diff --git a/packages/agentenv-protocol/src/agentenv_protocol/client.py b/packages/agentenv-protocol/src/agentenv_protocol/client.py
index a1e352879..64dad1546 100644
--- a/packages/agentenv-protocol/src/agentenv_protocol/client.py
+++ b/packages/agentenv-protocol/src/agentenv_protocol/client.py
@@ -80,7 +80,7 @@ def granted_headers(url: str, grants: dict[str | None, dict[str, str]]) -> dict[
 async def _rpc(base_url: str, method: str, params: dict, timeout: int, verify: bool,
                headers: dict[str, str] | None) -> dict:
     request = {"jsonrpc": "2.0", "id": 1, "method": method, "params": params}
-    url = f"{base_url}{RPC_PATH}"
+    url = f"{base_url.rstrip('/')}{RPC_PATH}"
     async with httpx.AsyncClient(verify=verify) as client:
         response = await client.post(url, json=request, timeout=timeout, headers=request_headers(url, headers))
         response.raise_for_status()
@@ -117,7 +117,7 @@ async def get_data(
 
 async def get_card(base_url: str, timeout: int = 10, verify: bool = True) -> dict:
     async with httpx.AsyncClient(verify=verify) as client:
-        response = await client.get(f"{base_url}{WELL_KNOWN_PATH}", timeout=timeout)
+        response = await client.get(f"{base_url.rstrip('/')}{WELL_KNOWN_PATH}", timeout=timeout)
         response.raise_for_status()
         return response.json()
 
@@ -341,7 +341,10 @@ async def invoke_extension(base_url: str, card: dict, uri: str, params: dict | N
         raise ValueError(f"extension {uri} does not advertise method {method!r} (advertises: {offered})")
     if not op["endpoint"]:
         raise ValueError(f"extension {uri} has no endpoint in params")
-    url = f"{base_url}{op['endpoint']}"
+    endpoint = op["endpoint"]
+    if not endpoint.startswith("/"):
+        endpoint = f"/{endpoint}"
+    url = f"{base_url.rstrip('/')}{endpoint}"
     sent = request_headers(url, headers)
     async with httpx.AsyncClient(verify=verify) as client:
         if op["method"] == "GET":
diff --git a/packages/agentenv-protocol/tests/test_client.py b/packages/agentenv-protocol/tests/test_client.py
index bfc65e9a7..0b804a174 100644
--- a/packages/agentenv-protocol/tests/test_client.py
+++ b/packages/agentenv-protocol/tests/test_client.py
@@ -76,6 +76,20 @@ async def test_get_card_gets_well_known():
     assert client.get.call_args.args[0] == f"{_BASE}/.well-known/agent-env.json"
 
 
+@pytest.mark.asyncio
+async def test_rpc_and_card_strip_trailing_slashes_from_base_url():
+    base_with_slash = f"{_BASE}/"
+    client = _client(_resp({"jsonrpc": "2.0", "id": 1, "result": {}}))
+    with _patch(client):
+        await protocol_v1.reset_data(base_with_slash)
+    assert client.post.call_args.args[0] == f"{_BASE}/agentenv"
+
+    client = _client(_resp({"name": "items"}))
+    with _patch(client):
+        await protocol_v1.get_card(base_with_slash)
+    assert client.get.call_args.args[0] == f"{_BASE}/.well-known/agent-env.json"
+
+
 @pytest.mark.asyncio
 async def test_supports_v1_true_on_card():
     with _patch(_client(_resp({"name": "items"}))):
@@ -270,6 +284,36 @@ async def test_invoke_extension_posts_to_advertised_endpoint():
     assert kwargs["json"] == {"tool_name": "list_items", "error_rate": 0.75}
 
 
+@pytest.mark.asyncio
+async def test_invoke_extension_strips_trailing_slashes_from_base_url():
+    base_with_slash = f"{_BASE}/"
+    client = _client(_resp({"tool_name": "list_items", "error_rate": 0.75}))
+    with _patch(client):
+        await protocol_v1.invoke_extension(
+            base_with_slash, _CARD_SET_ERRORS, "urn:agentenv:set-errors/v1",
+            {"tool_name": "list_items", "error_rate": 0.75},
+        )
+    args, _ = client.request.call_args
+    assert args[1] == f"{_BASE}/agentenv/ext/set_errors"
+
+
+@pytest.mark.asyncio
+async def test_invoke_extension_ensures_leading_slash_on_relative_endpoint():
+    card = {"capabilities": {"extensions": [{
+        "uri": "urn:agentenv:set-errors/v1",
+        "params": {"endpoint": "agentenv/ext/set_errors",
+                   "methods": {"set_errors": {"method": "POST", "request": {}}}},
+    }]}}
+    client = _client(_resp({"tool_name": "list_items", "error_rate": 0.75}))
+    with _patch(client):
+        await protocol_v1.invoke_extension(
+            _BASE, card, "urn:agentenv:set-errors/v1",
+            {"tool_name": "list_items", "error_rate": 0.75},
+        )
+    args, _ = client.request.call_args
+    assert args[1] == f"{_BASE}/agentenv/ext/set_errors"
+
+
 @pytest.mark.asyncio
 async def test_invoke_extension_get_uses_query_params():
     card = {"capabilities": {"extensions": [{
@@ -396,3 +440,10 @@ async def test_invoke_extension_unknown_method_raises_without_calling():
 async def test_invoke_extension_named_method_on_a_methodless_extension_raises():
     with pytest.raises(ValueError, match=re.escape("does not advertise method 'set_time' (advertises: none)")):
         await protocol_v1.invoke_extension(_BASE, _CARD_WITH_EXT, "urn:agentenv:clock/v1", method="set_time")
+
+
+@pytest.mark.asyncio
+async def test_invoke_extension_missing_endpoint_raises_value_error():
+    card = {"capabilities": {"extensions": [{"uri": "urn:agentenv:test/v1", "params": {}}]}}
+    with pytest.raises(ValueError, match="has no endpoint in params"):
+        await protocol_v1.invoke_extension(_BASE, card, "urn:agentenv:test/v1")