feat(gateway): per-channel model and system prompt overrides (Fixes #1955)

- config: ChannelOverride + PlatformConfig.channel_overrides

- run: _resolve_model_for_channel, _get_system_prompt_for_channel, channel provider runtime

- tests: channel overrides + config guard for bare runner; conftest asyncio fix; slack/whatsapp warning filters

Made-with: Cursor

tests/gateway/test_channel_overrides.py

@@ -0,0 +1,128 @@
+"""Tests for per-channel model and system prompt overrides (Fixes #1955)."""
+
+import pytest
+
+from gateway.config import (
+    ChannelOverride,
+    GatewayConfig,
+    Platform,
+    PlatformConfig,
+)
+from gateway.run import _get_channel_override, GatewayRunner
+
+
+class TestGetChannelOverride:
+    def test_no_override_when_empty_config(self):
+        config = GatewayConfig()
+        assert _get_channel_override(config, Platform.DISCORD, "123") is None
+
+    def test_no_override_when_platform_not_configured(self):
+        config = GatewayConfig(platforms={})
+        assert _get_channel_override(config, Platform.DISCORD, "123") is None
+
+    def test_no_override_when_channel_not_in_overrides(self):
+        config = GatewayConfig(
+            platforms={
+                Platform.DISCORD: PlatformConfig(
+                    enabled=True,
+                    channel_overrides={
+                        "999": ChannelOverride(model="openrouter/healer-alpha"),
+                    },
+                ),
+            },
+        )
+        assert _get_channel_override(config, Platform.DISCORD, "123") is None
+
+    def test_returns_override_when_channel_matches(self):
+        ov = ChannelOverride(
+            model="openrouter/healer-alpha",
+            provider="openrouter",
+            system_prompt="You are a summarizer.",
+        )
+        config = GatewayConfig(
+            platforms={
+                Platform.DISCORD: PlatformConfig(
+                    enabled=True,
+                    channel_overrides={"1234567890": ov},
+                ),
+            },
+        )
+        result = _get_channel_override(config, Platform.DISCORD, "1234567890")
+        assert result is not None
+        assert result.model == "openrouter/healer-alpha"
+        assert result.provider == "openrouter"
+        assert result.system_prompt == "You are a summarizer."
+
+    def test_returns_override_when_chat_id_is_int_like(self):
+        """Caller may pass str(chat_id); override keys are normalized to str."""
+        config = GatewayConfig(
+            platforms={
+                Platform.DISCORD: PlatformConfig(
+                    enabled=True,
+                    channel_overrides={"123": ChannelOverride(model="gpt-4")},
+                ),
+            },
+        )
+        assert _get_channel_override(config, Platform.DISCORD, "123").model == "gpt-4"
+
+
+class TestResolveModelForChannel:
+    def test_uses_channel_override_when_present(self):
+        config = GatewayConfig(
+            platforms={
+                Platform.DISCORD: PlatformConfig(
+                    enabled=True,
+                    channel_overrides={
+                        "chan_1": ChannelOverride(model="anthropic/claude-opus-4.6"),
+                    },
+                ),
+            },
+        )
+        runner = object.__new__(GatewayRunner)
+        runner.config = config
+        model = runner._resolve_model_for_channel(Platform.DISCORD, "chan_1")
+        assert model == "anthropic/claude-opus-4.6"
+
+    def test_falls_back_to_global_when_no_override(self, monkeypatch):
+        monkeypatch.setattr(
+            "gateway.run._resolve_gateway_model",
+            lambda: "global-model/default",
+        )
+        config = GatewayConfig(
+            platforms={
+                Platform.DISCORD: PlatformConfig(enabled=True, channel_overrides={}),
+            },
+        )
+        runner = object.__new__(GatewayRunner)
+        runner.config = config
+        model = runner._resolve_model_for_channel(Platform.DISCORD, "unknown_channel")
+        assert model == "global-model/default"
+
+
+class TestGetSystemPromptForChannel:
+    def test_uses_channel_override_when_present(self):
+        config = GatewayConfig(
+            platforms={
+                Platform.DISCORD: PlatformConfig(
+                    enabled=True,
+                    channel_overrides={
+                        "chan_1": ChannelOverride(system_prompt="You are a coding assistant."),
+                    },
+                ),
+            },
+        )
+        runner = object.__new__(GatewayRunner)
+        runner.config = config
+        runner._ephemeral_system_prompt = "Global prompt"
+        prompt = runner._get_system_prompt_for_channel(Platform.DISCORD, "chan_1")
+        assert prompt == "You are a coding assistant."
+
+    def test_falls_back_to_global_when_no_override(self):
+        config = GatewayConfig(
+            platforms={Platform.DISCORD: PlatformConfig(enabled=True)},
+        )
+        runner = object.__new__(GatewayRunner)
+        runner.config = config
+        runner._ephemeral_system_prompt = "Global prompt"
+        prompt = runner._get_system_prompt_for_channel(Platform.DISCORD, "other")
+        assert prompt == "Global prompt"

tests/gateway/test_config.py

@@ -5,6 +5,7 @@ import os
 from unittest.mock import patch
 
 from gateway.config import (
+    ChannelOverride,
     GatewayConfig,
     HomeChannel,
     Platform,
@@ -89,6 +90,54 @@ class TestPlatformConfigRoundtrip:
         # extra; from_dict must honor it there too (mirrors _grn fallback).
         restored = PlatformConfig.from_dict({"extra": {"typing_indicator": False}})
         assert restored.typing_indicator is False
+    def test_channel_overrides_roundtrip(self):
+        pc = PlatformConfig(
+            enabled=True,
+            channel_overrides={
+                "1234567890": ChannelOverride(
+                    model="openrouter/healer-alpha",
+                    provider="openrouter",
+                    system_prompt="You are a daily news summarizer.",
+                ),
+                "9876543210": ChannelOverride(
+                    model="anthropic/claude-opus-4.6",
+                    provider="anthropic",
+                    system_prompt="You are a coding assistant.",
+                ),
+            },
+        )
+        d = pc.to_dict()
+        assert "channel_overrides" in d
+        assert d["channel_overrides"]["1234567890"]["model"] == "openrouter/healer-alpha"
+        assert d["channel_overrides"]["9876543210"]["system_prompt"] == "You are a coding assistant."
+        restored = PlatformConfig.from_dict(d)
+        assert restored.channel_overrides["1234567890"].model == "openrouter/healer-alpha"
+        assert restored.channel_overrides["9876543210"].provider == "anthropic"
+
+    def test_channel_overrides_from_dict_normalizes_channel_id_to_str(self):
+        """YAML may have numeric channel IDs; we store as str."""
+        data = {
+            "enabled": True,
+            "channel_overrides": {
+                1234567890: {"model": "openrouter/healer-alpha"},
+            },
+        }
+        pc = PlatformConfig.from_dict(data)
+        assert "1234567890" in pc.channel_overrides
+        assert pc.channel_overrides["1234567890"].model == "openrouter/healer-alpha"
+
+
+class TestChannelOverride:
+    def test_from_dict_empty(self):
+        assert ChannelOverride.from_dict({}).model is None
+        assert ChannelOverride.from_dict(None).model is None
+
+    def test_to_dict_omits_none(self):
+        ov = ChannelOverride(model="gpt-4", provider=None, system_prompt="Hi")
+        d = ov.to_dict()
+        assert d["model"] == "gpt-4"
+        assert "provider" not in d
+        assert d["system_prompt"] == "Hi"
 
 
 class TestGetConnectedPlatforms: