diff --git a/pkg/providers/openai_compat_adapter.go b/pkg/providers/openai_compat_adapter.go
index 9914c26..107fd1d 100644
--- a/pkg/providers/openai_compat_adapter.go
+++ b/pkg/providers/openai_compat_adapter.go
@@ -183,6 +183,7 @@ func (p *HTTPProvider) buildOpenAICompatChatRequest(messages []Message, tools []
 
 func openAICompatMessages(messages []Message) []map[string]interface{} {
 	out := make([]map[string]interface{}, 0, len(messages))
+	pendingCalls := map[string]struct{}{}
 	for _, msg := range messages {
 		role := strings.ToLower(strings.TrimSpace(msg.Role))
 		content := openAICompatMessageContent(msg)
@@ -199,6 +200,9 @@ func openAICompatMessages(messages []Message) []map[string]interface{} {
 			if len(msg.ToolCalls) > 0 {
 				toolCalls := make([]map[string]interface{}, 0, len(msg.ToolCalls))
 				for _, tc := range msg.ToolCalls {
+					if strings.TrimSpace(tc.ID) != "" {
+						pendingCalls[strings.TrimSpace(tc.ID)] = struct{}{}
+					}
 					args := ""
 					if tc.Function != nil {
 						args = tc.Function.Arguments
@@ -224,9 +228,17 @@ func openAICompatMessages(messages []Message) []map[string]interface{} {
 			}
 			out = append(out, item)
 		case "tool":
+			callID := strings.TrimSpace(msg.ToolCallID)
+			if callID == "" {
+				continue
+			}
+			if _, ok := pendingCalls[callID]; !ok {
+				continue
+			}
+			delete(pendingCalls, callID)
 			out = append(out, map[string]interface{}{
 				"role":         "tool",
-				"tool_call_id": msg.ToolCallID,
+				"tool_call_id": callID,
 				"content":      content,
 			})
 		default:
diff --git a/pkg/providers/openai_compat_provider_test.go b/pkg/providers/openai_compat_provider_test.go
index bba8220..e247e94 100644
--- a/pkg/providers/openai_compat_provider_test.go
+++ b/pkg/providers/openai_compat_provider_test.go
@@ -159,6 +159,37 @@ func TestOpenAICompatMessagesPreserveMultimodalContentParts(t *testing.T) {
 	}
 }
 
+func TestOpenAICompatMessagesDropsOrphanToolOutputs(t *testing.T) {
+	msgs := openAICompatMessages([]Message{
+		{Role: "user", Content: "hi"},
+		{Role: "tool", ToolCallID: "call-orphan", Content: "orphan output"},
+		{
+			Role: "assistant",
+			ToolCalls: []ToolCall{{
+				ID:   "call-1",
+				Name: "read_file",
+				Function: &FunctionCall{
+					Name:      "read_file",
+					Arguments: `{"path":"README.md"}`,
+				},
+			}},
+		},
+		{Role: "tool", ToolCallID: "call-1", Content: "file content"},
+	})
+	if len(msgs) != 3 {
+		t.Fatalf("messages = %#v, want 3 items", msgs)
+	}
+	if got := msgs[1]["role"]; got != "assistant" {
+		t.Fatalf("second role = %#v, want assistant", got)
+	}
+	if got := msgs[2]["role"]; got != "tool" {
+		t.Fatalf("third role = %#v, want tool", got)
+	}
+	if got := msgs[2]["tool_call_id"]; got != "call-1" {
+		t.Fatalf("tool_call_id = %#v, want call-1", got)
+	}
+}
+
 func TestBuildOpenAICompatChatRequestAppliesThinkingSuffix(t *testing.T) {
 	base := NewHTTPProvider("openai", "token", "https://example.com/v1", "gpt-5", false, "api_key", 5*time.Second, nil)
 	body := base.buildOpenAICompatChatRequest([]Message{{Role: "user", Content: "hi"}}, nil, "gpt-5(high)", nil)