diff --git a/pkg/providers/anthropic_messages/provider.go b/pkg/providers/anthropic_messages/provider.go
index 55bb41e66..6894b6d61 100644
--- a/pkg/providers/anthropic_messages/provider.go
+++ b/pkg/providers/anthropic_messages/provider.go
@@ -253,6 +253,14 @@ func buildRequestBody(
 				content = append(content, toolUse)
 			}
 
+			// Skip empty assistant messages that have no content and no valid tool calls.
+			// This prevents breaking the Anthropic API requirement where a user message
+			// containing tool results must immediately follow the assistant message
+			// containing the corresponding tool calls.
+			if len(content) == 0 {
+				continue
+			}
+
 			apiMessages = append(apiMessages, map[string]any{
 				"role":    "assistant",
 				"content": content,
diff --git a/pkg/providers/anthropic_messages/provider_test.go b/pkg/providers/anthropic_messages/provider_test.go
index 39bc48117..153843bb1 100644
--- a/pkg/providers/anthropic_messages/provider_test.go
+++ b/pkg/providers/anthropic_messages/provider_test.go
@@ -506,6 +506,25 @@ func TestBuildRequestBodyEdgeCases(t *testing.T) {
 			},
 			wantErr: false,
 		},
+		{
+			name: "skip empty assistant messages",
+			messages: []Message{
+				{Role: "user", Content: "hello"},
+				{Role: "assistant", Content: "", ToolCalls: []ToolCall{}}, // Should be skipped
+				{Role: "assistant", Content: "valid message", ToolCalls: []ToolCall{}},
+				{Role: "assistant", Content: "", ToolCalls: []ToolCall{
+					{ID: "tool-valid", Name: "test_tool", Arguments: map[string]any{"arg": "value"}},
+				}},
+				{Role: "assistant", Content: "", ToolCalls: []ToolCall{
+					{ID: "tool-empty", Name: "", Arguments: map[string]any{"ignored": true}},
+				}}, // Should be skipped because tool call is empty and content is empty
+			},
+			model: "test-model",
+			options: map[string]any{
+				"max_tokens": 8192,
+			},
+			wantErr: false,
+		},
 	}
 
 	for _, tt := range tests {
@@ -558,6 +577,20 @@ func TestBuildRequestBodyEdgeCases(t *testing.T) {
 					t.Fatalf("tool_use id = %v, want %q", gotID, "tool-valid")
 				}
 			}
+
+			if tt.name == "skip empty assistant messages" {
+				messages, ok := got["messages"].([]any)
+				if !ok {
+					t.Fatalf("messages is not []any")
+				}
+				if len(messages) != 3 {
+					t.Fatalf(
+						"expected 3 API messages (user, assistant with text, assistant with tool_use), got %d: %#v",
+						len(messages),
+						messages,
+					)
+				}
+			}
 		})
 	}
 }