diff --git a/BACKEND_PLAN.md b/BACKEND_PLAN.md
index c2d01ce..be8be32 100644
--- a/BACKEND_PLAN.md
+++ b/BACKEND_PLAN.md
@@ -112,8 +112,8 @@ adiuva-api/
   - `UserProfile`: `id: str`, `email: str`, `tier: BillingTier`
 - **Outcome:** All request/response models defined and validated.
 
-### Step 3 — Agent Registry + base classes
-- [ ] `app/core/agent_registry.py`:
+### Step 3 — Agent Registry + base classes ✅
+- [x] `app/core/agent_registry.py`:
   - `BaseAgent(ABC)`:
     - `user_id: str`, `shared_memory: dict`, `vector_store_context: list[str]`, `skills: list[str]`
     - Abstract `get_name() -> str`, `get_description() -> str`
@@ -127,7 +127,7 @@ adiuva-api/
     - `get(name) -> ChatAgent`
     - `list_agents() -> list[dict]` — returns `[{name, description}]` for orchestrator prompt
     - `async call_agent(name, query, context) -> str` — for inter-agent calls
-- [ ] Unit tests: register, get, list, call_agent with mock
+- [x] Unit tests: register, get, list, call_agent with mock
 - **Outcome:** Pluggable agent framework.
 
 ### Step 4 — Orchestrator
diff --git a/app/core/agent_registry.py b/app/core/agent_registry.py
new file mode 100644
index 0000000..1037c14
--- /dev/null
+++ b/app/core/agent_registry.py
@@ -0,0 +1,137 @@
+"""Agent Registry — base classes and singleton registry for chat agents."""
+
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from typing import Any
+
+
+class BaseAgent(ABC):
+    """Common base for all agents."""
+
+    def __init__(
+        self,
+        user_id: str = "",
+        shared_memory: dict[str, Any] | None = None,
+        vector_store_context: list[str] | None = None,
+    ) -> None:
+        self.user_id = user_id
+        self.shared_memory: dict[str, Any] = shared_memory or {}
+        self.vector_store_context: list[str] = vector_store_context or []
+
+    @abstractmethod
+    def get_name(self) -> str: ...
+
+    @abstractmethod
+    def get_description(self) -> str: ...
+
+    @property
+    def skills(self) -> list[str]:
+        """Override in subclasses to advertise capabilities."""
+        return []
+
+
+class ChatAgent(BaseAgent):
+    """Base class for LLM-powered chat agents."""
+
+    @abstractmethod
+    async def handle(self, query: str, context: dict[str, Any]) -> str:
+        """Process a user query and return a text response."""
+        ...
+
+    @abstractmethod
+    def get_tools(self) -> list[Any]:
+        """Return LangChain tool definitions available to this agent."""
+        ...
+
+    async def _tool_loop(
+        self,
+        llm: Any,
+        messages: list[Any],
+        tools: list[Any],
+        max_iter: int = 5,
+    ) -> str:
+        """Shared tool-calling loop.
+
+        Binds *tools* to *llm*, invokes iteratively until the model stops
+        requesting tool calls or *max_iter* is reached, and returns the
+        final text response.
+        """
+        from langchain_core.messages import AIMessage, ToolMessage
+
+        llm_with_tools = llm.bind_tools(tools) if tools else llm
+
+        for _ in range(max_iter):
+            response: AIMessage = await llm_with_tools.ainvoke(messages)
+            messages.append(response)
+
+            if not response.tool_calls:
+                return str(response.content)
+
+            # Execute each requested tool call
+            tool_map = {t.name: t for t in tools}
+            for call in response.tool_calls:
+                tool_fn = tool_map.get(call["name"])
+                if tool_fn is None:
+                    result = f"Unknown tool: {call['name']}"
+                else:
+                    result = await tool_fn.ainvoke(call["args"])
+                messages.append(
+                    ToolMessage(content=str(result), tool_call_id=call["id"])
+                )
+
+        # Exhausted iterations — ask model for a final answer without tools
+        response = await llm.ainvoke(messages)
+        return str(response.content)
+
+
+class AgentRegistry:
+    """Singleton registry for ChatAgent subclasses."""
+
+    _instance: AgentRegistry | None = None
+
+    def __init__(self) -> None:
+        self._agents: dict[str, type[ChatAgent]] = {}
+
+    def __new__(cls) -> AgentRegistry:
+        if cls._instance is None:
+            cls._instance = super().__new__(cls)
+            cls._instance._agents = {}
+        return cls._instance
+
+    # ── public API ───────────────────────────────────────────────────
+
+    def register(self, agent_class: type[ChatAgent]) -> type[ChatAgent]:
+        """Class decorator — registers an agent by its name."""
+        instance = agent_class()
+        name = instance.get_name()
+        self._agents[name] = agent_class
+        return agent_class
+
+    def get(self, name: str) -> ChatAgent:
+        """Return a fresh instance of the named agent."""
+        cls = self._agents.get(name)
+        if cls is None:
+            raise KeyError(f"Agent not found: {name}")
+        return cls()
+
+    def list_agents(self) -> list[dict[str, str]]:
+        """Return ``[{name, description}]`` for the orchestrator prompt."""
+        result: list[dict[str, str]] = []
+        for cls in self._agents.values():
+            inst = cls()
+            result.append(
+                {"name": inst.get_name(), "description": inst.get_description()}
+            )
+        return result
+
+    async def call_agent(
+        self, name: str, query: str, context: dict[str, Any]
+    ) -> str:
+        """Instantiate the named agent and call its ``handle`` method."""
+        agent = self.get(name)
+        return await agent.handle(query, context)
+
+
+# Module-level singleton
+registry = AgentRegistry()
diff --git a/tests/test_agent_registry.py b/tests/test_agent_registry.py
new file mode 100644
index 0000000..9fd9381
--- /dev/null
+++ b/tests/test_agent_registry.py
@@ -0,0 +1,214 @@
+"""Unit tests for the agent registry, base classes, and tool loop."""
+
+from __future__ import annotations
+
+from typing import Any
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from app.core.agent_registry import AgentRegistry, ChatAgent
+
+
+# ── Helpers ──────────────────────────────────────────────────────────
+
+class _StubAgent(ChatAgent):
+    """Minimal concrete agent for testing."""
+
+    def get_name(self) -> str:
+        return "stub"
+
+    def get_description(self) -> str:
+        return "A stub agent for tests"
+
+    def get_tools(self) -> list[Any]:
+        return []
+
+    async def handle(self, query: str, context: dict[str, Any]) -> str:
+        return f"echo: {query}"
+
+
+class _AnotherAgent(ChatAgent):
+    def get_name(self) -> str:
+        return "another"
+
+    def get_description(self) -> str:
+        return "Another stub"
+
+    def get_tools(self) -> list[Any]:
+        return []
+
+    async def handle(self, query: str, context: dict[str, Any]) -> str:
+        return "another"
+
+
+# ── Fixtures ─────────────────────────────────────────────────────────
+
+@pytest.fixture(autouse=True)
+def _fresh_registry():
+    """Reset the singleton between tests."""
+    AgentRegistry._instance = None
+    yield
+    AgentRegistry._instance = None
+
+
+@pytest.fixture()
+def reg() -> AgentRegistry:
+    return AgentRegistry()
+
+
+# ── Tests ────────────────────────────────────────────────────────────
+
+class TestRegisterAndGet:
+    def test_register_decorator(self, reg: AgentRegistry) -> None:
+        reg.register(_StubAgent)
+        agent = reg.get("stub")
+        assert isinstance(agent, _StubAgent)
+
+    def test_get_unknown_raises(self, reg: AgentRegistry) -> None:
+        with pytest.raises(KeyError, match="not found"):
+            reg.get("nonexistent")
+
+    def test_register_multiple(self, reg: AgentRegistry) -> None:
+        reg.register(_StubAgent)
+        reg.register(_AnotherAgent)
+        assert reg.get("stub").get_name() == "stub"
+        assert reg.get("another").get_name() == "another"
+
+
+class TestListAgents:
+    def test_empty(self, reg: AgentRegistry) -> None:
+        assert reg.list_agents() == []
+
+    def test_list_after_register(self, reg: AgentRegistry) -> None:
+        reg.register(_StubAgent)
+        agents = reg.list_agents()
+        assert len(agents) == 1
+        assert agents[0] == {"name": "stub", "description": "A stub agent for tests"}
+
+    def test_list_multiple(self, reg: AgentRegistry) -> None:
+        reg.register(_StubAgent)
+        reg.register(_AnotherAgent)
+        names = {a["name"] for a in reg.list_agents()}
+        assert names == {"stub", "another"}
+
+
+class TestCallAgent:
+    @pytest.mark.asyncio
+    async def test_call_agent(self, reg: AgentRegistry) -> None:
+        reg.register(_StubAgent)
+        result = await reg.call_agent("stub", "hello", {})
+        assert result == "echo: hello"
+
+    @pytest.mark.asyncio
+    async def test_call_unknown_raises(self, reg: AgentRegistry) -> None:
+        with pytest.raises(KeyError):
+            await reg.call_agent("nope", "hi", {})
+
+
+class TestSingleton:
+    def test_singleton_identity(self) -> None:
+        a = AgentRegistry()
+        b = AgentRegistry()
+        assert a is b
+
+
+class TestToolLoop:
+    @pytest.mark.asyncio
+    async def test_no_tool_calls(self) -> None:
+        """When the LLM responds without tool calls, return content directly."""
+        agent = _StubAgent()
+
+        ai_msg = MagicMock()
+        ai_msg.content = "final answer"
+        ai_msg.tool_calls = []
+
+        llm = AsyncMock()
+        llm.bind_tools = MagicMock(return_value=llm)
+        llm.ainvoke = AsyncMock(return_value=ai_msg)
+
+        result = await agent._tool_loop(llm, [], [])
+        assert result == "final answer"
+
+    @pytest.mark.asyncio
+    async def test_tool_call_then_answer(self) -> None:
+        """LLM requests one tool call, gets result, then answers."""
+        agent = _StubAgent()
+
+        # First response: tool call
+        tool_call_msg = MagicMock()
+        tool_call_msg.content = ""
+        tool_call_msg.tool_calls = [
+            {"id": "call_1", "name": "my_tool", "args": {"x": 1}}
+        ]
+
+        # Second response: final answer
+        final_msg = MagicMock()
+        final_msg.content = "done"
+        final_msg.tool_calls = []
+
+        llm = AsyncMock()
+        llm.bind_tools = MagicMock(return_value=llm)
+        llm.ainvoke = AsyncMock(side_effect=[tool_call_msg, final_msg])
+
+        # Mock tool
+        tool = AsyncMock()
+        tool.name = "my_tool"
+        tool.ainvoke = AsyncMock(return_value="tool_result")
+
+        result = await agent._tool_loop(llm, [], [tool])
+        assert result == "done"
+        tool.ainvoke.assert_called_once_with({"x": 1})
+
+    @pytest.mark.asyncio
+    async def test_unknown_tool_handled(self) -> None:
+        """Unknown tool names produce an error message instead of crashing."""
+        agent = _StubAgent()
+
+        tool_call_msg = MagicMock()
+        tool_call_msg.content = ""
+        tool_call_msg.tool_calls = [
+            {"id": "call_1", "name": "missing", "args": {}}
+        ]
+
+        final_msg = MagicMock()
+        final_msg.content = "recovered"
+        final_msg.tool_calls = []
+
+        llm = AsyncMock()
+        llm.bind_tools = MagicMock(return_value=llm)
+        llm.ainvoke = AsyncMock(side_effect=[tool_call_msg, final_msg])
+
+        result = await agent._tool_loop(llm, [], [])
+        assert result == "recovered"
+
+    @pytest.mark.asyncio
+    async def test_max_iter_reached(self) -> None:
+        """When max iterations are exhausted, a final no-tools call is made."""
+        agent = _StubAgent()
+
+        # Every response requests a tool call
+        loop_msg = MagicMock()
+        loop_msg.content = ""
+        loop_msg.tool_calls = [
+            {"id": "call_x", "name": "t", "args": {}}
+        ]
+
+        final_msg = MagicMock()
+        final_msg.content = "gave up"
+        final_msg.tool_calls = []
+
+        tool = AsyncMock()
+        tool.name = "t"
+        tool.ainvoke = AsyncMock(return_value="ok")
+
+        llm_with_tools = AsyncMock()
+        llm_with_tools.ainvoke = AsyncMock(return_value=loop_msg)
+
+        llm = AsyncMock()
+        llm.bind_tools = MagicMock(return_value=llm_with_tools)
+        llm.ainvoke = AsyncMock(return_value=final_msg)
+
+        result = await agent._tool_loop(llm, [], [tool], max_iter=2)
+        assert result == "gave up"
+        assert llm_with_tools.ainvoke.call_count == 2