diff --git a/src/qodev_apollo_api/client.py b/src/qodev_apollo_api/client.py
index e862cde..6ee6a70 100644
--- a/src/qodev_apollo_api/client.py
+++ b/src/qodev_apollo_api/client.py
@@ -46,6 +46,20 @@
 
 logger = logging.getLogger(__name__)
 
+# Filters accepted by POST /accounts/search (besides page/per_page, which are
+# passed explicitly). Apollo *silently drops* unrecognised keys and returns an
+# unfiltered default page, so we validate against this allowlist and raise
+# instead of returning wrong data. See ``ApolloClient.search_accounts``.
+ACCOUNT_SEARCH_FILTERS = frozenset(
+    {
+        "q_organization_name",
+        "account_stage_ids",
+        "account_label_ids",
+        "sort_by_field",
+        "sort_ascending",
+    }
+)
+
 
 class ApolloClient:
     """Async Apollo.io API client with context manager support."""
@@ -350,11 +364,32 @@ async def search_accounts(
         Args:
             page: Page number (default 1)
             limit: Results per page (default 100, max 100)
-            **filters: Additional filters (q_organization_name, account_stage_ids, etc.)
+            **filters: Search filters. Must be keys Apollo actually supports
+                (see ``ACCOUNT_SEARCH_FILTERS``): ``q_organization_name``,
+                ``account_stage_ids``, ``account_label_ids``, ``sort_by_field``,
+                ``sort_ascending``.
 
         Returns:
             Paginated response with Account items
+
+        Raises:
+            ValueError: If an unrecognised filter key is passed. Apollo silently
+                ignores unknown keys and returns an unfiltered default page (which
+                looks like a real match), so we fail loudly instead of returning the
+                wrong accounts. Note ``query=`` is **not** a valid filter — use
+                ``q_organization_name=`` to search by name.
         """
+        unknown = set(filters) - ACCOUNT_SEARCH_FILTERS
+        if unknown:
+            raise ValueError(
+                "Unknown account search filter(s): "
+                + ", ".join(sorted(unknown))
+                + ". Apollo silently ignores unrecognised keys and returns an unfiltered "
+                "default page. Supported filters: "
+                + ", ".join(sorted(ACCOUNT_SEARCH_FILTERS))
+                + " (to search by name use q_organization_name=)."
+            )
+
         data = {"page": page, "per_page": min(limit, 100), **filters}
         result = await self._post("/accounts/search", data)
 
diff --git a/tests/test_client.py b/tests/test_client.py
index 6406a78..1740718 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -251,6 +251,45 @@ async def test_search_accounts(client: ApolloClient):
     assert result.items[0].name == "Acme Corp"
 
 
+async def test_search_accounts_rejects_unknown_filter(client: ApolloClient):
+    """An unrecognised filter raises instead of returning a wrong default page.
+
+    Regression: Apollo silently drops unknown keys (e.g. ``query=``) and returns
+    an unfiltered default list that looks like a real match.
+    """
+    with pytest.raises(ValueError, match="Unknown account search filter"):
+        await client.search_accounts(query="Red and Bundle")
+
+    # The bad request must never reach Apollo.
+    client._client.request.assert_not_called()
+
+
+async def test_search_accounts_error_names_the_bad_key_and_suggests_fix(client: ApolloClient):
+    """The message names the offending key and points to q_organization_name."""
+    with pytest.raises(ValueError, match=r"query.*q_organization_name"):
+        await client.search_accounts(query="x")
+
+
+async def test_search_accounts_allows_documented_filters(client: ApolloClient):
+    """All allowlisted keys pass validation and reach the request body."""
+    client._client.request.return_value = _make_response(
+        {"accounts": [], "pagination": {"total_entries": 0}}
+    )
+
+    await client.search_accounts(
+        q_organization_name="Acme",
+        account_stage_ids=["s1"],
+        account_label_ids=["l1"],
+        sort_by_field="account_created_at",
+        sort_ascending=True,
+    )
+
+    body = client._client.request.call_args[1]["json"]
+    assert body["q_organization_name"] == "Acme"
+    assert body["account_stage_ids"] == ["s1"]
+    assert body["sort_by_field"] == "account_created_at"
+
+
 async def test_search_deals(client: ApolloClient):
     """Test POST /opportunities/search returns PaginatedResponse[Deal]."""
     client._client.request.return_value = _make_response(