diff --git a/pkg/providers/openai_compat/provider.go b/pkg/providers/openai_compat/provider.go
index ce6e56b37..0dbeba0b6 100644
--- a/pkg/providers/openai_compat/provider.go
+++ b/pkg/providers/openai_compat/provider.go
@@ -255,6 +255,14 @@ func filterDeepSeekReasoningMessages(messages []Message) []Message {
 }
 
 func filterDeepSeekReasoningTurn(messages []Message) []Message {
+	hasToolInteraction := false
+	for _, msg := range messages {
+		if msg.Role == "tool" || (msg.Role == "assistant" && len(msg.ToolCalls) > 0) {
+			hasToolInteraction = true
+			break
+		}
+	}
+
 	out := make([]Message, 0, len(messages))
 	for _, msg := range messages {
 		if messageutil.IsTransientAssistantThoughtMessage(msg) {
@@ -262,10 +270,13 @@ func filterDeepSeekReasoningTurn(messages []Message) []Message {
 		}
 
 		cloned := msg
-		// DeepSeek thinking-mode requires reasoning_content to be echoed
-		// back for ALL assistant turns. If missing the API returns 400:
-		// "The reasoning_content in the thinking mode must be passed back
-		// to the API."
+		// DeepSeek thinking-mode replay only requires reasoning_content for
+		// turns that participate in a tool interaction round. For plain
+		// assistant turns between two user messages, the docs say the API will
+		// ignore reasoning_content on replay, so we strip it here.
+		if cloned.Role == "assistant" && strings.TrimSpace(cloned.ReasoningContent) != "" && !hasToolInteraction {
+			cloned.ReasoningContent = ""
+		}
 		if assistantMessageEmpty(cloned) {
 			continue
 		}