Prevent double-sending of thinking content in ACP

Track whether a model sends proper ReasoningDeltaEvent events. If so,
skip parsing <thinking> tags from text to avoid sending reasoning content
twice (once as proper reasoning, once parsed from text).

Also reset the tracking state at the start of each new prompt turn.

internal/acpserver/agent.go

@@ -43,6 +43,11 @@ type Agent struct {
 	// inThinkingTag tracks whether we're currently inside a <thinking> tag
 	// when parsing streaming content from models that wrap reasoning in XML tags.
 	inThinkingTag bool
+
+	// hasProperReasoningEvents tracks whether the model is sending ReasoningDeltaEvent
+	// (proper reasoning events) vs wrapping reasoning in <thinking> tags in text.
+	// If true, we skip thinking tag parsing to avoid double-sending reasoning.
+	hasProperReasoningEvents bool
 }
 
 // NewAgent creates a new ACP agent backed by Kit.
@@ -141,6 +146,10 @@ func (a *Agent) Prompt(ctx context.Context, params acp.PromptRequest) (acp.Promp
 
 	log.Debug("acp: prompt", "session", sessionID, "prompt_len", len(promptText), "files", len(files))
 
+	// Reset reasoning tracking for this new prompt turn
+	a.hasProperReasoningEvents = false
+	a.inThinkingTag = false
+
 	// Create a cancellable context for this prompt turn.
 	promptCtx, cancel := context.WithCancel(ctx)
 	sess.setCancel(cancel)
@@ -206,26 +215,36 @@ func (a *Agent) subscribeEvents(ctx context.Context, k *kit.Kit, sessionID acp.S
 		var update *acp.SessionUpdate
 		switch ev := e.(type) {
 		case kit.MessageUpdateEvent:
-			// Handle models that wrap reasoning in <thinking> tags (Qwen, DeepSeek)
-			// Parse the chunk and separate reasoning from regular text
-			reasoning, text := a.parseThinkingTags(ev.Chunk)
-
-			// Send reasoning update if we have reasoning content
-			if reasoning != "" {
-				u := acp.UpdateAgentThoughtText(reasoning)
-				_ = a.conn.SessionUpdate(ctx, acp.SessionNotification{
-					SessionId: sessionID,
-					Update:    u,
-				})
-			}
-
-			// Send text update if we have text content
-			if text != "" {
-				u := acp.UpdateAgentMessageText(text)
+			// If the model sends proper ReasoningDeltaEvent, don't parse thinking tags
+			// from text to avoid double-sending reasoning content.
+			if a.hasProperReasoningEvents {
+				// Send text as-is without thinking tag parsing
+				u := acp.UpdateAgentMessageText(ev.Chunk)
 				update = &u
+			} else {
+				// Handle models that wrap reasoning in <thinking> tags (Qwen, DeepSeek)
+				// Parse the chunk and separate reasoning from regular text
+				reasoning, text := a.parseThinkingTags(ev.Chunk)
+
+				// Send reasoning update if we have reasoning content
+				if reasoning != "" {
+					u := acp.UpdateAgentThoughtText(reasoning)
+					_ = a.conn.SessionUpdate(ctx, acp.SessionNotification{
+						SessionId: sessionID,
+						Update:    u,
+					})
+				}
+
+				// Send text update if we have text content
+				if text != "" {
+					u := acp.UpdateAgentMessageText(text)
+					update = &u
+				}
 			}
 
 		case kit.ReasoningDeltaEvent:
+			// Track that this model sends proper reasoning events
+			a.hasProperReasoningEvents = true
 			u := acp.UpdateAgentThoughtText(ev.Delta)
 			update = &u