diff --git a/pkg/agent/loop.go b/pkg/agent/loop.go
index 48790b8..05eda47 100644
--- a/pkg/agent/loop.go
+++ b/pkg/agent/loop.go
@@ -101,11 +101,13 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 		// Stream the assistant's response, retrying on rate limits, transient
 		// backend errors (5xx, network), and mid-stream drops.
 		var content strings.Builder
+		var reasoning strings.Builder
 		var toolCalls []backend.ToolCall
 		var stopReason string
 
 		for attempt := range maxTransientRetries + 1 {
 			content.Reset()
+			reasoning.Reset()
 			toolCalls = nil
 
 			ch, err := cfg.Backend.ChatStream(ctx, history, cfg.Tools, cfg.GenerationParams)
@@ -144,6 +146,7 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 						emit(cfg, Event{Role: "reasoning", Content: "<think>\n"})
 						hadReasoning = true
 					}
+					reasoning.WriteString(ev.Reasoning)
 					emit(cfg, Event{Role: "reasoning", Content: ev.Reasoning})
 				}
 				if ev.Content != "" {
@@ -194,7 +197,7 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 				if hadReasoning {
 					emit(cfg, Event{Role: "reasoning", Content: "\n</think>\n"})
 				}
-				msg := backend.Message{Role: "assistant", Content: content.String(), ToolCalls: toolCalls}
+				msg := backend.Message{Role: "assistant", Content: content.String(), Reasoning: reasoning.String(), ToolCalls: toolCalls}
 				var results []toolResult
 				for _, tc := range toolCalls {
 					results = append(results, toolResult{
@@ -241,7 +244,7 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 		}
 
 		// Execute tool calls, running consecutive parallel-read-safe tools concurrently.
-		msg := backend.Message{Role: "assistant", Content: content.String(), ToolCalls: toolCalls}
+		msg := backend.Message{Role: "assistant", Content: content.String(), Reasoning: reasoning.String(), ToolCalls: toolCalls}
 		results := make([]toolResult, 0, len(toolCalls))
 		interrupted := false
 
diff --git a/pkg/backend/backend.go b/pkg/backend/backend.go
index 54e06cf..4f97028 100644
--- a/pkg/backend/backend.go
+++ b/pkg/backend/backend.go
@@ -32,6 +32,7 @@ type ImageSource struct {
 type Message struct {
 	Role          string         `json:"role"`                     // "system" | "user" | "assistant" | "tool"
 	Content       string         `json:"content"`
+	Reasoning     string         `json:"reasoning,omitempty"`      // thinking/reasoning content (DeepSeek, OpenAI o-series)
 	ContentBlocks []ContentBlock `json:"content_blocks,omitempty"` // when set, overrides Content
 	ToolCalls     []ToolCall     `json:"tool_calls,omitempty"`     // set by assistant when calling tools
 	ToolCallID    string         `json:"tool_call_id,omitempty"`   // set on role=tool replies (required by OpenAI)
diff --git a/pkg/backend/openai.go b/pkg/backend/openai.go
index 79690ff..37aebc9 100644
--- a/pkg/backend/openai.go
+++ b/pkg/backend/openai.go
@@ -132,11 +132,12 @@ type openAIContentBlock struct {
 }
 
 type openAIMessage struct {
-	Role          string               `json:"role"`
-	Content       *string              `json:"content"`
-	ContentBlocks []openAIContentBlock `json:"-"` // non-nil overrides Content during marshaling
-	ToolCalls     []openAIToolCall     `json:"tool_calls,omitempty"`
-	ToolCallID    string               `json:"tool_call_id,omitempty"`
+	Role             string               `json:"role"`
+	Content          *string              `json:"content"`
+	ReasoningContent *string              `json:"reasoning_content,omitempty"`
+	ContentBlocks    []openAIContentBlock `json:"-"` // non-nil overrides Content during marshaling
+	ToolCalls        []openAIToolCall     `json:"tool_calls,omitempty"`
+	ToolCallID       string               `json:"tool_call_id,omitempty"`
 }
 
 // MarshalJSON encodes content as an array of blocks when ContentBlocks is set
@@ -145,11 +146,12 @@ type openAIMessage struct {
 func (m openAIMessage) MarshalJSON() ([]byte, error) {
 	if len(m.ContentBlocks) > 0 {
 		return json.Marshal(struct {
-			Role       string               `json:"role"`
-			Content    []openAIContentBlock `json:"content"`
-			ToolCalls  []openAIToolCall     `json:"tool_calls,omitempty"`
-			ToolCallID string               `json:"tool_call_id,omitempty"`
-		}{m.Role, m.ContentBlocks, m.ToolCalls, m.ToolCallID})
+			Role             string               `json:"role"`
+			ReasoningContent *string              `json:"reasoning_content,omitempty"`
+			Content          []openAIContentBlock `json:"content"`
+			ToolCalls        []openAIToolCall     `json:"tool_calls,omitempty"`
+			ToolCallID       string               `json:"tool_call_id,omitempty"`
+		}{m.Role, m.ReasoningContent, m.ContentBlocks, m.ToolCalls, m.ToolCallID})
 	}
 	type plain openAIMessage
 	return json.Marshal(plain(m))
@@ -243,6 +245,9 @@ func encodeOpenAIMessages(messages []Message) []openAIMessage {
 			Role:       m.Role,
 			ToolCallID: m.ToolCallID,
 		}
+		if m.Reasoning != "" {
+			wm.ReasoningContent = &m.Reasoning
+		}
 		for _, tc := range m.ToolCalls {
 			wm.ToolCalls = append(wm.ToolCalls, openAIToolCall{
 				ID:   tc.ID,