diff --git a/desktop/src/shared/lib/createRemarkPrefixPlugin.test.mjs b/desktop/src/shared/lib/createRemarkPrefixPlugin.test.mjs
new file mode 100644
index 00000000000..6712fe70cdf
--- /dev/null
+++ b/desktop/src/shared/lib/createRemarkPrefixPlugin.test.mjs
@@ -0,0 +1,103 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import remarkChannelLinks from "./remarkChannelLinks.ts";
+import remarkMentions from "./remarkMentions.ts";
+
+/**
+ * These run the real plugins over a real mdast tree. The pattern-level tests in
+ * `mentionPattern.test.mjs` strip the leading-boundary capture group by hand,
+ * which cannot catch a factory that swallows the boundary character instead of
+ * emitting it back as text — that only shows up in the tree.
+ */
+
+function paragraph(...children) {
+  return { type: "root", children: [{ type: "paragraph", children }] };
+}
+function text(value) {
+  return { type: "text", value };
+}
+function kids(tree) {
+  return tree.children[0].children;
+}
+/** `[type, value]` for each child, so a swallowed space fails the assertion. */
+function shape(tree) {
+  return kids(tree).map((node) => [node.type, node.value]);
+}
+
+function runMentions(value, names = ["alice", "bob"]) {
+  const tree = paragraph(text(value));
+  remarkMentions({ mentionNames: names })(tree);
+  return tree;
+}
+
+function runChannels(value, names = ["general"]) {
+  const tree = paragraph(text(value));
+  remarkChannelLinks({ channelNames: names })(tree);
+  return tree;
+}
+
+test("the boundary character survives as text before the mention", () => {
+  assert.deepEqual(shape(runMentions("hi @alice")), [
+    ["text", "hi "],
+    ["mention", "@alice"],
+  ]);
+});
+
+test("adjacent mentions keep the space between them", () => {
+  assert.deepEqual(shape(runMentions("@alice @bob")), [
+    ["mention", "@alice"],
+    ["text", " "],
+    ["mention", "@bob"],
+  ]);
+});
+
+test("a mention at the start of the text produces no empty leading node", () => {
+  assert.deepEqual(shape(runMentions("@alice hi")), [
+    ["mention", "@alice"],
+    ["text", " hi"],
+  ]);
+});
+
+test("an opening paren before a mention stays text", () => {
+  // Team expansions render as `Team (@ana @bo)`, so `(` opens a mention.
+  assert.deepEqual(shape(runMentions("Team (@alice)")), [
+    ["text", "Team ("],
+    ["mention", "@alice"],
+    ["text", ")"],
+  ]);
+});
+
+test("an email address is left entirely alone", () => {
+  assert.deepEqual(shape(runMentions("mail bob@alice.dev now")), [
+    ["text", "mail bob@alice.dev now"],
+  ]);
+});
+
+test("a channel link keeps its preceding text", () => {
+  assert.deepEqual(shape(runChannels("see #general")), [
+    ["text", "see "],
+    ["channel-link", "#general"],
+  ]);
+});
+
+test("an opening paren does not open a channel link", () => {
+  // The composer's channel highlighter accepts only start-of-text or
+  // whitespace; rendered messages must not be more permissive, or `(#general)`
+  // shows no chip while typing and turns into a link once sent.
+  assert.deepEqual(shape(runChannels("ask in (#general)")), [
+    ["text", "ask in (#general)"],
+  ]);
+});
+
+test("the generic channel fallback also refuses a mid-word prefix", () => {
+  assert.deepEqual(shape(runChannels("issue-42#general", [])), [
+    ["text", "issue-42#general"],
+  ]);
+});
+
+test("inline code is left untouched", () => {
+  const tree = paragraph({ type: "inlineCode", value: "@alice" });
+  remarkMentions({ mentionNames: ["alice"] })(tree);
+  assert.deepEqual(shape(tree), [["inlineCode", "@alice"]]);
+});
diff --git a/desktop/src/shared/lib/createRemarkPrefixPlugin.ts b/desktop/src/shared/lib/createRemarkPrefixPlugin.ts
index 8fe1c91d24b..fcc246d186b 100644
--- a/desktop/src/shared/lib/createRemarkPrefixPlugin.ts
+++ b/desktop/src/shared/lib/createRemarkPrefixPlugin.ts
@@ -15,6 +15,14 @@ type NodeBuilderResult = Node | { node: Node; trailing?: string };
 
 type NodeBuilder = (matchText: string) => NodeBuilderResult;
 
+/**
+ * `leadGroup` names a capture group holding a leading boundary character that
+ * the pattern had to consume to assert one (WebKit before Safari 16.4 fails to
+ * *parse* lookbehind, blanking the whole app, so patterns capture instead).
+ * Its text is emitted back as plain text and is not part of the built node.
+ */
+type PrefixPluginOptions = { leadGroup?: number };
+
 /**
  * Create a remark plugin that walks the tree, finds regex matches in text
  * nodes, and replaces each match with a node produced by `buildNode`.
@@ -22,17 +30,23 @@ type NodeBuilder = (matchText: string) => NodeBuilderResult;
 export function createRemarkPrefixPlugin(
   pattern: RegExp,
   buildNode: NodeBuilder,
+  options?: PrefixPluginOptions,
 ) {
+  const leadGroup = options?.leadGroup;
   return (
     // biome-ignore lint/suspicious/noExplicitAny: remark tree types are not available
     tree: any,
   ) => {
-    walkChildren(tree, pattern, buildNode);
+    walkChildren(tree, pattern, buildNode, leadGroup);
   };
 }
 
-// biome-ignore lint/suspicious/noExplicitAny: remark tree types are not available
-function walkChildren(node: any, pattern: RegExp, buildNode: NodeBuilder) {
+function walkChildren(
+  node: Node,
+  pattern: RegExp,
+  buildNode: NodeBuilder,
+  leadGroup?: number,
+) {
   if (
     !node?.children ||
     !Array.isArray(node.children) ||
@@ -45,7 +59,7 @@ function walkChildren(node: any, pattern: RegExp, buildNode: NodeBuilder) {
     const child = node.children[i];
 
     if (child.type === "text") {
-      const parts = splitByPattern(child.value, pattern, buildNode);
+      const parts = splitByPattern(child.value, pattern, buildNode, leadGroup);
       if (
         parts.length > 1 ||
         (parts.length === 1 && parts[0].type !== "text")
@@ -53,19 +67,23 @@ function walkChildren(node: any, pattern: RegExp, buildNode: NodeBuilder) {
         node.children.splice(i, 1, ...parts);
       }
     } else {
-      walkChildren(child, pattern, buildNode);
+      walkChildren(child, pattern, buildNode, leadGroup);
     }
   }
 }
 
-// biome-ignore lint/suspicious/noExplicitAny: remark tree types are not available
-function shouldSkipNode(node: any): boolean {
+function shouldSkipNode(node: Node): boolean {
   return (
     node.type === "link" || node.type === "code" || node.type === "inlineCode"
   );
 }
 
-function splitByPattern(text: string, pattern: RegExp, buildNode: NodeBuilder) {
+function splitByPattern(
+  text: string,
+  pattern: RegExp,
+  buildNode: NodeBuilder,
+  leadGroup?: number,
+) {
   // Reset lastIndex — the pattern is reused across text nodes with the `g` flag
   pattern.lastIndex = 0;
   // biome-ignore lint/suspicious/noExplicitAny: building mdast-compatible nodes
@@ -79,11 +97,15 @@ function splitByPattern(text: string, pattern: RegExp, buildNode: NodeBuilder) {
       break;
     }
 
-    if (match.index > lastIndex) {
-      parts.push({ type: "text", value: text.slice(lastIndex, match.index) });
+    const lead = leadGroup === undefined ? "" : (match[leadGroup] ?? "");
+    const matchStart = match.index + lead.length;
+    if (matchStart > lastIndex) {
+      parts.push({ type: "text", value: text.slice(lastIndex, matchStart) });
     }
 
-    const result = normalizeBuildNodeResult(buildNode(match[0]));
+    const result = normalizeBuildNodeResult(
+      buildNode(match[0].slice(lead.length)),
+    );
     parts.push(result.node);
     if (result.trailing) {
       parts.push({ type: "text", value: result.trailing });
diff --git a/desktop/src/shared/lib/mentionPattern.test.mjs b/desktop/src/shared/lib/mentionPattern.test.mjs
new file mode 100644
index 00000000000..dcc608bf34e
--- /dev/null
+++ b/desktop/src/shared/lib/mentionPattern.test.mjs
@@ -0,0 +1,97 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import {
+  buildMentionPattern,
+  buildPrefixPattern,
+  PREFIX_LEAD_GROUP,
+} from "./mentionPattern.ts";
+
+/** All matches, with the leading boundary stripped the way the plugin does. */
+function matches(pattern, text) {
+  pattern.lastIndex = 0;
+  const found = [];
+  let match = pattern.exec(text);
+  while (match) {
+    const lead = match[PREFIX_LEAD_GROUP] ?? "";
+    found.push({
+      text: match[0].slice(lead.length),
+      index: match.index + lead.length,
+    });
+    match = pattern.exec(text);
+  }
+  return found;
+}
+
+test("a mention is matched at the start of the text", () => {
+  const pattern = buildMentionPattern(["alice"]);
+  assert.deepEqual(matches(pattern, "@alice hi"), [
+    { text: "@alice", index: 0 },
+  ]);
+});
+
+test("a mention is matched after whitespace and after an opening paren", () => {
+  const pattern = buildMentionPattern(["alice"]);
+  assert.deepEqual(matches(pattern, "hi @alice"), [
+    { text: "@alice", index: 3 },
+  ]);
+  assert.deepEqual(matches(pattern, "hi (@alice)"), [
+    { text: "@alice", index: 4 },
+  ]);
+});
+
+test("an email address is not a mention of its domain", () => {
+  // "@alice" here is part of bob@alice.dev — the composer's own highlighter
+  // draws the boundary in the same place, so the rendered message should too.
+  const pattern = buildMentionPattern(["alice"]);
+  assert.deepEqual(matches(pattern, "mail me at bob@alice.dev"), []);
+});
+
+test("the boundary does not eat the space between two adjacent mentions", () => {
+  const pattern = buildMentionPattern(["alice", "bob"]);
+  assert.deepEqual(matches(pattern, "@alice @bob"), [
+    { text: "@alice", index: 0 },
+    { text: "@bob", index: 7 },
+  ]);
+});
+
+test("longest known name still wins over a shorter prefix of it", () => {
+  const pattern = buildMentionPattern(["ali", "alice"]);
+  assert.deepEqual(matches(pattern, "hi @alice"), [
+    { text: "@alice", index: 3 },
+  ]);
+});
+
+test("unknown names are not matched", () => {
+  const pattern = buildMentionPattern(["alice"]);
+  assert.deepEqual(matches(pattern, "hi @carol"), []);
+});
+
+test("with no known names the mention pattern never matches", () => {
+  assert.deepEqual(matches(buildMentionPattern([]), "hi @alice"), []);
+});
+
+test("the generic channel fallback also requires a leading boundary", () => {
+  const pattern = buildPrefixPattern("#", [], { fallbackToGeneric: true });
+  assert.deepEqual(matches(pattern, "see #general"), [
+    { text: "#general", index: 4 },
+  ]);
+  assert.deepEqual(matches(pattern, "issue-42#general"), []);
+});
+
+test("an opening paren opens a mention but not a channel", () => {
+  // Exactly the split the composer's highlighter draws: `(?<=[\s(])@` versus
+  // `(?<=\s)#`. Team expansions render as `Team (@ana @bo)`; channels have no
+  // such form, so `(#general)` is not a channel link.
+  assert.deepEqual(matches(buildMentionPattern(["alice"]), "(@alice)"), [
+    { text: "@alice", index: 1 },
+  ]);
+  assert.deepEqual(
+    matches(buildPrefixPattern("#", ["general"]), "(#general)"),
+    [],
+  );
+  assert.deepEqual(
+    matches(buildPrefixPattern("#", [], { fallbackToGeneric: true }), "(#gen)"),
+    [],
+  );
+});
diff --git a/desktop/src/shared/lib/mentionPattern.ts b/desktop/src/shared/lib/mentionPattern.ts
index 73d12a7a44b..40aae79244d 100644
--- a/desktop/src/shared/lib/mentionPattern.ts
+++ b/desktop/src/shared/lib/mentionPattern.ts
@@ -7,6 +7,33 @@ export function escapeRegExp(str: string): string {
 
 const NEVER_MATCH = /(?!)/gi;
 
+/**
+ * Capture index of the leading boundary character in a pattern built by
+ * {@link buildPrefixPattern} — pass it to `createRemarkPrefixPlugin` as
+ * `leadGroup` so the character is re-emitted as text rather than swallowed
+ * into the mention.
+ */
+export const PREFIX_LEAD_GROUP = 1;
+
+/**
+ * A prefix only opens a mention or channel link at the start of the text or
+ * after whitespace — `bob@alice.dev` is an address, not a mention of `@alice`.
+ *
+ * The boundary is a capture group rather than a lookbehind on purpose: WebKit
+ * before Safari 16.4 fails to parse lookbehind and blanks the whole app
+ * (#5547).
+ */
+const LEADING_BOUNDARY = "(^|\\s)";
+
+/**
+ * Mentions additionally open after `(`, because team expansions render as
+ * `Team (@ana @bo)`. Channels deliberately do not — these two boundaries are
+ * exactly the ones the composer's highlighter applies to `@` and `#`
+ * respectively, and the rendered message has to agree with the composer or a
+ * `(#general)` that shows no chip while typing turns into a link once sent.
+ */
+const LEADING_BOUNDARY_WITH_PAREN = "(^|[\\s(])";
+
 /**
  * Build a regex that matches a given prefix followed by known multi-word names
  * (longest-first to avoid partial matches). When known names are provided,
@@ -19,28 +46,38 @@ const NEVER_MATCH = /(?!)/gi;
  * - Otherwise returns a never-matching regex, preventing arbitrary `@word`
  *   patterns from being highlighted as valid mentions when no p-tags are
  *   present (used by remarkMentions / buildMentionPattern).
+ *
+ * `options.allowOpeningParen` widens the leading boundary to include `(`; see
+ * {@link LEADING_BOUNDARY_WITH_PAREN}. It is off by default so a new caller
+ * gets the stricter rule.
  */
 export function buildPrefixPattern(
   prefix: string,
   knownNames: string[],
-  options?: { fallbackToGeneric?: boolean },
+  options?: { fallbackToGeneric?: boolean; allowOpeningParen?: boolean },
 ): RegExp {
   const sorted = [...new Set(knownNames)]
     .filter((name) => name.trim().length > 0)
     .sort((a, b) => b.length - a.length);
 
   const escapedPrefix = escapeRegExp(prefix);
+  const lead = options?.allowOpeningParen
+    ? LEADING_BOUNDARY_WITH_PAREN
+    : LEADING_BOUNDARY;
 
   if (sorted.length === 0) {
     if (options?.fallbackToGeneric) {
-      return new RegExp(`${escapedPrefix}\\S+`, "gi");
+      return new RegExp(`${lead}${escapedPrefix}\\S+`, "gi");
     }
     return NEVER_MATCH;
   }
 
   const nameAlternatives = sorted.map((name) => escapeRegExp(name)).join("|");
   const boundary = "(?=[\\s,;.!?:)\\]}]|$)";
-  return new RegExp(`${escapedPrefix}(?:${nameAlternatives})${boundary}`, "gi");
+  return new RegExp(
+    `${lead}${escapedPrefix}(?:${nameAlternatives})${boundary}`,
+    "gi",
+  );
 }
 
 /**
@@ -50,5 +87,5 @@ export function buildPrefixPattern(
  * they correspond to an actual p-tagged member.
  */
 export function buildMentionPattern(mentionNames: string[]): RegExp {
-  return buildPrefixPattern("@", mentionNames);
+  return buildPrefixPattern("@", mentionNames, { allowOpeningParen: true });
 }
diff --git a/desktop/src/shared/lib/remarkChannelLinks.ts b/desktop/src/shared/lib/remarkChannelLinks.ts
index 008a6854071..52af95ac344 100644
--- a/desktop/src/shared/lib/remarkChannelLinks.ts
+++ b/desktop/src/shared/lib/remarkChannelLinks.ts
@@ -8,7 +8,7 @@
  */
 
 import { createRemarkPrefixPlugin } from "./createRemarkPrefixPlugin";
-import { buildPrefixPattern } from "./mentionPattern";
+import { buildPrefixPattern, PREFIX_LEAD_GROUP } from "./mentionPattern";
 
 type RemarkChannelLinksOptions = {
   channelNames?: string[];
@@ -21,16 +21,20 @@ export default function remarkChannelLinks(
     fallbackToGeneric: true,
   });
 
-  return createRemarkPrefixPlugin(channelPattern, (matchText) => {
-    const channelName = matchText.slice(1);
-    return {
-      type: "channel-link",
-      value: matchText,
-      data: {
-        hName: "channel-link",
-        hChildren: [{ type: "text", value: matchText }],
-        channelName,
-      },
-    };
-  });
+  return createRemarkPrefixPlugin(
+    channelPattern,
+    (matchText) => {
+      const channelName = matchText.slice(1);
+      return {
+        type: "channel-link",
+        value: matchText,
+        data: {
+          hName: "channel-link",
+          hChildren: [{ type: "text", value: matchText }],
+          channelName,
+        },
+      };
+    },
+    { leadGroup: PREFIX_LEAD_GROUP },
+  );
 }
diff --git a/desktop/src/shared/lib/remarkMentions.ts b/desktop/src/shared/lib/remarkMentions.ts
index 7413df6f35c..ec8773af567 100644
--- a/desktop/src/shared/lib/remarkMentions.ts
+++ b/desktop/src/shared/lib/remarkMentions.ts
@@ -8,7 +8,7 @@
  */
 
 import { createRemarkPrefixPlugin } from "./createRemarkPrefixPlugin";
-import { buildMentionPattern } from "./mentionPattern";
+import { buildMentionPattern, PREFIX_LEAD_GROUP } from "./mentionPattern";
 
 type RemarkMentionsOptions = {
   mentionNames?: string[];
@@ -17,12 +17,16 @@ type RemarkMentionsOptions = {
 export default function remarkMentions(options?: RemarkMentionsOptions) {
   const mentionPattern = buildMentionPattern(options?.mentionNames ?? []);
 
-  return createRemarkPrefixPlugin(mentionPattern, (matchText) => ({
-    type: "mention",
-    value: matchText,
-    data: {
-      hName: "mention",
-      hChildren: [{ type: "text", value: matchText }],
-    },
-  }));
+  return createRemarkPrefixPlugin(
+    mentionPattern,
+    (matchText) => ({
+      type: "mention",
+      value: matchText,
+      data: {
+        hName: "mention",
+        hChildren: [{ type: "text", value: matchText }],
+      },
+    }),
+    { leadGroup: PREFIX_LEAD_GROUP },
+  );
 }