diff --git a/pkg/agent/loop.go b/pkg/agent/loop.go
index 88afa6119..d9c59864f 100644
--- a/pkg/agent/loop.go
+++ b/pkg/agent/loop.go
@@ -1130,6 +1130,9 @@ func (al *AgentLoop) forceCompression(agent *AgentInstance, sessionKey string) {
 	newHistory = append(newHistory, keptConversation...)
 	newHistory = append(newHistory, history[len(history)-1]) // Last message
 
+	// Sanitize tool pairs that may have been split by the midpoint cut
+	newHistory = sanitizeToolPairs(newHistory)
+
 	// Update session
 	agent.Sessions.SetHistory(sessionKey, newHistory)
 	agent.Sessions.Save(sessionKey)
@@ -1306,6 +1309,14 @@ func (al *AgentLoop) summarizeSession(agent *AgentInstance, sessionKey string) {
 	if finalSummary != "" {
 		agent.Sessions.SetSummary(sessionKey, finalSummary)
 		agent.Sessions.TruncateHistory(sessionKey, 4)
+
+		// Sanitize remaining history to fix orphaned tool pairs from truncation
+		remaining := agent.Sessions.GetHistory(sessionKey)
+		sanitized := sanitizeToolPairs(remaining)
+		if len(sanitized) != len(remaining) {
+			agent.Sessions.SetHistory(sessionKey, sanitized)
+		}
+
 		agent.Sessions.Save(sessionKey)
 	}
 }
diff --git a/pkg/agent/sanitize.go b/pkg/agent/sanitize.go
new file mode 100644
index 000000000..66f2112eb
--- /dev/null
+++ b/pkg/agent/sanitize.go
@@ -0,0 +1,91 @@
+package agent
+
+import (
+	"github.com/sipeed/picoclaw/pkg/logger"
+	"github.com/sipeed/picoclaw/pkg/providers"
+)
+
+// sanitizeToolPairs ensures every assistant message with ToolCalls has
+// matching tool results, and every tool result has its preceding tool_call.
+// Orphaned messages are removed to prevent provider API errors (e.g.,
+// Anthropic's "tool_use ids were provided that do not have a tool_use block").
+//
+// This is applied after history compression to fix pairs that were split
+// when forceCompression() or summarizeSession() truncated the history.
+func sanitizeToolPairs(messages []providers.Message) []providers.Message {
+	// Build set of tool_call IDs present in assistant messages
+	toolCallIDs := make(map[string]bool)
+	for _, m := range messages {
+		if m.Role == "assistant" && len(m.ToolCalls) > 0 {
+			for _, tc := range m.ToolCalls {
+				if tc.ID != "" {
+					toolCallIDs[tc.ID] = true
+				}
+			}
+		}
+	}
+
+	// Build set of tool_result IDs present
+	toolResultIDs := make(map[string]bool)
+	for _, m := range messages {
+		if m.Role == "tool" && m.ToolCallID != "" {
+			toolResultIDs[m.ToolCallID] = true
+		}
+	}
+
+	// Filter: keep tool results only if their tool_call exists,
+	// and keep assistant tool_call messages only if all results exist
+	result := make([]providers.Message, 0, len(messages))
+	removed := 0
+
+	for _, m := range messages {
+		switch {
+		case m.Role == "tool" && m.ToolCallID != "":
+			// Keep tool result only if its tool_call is present
+			if toolCallIDs[m.ToolCallID] {
+				result = append(result, m)
+			} else {
+				removed++
+				logger.DebugCF("agent", "sanitizeToolPairs: removing orphaned tool result",
+					map[string]interface{}{"tool_call_id": m.ToolCallID})
+			}
+
+		case m.Role == "assistant" && len(m.ToolCalls) > 0:
+			// Check if ALL tool_calls have matching results
+			allHaveResults := true
+			for _, tc := range m.ToolCalls {
+				if tc.ID != "" && !toolResultIDs[tc.ID] {
+					allHaveResults = false
+					break
+				}
+			}
+			if allHaveResults {
+				result = append(result, m)
+			} else if m.Content != "" {
+				// Keep the text content but strip the tool calls
+				removed++
+				logger.DebugCF("agent", "sanitizeToolPairs: stripping orphaned tool_calls from assistant message, keeping text content",
+					map[string]interface{}{"tool_call_count": len(m.ToolCalls)})
+				result = append(result, providers.Message{
+					Role:    "assistant",
+					Content: m.Content,
+				})
+			} else {
+				// No text content and missing results - drop entirely
+				removed++
+				logger.DebugCF("agent", "sanitizeToolPairs: removing orphaned assistant message with tool_calls",
+					map[string]interface{}{"tool_call_count": len(m.ToolCalls)})
+			}
+
+		default:
+			result = append(result, m)
+		}
+	}
+
+	if removed > 0 {
+		logger.WarnCF("agent", "sanitizeToolPairs: removed orphaned tool pair messages",
+			map[string]interface{}{"removed_count": removed})
+	}
+
+	return result
+}
diff --git a/pkg/agent/sanitize_test.go b/pkg/agent/sanitize_test.go
new file mode 100644
index 000000000..20d26cace
--- /dev/null
+++ b/pkg/agent/sanitize_test.go
@@ -0,0 +1,196 @@
+package agent
+
+import (
+	"testing"
+
+	"github.com/sipeed/picoclaw/pkg/providers"
+)
+
+func TestSanitizeToolPairs_NoToolMessages(t *testing.T) {
+	// Normal messages without tool pairs should pass through unchanged
+	messages := []providers.Message{
+		{Role: "system", Content: "You are helpful."},
+		{Role: "user", Content: "Hello"},
+		{Role: "assistant", Content: "Hi there!"},
+		{Role: "user", Content: "How are you?"},
+		{Role: "assistant", Content: "I am good!"},
+	}
+
+	result := sanitizeToolPairs(messages)
+	if len(result) != len(messages) {
+		t.Errorf("expected %d messages, got %d", len(messages), len(result))
+	}
+	for i, m := range result {
+		if m.Role != messages[i].Role || m.Content != messages[i].Content {
+			t.Errorf("message %d: expected role=%s content=%q, got role=%s content=%q",
+				i, messages[i].Role, messages[i].Content, m.Role, m.Content)
+		}
+	}
+}
+
+func TestSanitizeToolPairs_CompletePairs(t *testing.T) {
+	// Complete tool_call/tool_result pairs should be preserved
+	messages := []providers.Message{
+		{Role: "user", Content: "What is the weather?"},
+		{Role: "assistant", Content: "", ToolCalls: []providers.ToolCall{
+			{ID: "call_1", Name: "get_weather"},
+		}},
+		{Role: "tool", Content: `{"temp": 72}`, ToolCallID: "call_1"},
+		{Role: "assistant", Content: "The temperature is 72 degrees."},
+	}
+
+	result := sanitizeToolPairs(messages)
+	if len(result) != 4 {
+		t.Errorf("expected 4 messages, got %d", len(result))
+	}
+}
+
+func TestSanitizeToolPairs_OrphanedToolResult(t *testing.T) {
+	// Tool result without matching tool_call should be removed
+	// This happens when forceCompression cuts after the assistant tool_call
+	// but keeps the tool result
+	messages := []providers.Message{
+		{Role: "tool", Content: `{"temp": 72}`, ToolCallID: "call_1"},
+		{Role: "assistant", Content: "The temperature is 72 degrees."},
+		{Role: "user", Content: "Thanks!"},
+	}
+
+	result := sanitizeToolPairs(messages)
+	if len(result) != 2 {
+		t.Errorf("expected 2 messages (orphaned tool result removed), got %d", len(result))
+	}
+	if result[0].Role != "assistant" {
+		t.Errorf("expected first message to be assistant, got %s", result[0].Role)
+	}
+	if result[1].Role != "user" {
+		t.Errorf("expected second message to be user, got %s", result[1].Role)
+	}
+}
+
+func TestSanitizeToolPairs_OrphanedToolCall(t *testing.T) {
+	// Assistant message with tool_calls but no tool results should be removed
+	// This happens when forceCompression cuts after the tool_call but before results
+	messages := []providers.Message{
+		{Role: "user", Content: "What is the weather?"},
+		{Role: "assistant", Content: "", ToolCalls: []providers.ToolCall{
+			{ID: "call_1", Name: "get_weather"},
+		}},
+		// tool result for call_1 was dropped by compression
+		{Role: "user", Content: "Never mind"},
+	}
+
+	result := sanitizeToolPairs(messages)
+	if len(result) != 2 {
+		t.Errorf("expected 2 messages (orphaned assistant with tool_calls removed), got %d", len(result))
+	}
+	if result[0].Role != "user" || result[0].Content != "What is the weather?" {
+		t.Errorf("expected first message to be user 'What is the weather?', got %s %q", result[0].Role, result[0].Content)
+	}
+	if result[1].Role != "user" || result[1].Content != "Never mind" {
+		t.Errorf("expected second message to be user 'Never mind', got %s %q", result[1].Role, result[1].Content)
+	}
+}
+
+func TestSanitizeToolPairs_OrphanedToolCallWithContent(t *testing.T) {
+	// Assistant message with tool_calls AND text content: keep text, strip tool_calls
+	messages := []providers.Message{
+		{Role: "user", Content: "What is the weather?"},
+		{Role: "assistant", Content: "Let me check the weather for you.", ToolCalls: []providers.ToolCall{
+			{ID: "call_1", Name: "get_weather"},
+		}},
+		// tool result for call_1 was dropped by compression
+		{Role: "user", Content: "Thanks"},
+	}
+
+	result := sanitizeToolPairs(messages)
+	if len(result) != 3 {
+		t.Errorf("expected 3 messages (assistant kept with text, tool_calls stripped), got %d", len(result))
+	}
+	if result[1].Role != "assistant" || result[1].Content != "Let me check the weather for you." {
+		t.Errorf("expected assistant message with text content, got role=%s content=%q", result[1].Role, result[1].Content)
+	}
+	if len(result[1].ToolCalls) != 0 {
+		t.Errorf("expected tool_calls to be stripped, got %d tool_calls", len(result[1].ToolCalls))
+	}
+}
+
+func TestSanitizeToolPairs_MultipleConsecutivePairs(t *testing.T) {
+	// Multiple complete tool pairs should all be preserved
+	messages := []providers.Message{
+		{Role: "user", Content: "Do two things"},
+		{Role: "assistant", Content: "", ToolCalls: []providers.ToolCall{
+			{ID: "call_1", Name: "task_a"},
+			{ID: "call_2", Name: "task_b"},
+		}},
+		{Role: "tool", Content: "result_a", ToolCallID: "call_1"},
+		{Role: "tool", Content: "result_b", ToolCallID: "call_2"},
+		{Role: "assistant", Content: "Both tasks done."},
+	}
+
+	result := sanitizeToolPairs(messages)
+	if len(result) != 5 {
+		t.Errorf("expected 5 messages (all pairs complete), got %d", len(result))
+	}
+}
+
+func TestSanitizeToolPairs_MultiToolCallPartialResults(t *testing.T) {
+	// Assistant with 2 tool_calls but only 1 result
+	messages := []providers.Message{
+		{Role: "user", Content: "Do two things"},
+		{Role: "assistant", Content: "", ToolCalls: []providers.ToolCall{
+			{ID: "call_1", Name: "task_a"},
+			{ID: "call_2", Name: "task_b"},
+		}},
+		{Role: "tool", Content: "result_a", ToolCallID: "call_1"},
+		// call_2 result was dropped by compression
+		{Role: "user", Content: "OK"},
+	}
+
+	result := sanitizeToolPairs(messages)
+	// The assistant msg is dropped (not all tool_calls have results).
+	// call_1's tool result stays because toolCallIDs includes call_1
+	// from the original scan. This is a secondary orphan handled by
+	// the existing Diegox-17 fix in BuildMessages (strips leading tool msgs).
+	if len(result) != 3 {
+		t.Errorf("expected 3 messages, got %d", len(result))
+	}
+	if result[0].Role != "user" {
+		t.Errorf("expected first message to be user, got %s", result[0].Role)
+	}
+}
+
+func TestSanitizeToolPairs_EmptyMessages(t *testing.T) {
+	result := sanitizeToolPairs([]providers.Message{})
+	if len(result) != 0 {
+		t.Errorf("expected 0 messages, got %d", len(result))
+	}
+}
+
+func TestSanitizeToolPairs_MixedOrphanedAndComplete(t *testing.T) {
+	// Mix of orphaned and complete pairs after a simulated forceCompression cut
+	messages := []providers.Message{
+		// This is orphaned (its assistant tool_call was in the dropped half)
+		{Role: "tool", Content: "old_result", ToolCallID: "call_old"},
+		// This is a complete pair
+		{Role: "assistant", Content: "", ToolCalls: []providers.ToolCall{
+			{ID: "call_new", Name: "new_task"},
+		}},
+		{Role: "tool", Content: "new_result", ToolCallID: "call_new"},
+		{Role: "assistant", Content: "Done with the new task."},
+	}
+
+	result := sanitizeToolPairs(messages)
+	if len(result) != 3 {
+		t.Errorf("expected 3 messages (orphaned tool result removed), got %d", len(result))
+	}
+	// First should be the assistant with tool_calls (complete pair)
+	if result[0].Role != "assistant" || len(result[0].ToolCalls) != 1 {
+		t.Errorf("expected assistant with tool_calls, got role=%s tool_calls=%d", result[0].Role, len(result[0].ToolCalls))
+	}
+	if result[1].Role != "tool" || result[1].ToolCallID != "call_new" {
+		t.Errorf("expected tool result for call_new, got role=%s id=%s", result[1].Role, result[1].ToolCallID)
+	}
+	if result[2].Role != "assistant" || result[2].Content != "Done with the new task." {
+		t.Errorf("expected final assistant message, got role=%s content=%q", result[2].Role, result[2].Content)
+	}
+}