diff --git a/pkg/app/app.go b/pkg/app/app.go
index db899d9e2..a6fb92be5 100644
--- a/pkg/app/app.go
+++ b/pkg/app/app.go
@@ -1,6 +1,7 @@
 package app
 
 import (
+	"cmp"
 	"context"
 	"encoding/base64"
 	"errors"
@@ -997,12 +998,24 @@ func (a *App) mergeEvents(events []tea.Msg) []tea.Msg {
 			result = append(result, merged)
 
 		case *runtime.PartialToolCallEvent:
-			// For PartialToolCallEvent, keep only the latest one per tool call ID
-			// Only merge consecutive events with the same ID
+			// For PartialToolCallEvent, merge consecutive events with the same tool call ID
+			// by concatenating argument deltas
 			latest := ev
 			for i+1 < len(events) {
 				if next, ok := events[i+1].(*runtime.PartialToolCallEvent); ok && next.ToolCall.ID == ev.ToolCall.ID {
-					latest = next
+					latest = &runtime.PartialToolCallEvent{
+						Type: ev.Type,
+						ToolCall: tools.ToolCall{
+							ID:   ev.ToolCall.ID,
+							Type: ev.ToolCall.Type,
+							Function: tools.FunctionCall{
+								Name:      cmp.Or(next.ToolCall.Function.Name, latest.ToolCall.Function.Name),
+								Arguments: latest.ToolCall.Function.Arguments + next.ToolCall.Function.Arguments,
+							},
+						},
+						ToolDefinition: next.ToolDefinition,
+						AgentContext:   ev.AgentContext,
+					}
 					i++
 				} else {
 					break
diff --git a/pkg/runtime/streaming.go b/pkg/runtime/streaming.go
index 62b132076..eb80b1d70 100644
--- a/pkg/runtime/streaming.go
+++ b/pkg/runtime/streaming.go
@@ -152,10 +152,21 @@ func (r *LocalRuntime) handleStream(ctx context.Context, stream chat.MessageStre
 					tc.Function.Arguments += delta.Function.Arguments
 				}
 
-				// Emit PartialToolCall once we have a name, and on subsequent argument deltas
+				// Emit PartialToolCall once we have a name, and on subsequent argument deltas.
+				// Only the current token (delta.Function.Arguments) is sent, not the
+				// full accumulated arguments, to avoid re-transmitting the entire
+				// payload on every token.
 				if tc.Function.Name != "" && (learningName || delta.Function.Arguments != "") {
 					if !emittedPartial[delta.ID] || delta.Function.Arguments != "" {
-						events <- PartialToolCall(*tc, toolDefMap[tc.Function.Name], a.Name())
+						partial := tools.ToolCall{
+							ID:   tc.ID,
+							Type: tc.Type,
+							Function: tools.FunctionCall{
+								Name:      tc.Function.Name,
+								Arguments: delta.Function.Arguments,
+							},
+						}
+						events <- PartialToolCall(partial, toolDefMap[tc.Function.Name], a.Name())
 						emittedPartial[delta.ID] = true
 					}
 				}
diff --git a/pkg/tui/components/messages/messages.go b/pkg/tui/components/messages/messages.go
index 810ef4a66..531113bab 100644
--- a/pkg/tui/components/messages/messages.go
+++ b/pkg/tui/components/messages/messages.go
@@ -1301,7 +1301,11 @@ func (m *model) AddOrUpdateToolCall(agentName string, toolCall tools.ToolCall, t
 		if msg.Type == types.MessageTypeToolCall && msg.ToolCall.ID == toolCall.ID {
 			msg.ToolStatus = status
 			if toolCall.Function.Arguments != "" {
-				msg.ToolCall.Function.Arguments = toolCall.Function.Arguments
+				if status == types.ToolStatusPending {
+					msg.ToolCall.Function.Arguments += toolCall.Function.Arguments
+				} else {
+					msg.ToolCall.Function.Arguments = toolCall.Function.Arguments
+				}
 			}
 			m.invalidateItem(i)
 			return nil
diff --git a/pkg/tui/components/reasoningblock/reasoningblock.go b/pkg/tui/components/reasoningblock/reasoningblock.go
index c53a50cfc..cd4778df5 100644
--- a/pkg/tui/components/reasoningblock/reasoningblock.go
+++ b/pkg/tui/components/reasoningblock/reasoningblock.go
@@ -206,7 +206,11 @@ func (m *Model) UpdateToolCall(toolCallID string, status types.ToolStatus, args
 		}
 		entry.msg.ToolStatus = status
 		if args != "" {
-			entry.msg.ToolCall.Function.Arguments = args
+			if status == types.ToolStatusPending {
+				entry.msg.ToolCall.Function.Arguments += args
+			} else {
+				entry.msg.ToolCall.Function.Arguments = args
+			}
 		}
 		m.toolEntries[i] = entry
 		return