[MCP] include assistant tool call before tool response + bug fix (#1618)

Fixes #1614.

Also, while experimenting with tiny-agents using OpenAI's gpt-4o, I
noticed that the assistant's tool call wasn t being appended to the
message list before the tool call result. This didn't cause issues
before, _likely_ because the models inferred the missing function call
context from the tool response. However, it seems that OpenAI is
stricter about message ordering and expect the assistant's tool call to
be explicitly in the conversation history.

Note: the fix is already implemented in Python tiny-agents :
huggingface/huggingface_hub#3159

Author: hanouticelina

packages/mcp-client/src/McpClient.ts

@@ -125,14 +125,18 @@ export class McpClient {
 	): AsyncGenerator<ChatCompletionStreamOutput | ChatCompletionInputMessageTool> {
 		debug("start of single turn");
 
-		const stream = this.client.chatCompletionStream({
-			provider: this.provider,
-			model: this.model,
-			messages,
-			tools: opts.exitLoopTools ? [...opts.exitLoopTools, ...this.availableTools] : this.availableTools,
-			tool_choice: "auto",
-			signal: opts.abortSignal,
-		});
+		const stream = this.client.chatCompletionStream(
+			{
+				provider: this.provider,
+				model: this.model,
+				messages,
+				tools: opts.exitLoopTools ? [...opts.exitLoopTools, ...this.availableTools] : this.availableTools,
+				tool_choice: "auto",
+			},
+			{
+				signal: opts.abortSignal,
+			}
+		);
 
 		const message = {
 			role: "unknown",
@@ -181,9 +185,26 @@ export class McpClient {
 			yield chunk;
 		}
 
-		messages.push(message);
+		const assistantMessage: ChatCompletionInputMessage = {
+			role: "assistant",
+			content: message.content,
+		};
+
+		const finalToolCallValues = Object.values(finalToolCalls);
+
+		if (finalToolCallValues.length > 0) {
+			assistantMessage.tool_calls = finalToolCallValues.map((toolCall) => ({
+				id: toolCall.id,
+				type: "function",
+				function: {
+					name: toolCall.function.name ?? "unknown",
+					arguments: toolCall.function.arguments,
+				},
+			}));
+		}
+		messages.push(assistantMessage);
 
-		for (const toolCall of Object.values(finalToolCalls)) {
+		for (const toolCall of finalToolCallValues) {
 			const toolName = toolCall.function.name ?? "unknown";
 			/// TODO(Fix upstream type so this is always a string)^
 			const toolMessage: ChatCompletionInputMessageTool = {