptyInbox();
+            bindRefresh();
+            return;
+        }
+
+        var items = state.messages
+            .map(function (msg) {
+                var unreadClass = msg.is_read ? "" : " unread";
+                var dotClass = msg.is_read ? "unread-dot read" : "unread-dot";
+                return (
+                    '<div class="message-item' + unreadClass + '" data-id="' + msg.id + '">' +
+                    '<div class="' + dotClass + '"></div>' +
+                    '<div class="message-content">' +
+                    '<div class="message-sender">' + escapeHtml(msg.sender) + "</div>" +
+                    '<div class="message-subject">' + escapeHtml(msg.subject || "(no subject)") + "</div>" +
+                    '<div class="message-preview">' + escapeHtml(truncate(msg.body_text, 100)) + "</div>" +
+                    "</div>" +
+                    '<div class="message-time">' + formatDate(msg.received_at) + "</div>" +
+                    "</div>"
+                );
+            })
+            .join("");
+
+        var pagination = "";
+        if (state.total > state.pageSize) {
+            var totalPages = Math.ceil(state.total / state.pageSize);
+            pagination =
+                '<div style="display:flex;justify-content:center;gap:0.5rem;margin-top:1rem;">' +
+                '<button class="btn btn-secondary" id="prev-page"' +
+                (state.currentPage === 0 ? " disabled" : "") +
+                ">Previous</button>" +
+                '<span style="padding:0.5rem;color:var(--text-secondary);font-size:0.875rem;">' +
+                "Page " + (state.currentPage + 1) + " of " + totalPages +
+                "</span>" +
+                '<button class="btn btn-secondary" id="next-page"' +
+                (state.currentPage >= totalPages - 1 ? " disabled" : "") +
+                ">Next</button>" +
+                "</div>";
+        }
+
+        appEl.innerHTML =
+            header +
+            '<div class="message-list">' + items + "</div>" +
+            pagination;
+
+        bindRefresh();
+        bindPagination();
+        bindMessageClicks();
+    }
+
+    function bindRefresh() {
+        var refreshBtn = document.getElementById("refresh-btn");
+        if (refreshBtn) {
+            refreshBtn.addEventListener("click", function () {
+                loadMessages();
+            });
+        }
+    }
+
+    function bindPagination() {
+        var prevBtn = document.getElementById("prev-page");
+        var nextBtn = document.getElementById("next-page");
+        if (prevBtn) {
+            prevBtn.addEventListener("click", function () {
+                if (state.currentPage > 0) {
+                    state.currentPage--;
+                    loadMessages();
+                }
+            });
+        }
+        if (nextBtn) {
+            nextBtn.addEventListener("click", function () {
+                state.currentPage++;
+                loadMessages();
+            });
+        }
+    }
+
+    function bindMessageClicks() {
+        var items = appEl.querySelectorAll(".message-item");
+        items.forEach(function (item) {
+            item.addEventListener("click", function () {
+                var id = item.getAttribute("data-id");
+                navigate("/inbox/" + id);
+            });
+        });
+    }
+
+    // ─── Message Detail View ─────────────────────────────────
+
+    function renderMessageDetailView(messageId) {
+        updateActiveNav("/inbox");
+
+        // Find message in current state
+        var msg = state.messages.find(function (m) {
+            return m.id === messageId;
+        });
+
+        if (!msg) {
+            // If message not in state, try to reload messages first
+            if (!state.apiKey) {
+                appEl.innerHTML = renderNoApiKey();
+                return;
+            }
+            appEl.innerHTML = renderLoading();
+            api.getMessages(0, 200)
+                .then(function (data) {
+                    state.messages = data.messages;
+                    state.total = data.total;
+                    msg = state.messages.find(function (m) {
+                        return m.id === messageId;
+                    });
+                    if (msg) {
+                        showMessageDetail(msg);
+                    } else {
+                        appEl.innerHTML =
+                            renderError("Message not found.") +
+                            '<a href="/inbox" data-link class="back-link">&larr; Back to Inbox</a>';
+                        bindLinks();
+                    }
+                })
+                .catch(function (err) {
+                    appEl.innerHTML = renderError(err.message);
+                });
+            return;
+        }
+
+        showMessageDetail(msg);
+    }
+
+    function showMessageDetail(msg) {
+        appEl.innerHTML =
+            '<a href="/inbox" data-link class="back-link">&larr; Back to Inbox</a>' +
+            '<div class="message-detail">' +
+            '<div class="message-detail-header">' +
+            "<h2>" + escapeHtml(msg.subject || "(no subject)") + "</h2>" +
+            '<div class="message-meta">' +
+            "<span><span class=\"label\">From</span> " + escapeHtml(msg.sender) + "</span>" +
+            "<span><span class=\"label\">To</span> " + escapeHtml(msg.recipient) + "</span>" +
+            "<span><span class=\"label\">Date</span> " + new Date(msg.received_at).toLocaleString() + "</span>" +
+            "</div>" +
+            "</div>" +
+            '<div class="message-detail-body">' +
+            escapeHtml(msg.body_text || "(no content)") +
+            "</div>" +
+            "</div>";
+
+        bindLinks();
+    }
+
+    // ─── Compose View ────────────────────────────────────────
+
+    function renderComposeView() {
+        updateActiveNav("/compose");
+
+        if (!state.apiKey) {
+            appEl.innerHTML = renderNoApiKey();
+            return;
+        }
+
+        appEl.innerHTML =
+            '<div class="page-header"><h1>Compose</h1></div>' +
+            '<form id="compose-form" class="compose-form">' +
+            '<div class="form-group">' +
+            '<label for="compose-to">To</label>' +
+            '<input type="email" id="compose-to" placeholder="recipient@example.com" required>' +
+            "</div>" +
+            '<div class="form-group">' +
+            '<label for="compose-subject">Subject</label>' +
+            '<input type="text" id="compose-subject" placeholder="Email subject">' +
+            "</div>" +
+            '<div class="form-group">' +
+            '<label for="compose-body">Body</label>' +
+            '<textarea id="compose-body" placeholder="Write your message..." required></textarea>' +
+            "</div>" +
+            '<div class="form-actions">' +
+            '<button type="button" class="btn btn-secondary" id="discard-btn">Discard</button>' +
+            '<button type="submit" class="btn btn-primary" id="send-btn">Send</button>' +
+            "</div>" +
+            "</form>";
+
+        var form = document.getElementById("compose-form");
+        var sendBtn = document.getElementById("send-btn");
+        var discardBtn = document.getElementById("discard-btn");
+
+        form.addEventListener("submit", function (e) {
+            e.preventDefault();
+            var to = document.getElementById("compose-to").value.trim();
+            var subject = document.getElementById("compose-subject").value.trim();
+            var body = document.getElementById("compose-body").value.trim();
+
+            if (!to || !body) {
+                showToast("Please fill in the recipient and message body.", "error");
+                return;
+            }
+
+            sendBtn.disabled = true;
+            sendBtn.textContent = "Sending...";
+
+            api.sendEmail(to, subject, body)
+                .then(function (res) {
+                    showToast("Email sent to " + res.to);
+                    navigate("/inbox");
+                })
+                .catch(function (err) {
+                    showToast(err.message, "error");
+                    sendBtn.disabled = false;
+                    sendBtn.textContent = "Send";
+                });
+        });
+
+        discardBtn.addEventListener("click", function () {
+            navigate("/inbox");
+        });
+    }
+
+    // ─── Router ──────────────────────────────────────────────
+
+    function navigate(path) {
+        window.history.pushState({}, "", path);
+        route();
+    }
+
+    function route() {
+        var path = window.location.pathname;
+
+        if (path === "/compose") {
+            renderComposeView();
+        } else if (path.match(/^\/inbox\/[a-f0-9-]+$/)) {
+            var id = path.split("/inbox/")[1];
+            renderMessageDetailView(id);
+        } else {
+            // Default to inbox (covers /inbox, /, etc.)
+            renderInboxView();
+        }
+    }
+
+    function updateActiveNav(activePath) {
+        var links = document.querySelectorAll(".nav-link");
+        links.forEach(function (link) {
+            if (link.getAttribute("href") === activePath) {
+                link.classList.add("active");
+            } else {
+                link.classList.remove("active");
+            }
+        });
+    }
+
+    function bindLinks() {
+        document.querySelectorAll("[data-link]").forEach(function (link) {
+            // Remove existing listener by cloning
+            var newLink = link.cloneNode(true);
+            link.parentNode.replaceChild(newLink, link);
+            newLink.addEventListener("click", function (e) {
+                e.preventDefault();
+                navigate(newLink.getAttribute("href"));
+            });
+        });
+    }
+
+    // ─── Settings Modal ──────────────────────────────────────
+
+    function openModal() {
+        apiKeyInput.value = state.apiKey;
+        apiKeyInput.type = "password";
+        showKeyCheckbox.checked = false;
+        modalOverlay.classList.remove("hidden");
+        apiKeyInput.focus();
+    }
+
+    function closeModal() {
+        modalOverlay.classList.add("hidden");
+    }
+
+    function saveApiKey() {
+        var key = apiKeyInput.value.trim();
+        state.apiKey = key;
+        if (key) {
+            localStorage.setItem("agent_suite_api_key", key);
+            showToast("API key saved");
+        } else {
+            localStorage.removeItem("agent_suite_api_key");
+            showToast("API key cleared");
+        }
+        closeModal();
+        route(); // Re-render current view
+    }
+
+    // ─── Event Bindings ──────────────────────────────────────
+
+    // Navigation links (client-side routing)
+    document.addEventListener("click", function (e) {
+        var link = e.target.closest("[data-link]");
+        if (link) {
+            e.preventDefault();
+            navigate(link.getAttribute("href"));
+        }
+    });
+
+    // Browser back/forward
+    window.addEventListener("popstate", function () {
+        route();
+    });
+
+    // Settings modal
+    settingsBtn.addEventListener("click", openModal);
+    saveKeyBtn.addEventListener("click", saveApiKey);
+    apiKeyInput.addEventListener("keydown", function (e) {
+        if (e.key === "Enter") saveApiKey();
+    });
+
+    // Close modal on overlay click or close button
+    modalOverlay.addEventListener("click", function (e) {
+        if (e.target === modalOverlay || e.target.classList.contains("modal-close")) {
+            closeModal();
+        }
+    });
+
+    // Close modal on Escape
+    document.addEventListener("keydown", function (e) {
+        if (e.key === "Escape" && !modalOverlay.classList.contains("hidden")) {
+            closeModal();
+        }
+    });
+
+    // Show/hide API key
+    showKeyCheckbox.addEventListener("change", function () {
+        apiKeyInput.type = showKeyCheckbox.checked ? "text" : "password";
+    });
+
+    // ─── Initialize ──────────────────────────────────────────
+
+    route();
+})();
diff --git a/app/static/index.html b/app/static/index.html
new file mode 100644
index 0000000..87c2db3
--- /dev/null
+++ b/app/static/index.html
@@ -0,0 +1,50 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Agent Suite - Inbox</title>
+    <link rel="stylesheet" href="/static/style.css">
+</head>
+<body>
+    <header>
+        <nav>
+            <a href="/inbox" class="brand" data-link>&#128231; Agent Suite</a>
+            <div class="nav-links">
+                <a href="/inbox" data-link class="nav-link">Inbox</a>
+                <a href="/compose" data-link class="nav-link">Compose</a>
+                <button id="settings-btn" class="btn btn-icon" title="API Key Settings">&#9881;</button>
+            </div>
+        </nav>
+    </header>
+
+    <main id="app"></main>
+
+    <!-- Settings Modal -->
+    <div id="modal-overlay" class="modal-overlay hidden">
+        <div class="modal">
+            <div class="modal-header">
+                <h2>API Key Settings</h2>
+                <button class="btn btn-icon modal-close">&times;</button>
+            </div>
+            <div class="modal-body">
+                <p class="hint">Enter your API key to access your inbox. Create one via <code>POST /v1/inboxes</code>.</p>
+                <label for="api-key-input">API Key</label>
+                <input type="password" id="api-key-input" placeholder="as_..." autocomplete="off">
+                <label class="checkbox-label">
+                    <input type="checkbox" id="show-key"> Show key
+                </label>
+            </div>
+            <div class="modal-footer">
+                <button id="save-key-btn" class="btn btn-primary">Save</button>
+                <button class="btn btn-secondary modal-close">Cancel</button>
+            </div>
+        </div>
+    </div>
+
+    <!-- Toast container for notifications -->
+    <div id="toast-container"></div>
+
+    <script src="/static/app.js"></script>
+</body>
+</html>
diff --git a/app/static/style.css b/app/static/style.css
new file mode 100644
index 0000000..aa9317a
--- /dev/null
+++ b/app/static/style.css
@@ -0,0 +1,684 @@
+/* === Agent Suite Dark Theme === */
+
+:root {
+    --bg-primary: #0d1117;
+    --bg-secondary: #161b22;
+    --bg-tertiary: #21262d;
+    --border-color: #30363d;
+    --border-hover: #484f58;
+    --text-primary: #c9d1d9;
+    --text-secondary: #8b949e;
+    --text-muted: #6e7681;
+    --accent: #58a6ff;
+    --accent-hover: #79c0ff;
+    --success: #3fb950;
+    --error: #f85149;
+    --warning: #d29922;
+    --radius: 6px;
+    --radius-lg: 10px;
+    --shadow: 0 1px 3px rgba(0, 0, 0, 0.3);
+    --shadow-lg: 0 8px 24px rgba(0, 0, 0, 0.4);
+    --transition: 150ms ease;
+}
+
+/* === Reset & Base === */
+
+*,
+*::before,
+*::after {
+    box-sizing: border-box;
+    margin: 0;
+    padding: 0;
+}
+
+html {
+    font-size: 16px;
+    -webkit-text-size-adjust: 100%;
+}
+
+body {
+    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial,
+        sans-serif, "Apple Color Emoji", "Segoe UI Emoji";
+    background: var(--bg-primary);
+    color: var(--text-primary);
+    line-height: 1.5;
+    min-height: 100vh;
+}
+
+a {
+    color: var(--accent);
+    text-decoration: none;
+    transition: color var(--transition);
+}
+
+a:hover {
+    color: var(--accent-hover);
+}
+
+code {
+    background: var(--bg-tertiary);
+    padding: 2px 6px;
+    border-radius: 3px;
+    font-size: 0.875em;
+}
+
+/* === Navigation === */
+
+header {
+    background: var(--bg-secondary);
+    border-bottom: 1px solid var(--border-color);
+    position: sticky;
+    top: 0;
+    z-index: 100;
+}
+
+nav {
+    max-width: 960px;
+    margin: 0 auto;
+    padding: 0 1rem;
+    height: 56px;
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+}
+
+.brand {
+    font-size: 1.125rem;
+    font-weight: 600;
+    color: var(--text-primary);
+    white-space: nowrap;
+}
+
+.brand:hover {
+    color: var(--accent);
+}
+
+.nav-links {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+}
+
+.nav-link {
+    padding: 0.375rem 0.75rem;
+    border-radius: var(--radius);
+    color: var(--text-secondary);
+    font-size: 0.875rem;
+    font-weight: 500;
+    transition: background var(--transition), color var(--transition);
+}
+
+.nav-link:hover,
+.nav-link.active {
+    background: var(--bg-tertiary);
+    color: var(--text-primary);
+}
+
+/* === Main Content === */
+
+main {
+    max-width: 960px;
+    margin: 0 auto;
+    padding: 1.5rem 1rem;
+}
+
+/* === Buttons === */
+
+.btn {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    gap: 0.375rem;
+    padding: 0.5rem 1rem;
+    border: 1px solid var(--border-color);
+    border-radius: var(--radius);
+    background: var(--bg-tertiary);
+    color: var(--text-primary);
+    font-size: 0.875rem;
+    font-weight: 500;
+    cursor: pointer;
+    transition: background var(--transition), border-color var(--transition);
+    white-space: nowrap;
+}
+
+.btn:hover {
+    background: var(--bg-secondary);
+    border-color: var(--border-hover);
+}
+
+.btn:disabled {
+    opacity: 0.5;
+    cursor: not-allowed;
+}
+
+.btn-primary {
+    background: var(--accent);
+    border-color: var(--accent);
+    color: #fff;
+}
+
+.btn-primary:hover {
+    background: var(--accent-hover);
+    border-color: var(--accent-hover);
+}
+
+.btn-secondary {
+    background: transparent;
+    border-color: var(--border-color);
+}
+
+.btn-icon {
+    padding: 0.375rem;
+    border: none;
+    background: transparent;
+    font-size: 1.25rem;
+    cursor: pointer;
+    color: var(--text-secondary);
+    border-radius: var(--radius);
+    transition: background var(--transition), color var(--transition);
+    line-height: 1;
+}
+
+.btn-icon:hover {
+    background: var(--bg-tertiary);
+    color: var(--text-primary);
+}
+
+/* === Page Header === */
+
+.page-header {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    margin-bottom: 1rem;
+    flex-wrap: wrap;
+    gap: 0.5rem;
+}
+
+.page-header h1 {
+    font-size: 1.5rem;
+    font-weight: 600;
+}
+
+.badge {
+    background: var(--bg-tertiary);
+    color: var(--text-secondary);
+    padding: 0.125rem 0.5rem;
+    border-radius: 10px;
+    font-size: 0.75rem;
+    font-weight: 500;
+    margin-left: 0.5rem;
+}
+
+/* === Message List === */
+
+.message-list {
+    border: 1px solid var(--border-color);
+    border-radius: var(--radius-lg);
+    overflow: hidden;
+}
+
+.message-item {
+    display: flex;
+    align-items: flex-start;
+    gap: 0.75rem;
+    padding: 0.875rem 1rem;
+    border-bottom: 1px solid var(--border-color);
+    cursor: pointer;
+    transition: background var(--transition);
+}
+
+.message-item:last-child {
+    border-bottom: none;
+}
+
+.message-item:hover {
+    background: var(--bg-secondary);
+}
+
+.message-item.unread {
+    background: var(--bg-secondary);
+}
+
+.message-item.unread .message-subject {
+    font-weight: 600;
+    color: var(--text-primary);
+}
+
+.unread-dot {
+    width: 8px;
+    height: 8px;
+    border-radius: 50%;
+    background: var(--accent);
+    flex-shrink: 0;
+    margin-top: 0.5rem;
+}
+
+.unread-dot.read {
+    background: transparent;
+}
+
+.message-content {
+    flex: 1;
+    min-width: 0;
+}
+
+.message-sender {
+    font-size: 0.875rem;
+    color: var(--text-secondary);
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+}
+
+.message-subject {
+    font-size: 0.9375rem;
+    color: var(--text-secondary);
+    font-weight: 400;
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+}
+
+.message-preview {
+    font-size: 0.8125rem;
+    color: var(--text-muted);
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    margin-top: 0.125rem;
+}
+
+.message-time {
+    font-size: 0.75rem;
+    color: var(--text-muted);
+    white-space: nowrap;
+    flex-shrink: 0;
+}
+
+/* === Message Detail === */
+
+.message-detail {
+    border: 1px solid var(--border-color);
+    border-radius: var(--radius-lg);
+    overflow: hidden;
+}
+
+.message-detail-header {
+    padding: 1.25rem;
+    border-bottom: 1px solid var(--border-color);
+    background: var(--bg-secondary);
+}
+
+.message-detail-header h2 {
+    font-size: 1.25rem;
+    font-weight: 600;
+    margin-bottom: 0.5rem;
+    word-break: break-word;
+}
+
+.message-meta {
+    display: flex;
+    flex-direction: column;
+    gap: 0.25rem;
+    font-size: 0.8125rem;
+    color: var(--text-secondary);
+}
+
+.message-meta span {
+    display: flex;
+    gap: 0.5rem;
+}
+
+.message-meta .label {
+    color: var(--text-muted);
+    min-width: 3rem;
+}
+
+.message-detail-body {
+    padding: 1.25rem;
+    font-size: 0.9375rem;
+    line-height: 1.7;
+    white-space: pre-wrap;
+    word-break: break-word;
+}
+
+.back-link {
+    display: inline-flex;
+    align-items: center;
+    gap: 0.375rem;
+    margin-bottom: 1rem;
+    font-size: 0.875rem;
+    color: var(--text-secondary);
+}
+
+.back-link:hover {
+    color: var(--accent);
+}
+
+/* === Compose Form === */
+
+.compose-form {
+    border: 1px solid var(--border-color);
+    border-radius: var(--radius-lg);
+    overflow: hidden;
+}
+
+.form-group {
+    padding: 0 1.25rem;
+    border-bottom: 1px solid var(--border-color);
+}
+
+.form-group:last-of-type {
+    border-bottom: none;
+}
+
+.form-group label {
+    display: block;
+    font-size: 0.75rem;
+    font-weight: 500;
+    color: var(--text-muted);
+    text-transform: uppercase;
+    letter-spacing: 0.05em;
+    padding-top: 0.75rem;
+}
+
+.form-group input,
+.form-group textarea {
+    width: 100%;
+    background: transparent;
+    border: none;
+    color: var(--text-primary);
+    font-size: 0.9375rem;
+    font-family: inherit;
+    padding: 0.5rem 0 0.75rem;
+    outline: none;
+    resize: vertical;
+}
+
+.form-group textarea {
+    min-height: 200px;
+    line-height: 1.6;
+}
+
+.form-group input::placeholder,
+.form-group textarea::placeholder {
+    color: var(--text-muted);
+}
+
+.form-actions {
+    padding: 1rem 1.25rem;
+    border-top: 1px solid var(--border-color);
+    background: var(--bg-secondary);
+    display: flex;
+    justify-content: flex-end;
+    gap: 0.5rem;
+}
+
+/* === States === */
+
+.empty-state {
+    text-align: center;
+    padding: 3rem 1rem;
+    color: var(--text-secondary);
+}
+
+.empty-state .icon {
+    font-size: 2.5rem;
+    margin-bottom: 0.75rem;
+}
+
+.empty-state h3 {
+    font-size: 1.125rem;
+    font-weight: 600;
+    margin-bottom: 0.375rem;
+    color: var(--text-primary);
+}
+
+.empty-state p {
+    font-size: 0.875rem;
+    max-width: 320px;
+    margin: 0 auto;
+}
+
+.loading {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    padding: 3rem;
+    color: var(--text-secondary);
+}
+
+.spinner {
+    width: 20px;
+    height: 20px;
+    border: 2px solid var(--border-color);
+    border-top-color: var(--accent);
+    border-radius: 50%;
+    animation: spin 0.6s linear infinite;
+    margin-right: 0.75rem;
+}
+
+@keyframes spin {
+    to { transform: rotate(360deg); }
+}
+
+.error-banner {
+    background: rgba(248, 81, 73, 0.1);
+    border: 1px solid rgba(248, 81, 73, 0.3);
+    color: var(--error);
+    padding: 0.75rem 1rem;
+    border-radius: var(--radius);
+    font-size: 0.875rem;
+    margin-bottom: 1rem;
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+}
+
+.success-banner {
+    background: rgba(63, 185, 80, 0.1);
+    border: 1px solid rgba(63, 185, 80, 0.3);
+    color: var(--success);
+    padding: 0.75rem 1rem;
+    border-radius: var(--radius);
+    font-size: 0.875rem;
+    margin-bottom: 1rem;
+}
+
+/* === Modal === */
+
+.modal-overlay {
+    position: fixed;
+    inset: 0;
+    background: rgba(0, 0, 0, 0.6);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    z-index: 200;
+    padding: 1rem;
+}
+
+.modal-overlay.hidden {
+    display: none;
+}
+
+.modal {
+    background: var(--bg-secondary);
+    border: 1px solid var(--border-color);
+    border-radius: var(--radius-lg);
+    width: 100%;
+    max-width: 440px;
+    box-shadow: var(--shadow-lg);
+}
+
+.modal-header {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    padding: 1rem 1.25rem;
+    border-bottom: 1px solid var(--border-color);
+}
+
+.modal-header h2 {
+    font-size: 1rem;
+    font-weight: 600;
+}
+
+.modal-body {
+    padding: 1.25rem;
+}
+
+.modal-body label {
+    display: block;
+    font-size: 0.75rem;
+    font-weight: 500;
+    color: var(--text-muted);
+    text-transform: uppercase;
+    letter-spacing: 0.05em;
+    margin-bottom: 0.375rem;
+    margin-top: 0.75rem;
+}
+
+.modal-body input[type="password"],
+.modal-body input[type="text"] {
+    width: 100%;
+    padding: 0.5rem 0.75rem;
+    background: var(--bg-primary);
+    border: 1px solid var(--border-color);
+    border-radius: var(--radius);
+    color: var(--text-primary);
+    font-size: 0.875rem;
+    font-family: monospace;
+    outline: none;
+    transition: border-color var(--transition);
+}
+
+.modal-body input:focus {
+    border-color: var(--accent);
+}
+
+.modal-body .hint {
+    font-size: 0.8125rem;
+    color: var(--text-secondary);
+    margin-bottom: 0.25rem;
+}
+
+.checkbox-label {
+    display: flex !important;
+    align-items: center;
+    gap: 0.375rem;
+    font-size: 0.8125rem !important;
+    color: var(--text-secondary) !important;
+    text-transform: none !important;
+    letter-spacing: normal !important;
+    cursor: pointer;
+    margin-top: 0.5rem !important;
+}
+
+.checkbox-label input[type="checkbox"] {
+    width: auto;
+}
+
+.modal-footer {
+    display: flex;
+    justify-content: flex-end;
+    gap: 0.5rem;
+    padding: 1rem 1.25rem;
+    border-top: 1px solid var(--border-color);
+}
+
+/* === Toast Notifications === */
+
+#toast-container {
+    position: fixed;
+    bottom: 1.5rem;
+    right: 1.5rem;
+    z-index: 300;
+    display: flex;
+    flex-direction: column;
+    gap: 0.5rem;
+}
+
+.toast {
+    background: var(--bg-secondary);
+    border: 1px solid var(--border-color);
+    border-radius: var(--radius);
+    padding: 0.75rem 1rem;
+    font-size: 0.875rem;
+    box-shadow: var(--shadow-lg);
+    animation: slideIn 200ms ease;
+    max-width: 360px;
+}
+
+.toast.success {
+    border-color: rgba(63, 185, 80, 0.4);
+}
+
+.toast.error {
+    border-color: rgba(248, 81, 73, 0.4);
+}
+
+@keyframes slideIn {
+    from {
+        opacity: 0;
+        transform: translateY(10px);
+    }
+    to {
+        opacity: 1;
+        transform: translateY(0);
+    }
+}
+
+/* === Responsive === */
+
+@media (max-width: 640px) {
+    nav {
+        padding: 0 0.75rem;
+        height: 48px;
+    }
+
+    .brand {
+        font-size: 1rem;
+    }
+
+    .nav-link {
+        padding: 0.25rem 0.5rem;
+        font-size: 0.8125rem;
+    }
+
+    main {
+        padding: 1rem 0.75rem;
+    }
+
+    .page-header h1 {
+        font-size: 1.25rem;
+    }
+
+    .message-item {
+        padding: 0.75rem;
+    }
+
+    .message-detail-header,
+    .message-detail-body {
+        padding: 1rem;
+    }
+
+    .form-group {
+        padding: 0 1rem;
+    }
+
+    .form-actions {
+        padding: 0.75rem 1rem;
+    }
+
+    .modal {
+        max-width: 100%;
+    }
+}
+
+@media (max-width: 400px) {
+    .message-time {
+        display: none;
+    }
+}
diff --git a/tests/test_api.py b/tests/test_api.py
index 08f4976..4135218 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -75,7 +75,7 @@ def test_list_messages(setup_db):
     create_resp = client.post("/v1/inboxes")
     api_key = create_resp.json()["api_key"]
     email = create_resp.json()["email_address"]
-    
+
     # Simulate incoming message via webhook
     client.post(
         "/v1/webhooks/mailgun",
@@ -87,7 +87,7 @@ def test_list_messages(setup_db):
             "message_id": "test123"
         }
     )
-    
+
     # List messages
     response = client.get(
         "/v1/inboxes/me/messages",
@@ -97,3 +97,60 @@ def test_list_messages(setup_db):
     data = response.json()
     assert data["total"] == 1
     assert data["messages"][0]["subject"] == "Test Subject"
+
+
+# ─── Web UI Tests ─────────────────────────────────────────
+
+
+def test_inbox_page():
+    """GET /inbox returns the SPA HTML page."""
+    response = client.get("/inbox")
+    assert response.status_code == 200
+    assert "text/html" in response.headers["content-type"]
+    assert "Agent Suite" in response.text
+
+
+def test_compose_page():
+    """GET /compose returns the SPA HTML page."""
+    response = client.get("/compose")
+    assert response.status_code == 200
+    assert "text/html" in response.headers["content-type"]
+    assert "Agent Suite" in response.text
+
+
+def test_message_detail_page():
+    """GET /inbox/{id} returns the SPA HTML page."""
+    response = client.get("/inbox/00000000-0000-0000-0000-000000000000")
+    assert response.status_code == 200
+    assert "text/html" in response.headers["content-type"]
+
+
+def test_static_css():
+    """Static CSS file is served correctly."""
+    response = client.get("/static/style.css")
+    assert response.status_code == 200
+    assert "text/css" in response.headers["content-type"]
+
+
+def test_static_js():
+    """Static JS file is served correctly."""
+    response = client.get("/static/app.js")
+    assert response.status_code == 200
+    assert "javascript" in response.headers["content-type"]
+
+
+def test_inbox_page_contains_key_elements():
+    """Verify the HTML page includes essential UI elements."""
+    response = client.get("/inbox")
+    html = response.text
+    # Navigation links
+    assert 'href="/inbox"' in html
+    assert 'href="/compose"' in html
+    # Settings modal
+    assert 'id="modal-overlay"' in html
+    assert 'id="api-key-input"' in html
+    # App container
+    assert 'id="app"' in html
+    # Static asset references
+    assert "/static/style.css" in html
+    assert "/static/app.js" in html