diff --git a/extensions/slack/src/monitor/relay-source.test.ts b/extensions/slack/src/monitor/relay-source.test.ts
index 8b15c60a4572..503526282c56 100644
--- a/extensions/slack/src/monitor/relay-source.test.ts
+++ b/extensions/slack/src/monitor/relay-source.test.ts
@@ -6,6 +6,8 @@ import {
   buildRelayWebSocketOptions,
   buildRelayWebSocketUrl,
   monitorSlackRelaySource,
+  parseRelayFrame,
+  SlackRelayMalformedFrameError,
   SLACK_RELAY_MAX_PAYLOAD_BYTES,
   type SlackRelayIdentity,
 } from "./relay-source.js";
@@ -20,6 +22,10 @@ function deferred<T>() {
   return { promise, reject, resolve };
 }
 
+function relayFrame(text: string): Buffer {
+  return Buffer.from(text, "utf8");
+}
+
 describe("Slack relay source", () => {
   it("builds authenticated relay websocket URLs safely", () => {
     expect(
@@ -198,4 +204,39 @@ describe("Slack relay source", () => {
       });
     });
   });
+
+  describe("parseRelayFrame", () => {
+    it("parses valid JSON frames", () => {
+      const frame = parseRelayFrame(
+        relayFrame(JSON.stringify({ type: "slack_event", data: { text: "hello" } })),
+      );
+      expect(frame).toEqual({ type: "slack_event", data: { text: "hello" } });
+    });
+
+    it("throws SlackRelayMalformedFrameError for malformed JSON", () => {
+      expect(() => parseRelayFrame(relayFrame("NOT JSON {{{"))).toThrow(
+        SlackRelayMalformedFrameError,
+      );
+    });
+
+    it("wraps the original SyntaxError as the cause", () => {
+      let error: unknown;
+      try {
+        parseRelayFrame(relayFrame("NOT JSON {{{"));
+      } catch (err: unknown) {
+        error = err;
+      }
+      expect(error).toBeInstanceOf(SlackRelayMalformedFrameError);
+      expect((error as SlackRelayMalformedFrameError).message).toContain("malformed JSON frame");
+      expect((error as SlackRelayMalformedFrameError).cause).toBeDefined();
+    });
+
+    it("parses empty object frames", () => {
+      expect(parseRelayFrame(relayFrame("{}"))).toEqual({});
+    });
+
+    it("parses array frames", () => {
+      expect(parseRelayFrame(relayFrame("[1, 2, 3]"))).toEqual([1, 2, 3]);
+    });
+  });
 });
diff --git a/extensions/slack/src/monitor/relay-source.ts b/extensions/slack/src/monitor/relay-source.ts
index 7ef7ba75341c..6dd118c4aa21 100644
--- a/extensions/slack/src/monitor/relay-source.ts
+++ b/extensions/slack/src/monitor/relay-source.ts
@@ -293,9 +293,20 @@ function isLocalRelayHost(hostname: string): boolean {
   return isIP(host) === 4 && host.startsWith("127.");
 }
 
-function parseRelayFrame(data: RawData): unknown {
+export class SlackRelayMalformedFrameError extends Error {
+  constructor(message: string, options?: ErrorOptions) {
+    super(message, options);
+    this.name = "SlackRelayMalformedFrameError";
+  }
+}
+
+export function parseRelayFrame(data: RawData): unknown {
   const text = rawDataToString(data);
-  return JSON.parse(text) as unknown;
+  try {
+    return JSON.parse(text) as unknown;
+  } catch (cause) {
+    throw new SlackRelayMalformedFrameError("Slack relay received malformed JSON frame", { cause });
+  }
 }
 
 function rawDataToString(data: RawData): string {