diff --git a/packages/core/src/event-stream/encoder.test.ts b/packages/core/src/event-stream/encoder.test.ts
index f33fe6f..164c3fd 100644
--- a/packages/core/src/event-stream/encoder.test.ts
+++ b/packages/core/src/event-stream/encoder.test.ts
@@ -8,8 +8,40 @@ import {
   encodeEventStreamMessage,
   encodeEventStreamMessageComments,
   encodeEventStreamMessageData,
+  isEventStreamMessageComment,
+  isEventStreamMessageId,
+  isEventStreamMessageRetry,
 } from './encoder'
 
+describe('predicates', () => {
+  it('accept valid values', () => {
+    expect(isEventStreamMessageId('')).toBe(true)
+    expect(isEventStreamMessageId('123')).toBe(true)
+    expect(isEventStreamMessageComment('hi')).toBe(true)
+    expect(isEventStreamMessageRetry(0)).toBe(true)
+    expect(isEventStreamMessageRetry(10000)).toBe(true)
+  })
+
+  it('reject strings containing line breaks', () => {
+    for (const lineBreak of ['\n', '\r', '\r\n']) {
+      expect(isEventStreamMessageId(`hi${lineBreak}`)).toBe(false)
+      expect(isEventStreamMessageComment(`hi${lineBreak}`)).toBe(false)
+    }
+  })
+
+  it('reject non-integer or negative retry values', () => {
+    for (const retry of [Number.NaN, -1, 1.5, Number.POSITIVE_INFINITY]) {
+      expect(isEventStreamMessageRetry(retry)).toBe(false)
+    }
+  })
+
+  it('reject wrong types', () => {
+    expect(isEventStreamMessageId(123)).toBe(false)
+    expect(isEventStreamMessageComment(undefined)).toBe(false)
+    expect(isEventStreamMessageRetry('3000')).toBe(false)
+  })
+})
+
 describe('assertions', () => {
   it('accept values without line breaks', () => {
     expect(() => assertEventStreamMessageId('123')).not.toThrow()
diff --git a/packages/core/src/event-stream/encoder.ts b/packages/core/src/event-stream/encoder.ts
index 12b778f..ffa7646 100644
--- a/packages/core/src/event-stream/encoder.ts
+++ b/packages/core/src/event-stream/encoder.ts
@@ -8,8 +8,20 @@ function containsEventStreamLineBreak(value: string): boolean {
   return EVENT_STREAM_LINE_ENDING_REGEX.test(value)
 }
 
+export function isEventStreamMessageId(maybe: unknown): maybe is string {
+  return typeof maybe === 'string' && !containsEventStreamLineBreak(maybe)
+}
+
+export function isEventStreamMessageRetry(maybe: unknown): maybe is number {
+  return Number.isInteger(maybe) && (maybe as number) >= 0
+}
+
+export function isEventStreamMessageComment(maybe: unknown): maybe is string {
+  return typeof maybe === 'string' && !containsEventStreamLineBreak(maybe)
+}
+
 export function assertEventStreamMessageId(id: string): void {
-  if (containsEventStreamLineBreak(id)) {
+  if (!isEventStreamMessageId(id)) {
     throw new EventStreamEncoderError('Event\'s id must not contain a carriage return or newline character')
   }
 }
@@ -21,13 +33,13 @@ export function assertEventStreamMessageName(event: string): void {
 }
 
 export function assertEventStreamMessageRetry(retry: number): void {
-  if (!Number.isInteger(retry) || retry < 0) {
+  if (!isEventStreamMessageRetry(retry)) {
     throw new EventStreamEncoderError('Event\'s retry must be a integer and >= 0')
   }
 }
 
 export function assertEventStreamMessageComment(comment: string): void {
-  if (containsEventStreamLineBreak(comment)) {
+  if (!isEventStreamMessageComment(comment)) {
     throw new EventStreamEncoderError('Event\'s comment must not contain a carriage return or newline character')
   }
 }
diff --git a/packages/peer/src/validators.test.ts b/packages/peer/src/validators.test.ts
index a7bacc2..e64bfd3 100644
--- a/packages/peer/src/validators.test.ts
+++ b/packages/peer/src/validators.test.ts
@@ -132,7 +132,11 @@ describe('isPeerEventStreamMessage', () => {
     ['non-string id', { id: 42 }],
     ['non-string event', { event: true }],
     ['non-finite retry', { retry: Infinity }],
+    ['negative retry', { retry: -1 }],
+    ['fractional retry', { retry: 1.5 }],
+    ['id with line break', { id: 'a\nb' }],
     ['non-string-array comments', { comments: [1, 2] }],
+    ['comment with line break', { comments: ['ok', 'a\rb'] }],
   ])('rejects %s', (_, json) => expect(isPeerEventStreamMessage(msg(json))).toBe(false))
 })
 
diff --git a/packages/peer/src/validators.ts b/packages/peer/src/validators.ts
index 083c037..f915f24 100644
--- a/packages/peer/src/validators.ts
+++ b/packages/peer/src/validators.ts
@@ -1,5 +1,5 @@
 import type { ClientPeerSendMessage, PeerCancelMessage, PeerEventStreamMessage, PeerMessage, PeerOctetStreamMessage, PeerRequestMessage, PeerResponseMessage, PeerStreamCancelMessage, ServerPeerSendMessage } from './types'
-import { isStandardHeaders, isStandardMethod, isStandardStatus, isStandardUrl } from '@standard-server/core'
+import { isEventStreamMessageComment, isEventStreamMessageId, isEventStreamMessageRetry, isStandardHeaders, isStandardMethod, isStandardStatus, isStandardUrl } from '@standard-server/core'
 import { isTypescriptObject } from '@standard-server/shared'
 
 export function isPeerMessage(maybe: unknown): maybe is PeerMessage {
@@ -75,7 +75,7 @@ export function isPeerEventStreamMessage(maybe: PeerMessage): maybe is PeerEvent
     return false
   }
 
-  if (maybe.json.id !== undefined && typeof maybe.json.id !== 'string') {
+  if (maybe.json.id !== undefined && !isEventStreamMessageId(maybe.json.id)) {
     return false
   }
 
@@ -83,13 +83,13 @@ export function isPeerEventStreamMessage(maybe: PeerMessage): maybe is PeerEvent
     return false
   }
 
-  if (maybe.json.retry !== undefined && !Number.isFinite(maybe.json.retry)) {
+  if (maybe.json.retry !== undefined && !isEventStreamMessageRetry(maybe.json.retry)) {
     return false
   }
 
   if (
     maybe.json.comments !== undefined
-    && !(Array.isArray(maybe.json.comments) && maybe.json.comments.every(v => typeof v === 'string'))
+    && !(Array.isArray(maybe.json.comments) && maybe.json.comments.every(isEventStreamMessageComment))
   ) {
     return false
   }