diff --git a/CHANGELOG.md b/CHANGELOG.md
index c7faf53..e0c46fb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,27 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/) and
 
 ## [Unreleased]
 
+### Changed (schema)
+
+- `EnvVar.inject_into_auth_data: bool = False` added — additive,
+  defaults to `False` (today's behavior preserved). When `True`, the
+  modulex runtime surfaces the value in `auth_data` at action time:
+  per-credential user input (`only_for_custom=False`) is persisted at
+  OAuth2 creation; server-level secrets (`only_for_custom=True`) are
+  injected from the server environment at tool execution. Fully
+  backward-compatible — every other integration dumps it as `False`
+  and the runtime injection is a no-op for them.
+
+### Fixed
+
+- `google_ads` / `google_merchant_center` — flagged
+  `GOOGLE_ADS_DEVELOPER_TOKEN` (`only_for_custom=True`) and
+  `GOOGLE_MERCHANT_CENTER_MERCHANT_ID` (`only_for_custom=False`) with
+  `inject_into_auth_data=True` so the developer token and merchant ID
+  reach `auth_data` at action time, fixing the "missing from auth_data"
+  errors on `list_account_id_options` / `create_product`. Requires the
+  matching modulex runtime change (external brief #021).
+
 ### Added
 
 - `revolt` integration — 3 actions, auth: bearer_token. Revolt open-source
diff --git a/src/modulex_integrations/schema.py b/src/modulex_integrations/schema.py
index 763d80a..b64da0c 100644
--- a/src/modulex_integrations/schema.py
+++ b/src/modulex_integrations/schema.py
@@ -98,6 +98,21 @@ class EnvVar(BaseModel):
     required: bool = True
     sensitive: bool = False
     only_for_custom: bool = False
+    # When True, the runtime must guarantee this value is present in the
+    # credential's ``auth_data`` at action-execution time, so a ``tools.py``
+    # function can read it. The source is *derived*, not declared:
+    #   - ``only_for_custom=False`` -> per-credential user input; the runtime
+    #     persists the user-entered value into ``auth_data`` at credential
+    #     creation (e.g. Google Merchant Center ``merchant_id``).
+    #   - ``only_for_custom=True``  -> server-level secret; for the managed
+    #     app the runtime resolves it from the server environment and injects
+    #     it at credential-resolution time, while a bring-your-own-app user
+    #     supplies their own (e.g. Google Ads ``developer_token``).
+    # Tools read the value via the normalized (prefix-stripped, lowercased)
+    # key. Default False preserves today's behavior: the EnvVar is used only
+    # for OAuth provider config and the credential test endpoint, never for
+    # action calls.
+    inject_into_auth_data: bool = False
     sample_format: str | None = None
     about_url: str | None = None
 
diff --git a/src/modulex_integrations/tools/google_ads/manifest.py b/src/modulex_integrations/tools/google_ads/manifest.py
index d313835..2b76397 100644
--- a/src/modulex_integrations/tools/google_ads/manifest.py
+++ b/src/modulex_integrations/tools/google_ads/manifest.py
@@ -567,6 +567,12 @@
                     ),
                     required=True,
                     sensitive=True,
+                    # Server-level secret for the managed app (one token for all
+                    # users; per-user access is via OAuth). The runtime resolves
+                    # it from the server environment and injects it into
+                    # auth_data; a bring-your-own-app user supplies their own.
+                    only_for_custom=True,
+                    inject_into_auth_data=True,
                     sample_format="xxxxxxxxxxxxxxxxxxxxxx",
                     about_url="https://developers.google.com/google-ads/api/docs/get-started/dev-token",
                 ),
diff --git a/src/modulex_integrations/tools/google_merchant_center/manifest.py b/src/modulex_integrations/tools/google_merchant_center/manifest.py
index a0f37a6..c4e5518 100644
--- a/src/modulex_integrations/tools/google_merchant_center/manifest.py
+++ b/src/modulex_integrations/tools/google_merchant_center/manifest.py
@@ -113,7 +113,12 @@
                     description="Your Google Merchant Center account ID (numeric)",
                     required=True,
                     sensitive=False,
+                    # Per-credential user input (every merchant has their own
+                    # ID, so it can't be a server global). The runtime persists
+                    # the user-entered value into auth_data at credential
+                    # creation; tools.py reads it as auth_data["merchant_id"].
                     only_for_custom=False,
+                    inject_into_auth_data=True,
                     sample_format="123456789",
                     about_url="https://merchants.google.com/mc/overview",
                 ),