diff --git a/src/archastro/platform/runtime/http_client.py b/src/archastro/platform/runtime/http_client.py
index 2ba290d..27c52cb 100644
--- a/src/archastro/platform/runtime/http_client.py
+++ b/src/archastro/platform/runtime/http_client.py
@@ -18,6 +18,27 @@
 T = TypeVar("T")
 
 
+def _encode_query(query: dict[str, Any] | None) -> dict[str, Any] | None:
+    """Drop unset parameters and encode list values as ``key[]=item``.
+
+    httpx writes a list under a bare repeated key (``source=a&source=b``),
+    and Plug's query parser keeps only the last value — the server would
+    silently filter on one element of a multi-value filter. The bracket
+    suffix parses as a list and matches the TypeScript SDK's encoding.
+    """
+    if not query:
+        return None
+    params: dict[str, Any] = {}
+    for key, value in query.items():
+        if value is None:
+            continue
+        if isinstance(value, (list, tuple)):
+            params[f"{key}[]"] = [str(item) for item in value]
+        else:
+            params[key] = value
+    return params or None
+
+
 @cache
 def _type_adapter(tp: Any) -> TypeAdapter[Any]:
     """Cache adapters per response type; TypeAdapter construction is costly."""
@@ -101,9 +122,7 @@ async def _do_fetch(
         if headers:
             req_headers.update(headers)
 
-        params = None
-        if query:
-            params = {k: v for k, v in query.items() if v is not None}
+        params = _encode_query(query)
 
         return await self._client.request(
             method,
@@ -278,7 +297,7 @@ async def stream_sse(
             req_headers["Authorization"] = f"Bearer {token}"
         if headers:
             req_headers.update(headers)
-        params = {k: v for k, v in (query or {}).items() if v is not None} or None
+        params = _encode_query(query)
 
         async with self._client.stream(
             method,
@@ -378,9 +397,7 @@ def _do_fetch(
         if headers:
             req_headers.update(headers)
 
-        params = None
-        if query:
-            params = {k: v for k, v in query.items() if v is not None}
+        params = _encode_query(query)
 
         return self._client.request(
             method,
@@ -537,7 +554,7 @@ def stream_sse_sync(
             req_headers["Authorization"] = f"Bearer {token}"
         if headers:
             req_headers.update(headers)
-        params = {k: v for k, v in (query or {}).items() if v is not None} or None
+        params = _encode_query(query)
 
         with self._client.stream(
             method,
diff --git a/tests/test_http_client.py b/tests/test_http_client.py
index 602898b..ee6183d 100644
--- a/tests/test_http_client.py
+++ b/tests/test_http_client.py
@@ -856,3 +856,90 @@ def test_stream_sse_sync_raises_apierror_on_non_2xx():
     with patch.object(client._client, "stream", return_value=_FakeSyncStream(resp)):
         with pytest.raises(ApiError):
             list(client.stream_sse_sync("/api/v1/x/stream", method="POST", body={}))
+
+
+# --- query encoding contract -------------------------------------------------
+#
+# The platform reads query parameters through Plug, whose parser keeps only the
+# last value of a repeated bare key. A multi-value filter sent as
+# `source=a&source=b` therefore silently narrows to one value server-side. These
+# tests pin the wire bytes rather than the dict handed to httpx, because the
+# defect only becomes visible after httpx encodes.
+
+
+async def test_async_list_query_params_encode_as_bracket_suffixed_repeats():
+    seen: list[httpx.Request] = []
+
+    async def handler(request: httpx.Request) -> httpx.Response:
+        seen.append(request)
+        return httpx.Response(200, json={})
+
+    client = HttpClient(base_url="https://api.test")
+    client._client = httpx.AsyncClient(transport=httpx.MockTransport(handler))
+
+    await client.request(
+        "/api/v1/knowledge_documents",
+        query={"source": ["cso_a", "cso_b", "cso_c"], "page_size": 100},
+    )
+
+    assert seen[0].url.query == (
+        b"source%5B%5D=cso_a&source%5B%5D=cso_b&source%5B%5D=cso_c&page_size=100"
+    )
+    # Every element survives the round trip as a distinct value under one key.
+    assert seen[0].url.params.get_list("source[]") == ["cso_a", "cso_b", "cso_c"]
+
+
+async def test_async_scalar_query_params_are_unchanged_and_none_is_dropped():
+    seen: list[httpx.Request] = []
+
+    async def handler(request: httpx.Request) -> httpx.Response:
+        seen.append(request)
+        return httpx.Response(200, json={})
+
+    client = HttpClient(base_url="https://api.test")
+    client._client = httpx.AsyncClient(transport=httpx.MockTransport(handler))
+
+    await client.request(
+        "/api/v1/knowledge_documents",
+        query={"q": "notes", "page_size": 25, "agent": None},
+    )
+
+    assert seen[0].url.query == b"q=notes&page_size=25"
+
+
+def test_sync_list_query_params_encode_as_bracket_suffixed_repeats():
+    seen: list[httpx.Request] = []
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        seen.append(request)
+        return httpx.Response(200, json={})
+
+    client = SyncHttpClient(base_url="https://api.test")
+    client._client = httpx.Client(transport=httpx.MockTransport(handler))
+
+    client.request(
+        "/api/v1/knowledge_documents",
+        query={"source": ["cso_a", "cso_b", "cso_c"], "page_size": 100},
+    )
+
+    assert seen[0].url.query == (
+        b"source%5B%5D=cso_a&source%5B%5D=cso_b&source%5B%5D=cso_c&page_size=100"
+    )
+
+
+def test_sync_scalar_query_params_are_unchanged_and_none_is_dropped():
+    seen: list[httpx.Request] = []
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        seen.append(request)
+        return httpx.Response(200, json={})
+
+    client = SyncHttpClient(base_url="https://api.test")
+    client._client = httpx.Client(transport=httpx.MockTransport(handler))
+
+    client.request(
+        "/api/v1/knowledge_documents",
+        query={"q": "notes", "page_size": 25, "agent": None},
+    )
+
+    assert seen[0].url.query == b"q=notes&page_size=25"