diff --git a/apps/web/src/app/opencode/page.tsx b/apps/web/src/app/opencode/page.tsx
index 1291295..be5a981 100644
--- a/apps/web/src/app/opencode/page.tsx
+++ b/apps/web/src/app/opencode/page.tsx
@@ -9,6 +9,7 @@ import { opencodePluginVersion } from "@/lib/opencode-version";
 const npmUrl = "https://www.npmjs.com/package/@sketchi-app/opencode-excalidraw";
 const githubUrl =
   "https://github.com/anand-testcompare/sketchi/tree/main/packages/opencode-excalidraw";
+const authCommand = "opencode auth login";
 const webCommand = "opencode web";
 const cliCommand = "opencode";
 
@@ -284,11 +285,15 @@ export default function OpenCodeDocsPage() {
                 .
               </li>
               <li>
-                Run <code>opencode web</code>.
+                Run <code>{authCommand}</code> and select <code>sketchi</code>{" "}
+                (device-flow sign-in).
               </li>
               <li>
-                Then switch to <code>opencode</code> and run again in terminal
-                mode.
+                Run <code>{webCommand}</code>.
+              </li>
+              <li>
+                Then switch to <code>{cliCommand}</code> and run again in
+                terminal mode.
               </li>
             </ol>
           </div>
diff --git a/packages/opencode-excalidraw/README.md b/packages/opencode-excalidraw/README.md
index ddc8e84..f8d3203 100644
--- a/packages/opencode-excalidraw/README.md
+++ b/packages/opencode-excalidraw/README.md
@@ -44,7 +44,7 @@ When this plugin is loaded, it registers a `sketchi-diagram` subagent (without d
 
 Optional env override:
 
-- `SKETCHI_API_URL` (defaults to `https://sketchi.app`)
+- `SKETCHI_API_URL` (defaults to `https://www.sketchi.app`)
 - `SKETCHI_ALLOW_UNSAFE_OUTPUT_PATH=1` to opt out of default output-path containment (power users only)
 
 Authentication is required for diagram APIs. In OpenCode, select auth provider `sketchi`:
diff --git a/packages/opencode-excalidraw/src/index.test.ts b/packages/opencode-excalidraw/src/index.test.ts
index b004afb..7863cd0 100644
--- a/packages/opencode-excalidraw/src/index.test.ts
+++ b/packages/opencode-excalidraw/src/index.test.ts
@@ -169,6 +169,58 @@ describe("SketchiPlugin", () => {
     expect(method?.label.toLowerCase()).toContain("device flow");
   });
 
+  test("canonicalizes sketchi.app API base to www for auth device start", async () => {
+    const originalFetch = globalThis.fetch;
+    const originalApiBase = process.env.SKETCHI_API_URL;
+    const requestedUrls: string[] = [];
+
+    globalThis.fetch = ((input) => {
+      let url: string;
+      if (typeof input === "string") {
+        url = input;
+      } else if (input instanceof URL) {
+        url = input.toString();
+      } else {
+        url = input.url;
+      }
+      requestedUrls.push(url);
+      return new Response(
+        JSON.stringify({
+          deviceCode: "device-code",
+          userCode: "ABCD-EFGH",
+          interval: 5,
+          expiresIn: 600,
+          verificationUrl: "https://www.sketchi.app/device",
+        }),
+        {
+          status: 200,
+          headers: { "content-type": "application/json" },
+        }
+      );
+    }) as typeof fetch;
+
+    process.env.SKETCHI_API_URL = "https://sketchi.app/";
+
+    try {
+      const plugin = await SketchiPlugin(createPluginInput());
+      const method = plugin.auth?.methods?.[0];
+      expect(method?.type).toBe("oauth");
+
+      const authStart = await method?.authorize();
+      expect(authStart?.url).toBe("https://www.sketchi.app/device");
+      expect(requestedUrls[0]).toBe(
+        "https://www.sketchi.app/api/auth/device/start"
+      );
+    } finally {
+      globalThis.fetch = originalFetch;
+      if (originalApiBase === undefined) {
+        process.env.SKETCHI_API_URL = undefined;
+      } else {
+        process.env.SKETCHI_API_URL = originalApiBase;
+      }
+    }
+  });
+
   test("diagram_grade blocks concurrent calls for the same message", async () => {
     const deferred = createDeferred<{
       data: { parts: Array<{ type: string; text: string }> };
diff --git a/packages/opencode-excalidraw/src/index.ts b/packages/opencode-excalidraw/src/index.ts
index c46460a..2cec2bf 100644
--- a/packages/opencode-excalidraw/src/index.ts
+++ b/packages/opencode-excalidraw/src/index.ts
@@ -13,7 +13,7 @@ import { closeBrowser, renderElementsToPng } from "./lib/render";
 import { resolveExcalidrawFromShareUrl } from "./lib/resolve-share-url";
 import { createToolTraceId } from "./lib/trace";
 
-const DEFAULT_API_BASE = "https://sketchi.app";
+const DEFAULT_API_BASE = "https://www.sketchi.app";
 const DEFAULT_OAUTH_TOKEN_TTL_MS = 60 * 60 * 1000;
 const DEVICE_FLOW_POLLING_SAFETY_MARGIN_MS = 1000;
 const TRAILING_SLASH_PATTERN = /\/$/;
@@ -149,7 +149,18 @@ function completeGradeCall(key: string): void {
 }
 
 function normalizeApiBase(value: string): string {
-  return value.replace(TRAILING_SLASH_PATTERN, "");
+  const trimmed = value.trim();
+  const withoutTrailingSlash = trimmed.replace(TRAILING_SLASH_PATTERN, "");
+
+  try {
+    const parsed = new URL(withoutTrailingSlash);
+    if (parsed.hostname === "sketchi.app") {
+      parsed.hostname = "www.sketchi.app";
+    }
+    return parsed.toString().replace(TRAILING_SLASH_PATTERN, "");
+  } catch {
+    return withoutTrailingSlash;
+  }
 }
 
 function extractMessageText(