feat: add support for built-in AI prefix continuation

- Introduced a new patched dependency for @built-in-ai/core@2.0.1 in both bun.lock and package.json.
- Enhanced message processing in sendMessage function to conditionally prefix assistant messages based on provider type.

Author: CoolSpring8

.bun-patches/@built-in-ai%2Fcore@2.0.1.patch

@@ -0,0 +1,75 @@
+diff --git a/node_modules/@built-in-ai/core/.bun-tag-d546d55538164722 b/.bun-tag-d546d55538164722
+new file mode 100644
+index 0000000000000000000000000000000000000000..e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
+diff --git a/dist/index.js b/dist/index.js
+index 9398fde99b09707f7d62dfdbce5062784c665c3d..62146453379a454a8c78f87cbd6342b9dc772cc4 100644
+--- a/dist/index.js
++++ b/dist/index.js
+@@ -62,7 +62,13 @@ function convertFileData(data, mediaType) {
+ function convertToBuiltInAIMessages(prompt) {
+   let systemMessage;
+   const messages = [];
+-  for (const message of prompt) {
++  for (const [index, message] of prompt.entries()) {
++    const prefix = !!message.providerOptions?.["browser-ai"]?.prefix;
++    if (prefix && (message.role !== "assistant" || index !== prompt.length - 1)) {
++      throw new import_provider.UnsupportedFunctionalityError({
++        functionality: "prefix on non-final assistant message"
++      });
++    }
+     switch (message.role) {
+       case "system": {
+         systemMessage = message.content;
+@@ -124,10 +130,14 @@ function convertToBuiltInAIMessages(prompt) {
+             }
+           }
+         }
+-        messages.push({
++        const assistantMessage = {
+           role: "assistant",
+           content: text
+-        });
++        };
++        if (prefix) {
++          assistantMessage.prefix = true;
++        }
++        messages.push(assistantMessage);
+         break;
+       }
+       case "tool": {
+diff --git a/dist/index.mjs b/dist/index.mjs
+index 4c346c33f370c5682b911734cda1402f40603afb..42fa23c6b9be6415b73b9d097627a8d1a3d91e54 100644
+--- a/dist/index.mjs
++++ b/dist/index.mjs
+@@ -35,7 +35,13 @@ function convertFileData(data, mediaType) {
+ function convertToBuiltInAIMessages(prompt) {
+   let systemMessage;
+   const messages = [];
+-  for (const message of prompt) {
++  for (const [index, message] of prompt.entries()) {
++    const prefix = !!message.providerOptions?.["browser-ai"]?.prefix;
++    if (prefix && (message.role !== "assistant" || index !== prompt.length - 1)) {
++      throw new UnsupportedFunctionalityError({
++        functionality: "prefix on non-final assistant message"
++      });
++    }
+     switch (message.role) {
+       case "system": {
+         systemMessage = message.content;
+@@ -97,10 +103,14 @@ function convertToBuiltInAIMessages(prompt) {
+             }
+           }
+         }
+-        messages.push({
++        const assistantMessage = {
+           role: "assistant",
+           content: text
+-        });
++        };
++        if (prefix) {
++          assistantMessage.prefix = true;
++        }
++        messages.push(assistantMessage);
+         break;
+       }
+       case "tool": {

bun.lock

@@ -51,6 +51,7 @@
 		"typescript": "^5.6.3"
 	},
 	"patchedDependencies": {
-		"@ai-sdk/openai-compatible@1.0.27": ".bun-patches/@ai-sdk%2Fopenai-compatible@1.0.27.patch"
+		"@ai-sdk/openai-compatible@1.0.27": ".bun-patches/@ai-sdk%2Fopenai-compatible@1.0.27.patch",
+		"@built-in-ai/core@2.0.1": ".bun-patches/@built-in-ai%2Fcore@2.0.1.patch"
 	}
 }

package.json

@@ -70,16 +70,29 @@ export const sendMessage = async (
 	const assistantId = shouldAppendToAssistant
 		? lastMessage._metadata.uuid
 		: createAssistantAfter(resolvedParentId);
+	const shouldPrefixAssistant =
+		provider.kind === "built-in" && shouldAppendToAssistant;
 	setNodeStatus(assistantId, "streaming");
 	setActiveTarget(assistantId);
 	const abortController = new AbortController();
 	streamManager.register(assistantId, abortController);
 	const modelMessages = contextMessages
 		.filter((message) => message.role !== "tool")
-		.map((message) => ({
-			role: message.role as "system" | "user" | "assistant",
-			content: message.content,
-		})) as ModelMessage[];
+		.map((message) => {
+			const base = {
+				role: message.role as "system" | "user" | "assistant",
+				content: message.content,
+			};
+			if (shouldPrefixAssistant && message._metadata.uuid === assistantId) {
+				return {
+					...base,
+					providerOptions: {
+						"browser-ai": { prefix: true },
+					},
+				};
+			}
+			return base;
+		}) as ModelMessage[];
 	try {
 		const stream = streamText({
 			model,