diff --git a/extensions/mattermost/src/mattermost/monitor-onchar.test.ts b/extensions/mattermost/src/mattermost/monitor-onchar.test.ts
new file mode 100644
index 00000000000..b2e43e6ba33
--- /dev/null
+++ b/extensions/mattermost/src/mattermost/monitor-onchar.test.ts
@@ -0,0 +1,32 @@
+import { describe, expect, it } from "vitest";
+import { resolveOncharPrefixes, stripOncharPrefix } from "./monitor-onchar.js";
+
+describe("mattermost monitor onchar", () => {
+  it("uses defaults when prefixes are missing or empty after trimming", () => {
+    expect(resolveOncharPrefixes(undefined)).toEqual([">", "!"]);
+    expect(resolveOncharPrefixes([" ", ""])).toEqual([">", "!"]);
+  });
+
+  it("trims configured prefixes and preserves order", () => {
+    expect(resolveOncharPrefixes(["  ?? ", " !", " /bot "] )).toEqual(["??", "!", "/bot"]);
+  });
+
+  it("strips the first matching prefix after leading whitespace", () => {
+    expect(stripOncharPrefix("   ! hello world", ["!", ">"])).toEqual({
+      triggered: true,
+      stripped: "hello world",
+    });
+
+    expect(stripOncharPrefix("??multi prefix", ["??", "?"])).toEqual({
+      triggered: true,
+      stripped: "multi prefix",
+    });
+  });
+
+  it("returns the original text when no prefix matches", () => {
+    expect(stripOncharPrefix("hello world", ["!", ">"])).toEqual({
+      triggered: false,
+      stripped: "hello world",
+    });
+  });
+});