@@ -44,6 +44,16 @@
 # We hash long names down to this many hex characters.
 COMPRESSED_NAME_LENGTH: int = 12
 
+# Gemini requires a function name to begin with a letter or an underscore, and
+# a hex digest begins with a digit ten times in sixteen. Left bare, a namespace
+# yields tool names such as ``8fb2adfa03efcontainer_shell_exec``, which Gemini
+# refuses with 400 ``invalid_request_body`` for the whole request, so one
+# unlucky digest takes down every tool in the run and not just that server's.
+# OpenAI and Anthropic document the laxer ``^[a-zA-Z0-9_-]{1,64}$`` and do
+# accept a leading digit, which is why this presents as a model-specific fault.
+# https://ai.google.dev/api/caching#FunctionDeclaration
+COMPRESSED_NAME_PREFIX: str = "ns"
+
 
 def compress_name(name: str) -> str:
     """Return a short hash of *name* to fit the OpenAI 64-char tool-name limit.
@@ -52,11 +62,12 @@ def compress_name(name: str) -> str:
         name: The original tool / toolbox name.
 
     Returns:
-        A 12-character lowercase hex digest.
+        A lowercase hex digest of ``COMPRESSED_NAME_LENGTH`` characters, behind
+        a fixed prefix so the result always starts with a letter.
     """
     m = hashlib.sha256()
     m.update(name.encode("utf-8"))
-    return m.hexdigest()[:COMPRESSED_NAME_LENGTH]
+    return COMPRESSED_NAME_PREFIX + m.hexdigest()[:COMPRESSED_NAME_LENGTH]
 
 
 class MCPNamespaceWrap:
 
@@ -134,6 +134,36 @@ def test_list_tools_unfiltered_idempotent_on_prefixed_input():
     assert not result[0].name.startswith(f"{ns}{ns}")
 
 
+# -- compress_name() --
+
+
+@pytest.mark.parametrize(
+    "server_name",
+    # "ContainerShell" is one whose sha256 starts with a digit, which is what
+    # made this show up as an intermittent failure of unrelated toolboxes.
+    ["ContainerShell", "RepoContext", "FindingLedger", "RepoSurvey", "", "x" * 200],
+)
+def test_compress_name_starts_with_a_letter(server_name):
+    """Gemini rejects a function name that starts with a digit.
+
+    A namespace is prefixed to every tool a server exposes, so a digest
+    beginning with a digit produces names like `8fb2adfa03efcontainer_shell_exec`.
+    Gemini answers 400 `invalid_request_body` to the whole request, taking down
+    every tool in the run and not just the offending server's.
+    """
+    assert compress_name(server_name)[0].isalpha()
+
+
+def test_compress_name_is_stable_and_distinct():
+    assert compress_name("RepoContext") == compress_name("RepoContext")
+    assert compress_name("RepoContext") != compress_name("RepoSurvey")
+
+
+def test_compress_name_leaves_room_under_the_64_character_limit():
+    """The prefix must not eat the headroom the compression exists to create."""
+    assert len(compress_name("x" * 200)) + len("a_very_long_tool_name_indeed") <= 64
+
+
 # -- list_tools() (regression) --