feat: Support XML tool call fallback when using native tool protocol

When native tool protocol is enabled, some models may still output XML-formatted
tool calls in their text stream instead of using the native tool_call chunks.
This change adds an XML fallback parser that runs alongside the native protocol
handler to detect and parse these XML tool calls.

The implementation:
1. Initializes an XML fallback parser when in native protocol mode
2. Feeds text chunks to the fallback parser during streaming
3. After stream completes, if no native tool_call chunks were received but
   the XML fallback parser found tool calls, uses those parsed blocks instead

This ensures tool calls are properly detected and executed regardless of whether
the model outputs them in native or XML format.

Fixes #10325

src/core/task/Task.ts

@@ -342,6 +342,8 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 	didToolFailInCurrentTurn = false
 	didCompleteReadingStream = false
 	assistantMessageParser?: AssistantMessageParser
+	// XML fallback parser for native protocol - detects and parses XML tool calls in text when model outputs them
+	private xmlFallbackParser?: AssistantMessageParser
 	private providerProfileChangeListener?: (config: { name: string; provider?: string }) => void
 
 	// Native tool call streaming state (track which index each tool is at)
@@ -2535,6 +2537,14 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 				)
 				const shouldUseXmlParser = streamProtocol === "xml"
 
+				// Initialize XML fallback parser for native protocol
+				// This allows detecting XML tool calls in text when models output them despite native tools being enabled
+				if (!shouldUseXmlParser) {
+					this.xmlFallbackParser = new AssistantMessageParser()
+				} else {
+					this.xmlFallbackParser = undefined
+				}
+
 				// Yields only if the first chunk is successful, otherwise will
 				// allow the user to retry the request (most likely due to rate
 				// limit error, which gets thrown on the first chunk).
@@ -2789,6 +2799,12 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 										this.userMessageContentReady = false
 									}
 
+									// Also feed text to XML fallback parser to detect XML tool calls
+									// in case the model outputs them despite native tools being enabled
+									if (this.xmlFallbackParser) {
+										this.xmlFallbackParser.processChunk(chunk.text)
+									}
+
 									// Present content to user
 									presentAssistantMessage(this)
 								}
@@ -3141,6 +3157,41 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 					this.assistantMessageContent = parsedBlocks
 				}
 
+				// XML Fallback for Native Protocol:
+				// If we're in native protocol mode but no native tool_call chunks were received,
+				// check if the XML fallback parser found any tool calls in the text stream.
+				// This handles models that output XML tool calls despite native tools being enabled.
+				if (!shouldUseXmlParser && this.xmlFallbackParser) {
+					this.xmlFallbackParser.finalizeContentBlocks()
+					const fallbackBlocks = this.xmlFallbackParser.getContentBlocks()
+
+					// Check if fallback parser found any tool uses
+					const fallbackHasToolUses = fallbackBlocks.some(
+						(block) => block.type === "tool_use" || block.type === "mcp_tool_use",
+					)
+
+					// Check if native protocol already found tool uses
+					const nativeHasToolUses = this.assistantMessageContent.some(
+						(block) => block.type === "tool_use" || block.type === "mcp_tool_use",
+					)
+
+					// If native protocol didn't find tools but XML fallback did, use the fallback results
+					if (!nativeHasToolUses && fallbackHasToolUses) {
+						console.log(
+							`[Task#${this.taskId}] XML fallback detected tool calls in native protocol text stream. Using fallback parser results.`,
+						)
+						this.assistantMessageContent = fallbackBlocks
+						// Mark that we have new content to process
+						this.userMessageContentReady = false
+						// Present the fallback tool calls
+						presentAssistantMessage(this)
+					}
+
+					// Reset the fallback parser for next request
+					this.xmlFallbackParser.reset()
+					this.xmlFallbackParser = undefined
+				}
+
 				// Present any partial blocks that were just completed
 				// For XML protocol: includes both text and tool_use blocks parsed from the text stream
 				// For native protocol: tool_use blocks were already presented during streaming via