fix(provider): deduplicate tool results and merge consecutive tool_result blocks for Anthropic API (#1793)

Anthropic API returns 400 when multiple tool_result blocks share the same
tool_use_id, or when consecutive tool results are sent as separate user
messages. This fix:

1. Adds ToolCallID deduplication in sanitizeHistoryForProvider (context.go)
   to drop duplicate tool results before sending to any provider.
2. Merges consecutive tool result messages into a single user message with
   multiple tool_result content blocks in Anthropic's buildRequestBody,
   for both "user" (with ToolCallID) and "tool" role messages.
3. Adds tests for both behaviors.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/agent/context.go

@@ -678,8 +678,21 @@ func sanitizeHistoryForProvider(history []providers.Message) []providers.Message
 	// like DeepSeek that enforce: "An assistant message with 'tool_calls' must
 	// be followed by tool messages responding to each 'tool_call_id'."
 	final := make([]providers.Message, 0, len(sanitized))
+	seenToolCallID := make(map[string]bool)
 	for i := 0; i < len(sanitized); i++ {
 		msg := sanitized[i]
+
+		// Deduplicate tool results by ToolCallID
+		if msg.Role == "tool" && msg.ToolCallID != "" {
+			if seenToolCallID[msg.ToolCallID] {
+				logger.DebugCF("agent", "Dropping duplicate tool result", map[string]any{
+					"tool_call_id": msg.ToolCallID,
+				})
+				continue
+			}
+			seenToolCallID[msg.ToolCallID] = true
+		}
+
 		if msg.Role == "assistant" && len(msg.ToolCalls) > 0 {
 			// Collect expected tool_call IDs
 			expected := make(map[string]bool, len(msg.ToolCalls))

pkg/agent/context_test.go

@@ -188,6 +188,31 @@ func TestSanitizeHistoryForProvider_PlainConversation(t *testing.T) {
 	assertRoles(t, result, "user", "assistant", "user", "assistant")
 }
 
+func TestSanitizeHistoryForProvider_DuplicateToolResults(t *testing.T) {
+	history := []providers.Message{
+		msg("user", "do something"),
+		assistantWithTools("A", "B"),
+		toolResult("A"),
+		toolResult("B"),
+		toolResult("A"), // duplicate
+		toolResult("B"), // duplicate
+		msg("assistant", "done"),
+	}
+
+	result := sanitizeHistoryForProvider(history)
+	if len(result) != 5 {
+		t.Fatalf("expected 5 messages, got %d: %+v", len(result), roles(result))
+	}
+	assertRoles(t, result, "user", "assistant", "tool", "tool", "assistant")
+	// Verify the kept tool results have the correct IDs
+	if result[2].ToolCallID != "A" {
+		t.Errorf("expected tool result A, got %q", result[2].ToolCallID)
+	}
+	if result[3].ToolCallID != "B" {
+		t.Errorf("expected tool result B, got %q", result[3].ToolCallID)
+	}
+}
+
 func roles(msgs []providers.Message) []string {
 	r := make([]string, len(msgs))
 	for i, m := range msgs {

pkg/providers/anthropic_messages/provider.go

@@ -188,17 +188,23 @@ func buildRequestBody(
 
 		case "user":
 			if msg.ToolCallID != "" {
-				// Tool result message
-				content := []map[string]any{
-					{
-						"type":        "tool_result",
-						"tool_use_id": msg.ToolCallID,
-						"content":     msg.Content,
-					},
+				// Tool result message — merge into previous user message if it contains tool_results
+				toolResultBlock := map[string]any{
+					"type":        "tool_result",
+					"tool_use_id": msg.ToolCallID,
+					"content":     msg.Content,
+				}
+				if len(apiMessages) > 0 {
+					if prev, ok := apiMessages[len(apiMessages)-1].(map[string]any); ok && prev["role"] == "user" {
+						if content, ok := prev["content"].([]map[string]any); ok {
+							prev["content"] = append(content, toolResultBlock)
+							continue
+						}
+					}
 				}
 				apiMessages = append(apiMessages, map[string]any{
 					"role":    "user",
-					"content": content,
+					"content": []map[string]any{toolResultBlock},
 				})
 			} else {
 				// Regular user message
@@ -246,17 +252,23 @@ func buildRequestBody(
 			})
 
 		case "tool":
-			// Tool result (alternative format)
-			content := []map[string]any{
-				{
-					"type":        "tool_result",
-					"tool_use_id": msg.ToolCallID,
-					"content":     msg.Content,
-				},
+			// Tool result (alternative format) — merge into previous user message if it contains tool_results
+			toolResultBlock := map[string]any{
+				"type":        "tool_result",
+				"tool_use_id": msg.ToolCallID,
+				"content":     msg.Content,
+			}
+			if len(apiMessages) > 0 {
+				if prev, ok := apiMessages[len(apiMessages)-1].(map[string]any); ok && prev["role"] == "user" {
+					if content, ok := prev["content"].([]map[string]any); ok {
+						prev["content"] = append(content, toolResultBlock)
+						continue
+					}
+				}
 			}
 			apiMessages = append(apiMessages, map[string]any{
 				"role":    "user",
-				"content": content,
+				"content": []map[string]any{toolResultBlock},
 			})
 		}
 	}

pkg/providers/anthropic_messages/provider_test.go

@@ -562,6 +562,96 @@ func TestBuildRequestBodyEdgeCases(t *testing.T) {
 	}
 }
 
+func TestBuildRequestBody_ConsecutiveToolResultsMerged(t *testing.T) {
+	// Consecutive tool results (role "tool") should be merged into a single "user" message
+	messages := []Message{
+		{Role: "user", Content: "Use tools"},
+		{Role: "assistant", Content: "", ToolCalls: []ToolCall{
+			{ID: "t1", Name: "tool_a", Arguments: map[string]any{"x": 1}},
+			{ID: "t2", Name: "tool_b", Arguments: map[string]any{"y": 2}},
+		}},
+		{Role: "tool", ToolCallID: "t1", Content: "result1"},
+		{Role: "tool", ToolCallID: "t2", Content: "result2"},
+	}
+
+	got, err := buildRequestBody(messages, nil, "test-model", map[string]any{"max_tokens": 8192})
+	if err != nil {
+		t.Fatalf("buildRequestBody() error: %v", err)
+	}
+
+	apiMessages, ok := got["messages"].([]any)
+	if !ok {
+		t.Fatalf("messages is not []any")
+	}
+
+	// Expect: user, assistant, user (merged tool results)
+	if len(apiMessages) != 3 {
+		for i, m := range apiMessages {
+			t.Logf("message[%d]: %+v", i, m)
+		}
+		t.Fatalf("expected 3 API messages, got %d", len(apiMessages))
+	}
+
+	// The third message should be a user message with 2 tool_result blocks
+	toolResultMsg, ok := apiMessages[2].(map[string]any)
+	if !ok {
+		t.Fatalf("tool result message is not map[string]any")
+	}
+	if toolResultMsg["role"] != "user" {
+		t.Errorf("expected role 'user', got %v", toolResultMsg["role"])
+	}
+	content, ok := toolResultMsg["content"].([]map[string]any)
+	if !ok {
+		t.Fatalf("content is not []map[string]any: %T", toolResultMsg["content"])
+	}
+	if len(content) != 2 {
+		t.Fatalf("expected 2 tool_result blocks, got %d", len(content))
+	}
+	if content[0]["tool_use_id"] != "t1" {
+		t.Errorf("first tool_result tool_use_id = %v, want t1", content[0]["tool_use_id"])
+	}
+	if content[1]["tool_use_id"] != "t2" {
+		t.Errorf("second tool_result tool_use_id = %v, want t2", content[1]["tool_use_id"])
+	}
+}
+
+func TestBuildRequestBody_UserToolResultsMerged(t *testing.T) {
+	// Consecutive tool results using role "user" with ToolCallID should also be merged
+	messages := []Message{
+		{Role: "user", Content: "Use tools"},
+		{Role: "assistant", Content: "", ToolCalls: []ToolCall{
+			{ID: "t1", Name: "tool_a", Arguments: map[string]any{"x": 1}},
+			{ID: "t2", Name: "tool_b", Arguments: map[string]any{"y": 2}},
+		}},
+		{Role: "user", ToolCallID: "t1", Content: "result1"},
+		{Role: "user", ToolCallID: "t2", Content: "result2"},
+	}
+
+	got, err := buildRequestBody(messages, nil, "test-model", map[string]any{"max_tokens": 8192})
+	if err != nil {
+		t.Fatalf("buildRequestBody() error: %v", err)
+	}
+
+	apiMessages, ok := got["messages"].([]any)
+	if !ok {
+		t.Fatalf("messages is not []any")
+	}
+
+	// Expect: user, assistant, user (merged tool results)
+	if len(apiMessages) != 3 {
+		t.Fatalf("expected 3 API messages, got %d", len(apiMessages))
+	}
+
+	toolResultMsg := apiMessages[2].(map[string]any)
+	content, ok := toolResultMsg["content"].([]map[string]any)
+	if !ok {
+		t.Fatalf("content is not []map[string]any: %T", toolResultMsg["content"])
+	}
+	if len(content) != 2 {
+		t.Fatalf("expected 2 tool_result blocks, got %d", len(content))
+	}
+}
+
 // TestParseResponseBodyEdgeCases tests edge cases for parseResponseBody.
 func TestParseResponseBodyEdgeCases(t *testing.T) {
 	tests := []struct {