diff --git a/pkg/agent/loop.go b/pkg/agent/loop.go
index b36f4a0c4..9a66c6cd1 100644
--- a/pkg/agent/loop.go
+++ b/pkg/agent/loop.go
@@ -635,9 +635,10 @@ func (al *AgentLoop) runLLMIteration(
 			}
 
 			assistantMsg.ToolCalls = append(assistantMsg.ToolCalls, providers.ToolCall{
-				ID:   tc.ID,
-				Type: "function",
-				Name: tc.Name,
+				ID:        tc.ID,
+				Type:      "function",
+				Name:      tc.Name,
+				Arguments: tc.Arguments,
 				Function: &providers.FunctionCall{
 					Name:             tc.Name,
 					Arguments:        string(argumentsJSON),
diff --git a/pkg/providers/anthropic/provider.go b/pkg/providers/anthropic/provider.go
index b23236869..39e523ba4 100644
--- a/pkg/providers/anthropic/provider.go
+++ b/pkg/providers/anthropic/provider.go
@@ -146,7 +146,20 @@ func buildParams(
 					blocks = append(blocks, anthropic.NewTextBlock(msg.Content))
 				}
 				for _, tc := range msg.ToolCalls {
-					blocks = append(blocks, anthropic.NewToolUseBlock(tc.ID, tc.Arguments, tc.Name))
+					input := tc.Arguments
+					if input == nil {
+						// Recover from nil Arguments (e.g. loaded from older sessions)
+						if tc.Function != nil && tc.Function.Arguments != "" {
+							var parsed map[string]interface{}
+							if err := json.Unmarshal([]byte(tc.Function.Arguments), &parsed); err == nil {
+								input = parsed
+							}
+						}
+						if input == nil {
+							input = map[string]interface{}{}
+						}
+					}
+					blocks = append(blocks, anthropic.NewToolUseBlock(tc.ID, input, tc.Name))
 				}
 				anthropicMessages = append(anthropicMessages, anthropic.NewAssistantMessage(blocks...))
 			} else {
diff --git a/pkg/session/manager.go b/pkg/session/manager.go
index 08f0b0ad2..09e494bfd 100644
--- a/pkg/session/manager.go
+++ b/pkg/session/manager.go
@@ -259,6 +259,14 @@ func (sm *SessionManager) loadSessions() error {
 			continue
 		}
 
+		// Normalize tool calls to ensure Arguments map is populated
+		// from Function.Arguments for sessions saved before this fix.
+		for i, msg := range session.Messages {
+			for j, tc := range msg.ToolCalls {
+				session.Messages[i].ToolCalls[j] = providers.NormalizeToolCall(tc)
+			}
+		}
+
 		sm.sessions[session.Key] = &session
 	}