diff --git a/agent/transports/types.py b/agent/transports/types.py
index 5199a5db1d..74481f85cd 100644
--- a/agent/transports/types.py
+++ b/agent/transports/types.py
@@ -61,6 +61,20 @@ class ToolCall:
         """Codex response_item_id from provider_data."""
         return (self.provider_data or {}).get("response_item_id")
 
+    @property
+    def extra_content(self) -> Optional[Dict[str, Any]]:
+        """Gemini extra_content (thought_signature) from provider_data.
+
+        Gemini 3 thinking models attach ``extra_content`` with a
+        ``thought_signature`` to each tool call.  This signature must be
+        replayed on subsequent API calls — without it the API rejects the
+        request with HTTP 400.  The chat_completions transport stores this
+        in ``provider_data["extra_content"]``; this property exposes it so
+        ``_build_assistant_message`` can ``getattr(tc, "extra_content")``
+        uniformly.
+        """
+        return (self.provider_data or {}).get("extra_content")
+
 
 @dataclass
 class Usage:
diff --git a/tests/agent/transports/test_types.py b/tests/agent/transports/test_types.py
index 8391342496..dd3aadf1e1 100644
--- a/tests/agent/transports/test_types.py
+++ b/tests/agent/transports/test_types.py
@@ -200,6 +200,35 @@ class TestToolCallBackwardCompat:
         tc_no_pd = ToolCall(id="1", name="fn", arguments="{}")
         assert getattr(tc_no_pd, "call_id", None) is None
 
+    def test_extra_content_from_provider_data(self):
+        """Gemini thought_signature stored in provider_data is exposed via property."""
+        ec = {"google": {"thought_signature": "SIG_ABC123"}}
+        tc = ToolCall(id="1", name="fn", arguments="{}", provider_data={"extra_content": ec})
+        assert tc.extra_content == ec
+
+    def test_extra_content_none_when_no_provider_data(self):
+        tc = ToolCall(id="1", name="fn", arguments="{}", provider_data=None)
+        assert tc.extra_content is None
+
+    def test_extra_content_none_when_key_absent(self):
+        tc = ToolCall(id="1", name="fn", arguments="{}", provider_data={"call_id": "c1"})
+        assert tc.extra_content is None
+
+    def test_extra_content_getattr_pattern(self):
+        """_build_assistant_message uses getattr(tc, 'extra_content', None).
+
+        This is the exact pattern that was broken before the extra_content
+        property was added — ToolCall lacked the property so getattr always
+        returned None, silently dropping the Gemini thought_signature and
+        causing HTTP 400 on subsequent turns (issue #14488).
+        """
+        ec = {"google": {"thought_signature": "SIG_ABC123"}}
+        tc = ToolCall(id="1", name="fn", arguments="{}", provider_data={"extra_content": ec})
+        assert getattr(tc, "extra_content", None) == ec
+
+        tc_no_extra = ToolCall(id="1", name="fn", arguments="{}")
+        assert getattr(tc_no_extra, "extra_content", None) is None
+
 
 class TestNormalizedResponseBackwardCompat:
     """Test properties that replaced _nr_to_assistant_message() shim."""