diff --git a/gateway/display_config.py b/gateway/display_config.py
index 78e8bc9afa..832f5cb2f2 100644
--- a/gateway/display_config.py
+++ b/gateway/display_config.py
@@ -79,7 +79,9 @@ _PLATFORM_DEFAULTS: dict[str, dict[str, Any]] = {
     "discord":     _TIER_HIGH,
 
     # Tier 2 — edit support, often customer/workspace channels
-    "slack":           _TIER_MEDIUM,
+    # Slack: tool_progress off by default — Bolt posts cannot be edited like CLI;
+    # "new"/"all" spam permanent lines in channels (hermes-agent#14663).
+    "slack":           {**_TIER_MEDIUM, "tool_progress": "off"},
     "mattermost":      _TIER_MEDIUM,
     "matrix":          _TIER_MEDIUM,
     "feishu":          _TIER_MEDIUM,
diff --git a/tests/gateway/test_display_config.py b/tests/gateway/test_display_config.py
index 2192d67bc9..07d5c82a5f 100644
--- a/tests/gateway/test_display_config.py
+++ b/tests/gateway/test_display_config.py
@@ -186,12 +186,18 @@ class TestPlatformDefaults:
             assert resolve_display_setting({}, plat, "tool_progress") == "all", plat
 
     def test_medium_tier_platforms(self):
-        """Slack, Mattermost, Matrix default to 'new' tool progress."""
+        """Mattermost, Matrix, Feishu, WhatsApp default to 'new' tool progress."""
         from gateway.display_config import resolve_display_setting
 
-        for plat in ("slack", "mattermost", "matrix", "feishu", "whatsapp"):
+        for plat in ("mattermost", "matrix", "feishu", "whatsapp"):
             assert resolve_display_setting({}, plat, "tool_progress") == "new", plat
 
+    def test_slack_defaults_tool_progress_off(self):
+        """Slack defaults to quiet tool progress (permanent chat noise otherwise)."""
+        from gateway.display_config import resolve_display_setting
+
+        assert resolve_display_setting({}, "slack", "tool_progress") == "off"
+
     def test_low_tier_platforms(self):
         """Signal, BlueBubbles, etc. default to 'off' tool progress."""
         from gateway.display_config import resolve_display_setting
@@ -241,7 +247,7 @@ class TestConfigMigration:
                 },
             },
         }
-        config_path.write_text(yaml.dump(config))
+        config_path.write_text(yaml.dump(config), encoding="utf-8")
 
         monkeypatch.setenv("HERMES_HOME", str(tmp_path))
         # Re-import to pick up the new HERMES_HOME
@@ -251,7 +257,7 @@ class TestConfigMigration:
 
         result = cfg_mod.migrate_config(interactive=False, quiet=True)
         # Re-read config
-        updated = yaml.safe_load(config_path.read_text())
+        updated = yaml.safe_load(config_path.read_text(encoding="utf-8"))
         platforms = updated.get("display", {}).get("platforms", {})
         assert platforms.get("signal", {}).get("tool_progress") == "off"
         assert platforms.get("telegram", {}).get("tool_progress") == "all"
@@ -268,7 +274,7 @@ class TestConfigMigration:
                 "platforms": {"telegram": {"tool_progress": "verbose"}},
             },
         }
-        config_path.write_text(yaml.dump(config))
+        config_path.write_text(yaml.dump(config), encoding="utf-8")
 
         monkeypatch.setenv("HERMES_HOME", str(tmp_path))
         import importlib
@@ -276,7 +282,7 @@ class TestConfigMigration:
         importlib.reload(cfg_mod)
 
         cfg_mod.migrate_config(interactive=False, quiet=True)
-        updated = yaml.safe_load(config_path.read_text())
+        updated = yaml.safe_load(config_path.read_text(encoding="utf-8"))
         # Existing "verbose" should NOT be overwritten by legacy "off"
         assert updated["display"]["platforms"]["telegram"]["tool_progress"] == "verbose"
 
diff --git a/tests/gateway/test_verbose_command.py b/tests/gateway/test_verbose_command.py
index c34167b2e4..c3743e5915 100644
--- a/tests/gateway/test_verbose_command.py
+++ b/tests/gateway/test_verbose_command.py
@@ -134,7 +134,7 @@ class TestVerboseCommand:
         """Cycling /verbose on Telegram doesn't change Slack's setting.
 
         Without a global tool_progress, each platform uses its built-in
-        default: Telegram = 'all' (high tier), Slack = 'new' (medium tier).
+        default: Telegram = 'all' (high tier), Slack = 'off' (quiet Slack default).
         """
         hermes_home = tmp_path / "hermes"
         hermes_home.mkdir()
@@ -161,8 +161,8 @@ class TestVerboseCommand:
         platforms = saved["display"]["platforms"]
         # Telegram: all -> verbose (high tier default = all)
         assert platforms["telegram"]["tool_progress"] == "verbose"
-        # Slack: new -> all (medium tier default = new, cycle to all)
-        assert platforms["slack"]["tool_progress"] == "all"
+        # Slack: off -> new (first /verbose cycle from quiet default)
+        assert platforms["slack"]["tool_progress"] == "new"
 
     @pytest.mark.asyncio
     async def test_no_config_file_returns_disabled(self, tmp_path, monkeypatch):