feat(executor): sanitize web search tool domains to meet Anthropic requirements

- Added `sanitizeClaudeWebSearchDomains` to remove empty `allowed_domains` and `blocked_domains` fields for built-in web_search tools, addressing ambiguity errors from Anthropic.
- Integrated domain sanitization into the Claude message preparation pipeline.
- Added test cases to validate correct handling of empty and non-empty domain fields across various tool types.

Closes: #2681

internal/runtime/executor/claude_executor.go

@@ -48,9 +48,38 @@ const claudeToolPrefix = ""
 func sanitizeClaudeMessagesForClaudeUpstreamWithDebug(ctx context.Context, body []byte, baseModel string) []byte {
 	sanitized, report := sigcompat.SanitizeClaudeMessagesForClaudeUpstream(body, baseModel)
 	logClaudeSignatureSanitizeReport(ctx, baseModel, report)
+	sanitized = sanitizeClaudeWebSearchDomains(sanitized)
 	return sanitized
 }
 
+// sanitizeClaudeWebSearchDomains removes empty allowed_domains/blocked_domains
+// arrays from built-in web_search tools. Some clients (e.g. litellm) emit an
+// empty array instead of omitting the field, and Anthropic rejects it with
+// "Empty list of domains is ambiguous. Provide at least one domain or null.".
+// Deleting the key is equivalent to leaving it unset.
+func sanitizeClaudeWebSearchDomains(body []byte) []byte {
+	tools := gjson.GetBytes(body, "tools")
+	if !tools.Exists() || !tools.IsArray() {
+		return body
+	}
+	tools.ForEach(func(index, tool gjson.Result) bool {
+		if !strings.HasPrefix(tool.Get("type").String(), "web_search_") {
+			return true
+		}
+		for _, field := range []string{"allowed_domains", "blocked_domains"} {
+			value := tool.Get(field)
+			if value.Exists() && value.IsArray() && len(value.Array()) == 0 {
+				path := fmt.Sprintf("tools.%d.%s", index.Int(), field)
+				if updated, errDelete := sjson.DeleteBytes(body, path); errDelete == nil {
+					body = updated
+				}
+			}
+		}
+		return true
+	})
+	return body
+}
+
 func logClaudeSignatureSanitizeReport(ctx context.Context, baseModel string, report sigcompat.SignatureSanitizeReport) {
 	if report.DroppedBlocks == 0 && report.DroppedSignatures == 0 && report.ReplacedSignatures == 0 {
 		return

internal/runtime/executor/claude_executor_test.go

@@ -636,6 +636,36 @@ func TestApplyClaudeToolPrefix_WithToolReference(t *testing.T) {
 	}
 }
 
+func TestSanitizeClaudeWebSearchDomains(t *testing.T) {
+	// Mirrors the litellm payload from issue #2681: a non-empty allowed_domains
+	// alongside an empty blocked_domains, which Anthropic rejects as ambiguous.
+	input := []byte(`{"tools":[{"type":"web_search_20250305","name":"web_search","allowed_domains":["anthropic.com"],"blocked_domains":[],"max_uses":8}]}`)
+	out := sanitizeClaudeWebSearchDomains(input)
+
+	if gjson.GetBytes(out, "tools.0.blocked_domains").Exists() {
+		t.Fatalf("empty blocked_domains should be removed: %s", string(out))
+	}
+	if got := gjson.GetBytes(out, "tools.0.allowed_domains").Array(); len(got) != 1 || got[0].String() != "anthropic.com" {
+		t.Fatalf("non-empty allowed_domains should be preserved: %s", string(out))
+	}
+	if got := gjson.GetBytes(out, "tools.0.max_uses").Int(); got != 8 {
+		t.Fatalf("max_uses should be preserved: got %d", got)
+	}
+}
+
+func TestSanitizeClaudeWebSearchDomains_LeavesNonBuiltinAndNonEmpty(t *testing.T) {
+	// Empty arrays on non-web_search tools must be left untouched.
+	input := []byte(`{"tools":[{"type":"custom","name":"x","blocked_domains":[]},{"type":"web_search_20250305","name":"web_search","blocked_domains":["evil.com"]}]}`)
+	out := sanitizeClaudeWebSearchDomains(input)
+
+	if !gjson.GetBytes(out, "tools.0.blocked_domains").Exists() {
+		t.Fatalf("non-web_search tool fields should be untouched: %s", string(out))
+	}
+	if got := gjson.GetBytes(out, "tools.1.blocked_domains").Array(); len(got) != 1 || got[0].String() != "evil.com" {
+		t.Fatalf("non-empty blocked_domains should be preserved: %s", string(out))
+	}
+}
+
 func TestApplyClaudeToolPrefix_SkipsBuiltinTools(t *testing.T) {
 	input := []byte(`{"tools":[{"type":"web_search_20250305","name":"web_search"},{"name":"my_custom_tool","input_schema":{"type":"object"}}]}`)
 	out := applyClaudeToolPrefix(input, "proxy_")