diff --git a/agent_builder/tools.py b/agent_builder/tools.py
index 4fd2681..a7f4a5b 100644
--- a/agent_builder/tools.py
+++ b/agent_builder/tools.py
@@ -202,11 +202,13 @@ def build_tools(ctx: ToolContext) -> list[Any]:
         # Read the entrypoint from a2a.yaml so we can pass it verbatim.
         entrypoint = _read_entrypoint(bundle_bytes) or f"agent:{_class_name(name)}"
 
-        files = {"tarball": (f"{name}.tar.gz", bundle_bytes, "application/gzip")}
+        # NOTE: the CP from-tarball endpoint names the file field
+        # ``source`` (see apps/control-plane/control_plane/routes/agents.py).
+        files = {"source": (f"{name}.tar.gz", bundle_bytes, "application/gzip")}
         data = {
             "name": name, "version": version,
             "entrypoint": entrypoint, "public": "true" if public else "false",
-            "description": f"Built by agent-builder.",
+            "description": "Built by agent-builder.",
         }
         try:
             async with httpx.AsyncClient(timeout=60.0) as c: