diff --git a/desktop/src/shared/api/relayMeshSignaling.ts b/desktop/src/shared/api/relayMeshSignaling.ts
index 906b7b1e2d..aaec24150e 100644
--- a/desktop/src/shared/api/relayMeshSignaling.ts
+++ b/desktop/src/shared/api/relayMeshSignaling.ts
@@ -1,6 +1,7 @@
 import { signRelayEvent } from "@/shared/api/tauri";
 import type { RelayEvent } from "@/shared/api/types";
 import { relayClient } from "@/shared/api/relayClient";
+import { canonicalPubkeyOrThrow } from "@/shared/lib/pubkey";
 import {
   KIND_MESH_CALL_ME_NOW,
   KIND_MESH_CONNECT_REQUEST,
@@ -43,7 +44,7 @@ export async function publishMeshConnectRequest(input: {
   const event = await signRelayEvent({
     kind: KIND_MESH_CONNECT_REQUEST,
     content: JSON.stringify(content),
-    tags: [["p", input.targetPubkey]],
+    tags: [["p", canonicalPubkeyOrThrow(input.targetPubkey)]],
   });
   await relayClient.publishEvent(
     event,
diff --git a/desktop/src/shared/lib/pubkey.test.mjs b/desktop/src/shared/lib/pubkey.test.mjs
new file mode 100644
index 0000000000..85aa823c2d
--- /dev/null
+++ b/desktop/src/shared/lib/pubkey.test.mjs
@@ -0,0 +1,43 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { canonicalPubkeyOrThrow } from "./pubkey.ts";
+
+const HEX = "35c18ae273fccfaf80d629e20e7f8721b90499379addff533054acc2504c12b4";
+const NPUB = "npub1xhqc4cnnln86lqxk983qulu8yxusfxfhntwl75es2jkvy5zvz26qzr0685";
+
+test("passes through canonical lowercase hex unchanged", () => {
+  assert.equal(canonicalPubkeyOrThrow(HEX), HEX);
+});
+
+test("lowercases uppercase hex", () => {
+  assert.equal(canonicalPubkeyOrThrow(HEX.toUpperCase()), HEX);
+});
+
+test("trims surrounding whitespace", () => {
+  assert.equal(canonicalPubkeyOrThrow(`  ${HEX}\n`), HEX);
+});
+
+test("decodes npub to hex", () => {
+  assert.equal(canonicalPubkeyOrThrow(NPUB), HEX);
+});
+
+test("trims then decodes npub", () => {
+  assert.equal(canonicalPubkeyOrThrow(`  ${NPUB} `), HEX);
+});
+
+test("throws on empty string", () => {
+  assert.throws(() => canonicalPubkeyOrThrow(""), /invalid pubkey/);
+});
+
+test("throws on too-short hex", () => {
+  assert.throws(() => canonicalPubkeyOrThrow("35c18ae2"), /invalid pubkey/);
+});
+
+test("throws on non-hex garbage", () => {
+  assert.throws(() => canonicalPubkeyOrThrow("not-a-pubkey"), /invalid pubkey/);
+});
+
+test("throws on malformed npub", () => {
+  assert.throws(() => canonicalPubkeyOrThrow("npub1zzzz"));
+});
diff --git a/desktop/src/shared/lib/pubkey.ts b/desktop/src/shared/lib/pubkey.ts
index 072140374c..b13fae343e 100644
--- a/desktop/src/shared/lib/pubkey.ts
+++ b/desktop/src/shared/lib/pubkey.ts
@@ -1,3 +1,5 @@
+import { decode } from "nostr-tools/nip19";
+
 /**
  * Canonical pubkey normalisation.
  *
@@ -7,3 +9,32 @@
 export function normalizePubkey(pubkey: string): string {
   return pubkey.trim().toLowerCase();
 }
+
+const HEX64 = /^[0-9a-f]{64}$/;
+
+/**
+ * Coerce a pubkey to canonical 64-char lowercase hex, accepting either hex
+ * (any case, padded) or `npub1…`. Throws if the result is not a valid pubkey.
+ *
+ * Use at signing boundaries where a malformed value would otherwise be signed
+ * into a tag and rejected downstream with an opaque error (e.g. the relay's
+ * "mesh connect request missing #p target" when the tag value is unusable).
+ * Failing here turns that into a clear, local error before the event is sent.
+ */
+export function canonicalPubkeyOrThrow(pubkey: string): string {
+  const trimmed = pubkey.trim();
+  if (trimmed.startsWith("npub1")) {
+    const decoded = decode(trimmed);
+    if (decoded.type !== "npub") {
+      throw new Error(`invalid pubkey: ${trimmed}`);
+    }
+    return decoded.data;
+  }
+  const hex = trimmed.toLowerCase();
+  if (!HEX64.test(hex)) {
+    throw new Error(
+      `invalid pubkey: expected 64-char hex or npub, got ${trimmed}`,
+    );
+  }
+  return hex;
+}
diff --git a/desktop/src/testing/e2eBridge.ts b/desktop/src/testing/e2eBridge.ts
index 27544bb79c..5f3b045fa9 100644
--- a/desktop/src/testing/e2eBridge.ts
+++ b/desktop/src/testing/e2eBridge.ts
@@ -4981,10 +4981,23 @@ function sendToMockSocket(args: {
 
     // Mesh control events (24620 status report, 24621 connect request) are not
     // channel messages — they carry a `p` tag, not an `h` tag. The real relay
-    // accepts them after membership/shape checks; the mock just ACKs so the
-    // desktop mesh flow (publishMeshConnectRequest) can proceed. We do not model
-    // the paired 24622 here; that belongs in a dedicated call-me-now test.
+    // accepts them after membership/shape checks; the mock mirrors the one shape
+    // check that matters for regression coverage: a 24621 MUST carry a `#p`
+    // target, else the real relay rejects it with "missing #p target". ACKing a
+    // tagless 24621 here would hide exactly that bug, so reject it instead.
     if (event.kind === 24620 || event.kind === 24621) {
+      if (
+        event.kind === 24621 &&
+        !event.tags.some((tag) => tag[0] === "p" && tag[1])
+      ) {
+        sendWsText(socket.handler, [
+          "OK",
+          event.id,
+          false,
+          "invalid: mesh connect request missing #p target",
+        ]);
+        return;
+      }
       sendWsText(socket.handler, ["OK", event.id, true, ""]);
       return;
     }