diff --git a/src/EventSource.ts b/src/EventSource.ts
index 779740a..476b3b5 100644
--- a/src/EventSource.ts
+++ b/src/EventSource.ts
@@ -1,6 +1,6 @@
 import {createParser, type EventSourceMessage, type EventSourceParser} from 'eventsource-parser'
 
-import {ErrorEvent, syntaxError} from './errors.js'
+import {ErrorEvent, flattenError, syntaxError} from './errors.js'
 import type {
   AddEventListenerOptions,
   EventListenerOptions,
@@ -438,7 +438,7 @@ export class EventSource extends EventTarget {
       return
     }
 
-    this.#scheduleReconnect()
+    this.#scheduleReconnect(flattenError(err))
   }
 
   /**
@@ -514,6 +514,7 @@ export class EventSource extends EventTarget {
    * Handles the process referred to in the EventSource specification as "failing a connection".
    *
    * @param error - The error causing the connection to fail
+   * @param code - The HTTP status code, if available
    * @internal
    */
   #failConnection(error?: string, code?: number) {
@@ -525,14 +526,11 @@ export class EventSource extends EventTarget {
 
     // [spec] …and fires an event named `error` at the `EventSource` object.
     // [spec] Once the user agent has failed the connection, it does not attempt to reconnect.
-    const errorEvent = new ErrorEvent('error')
-
     // [spec] > Implementations are especially encouraged to report detailed information
     // [spec] > to their development consoles whenever an error event is fired, since little
     // [spec] > to no information can be made available in the events themselves.
     // Printing to console is not very programatically helpful, though, so we emit a custom event.
-    errorEvent.code = code
-    errorEvent.message = error
+    const errorEvent = new ErrorEvent('error', code, error)
 
     this.#onError?.(errorEvent)
     this.dispatchEvent(errorEvent)
@@ -541,9 +539,11 @@ export class EventSource extends EventTarget {
   /**
    * Schedules a reconnection attempt against the EventSource endpoint.
    *
+   * @param error - The error causing the connection to fail
+   * @param code - The HTTP status code, if available
    * @internal
    */
-  #scheduleReconnect() {
+  #scheduleReconnect(error?: string, code?: number) {
     // [spec] If the readyState attribute is set to CLOSED, abort the task.
     if (this.#readyState === this.CLOSED) {
       return
@@ -553,7 +553,7 @@ export class EventSource extends EventTarget {
     this.#readyState = this.CONNECTING
 
     // [spec] Fire an event named `error` at the EventSource object.
-    const errorEvent = new ErrorEvent('error')
+    const errorEvent = new ErrorEvent('error', code, error)
     this.#onError?.(errorEvent)
     this.dispatchEvent(errorEvent)
 
diff --git a/src/errors.ts b/src/errors.ts
index eb79470..18a7ec0 100644
--- a/src/errors.ts
+++ b/src/errors.ts
@@ -21,6 +21,12 @@ export class ErrorEvent extends Event {
    * @public
    */
   public message?: string | undefined
+
+  constructor(type: string, code?: number, message?: string) {
+    super(type)
+    this.code = code ?? undefined
+    this.message = message ?? undefined
+  }
 }
 
 /**
@@ -43,3 +49,27 @@ export function syntaxError(message: string): SyntaxError {
 
   return new SyntaxError(message)
 }
+
+/**
+ * Flatten an error into a single error message string.
+ * Unwraps nested errors and joins them with a comma.
+ *
+ * @param err - The error to flatten
+ * @returns A string representation of the error
+ * @internal
+ */
+export function flattenError(err: unknown): string {
+  if (!(err instanceof Error)) {
+    return `${err}`
+  }
+
+  if ('errors' in err && Array.isArray(err.errors)) {
+    return err.errors.map(flattenError).join(', ')
+  }
+
+  if ('cause' in err && err.cause instanceof Error) {
+    return `${err}: ${flattenError(err.cause)}`
+  }
+
+  return err.message
+}
diff --git a/test/helpers.ts b/test/helpers.ts
index 8e29db6..81070aa 100644
--- a/test/helpers.ts
+++ b/test/helpers.ts
@@ -7,6 +7,7 @@ type MessageReceiver = SinonSpy & {
 }
 
 const TYPE_ASSERTER = Symbol.for('waffle.type-asserter')
+const PATTERN_ASSERTER = Symbol.for('waffle.pattern-asserter')
 
 export class ExpectationError extends Error {
   type = 'ExpectationError'
@@ -146,6 +147,38 @@ export function expect(
           return
         }
 
+        if (
+          typeof expected[key] === 'object' &&
+          expected[key] !== null &&
+          PATTERN_ASSERTER in expected[key]
+        ) {
+          if (typeof thing[key] !== 'string') {
+            throw new ExpectationError(
+              `Expected key "${key}" of ${descriptor || 'object'} to be a string, got ${typeof thing[key]}`,
+            )
+          }
+
+          if (typeof expected[key][PATTERN_ASSERTER] === 'string') {
+            if (!thing[key].includes(expected[key][PATTERN_ASSERTER])) {
+              throw new ExpectationError(
+                `Expected key "${key}" of ${descriptor || 'object'} to include "${expected[key][PATTERN_ASSERTER]}", got "${thing[key]}"`,
+              )
+            }
+            return
+          }
+
+          if (expected[key][PATTERN_ASSERTER] instanceof RegExp) {
+            if (!expected[key][PATTERN_ASSERTER].test(thing[key])) {
+              throw new ExpectationError(
+                `Expected key "${key}" of ${descriptor || 'object'} to match pattern ${expected[key][PATTERN_ASSERTER]}, got "${thing[key]}"`,
+              )
+            }
+            return
+          }
+
+          throw new Error('Invalid pattern asserter')
+        }
+
         if (thing[key] !== expected[key]) {
           throw new ExpectationError(
             `Expected key "${key}" of ${descriptor || 'object'} to be ${JSON.stringify(expected[key])}, was ${JSON.stringify(
@@ -188,6 +221,12 @@ expect.any = (
   }
 }
 
+expect.stringMatching = (expected: string | RegExp) => {
+  return {
+    [PATTERN_ASSERTER]: expected,
+  }
+}
+
 function isPlainObject(obj: unknown): obj is Record<string, unknown> {
   return typeof obj === 'object' && obj !== null && !Array.isArray(obj)
 }
diff --git a/test/tests.ts b/test/tests.ts
index c128da0..f73a960 100644
--- a/test/tests.ts
+++ b/test/tests.ts
@@ -697,7 +697,27 @@ export function registerTests(options: {
     await deferClose(es)
   })
 
-  test('[NON-SPEC] message event contains extended properties', async () => {
+  test('[NON-SPEC] message event contains extended properties (failed connection)', async () => {
+    const onError = getCallCounter({name: 'onError'})
+    const es = new OurEventSource(`${baseUrl}:9999/should-not-connect`, {fetch})
+
+    es.addEventListener('error', onError)
+    await onError.waitForCallCount(1)
+
+    expect(onError.lastCall.lastArg).toMatchObject({
+      type: 'error',
+      defaultPrevented: false,
+      cancelable: false,
+      timeStamp: expect.any('number'),
+      message: expect.stringMatching(
+        /fetch failed|failed to fetch|load failed|attempting to fetch/i,
+      ),
+      code: undefined,
+    })
+    await deferClose(es)
+  })
+
+  test('[NON-SPEC] message event contains extended properties (invalid http response)', async () => {
     const onError = getCallCounter({name: 'onError'})
     const es = new OurEventSource(`${baseUrl}:${port}/end-after-one`, {fetch})