diff --git a/src/backend/workflows/mcp_tool_bridge.py b/src/backend/workflows/mcp_tool_bridge.py
index ad40437cd..e1a1acca2 100644
--- a/src/backend/workflows/mcp_tool_bridge.py
+++ b/src/backend/workflows/mcp_tool_bridge.py
@@ -216,25 +216,34 @@ def apply_runtime_defaults_to_mcp_payload(
             if isinstance(current_profile, str)
             else ""
         )
-        if default_profile and (
-            not current_profile_text
-            or (
-                current_profile_text in {"default", "primary"}
-                and default_profile != current_profile_text
-            )
-        ):
+        # Enforce the authoritative Gmail profile (request-selected, else the
+        # configured default) over whatever was supplied, INCLUDING an explicit
+        # profile the model chose itself. Which Gmail account to use is an
+        # identity/credential decision the caller's selection owns, not the
+        # model: models have picked stale/unauthorised profiles (e.g. an account
+        # with an expired/revoked token) even when a valid profile was selected,
+        # which hard-fails the whole turn (invalid_grant). When no authoritative
+        # default is resolved (e.g. deterministic/scheduled workflows where it is
+        # None) the supplied value is left untouched — nothing to enforce.
+        if default_profile and current_profile_text != default_profile:
             payload["profile"] = default_profile
-            bindings.append(
-                {
-                    "field": "profile",
-                    "source": (
-                        "default_gmail_profile_placeholder_replacement"
-                        if current_profile_text in {"default", "primary"}
-                        else "default_gmail_profile"
-                    ),
-                    "value_present": True,
-                }
-            )
+            if not current_profile_text:
+                profile_source = "default_gmail_profile"
+            elif current_profile_text in {"default", "primary"}:
+                profile_source = "default_gmail_profile_placeholder_replacement"
+            else:
+                profile_source = "default_gmail_profile_enforced_override"
+            profile_binding: dict[str, Any] = {
+                "field": "profile",
+                "source": profile_source,
+                "value_present": True,
+            }
+            if (
+                current_profile_text
+                and current_profile_text not in {"default", "primary"}
+            ):
+                profile_binding["overridden_profile"] = current_profile_text
+            bindings.append(profile_binding)
 
     namespace_binding = apply_namespace_to_mcp_payload(
         payload,
diff --git a/tests/backend/test_mcp_tool_bridge_gmail_profile.py b/tests/backend/test_mcp_tool_bridge_gmail_profile.py
new file mode 100644
index 000000000..9c22acdb6
--- /dev/null
+++ b/tests/backend/test_mcp_tool_bridge_gmail_profile.py
@@ -0,0 +1,84 @@
+"""Gmail profile enforcement in apply_runtime_defaults_to_mcp_payload.
+
+Which Gmail account a tool call uses is an identity/credential decision the
+caller's selection owns — not the model. A turn was observed failing because
+qwen3:8b chose an explicit stale profile ("zhan-gmail", expired/revoked token)
+even though a valid profile was selected; the old defaulting only filled blank
+or "default"/"primary" placeholders, so the model's explicit (wrong) profile
+won and the turn hard-failed with invalid_grant. The authoritative profile must
+override any supplied one.
+"""
+
+from __future__ import annotations
+
+from src.backend.workflows.mcp_tool_bridge import (
+    apply_runtime_defaults_to_mcp_payload,
+)
+
+_AUTH = "vonwitbrock-gmail"
+
+
+def _apply(payload: dict, *, default: str | None):
+    return apply_runtime_defaults_to_mcp_payload(
+        payload,
+        tool_name="gmail_list_messages",
+        input_schema=None,  # None => schema accepts any field (profile included)
+        user_namespace="#V#michael_witbrock@university_of_auckland_strong_ai_lab",
+        default_gmail_profile=default,
+    )
+
+
+def _binding_sources(bindings) -> set[str]:
+    return {b.get("source") for b in bindings if b.get("field") == "profile"}
+
+
+def test_model_supplied_wrong_profile_is_overridden() -> None:
+    payload = {"profile": "zhan-gmail", "max_results": 8}
+    bindings = _apply(payload, default=_AUTH)
+    assert payload["profile"] == _AUTH  # the bug: previously stayed zhan-gmail
+    assert "default_gmail_profile_enforced_override" in _binding_sources(bindings)
+    override = next(b for b in bindings if b.get("field") == "profile")
+    assert override["overridden_profile"] == "zhan-gmail"
+
+
+def test_blank_profile_is_filled() -> None:
+    payload = {"max_results": 5}
+    _apply(payload, default=_AUTH)
+    assert payload["profile"] == _AUTH
+
+
+def test_placeholder_profile_is_replaced() -> None:
+    payload = {"profile": "default"}
+    bindings = _apply(payload, default=_AUTH)
+    assert payload["profile"] == _AUTH
+    assert (
+        "default_gmail_profile_placeholder_replacement"
+        in _binding_sources(bindings)
+    )
+
+
+def test_profile_matching_default_is_untouched() -> None:
+    payload = {"profile": _AUTH}
+    bindings = _apply(payload, default=_AUTH)
+    assert payload["profile"] == _AUTH
+    assert _binding_sources(bindings) == set()  # no profile binding emitted
+
+
+def test_no_authoritative_default_leaves_supplied_profile() -> None:
+    # Deterministic/scheduled contexts pass default_gmail_profile=None; there is
+    # nothing to enforce, so an explicit profile must be preserved.
+    payload = {"profile": "zhan-gmail"}
+    _apply(payload, default=None)
+    assert payload["profile"] == "zhan-gmail"
+
+
+def test_non_gmail_tool_profile_is_untouched() -> None:
+    payload = {"profile": "zhan-gmail"}
+    apply_runtime_defaults_to_mcp_payload(
+        payload,
+        tool_name="search_web",
+        input_schema=None,
+        user_namespace=None,
+        default_gmail_profile=_AUTH,
+    )
+    assert payload["profile"] == "zhan-gmail"  # enforcement is gmail-only