diff --git a/src-tauri/src/sidecar_discovery.rs b/src-tauri/src/sidecar_discovery.rs
index 6dca01b44..0d9503933 100644
--- a/src-tauri/src/sidecar_discovery.rs
+++ b/src-tauri/src/sidecar_discovery.rs
@@ -154,7 +154,16 @@ pub(super) fn find_coven() -> Option<PathBuf> {
     #[cfg(target_os = "windows")]
     {
         let home = std::env::var("USERPROFILE").unwrap_or_default();
+        // npm global installs land in %APPDATA%\\npm as a .cmd shim (not .exe),
+        // which is the most common way users install the Coven CLI. A GUI-launched
+        // Tauri app frequently does not inherit that dir on PATH, so we must probe
+        // it explicitly or `where.exe` below will miss it. Fall back to
+        // %USERPROFILE%\\AppData\\Roaming\\npm when APPDATA is unset.
+        let appdata = std::env::var("APPDATA")
+            .unwrap_or_else(|_| format!("{}\\AppData\\Roaming", home));
         let candidates = [
+            PathBuf::from(format!("{}\\npm\\coven.cmd", appdata)),
+            PathBuf::from(format!("{}\\npm\\coven.exe", appdata)),
             PathBuf::from(format!("{}\\.volta\\bin\\coven.exe", home)),
             PathBuf::from(format!("{}\\.bun\\bin\\coven.exe", home)),
             PathBuf::from(format!("{}\\.cargo\\bin\\coven.exe", home)),
diff --git a/src/lib/server/assist-runner.test.ts b/src/lib/server/assist-runner.test.ts
index 778128da4..6a6f944e0 100644
--- a/src/lib/server/assist-runner.test.ts
+++ b/src/lib/server/assist-runner.test.ts
@@ -1,6 +1,11 @@
 // @ts-nocheck
 import assert from "node:assert/strict";
-import { buildAssistInvocation, ASSIST_TIMEOUT_MS } from "./assist-runner.ts";
+import {
+  buildAssistInvocation,
+  describeEmptyAssistOutput,
+  stderrReason,
+  ASSIST_TIMEOUT_MS,
+} from "./assist-runner.ts";
 
 const prevBin = process.env.COVEN_CODEX_BIN;
 
@@ -36,6 +41,50 @@ try {
   // The default budget matches the sew's historical bound.
   assert.equal(ASSIST_TIMEOUT_MS, 180_000);
 
+  // ── stderr reason collapsing ───────────────────────────────────────────────
+  assert.equal(stderrReason(""), "");
+  assert.equal(stderrReason("  \n\t\n"), "", "whitespace-only tail collapses to nothing");
+  assert.equal(
+    stderrReason("line1\r\nline2\nline3\n\nline4\n"),
+    "line2 · line3 · line4",
+    "keeps the last 3 non-empty lines across CRLF",
+  );
+  assert.equal(stderrReason("x".repeat(400)), "x".repeat(300), "caps at the final 300 chars");
+
+  // ── empty-output diagnostics ───────────────────────────────────────────────
+  // Auth-shaped stderr → explicit sign-in guidance with a runnable command.
+  let msg = describeEmptyAssistOutput(
+    "codex",
+    "ERROR: Not logged in. Run `codex login` to authenticate.",
+  );
+  assert.ok(msg.includes("isn't signed in"), msg);
+  assert.ok(msg.includes("`codex login`"), msg);
+  assert.ok(msg.includes("Not logged in"), "carries the stderr tail");
+
+  msg = describeEmptyAssistOutput("codex", "request failed: 401 Unauthorized");
+  assert.ok(msg.includes("`codex login`"), msg);
+
+  // A COVEN_CODEX_BIN path with spaces still yields a runnable login command,
+  // with backslashes escaped ahead of quotes (js/incomplete-sanitization).
+  msg = describeEmptyAssistOutput("C:\\Program Files\\codex.exe", "stream error: unauthorized");
+  assert.ok(msg.includes('`"C:\\\\Program Files\\\\codex.exe" login`'), msg);
+
+  // Non-auth stderr must NOT claim a sign-in problem — loose `token` / `auth`
+  // / `sign ?in` substring matching used to misfire on lines like these.
+  for (const stderr of [
+    "SyntaxError: Unexpected token < in JSON at position 0",
+    "error: not inside a trusted directory; pass --skip-git-repo-check",
+    "warning: redesign in progress for the author page catalog index",
+  ]) {
+    msg = describeEmptyAssistOutput("codex", stderr);
+    assert.ok(!msg.includes("signed in"), `false auth positive for: ${stderr}`);
+    assert.ok(msg.startsWith("assist produced no output"), msg);
+    assert.ok(msg.includes(stderrReason(stderr)), "still surfaces the stderr tail");
+  }
+
+  // No stderr at all → the original terse message, no dangling separator.
+  assert.equal(describeEmptyAssistOutput("codex", ""), "assist produced no output");
+
   console.log("assist-runner.test.ts OK");
 } finally {
   if (prevBin === undefined) delete process.env.COVEN_CODEX_BIN;
diff --git a/src/lib/server/assist-runner.ts b/src/lib/server/assist-runner.ts
index abda387a6..96f0b667c 100644
--- a/src/lib/server/assist-runner.ts
+++ b/src/lib/server/assist-runner.ts
@@ -57,11 +57,52 @@ export type AssistRunResult =
   | { ok: true; lastMessage: string }
   | { ok: false; error: string };
 
+/**
+ * Pure: collapse a bounded stderr capture into a one-line reason suitable
+ * for appending to an error message — last 3 non-empty lines, joined, capped
+ * at the final 300 chars. Unit-tested.
+ */
+export function stderrReason(stderrTail: string): string {
+  return stderrTail.trim().split(/\r?\n/).filter(Boolean).slice(-3).join(" · ").slice(-300);
+}
+
+// Signals that codex exited without output because it isn't signed in. Kept
+// deliberately precise: loose substrings like `auth`, `token`, or `sign ?in`
+// would misread "unexpected token" / "author" / "design in…" stderr as a
+// sign-in problem and send the user to `codex login` for an unrelated
+// failure. A missed match is benign — the generic path below still surfaces
+// the stderr tail verbatim.
+const AUTH_STDERR_RE =
+  /not (?:logged|signed) in|codex login|please (?:log|sign) ?in|unauthorized|authentication (?:required|failed|error)|(?:invalid|missing|expired) (?:credentials?|api key)|credentials? (?:invalid|missing|expired)|token (?:expired|revoked)|\b401\b/i;
+
+/**
+ * Pure: error message for the "exited 0 but wrote no last message" case.
+ * codex does this when it is not signed in, leaving the real reason on
+ * stderr — so surface the stderr tail, plus an explicit sign-in hint (with a
+ * runnable login command) when that tail looks like an auth failure.
+ * Unit-tested.
+ */
+export function describeEmptyAssistOutput(command: string, stderrTail: string): string {
+  const tail = stderrReason(stderrTail);
+  if (AUTH_STDERR_RE.test(stderrTail)) {
+    const loginCommand = /\s/.test(command)
+      ? // Escape backslashes before quotes so a Windows path (C:\…\coven.cmd)
+        // can't break out of the double-quoted suggestion (js/incomplete-sanitization).
+        `"${command.replace(/\\/g, "\\\\").replace(/"/g, '\\"')}" login`
+      : `${command} login`;
+    return `${command} isn't signed in, so this assist produced no output. Run \`${loginCommand}\` in a terminal, then try again${
+      tail ? ` (${tail})` : ""
+    }.`;
+  }
+  return `assist produced no output${tail ? ` — ${tail}` : ""}`;
+}
+
 /**
  * Run one bounded assist end-to-end: spawn `codex exec`, wait (bounded), and
  * return the final message for the caller to parse against its own output
  * contract. Never throws. Spawn behavior is exercised manually — CI covers
- * `buildAssistInvocation` only.
+ * the pure pieces only (`buildAssistInvocation`, `stderrReason`,
+ * `describeEmptyAssistOutput`).
  */
 export async function runBoundedAssist(opts: {
   prompt: string;
@@ -140,7 +181,7 @@ export async function runBoundedAssist(opts: {
     });
     if (spawned.error) return { ok: false, error: spawned.error };
     if (spawned.code !== 0) {
-      const reason = stderrTail.trim().split(/\r?\n/).filter(Boolean).slice(-3).join(" · ").slice(-300);
+      const reason = stderrReason(stderrTail);
       return {
         ok: false,
         error: `codex exec exited with ${spawned.code}${reason ? ` — ${reason}` : ""}`,
@@ -150,7 +191,11 @@ export async function runBoundedAssist(opts: {
     try {
       lastMessage = await readFile(/* turbopackIgnore: true */ lastMessagePath, "utf8");
     } catch {
-      return { ok: false, error: "assist produced no output" };
+      // codex can exit 0 yet write no last-message file when it is not signed
+      // in. The real reason lands on stderr, so surface that (and an explicit
+      // sign-in hint for the auth case) instead of an opaque "produced no
+      // output".
+      return { ok: false, error: describeEmptyAssistOutput(inv.command, stderrTail) };
     }
     return { ok: true, lastMessage };
   } catch (err) {