diff --git a/molecule_runtime/a2a_client.py b/molecule_runtime/a2a_client.py
index 3b7882c..76fb205 100644
--- a/molecule_runtime/a2a_client.py
+++ b/molecule_runtime/a2a_client.py
@@ -76,10 +76,20 @@ async def send_a2a_message(target_url: str, message: str) -> str:
                     return f"{_A2A_ERROR_PREFIX}{text}"
                 return text
             elif "error" in data:
-                return f"{_A2A_ERROR_PREFIX}{data['error'].get('message', 'unknown')}"
+                err = data["error"]
+                msg = err.get("message") or "unknown"
+                code = err.get("code")
+                if code is not None:
+                    return f"{_A2A_ERROR_PREFIX}[code={code}] {msg}"
+                return f"{_A2A_ERROR_PREFIX}{msg}"
             return str(data)
         except Exception as e:
-            return f"{_A2A_ERROR_PREFIX}{e}"
+            # #51: str(e) is empty for bare TimeoutError(), BrokenPipeError(),
+            # and several httpx transport errors — leaving "[A2A_ERROR] " with
+            # no diagnostic. Fall back to the exception class name so logs
+            # always carry at least one actionable breadcrumb.
+            detail = str(e) or type(e).__name__
+            return f"{_A2A_ERROR_PREFIX}{detail}"
 
 
 async def get_peers() -> list[dict]:
diff --git a/tests/test_a2a_error_observability.py b/tests/test_a2a_error_observability.py
new file mode 100644
index 0000000..3df8104
--- /dev/null
+++ b/tests/test_a2a_error_observability.py
@@ -0,0 +1,86 @@
+"""Regression tests for issue #51.
+
+The `[A2A_ERROR]` prefix from ``send_a2a_message`` must always carry a
+diagnostic suffix. Before #51 an exception whose ``str(e)`` was empty
+(bare ``TimeoutError()``, ``BrokenPipeError()``, several httpx transport
+errors) produced ``"[A2A_ERROR] "`` with a trailing space and zero
+context, masking the real cause of peer-delegation failures.
+
+CI does not install pytest-asyncio — use the local _run helper pattern
+established in test_shared_runtime.py.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import os
+
+# Set WORKSPACE_ID before importing molecule_runtime modules — platform_auth
+# evaluates it at import time and refuses to load otherwise.
+os.environ.setdefault("WORKSPACE_ID", "test-workspace")
+
+from molecule_runtime import a2a_client  # noqa: E402
+from molecule_runtime.a2a_client import _A2A_ERROR_PREFIX  # noqa: E402
+
+
+def _run(coro):
+    """Run an async coroutine synchronously (no pytest-asyncio available)."""
+    loop = asyncio.new_event_loop()
+    try:
+        return loop.run_until_complete(coro)
+    finally:
+        loop.close()
+
+
+class _BareException(Exception):
+    """Exception whose str() is empty — mimics bare TimeoutError()."""
+
+    def __str__(self) -> str:  # noqa: D401
+        return ""
+
+
+class _StubAsyncClient:
+    """Async context manager that raises a supplied exception on .post()."""
+
+    def __init__(self, exc: BaseException) -> None:
+        self._exc = exc
+
+    async def __aenter__(self) -> "_StubAsyncClient":
+        return self
+
+    async def __aexit__(self, *_exc) -> bool:
+        return False
+
+    async def post(self, *_args, **_kwargs):
+        raise self._exc
+
+
+def test_bare_exception_yields_class_name(monkeypatch):
+    """When str(e) is empty the result must still include the exception class."""
+
+    def _factory(*_a, **_kw):
+        return _StubAsyncClient(_BareException())
+
+    monkeypatch.setattr(a2a_client.httpx, "AsyncClient", _factory)
+    monkeypatch.setattr(a2a_client, "PLATFORM_URL", "http://stub")
+    monkeypatch.setattr(a2a_client, "auth_headers", lambda: {})
+
+    result = _run(a2a_client.send_a2a_message("peer-ws-id", "hi"))
+    assert result.startswith(_A2A_ERROR_PREFIX)
+    suffix = result[len(_A2A_ERROR_PREFIX):]
+    assert suffix.strip() != "", f"expected non-empty suffix, got {result!r}"
+    assert "BareException" in suffix
+
+
+def test_exception_with_message_passes_through(monkeypatch):
+    """Regular exception messages are preserved."""
+
+    def _factory(*_a, **_kw):
+        return _StubAsyncClient(RuntimeError("upstream 429"))
+
+    monkeypatch.setattr(a2a_client.httpx, "AsyncClient", _factory)
+    monkeypatch.setattr(a2a_client, "PLATFORM_URL", "http://stub")
+    monkeypatch.setattr(a2a_client, "auth_headers", lambda: {})
+
+    result = _run(a2a_client.send_a2a_message("peer-ws-id", "hi"))
+    assert result == f"{_A2A_ERROR_PREFIX}upstream 429"