diff --git a/types/dom-chromium-ai/dom-chromium-ai-tests.ts b/types/dom-chromium-ai/dom-chromium-ai-tests.ts
index a443bc82c111f2..a4b814a6c1161f 100644
--- a/types/dom-chromium-ai/dom-chromium-ai-tests.ts
+++ b/types/dom-chromium-ai/dom-chromium-ai-tests.ts
@@ -2,10 +2,15 @@ async function topLevel() {
     // Language Model
 
     await LanguageModel.create({
-        // @ts-expect-error - System prompt must be first element of the initialPrompt array.
+        // @ts-expect-error - System prompt must be first element of the initialPrompts array.
         initialPrompts: [{ role: "user", content: "foo" }, { role: "system", content: "foo" }],
     });
 
+    await LanguageModel.create({
+        // @ts-expect-error - Prefixes are not allowed in initialPrompts.
+        initialPrompts: [{ role: "assistant", content: "foo", prefix: true }],
+    });
+
     const languageModel = await LanguageModel.create({
         topK: 1,
         temperature: 0,
@@ -434,11 +439,13 @@ async function topLevel() {
     });
     console.log(proofreaderAvailability2);
 
-    const proofreaderResult: ProofreadResult = await proofreader.proofread("foo");
+    const proofreaderResult: ProofreadResult = await proofreader.proofread("foo", {
+        signal: (new AbortController()).signal,
+    });
     console.log(proofreaderResult);
 
     // for await (
-    //     const chunk of proofreader.proofreadStreaming("foo")
+    //     const chunk of proofreader.proofreadStreaming("foo", { signal: (new AbortController()).signal })
     // ) {
     //     console.log(chunk);
     // }
diff --git a/types/dom-chromium-ai/index.d.ts b/types/dom-chromium-ai/index.d.ts
index b5028339c20b78..c539ae728ff60e 100644
--- a/types/dom-chromium-ai/index.d.ts
+++ b/types/dom-chromium-ai/index.d.ts
@@ -146,11 +146,17 @@ interface LanguageModelToolFunction {
     (...args: any[]): Promise<string>;
 }
 
-type LanguageModelPrompt = LanguageModelMessage[] | string;
+type LanguageModelPrompt = (LanguageModelMessage | LanguageModelAssistantMessage)[] | string;
 
 interface LanguageModelMessage {
     role: LanguageModelMessageRole;
     content: LanguageModelMessageContent[] | string;
+}
+
+// Not in IDL, split up here for allowing assistant messages with prefix in prompt() and promptStreaming() only
+interface LanguageModelAssistantMessage {
+    role: LanguageModelAssistantMessageRole;
+    content: LanguageModelMessageContent[] | string;
     prefix?: boolean;
 }
 
@@ -165,7 +171,9 @@ interface LanguageModelMessageContent {
     value: LanguageModelMessageValue;
 }
 
-type LanguageModelMessageRole = "user" | "assistant";
+type LanguageModelMessageRole = "user" | LanguageModelAssistantMessageRole;
+// Not in IDL, split up here for allowing assistant messages with prefix in prompt() and promptStreaming() only
+type LanguageModelAssistantMessageRole = "assistant";
 // Not in IDL, split up here for enforcing the system message as the first element
 type LanguageModelSystemMessageRole = "system";
 
@@ -397,8 +405,8 @@ declare abstract class Proofreader implements DestroyableModel {
     static create(options?: ProofreaderCreateOptions): Promise<Proofreader>;
     static availability(options?: ProofreaderCreateCoreOptions): Promise<Availability>;
 
-    proofread(input: string): Promise<ProofreadResult>;
-    // proofreadStreaming(input: string): ReadableStream<unknown>;
+    proofread(input: string, options?: ProofreaderProofreadOptions): Promise<ProofreadResult>;
+    // proofreadStreaming(input: string, options?: ProofreaderProofreadOptions): ReadableStream<unknown>;
 
     readonly includeCorrectionTypes: boolean;
     readonly includeCorrectionExplanations: boolean;
@@ -420,6 +428,10 @@ interface ProofreaderCreateOptions extends ProofreaderCreateCoreOptions {
     monitor?: CreateMonitorCallback;
 }
 
+interface ProofreaderProofreadOptions {
+    signal?: AbortSignal;
+}
+
 interface ProofreadResult {
     correctedInput: string;
     corrections: ProofreadCorrection[];