fix(slack): instruct gateway runtimes to reply over acp

Author: osolmaz

docs/2026-04-26-channel-delivery-feedback-architecture.md

@@ -256,6 +256,19 @@ protocol. In gateway-routed mode, it should not be given direct provider-channel
 send tools for Slack, Discord, Teams, or any future chat provider where Spritz
 owns delivery.
 
+Gateway prompts should carry the same boundary as an explicit runtime
+instruction:
+
+```text
+Spritz channel gateway will deliver your visible reply. Reply by returning
+normal assistant text over ACP. Do not call Slack, Discord, Teams, or other
+provider-channel send tools.
+```
+
+This instruction is a guardrail, not the only enforcement layer. Runtime config
+must still hide or disable direct provider-channel send tools in gateway-routed
+deployments.
+
 Spritz maps the typed runtime outcome to provider delivery behavior:
 
 ```json

integrations/slack-gateway/gateway_test.go

@@ -2911,6 +2911,9 @@ func TestSlackEventRoutesToConversationAndReplies(t *testing.T) {
 			if !strings.Contains(text, "\"actor_user_id\":\"U_1\"") {
 				t.Fatalf("expected actor metadata in prompt text, got %q", text)
 			}
+			if !strings.Contains(text, "Reply by returning normal assistant text over ACP") {
+				t.Fatalf("expected ACP reply instruction in prompt text, got %q", text)
+			}
 			if !strings.HasSuffix(text, "\n\nhello") {
 				t.Fatalf("expected normalized prompt body after metadata block, got %q", text)
 			}
@@ -3752,6 +3755,9 @@ func TestBuildSlackPromptTextPrependsTrustedContext(t *testing.T) {
 	if payload["direct_message"] != false {
 		t.Fatalf("expected non-DM metadata, got %#v", payload["direct_message"])
 	}
+	if !strings.Contains(prompt, "Reply by returning normal assistant text over ACP") {
+		t.Fatalf("expected ACP reply instruction in prompt, got %q", prompt)
+	}
 	if !strings.HasSuffix(prompt, "\n\ncreate a zeno for me") {
 		t.Fatalf("expected normalized user text after metadata block, got %q", prompt)
 	}

integrations/slack-gateway/slack_events.go

@@ -18,9 +18,10 @@ import (
 )
 
 const (
-	slackRecoveryStatusText  = "Still waking up. I will continue here shortly."
-	slackRecoveryFailureText = "I could not recover the channel runtime. Please try again."
-	slackAckReactionTimeout  = 2 * time.Second
+	slackRecoveryStatusText         = "Still waking up. I will continue here shortly."
+	slackRecoveryFailureText        = "I could not recover the channel runtime. Please try again."
+	slackAckReactionTimeout         = 2 * time.Second
+	slackGatewayACPReplyInstruction = "Spritz channel gateway will deliver your visible reply. Reply by returning normal assistant text over ACP. Do not call Slack, Discord, Teams, or other provider-channel send tools."
 )
 
 var slackMentionTokenPattern = regexp.MustCompile(`<@[^>]+>`)
@@ -1094,7 +1095,9 @@ func buildSlackPromptTextWithSynthetic(teamID string, event slackEventInner, bot
 	if err != nil {
 		return normalized
 	}
-	return "<spritz-channel-context>" + string(payload) + "</spritz-channel-context>\n\n" + normalized
+	return "<spritz-channel-context>" + string(payload) + "</spritz-channel-context>\n\n" +
+		"<spritz-channel-delivery-instructions>" + slackGatewayACPReplyInstruction + "</spritz-channel-delivery-instructions>\n\n" +
+		normalized
 }
 
 func slackReplyThreadTS(event slackEventInner) string {