diff --git a/tests/test_config.py b/tests/test_config.py
new file mode 100644
index 0000000..3d9b476
--- /dev/null
+++ b/tests/test_config.py
@@ -0,0 +1,93 @@
+"""Tests for configuration loading and merging."""
+
+import copy
+from pathlib import Path
+
+import pytest
+
+from derp.config import DEFAULTS, _merge, load, resolve_config
+
+
+class TestMerge:
+    """Test deep merge logic."""
+
+    def test_flat_override(self):
+        base = {"a": 1, "b": 2}
+        result = _merge(base, {"b": 99})
+        assert result == {"a": 1, "b": 99}
+
+    def test_nested_merge(self):
+        base = {"server": {"host": "a", "port": 1}}
+        result = _merge(base, {"server": {"port": 2}})
+        assert result == {"server": {"host": "a", "port": 2}}
+
+    def test_list_replacement(self):
+        base = {"channels": ["#old"]}
+        result = _merge(base, {"channels": ["#new"]})
+        assert result == {"channels": ["#new"]}
+
+    def test_empty_override(self):
+        base = {"a": 1, "b": 2}
+        result = _merge(base, {})
+        assert result == {"a": 1, "b": 2}
+
+    def test_new_keys(self):
+        base = {"a": 1}
+        result = _merge(base, {"b": 2})
+        assert result == {"a": 1, "b": 2}
+
+    def test_base_not_mutated(self):
+        base = {"server": {"host": "original"}}
+        original = copy.deepcopy(base)
+        _merge(base, {"server": {"host": "changed"}})
+        assert base == original
+
+
+class TestLoad:
+    """Test TOML file loading."""
+
+    def test_minimal_toml(self, tmp_path: Path):
+        config_file = tmp_path / "test.toml"
+        config_file.write_text('[server]\nnick = "testbot"\n')
+        result = load(config_file)
+        # Overridden value
+        assert result["server"]["nick"] == "testbot"
+        # Default preserved
+        assert result["server"]["host"] == "irc.libera.chat"
+        assert result["server"]["port"] == 6697
+
+    def test_full_override(self, tmp_path: Path):
+        config_file = tmp_path / "test.toml"
+        config_file.write_text(
+            '[server]\nhost = "custom.irc"\nport = 6667\n'
+            '[bot]\nprefix = "."\n'
+        )
+        result = load(config_file)
+        assert result["server"]["host"] == "custom.irc"
+        assert result["server"]["port"] == 6667
+        assert result["bot"]["prefix"] == "."
+        # Unset defaults still present
+        assert result["bot"]["channels"] == ["#test"]
+
+
+class TestResolveConfig:
+    """Test config path resolution and fallback."""
+
+    def test_explicit_path(self, tmp_path: Path):
+        config_file = tmp_path / "explicit.toml"
+        config_file.write_text('[server]\nnick = "explicit"\n')
+        result = resolve_config(str(config_file))
+        assert result["server"]["nick"] == "explicit"
+
+    def test_no_file_fallback(self, tmp_path: Path, monkeypatch: "pytest.MonkeyPatch"):
+        # Run from tmp_path so config/derp.toml isn't found
+        monkeypatch.chdir(tmp_path)
+        result = resolve_config(str(tmp_path / "nonexistent.toml"))
+        # Should fall back to DEFAULTS
+        assert result["server"]["host"] == DEFAULTS["server"]["host"]
+        assert result["bot"]["prefix"] == DEFAULTS["bot"]["prefix"]
+
+    def test_defaults_not_mutated(self):
+        original = copy.deepcopy(DEFAULTS)
+        resolve_config(None)
+        assert DEFAULTS == original
diff --git a/tests/test_irc.py b/tests/test_irc.py
index b67ee16..39f4674 100644
--- a/tests/test_irc.py
+++ b/tests/test_irc.py
@@ -128,3 +128,17 @@ class TestFormat:
 
     def test_pong(self):
         assert format_msg("PONG", "server.example.com") == "PONG :server.example.com"
+
+    def test_trailing_starts_with_colon(self):
+        result = format_msg("PRIVMSG", "#ch", ":)")
+        assert result == "PRIVMSG #ch ::)"
+
+    def test_empty_trailing(self):
+        # Empty string has no space and no leading colon, but head exists
+        result = format_msg("PRIVMSG", "#ch", "")
+        assert result == "PRIVMSG #ch "
+
+    def test_multi_param_mode(self):
+        # No space in tail, not starting with colon, head exists -> no colon
+        result = format_msg("MODE", "#ch", "+o", "nick")
+        assert result == "MODE #ch +o nick"
diff --git a/tests/test_plugin.py b/tests/test_plugin.py
index 116e27a..bf4689e 100644
--- a/tests/test_plugin.py
+++ b/tests/test_plugin.py
@@ -1,9 +1,10 @@
 """Tests for the plugin system."""
 
+import asyncio
 import textwrap
 from pathlib import Path
 
-from derp.bot import _AMBIGUOUS, Bot
+from derp.bot import _AMBIGUOUS, Bot, _split_utf8
 from derp.irc import Message
 from derp.plugin import PluginRegistry, command, event
 from derp.state import StateStore
@@ -545,3 +546,110 @@ class TestStateStore:
         store.close()
         store2 = StateStore(db_path)
         assert store2.get("plug", "persist") == "yes"
+
+
+class _FakeConnection:
+    """Minimal IRCConnection stand-in that captures sent lines."""
+
+    def __init__(self):
+        self.sent: list[str] = []
+
+    async def send(self, line: str) -> None:
+        self.sent.append(line)
+
+
+def _make_test_bot() -> Bot:
+    """Create a Bot with a FakeConnection for testing the public API."""
+    config = {
+        "server": {"host": "localhost", "port": 6667, "tls": False,
+                    "nick": "test", "user": "test", "realname": "test"},
+        "bot": {"prefix": "!", "channels": [], "plugins_dir": "plugins"},
+    }
+    bot = Bot(config, PluginRegistry())
+    bot.conn = _FakeConnection()  # type: ignore[assignment]
+    return bot
+
+
+class TestBotAPI:
+    """Test Bot public API methods via FakeConnection."""
+
+    def test_send(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.send("#ch", "hello world"))
+        assert bot.conn.sent == ["PRIVMSG #ch :hello world"]
+
+    def test_send_single_word(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.send("#ch", "hello"))
+        assert bot.conn.sent == ["PRIVMSG #ch hello"]
+
+    def test_send_multiline(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.send("#ch", "line one\nline two"))
+        assert len(bot.conn.sent) == 2
+        assert bot.conn.sent[0] == "PRIVMSG #ch :line one"
+        assert bot.conn.sent[1] == "PRIVMSG #ch :line two"
+
+    def test_reply_channel(self):
+        bot = _make_test_bot()
+        msg = Message(raw="", prefix="nick!u@h", nick="nick",
+                      command="PRIVMSG", params=["#ch", "hi"], tags={})
+        asyncio.run(bot.reply(msg, "hello there"))
+        assert bot.conn.sent == ["PRIVMSG #ch :hello there"]
+
+    def test_reply_pm(self):
+        bot = _make_test_bot()
+        msg = Message(raw="", prefix="nick!u@h", nick="nick",
+                      command="PRIVMSG", params=["testbot", "hi"], tags={})
+        asyncio.run(bot.reply(msg, "hello there"))
+        assert bot.conn.sent == ["PRIVMSG nick :hello there"]
+
+    def test_join(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.join("#test"))
+        assert bot.conn.sent == ["JOIN :#test"]
+
+    def test_kick(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.kick("#ch", "baduser", "bad behavior"))
+        assert bot.conn.sent == ["KICK #ch baduser :bad behavior"]
+
+    def test_mode(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.mode("#ch", "+o", "nick"))
+        assert bot.conn.sent == ["MODE #ch +o nick"]
+
+    def test_set_topic(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.set_topic("#ch", "new topic"))
+        assert bot.conn.sent == ["TOPIC #ch :new topic"]
+
+
+class TestSplitUtf8:
+    """Test UTF-8 safe message splitting."""
+
+    def test_short_text(self):
+        assert _split_utf8("hello", 100) == ["hello"]
+
+    def test_ascii_split(self):
+        text = "a" * 20
+        chunks = _split_utf8(text, 10)
+        assert len(chunks) == 2
+        assert all(len(c.encode("utf-8")) <= 10 for c in chunks)
+        assert "".join(chunks) == text
+
+    def test_multibyte_boundary(self):
+        # Each char is 3 bytes in UTF-8
+        text = "\u00e9" * 10  # e-acute, 2 bytes each -> 20 bytes total
+        chunks = _split_utf8(text, 7)
+        recombined = "".join(chunks)
+        assert recombined == text
+        for chunk in chunks:
+            assert len(chunk.encode("utf-8")) <= 7
+
+    def test_empty(self):
+        assert _split_utf8("", 10) == [""]
+
+    def test_exact_fit(self):
+        text = "abcde"
+        assert _split_utf8(text, 5) == ["abcde"]