diff --git a/tests/test_client.py b/tests/test_client.py
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+++ b/tests/test_client.py
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+"""
+tests/test_client.py
+
+Tests for src/starter/client.py (MCPClient).
+
+The file has two layers, because they prove different things:
+
+1. Unit tests with a fake ClientSession (class ``TestClientWithFakeSession``).
+   These check the logic MCPClient itself owns: turning SDK response objects
+   into plain dicts/strings, handling empty results, and refusing to work
+   when not connected. No subprocess is started, so they are instant and
+   cannot be broken by anything outside client.py.
+
+2. A real round trip over stdio (class ``TestRealRoundTrip``).
+   These launch ``python -m starter.server`` as a subprocess and talk to it
+   through MCPClient. This is the only layer that proves the client and
+   server actually agree on the protocol. The server is local and
+   deterministic, so no network or external service is involved.
+
+Covers the five behaviours listed in issue B01:
+  1. connect / initialise            4. a valid calculation returns the result
+  2. discover the calculate tool     5. read a registered resource
+  3. invoke the calculate tool
+"""
+
+from __future__ import annotations
+
+import asyncio
+from types import SimpleNamespace
+from typing import Any
+
+import pytest
+
+from starter.client import MCPClient
+
+# A hung subprocess would otherwise stall the whole test run silently.
+ROUND_TRIP_TIMEOUT_SECONDS = 30
+
+
+# ──────────────────────────────────────────────────────────────────────────────
+# Fake session — stands in for mcp.ClientSession, returns canned SDK-shaped data
+# ──────────────────────────────────────────────────────────────────────────────
+class FakeSession:
+    """Mimics the few ClientSession methods that MCPClient calls.
+
+    Each method returns a SimpleNamespace shaped like the real SDK response
+    (e.g. ``response.tools``), so MCPClient's attribute access works unchanged.
+    """
+
+    def __init__(
+        self,
+        *,
+        tools: list[Any] | None = None,
+        resources: list[Any] | None = None,
+        call_content: list[Any] | None = None,
+        read_contents: list[Any] | None = None,
+    ) -> None:
+        self._tools = tools or []
+        self._resources = resources or []
+        self._call_content = call_content if call_content is not None else []
+        self._read_contents = read_contents if read_contents is not None else []
+        self.calls: list[tuple[str, tuple[Any, ...]]] = []
+
+    async def list_tools(self) -> SimpleNamespace:
+        self.calls.append(("list_tools", ()))
+        return SimpleNamespace(tools=self._tools)
+
+    async def call_tool(self, name: str, arguments: dict[str, Any]) -> SimpleNamespace:
+        self.calls.append(("call_tool", (name, arguments)))
+        return SimpleNamespace(content=self._call_content)
+
+    async def list_resources(self) -> SimpleNamespace:
+        self.calls.append(("list_resources", ()))
+        return SimpleNamespace(resources=self._resources)
+
+    async def read_resource(self, uri: Any) -> SimpleNamespace:
+        self.calls.append(("read_resource", (str(uri),)))
+        return SimpleNamespace(contents=self._read_contents)
+
+
+def client_with(session: FakeSession) -> MCPClient:
+    """Build an MCPClient that believes it is connected to *session*."""
+    client = MCPClient()
+    client._session = session  # type: ignore[assignment]
+    return client
+
+
+def text_block(text: str) -> SimpleNamespace:
+    return SimpleNamespace(type="text", text=text)
+
+
+# ──────────────────────────────────────────────────────────────────────────────
+# Layer 1 — unit tests, no subprocess
+# ──────────────────────────────────────────────────────────────────────────────
+class TestNotConnected:
+    """Using the client before connect() must fail loudly and clearly."""
+
+    @pytest.mark.parametrize(
+        ("method", "args"),
+        [
+            ("list_tools", ()),
+            ("call_tool", ("calculate", {"expression": "1 + 1"})),
+            ("list_resources", ()),
+            ("read_resource", ("workshop://introduction",)),
+        ],
+    )
+    async def test_method_raises_runtime_error(self, method, args):
+        client = MCPClient()
+        with pytest.raises(RuntimeError, match="not connected"):
+            await getattr(client, method)(*args)
+
+    async def test_disconnect_without_connect_is_a_noop(self):
+        client = MCPClient()
+        await client.disconnect()  # must not raise
+        assert client._session is None
+
+
+class TestClientWithFakeSession:
+    async def test_list_tools_returns_plain_dicts(self):
+        session = FakeSession(
+            tools=[
+                SimpleNamespace(
+                    name="calculate",
+                    description="Evaluate arithmetic.",
+                    inputSchema={"type": "object", "required": ["expression"]},
+                )
+            ]
+        )
+        tools = await client_with(session).list_tools()
+
+        assert tools == [
+            {
+                "name": "calculate",
+                "description": "Evaluate arithmetic.",
+                "inputSchema": {"type": "object", "required": ["expression"]},
+            }
+        ]
+
+    async def test_list_tools_empty_server(self):
+        assert await client_with(FakeSession()).list_tools() == []
+
+    async def test_call_tool_forwards_name_and_arguments(self):
+        session = FakeSession(call_content=[text_block("5")])
+        result = await client_with(session).call_tool(
+            "calculate", {"expression": "2 + 3"}
+        )
+
+        assert result == "5"
+        assert session.calls == [("call_tool", ("calculate", {"expression": "2 + 3"}))]
+
+    async def test_call_tool_returns_only_the_first_block(self):
+        session = FakeSession(call_content=[text_block("first"), text_block("second")])
+        assert await client_with(session).call_tool("calculate", {}) == "first"
+
+    async def test_call_tool_with_no_content_returns_empty_string(self):
+        session = FakeSession(call_content=[])
+        assert await client_with(session).call_tool("calculate", {}) == ""
+
+    async def test_call_tool_falls_back_to_str_for_non_text_block(self):
+        # e.g. an image block has no .text attribute.
+        block = SimpleNamespace(type="image", data="abc")
+        session = FakeSession(call_content=[block])
+        result = await client_with(session).call_tool("calculate", {})
+        assert result == str(block)
+
+    async def test_list_resources_returns_plain_dicts(self):
+        session = FakeSession(
+            resources=[
+                SimpleNamespace(
+                    uri="workshop://introduction",
+                    name="Workshop Introduction",
+                    description="A brief introduction.",
+                    mimeType="text/plain",
+                )
+            ]
+        )
+        resources = await client_with(session).list_resources()
+
+        assert resources == [
+            {
+                "uri": "workshop://introduction",
+                "name": "Workshop Introduction",
+                "description": "A brief introduction.",
+                "mimeType": "text/plain",
+            }
+        ]
+
+    async def test_list_resources_applies_defaults_for_missing_fields(self):
+        session = FakeSession(
+            resources=[
+                SimpleNamespace(
+                    uri="workshop://bare", name=None, description=None, mimeType=None
+                )
+            ]
+        )
+        (resource,) = await client_with(session).list_resources()
+
+        assert resource["name"] == ""
+        assert resource["description"] == ""
+        assert resource["mimeType"] == "text/plain"
+
+    async def test_read_resource_returns_text_and_passes_uri(self):
+        session = FakeSession(read_contents=[SimpleNamespace(text="Welcome!")])
+        text = await client_with(session).read_resource("workshop://introduction")
+
+        assert text == "Welcome!"
+        assert session.calls == [("read_resource", ("workshop://introduction",))]
+
+    async def test_read_resource_with_no_contents_returns_empty_string(self):
+        session = FakeSession(read_contents=[])
+        assert await client_with(session).read_resource("workshop://x") == ""
+
+
+# ──────────────────────────────────────────────────────────────────────────────
+# Layer 2 — real round trip: MCPClient <-> stdio <-> starter.server
+# ──────────────────────────────────────────────────────────────────────────────
+class TestRealRoundTrip:
+    """Each test opens its own connection (about half a second each).
+
+    The client is entered with ``async with`` inside the test body, not in a
+    fixture: the SDK's stdio transport uses task groups that must be entered
+    and exited from the same task, which fixture setup/teardown does not
+    guarantee.
+    """
+
+    async def test_connects_and_exposes_a_session(self):
+        async with asyncio.timeout(ROUND_TRIP_TIMEOUT_SECONDS):
+            async with MCPClient() as client:
+                assert client._session is not None
+
+    async def test_session_is_closed_after_leaving_context(self):
+        async with asyncio.timeout(ROUND_TRIP_TIMEOUT_SECONDS):
+            async with MCPClient() as client:
+                pass
+            assert client._session is None
+            with pytest.raises(RuntimeError, match="not connected"):
+                await client.list_tools()
+
+    async def test_discovers_the_calculate_tool(self):
+        async with asyncio.timeout(ROUND_TRIP_TIMEOUT_SECONDS):
+            async with MCPClient() as client:
+                tools = await client.list_tools()
+
+        by_name = {tool["name"]: tool for tool in tools}
+        assert "calculate" in by_name
+        schema = by_name["calculate"]["inputSchema"]
+        assert "expression" in schema["properties"]
+        assert "expression" in schema["required"]
+
+    @pytest.mark.parametrize(
+        ("expression", "expected"),
+        [("2 + 3", "5"), ("10 * 5", "50"), ("(10 + 5) / 3", "5")],
+    )
+    async def test_valid_calculation_returns_expected_result(
+        self, expression, expected
+    ):
+        async with asyncio.timeout(ROUND_TRIP_TIMEOUT_SECONDS):
+            async with MCPClient() as client:
+                result = await client.call_tool("calculate", {"expression": expression})
+
+        assert result == expected
+
+    async def test_calculator_error_comes_back_as_text_not_exception(self):
+        # server.py catches ValueError and returns "Error: ..." as the result,
+        # so the client sees a normal string rather than a raised exception.
+        async with asyncio.timeout(ROUND_TRIP_TIMEOUT_SECONDS):
+            async with MCPClient() as client:
+                result = await client.call_tool("calculate", {"expression": "1 / 0"})
+
+        assert result.startswith("Error:")
+        assert "zero" in result.lower()
+
+    async def test_discovers_registered_resources(self):
+        async with asyncio.timeout(ROUND_TRIP_TIMEOUT_SECONDS):
+            async with MCPClient() as client:
+                resources = await client.list_resources()
+
+        uris = {resource["uri"] for resource in resources}
+        assert {"workshop://introduction", "workshop://architecture"} <= uris
+
+    async def test_reads_a_registered_resource(self):
+        from starter.resources import read_resource
+
+        async with asyncio.timeout(ROUND_TRIP_TIMEOUT_SECONDS):
+            async with MCPClient() as client:
+                text = await client.read_resource("workshop://introduction")
+
+        # Compare with the source of truth instead of hard-coding the prose,
+        # so editing the resource text does not break this test.
+        assert text == read_resource("workshop://introduction")
+        assert text.strip() != ""