diff --git a/.env.example b/.env.example
index b645e3c..b93ff00 100644
--- a/.env.example
+++ b/.env.example
@@ -140,6 +140,9 @@ GITHUB_OAUTH_HTTP_RECEIVE_TIMEOUT_MS=
 # https://YOUR_PHX_HOST/auth/google/callback
 GOOGLE_OAUTH_CLIENT_ID=
 GOOGLE_OAUTH_CLIENT_SECRET=
+# Comma-separated Android OAuth client IDs allowed as the verified azp claim
+# for Credential Manager cross-client ID tokens. Keep environments isolated.
+GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS=
 # Leave endpoint and timeout overrides empty for Google's discovery endpoint
 # and Req defaults. The base URL override exists for isolated protocol tests.
 GOOGLE_OAUTH_BASE_URL=
diff --git a/README.md b/README.md
index cf4c127..84ac64a 100644
--- a/README.md
+++ b/README.md
@@ -346,6 +346,12 @@ If both values are empty, the Google buttons remain visible but disabled with
 an explanation. A partial pair is rejected at startup and by the production
 environment validator.
 
+For Android Credential Manager, set `GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS` to the
+comma-separated OAuth client IDs registered for the Android package/signing
+certificates in that same environment. Phoenix still requires the Web client
+ID as the token audience; the Android client ID is accepted only as the
+verified `azp` (authorized party) claim.
+
 The flow requests `openid email profile`, verifies the provider email claim,
 uses state, nonce, and PKCE, and discards provider tokens. A new Google identity
 continues to a safe registration-completion page and creates a confirmed local
@@ -359,8 +365,9 @@ isolated protocol drill.
 The Android app does not open Google OAuth inside the WebView. Its visible
 Google button uses Android Credential Manager, asks this same server for a
 session-bound one-time nonce, and sends the resulting ID token directly back to
-the server. The server verifies the signature, issuer, audience, expiration,
-verified email, and nonce before it reuses the ordinary login, registration, or
+the server. The server verifies the signature, algorithm, issuer, Web audience,
+allowlisted Android authorized party, expiration, issued-at time, verified
+email, and nonce before it reuses the ordinary login, registration, or
 account-linking rules. `GOOGLE_OAUTH_CLIENT_SECRET` remains server-only; neither
 it nor the ID token is exposed to WebView JavaScript or compiled into the APK.
 
diff --git a/compose.yaml b/compose.yaml
index c15cfe9..cd9165e 100644
--- a/compose.yaml
+++ b/compose.yaml
@@ -41,6 +41,7 @@ x-app-environment: &app-environment
   GITHUB_OAUTH_HTTP_RECEIVE_TIMEOUT_MS: ${GITHUB_OAUTH_HTTP_RECEIVE_TIMEOUT_MS:-}
   GOOGLE_OAUTH_CLIENT_ID: ${GOOGLE_OAUTH_CLIENT_ID:-}
   GOOGLE_OAUTH_CLIENT_SECRET: ${GOOGLE_OAUTH_CLIENT_SECRET:-}
+  GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS: ${GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS:-}
   GOOGLE_OAUTH_BASE_URL: ${GOOGLE_OAUTH_BASE_URL:-}
   GOOGLE_OAUTH_HTTP_CONNECT_TIMEOUT_MS: ${GOOGLE_OAUTH_HTTP_CONNECT_TIMEOUT_MS:-}
   GOOGLE_OAUTH_HTTP_RECEIVE_TIMEOUT_MS: ${GOOGLE_OAUTH_HTTP_RECEIVE_TIMEOUT_MS:-}
diff --git a/config/runtime.exs b/config/runtime.exs
index 7939907..30aabb8 100644
--- a/config/runtime.exs
+++ b/config/runtime.exs
@@ -203,6 +203,12 @@ config :who_need_help, :social_oauth, github_oauth
         [
           client_id: client_id,
           client_secret: client_secret,
+          authorized_party_ids:
+            System.get_env("GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS", "")
+            |> String.split(",", trim: true)
+            |> Enum.map(&String.trim/1)
+            |> Enum.reject(&(&1 == ""))
+            |> Enum.uniq(),
           base_url: base_url,
           authorization_params: [scope: "email profile"],
           http_adapter: {Assent.HTTPAdapter.Req, oauth_http_options.("GOOGLE")}
diff --git a/docs/operations.md b/docs/operations.md
index 8751bc2..b175c89 100644
--- a/docs/operations.md
+++ b/docs/operations.md
@@ -183,7 +183,7 @@ test checkout. The generated file uses the ordinary runtime names:
 
 | Capability | Values kept in that checkout's `.env` |
 | --- | --- |
-| Google sign-in | `GOOGLE_OAUTH_CLIENT_ID`, `GOOGLE_OAUTH_CLIENT_SECRET` |
+| Google sign-in | `GOOGLE_OAUTH_CLIENT_ID`, `GOOGLE_OAUTH_CLIENT_SECRET`, `GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS` |
 | Browser push | three `WEB_PUSH_VAPID_*` values |
 | Android Firebase client | four public `WNH_FIREBASE_*` values |
 | Android delivery | `FCM_PROJECT_ID` and one private service-account source |
@@ -205,6 +205,9 @@ for development,
 `org.whoneedhelp.mobile.staging` plus the stable staging certificate for test,
 and `org.whoneedhelp.mobile` plus the Play-distributed certificate for
 production. Do not add a second Google secret file or Gradle property.
+Set `GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS` to the comma-separated Android OAuth
+client IDs from that environment. They are accepted only as the signed `azp`
+claim; the signed `aud` must still contain the environment's Web client ID.
 
 Check an environment without printing its secret values:
 
@@ -261,6 +264,7 @@ cd /srv/who_need_help-test
 TEST_CODEX_SESSION_ID=YOUR_MAIN_CODEX_SESSION_ID \
 TEST_GOOGLE_OAUTH_CLIENT_ID=YOUR_TEST_CLIENT_ID \
 TEST_GOOGLE_OAUTH_CLIENT_SECRET=YOUR_TEST_CLIENT_SECRET \
+TEST_GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS=YOUR_TEST_ANDROID_CLIENT_ID \
   ./scripts/init-test-env.sh test.whoneedhelp.com
 ./scripts/validate-test-env.sh .env test.whoneedhelp.com
 ./scripts/deploy-up.sh .env
@@ -293,6 +297,7 @@ PRODUCTION_SMTP_USERNAME=YOUR_PRODUCTION_SMTP_LOGIN \
 PRODUCTION_SMTP_PASSWORD=YOUR_PRODUCTION_SMTP_PASSWORD \
 PRODUCTION_GOOGLE_OAUTH_CLIENT_ID=YOUR_PRODUCTION_CLIENT_ID \
 PRODUCTION_GOOGLE_OAUTH_CLIENT_SECRET=YOUR_PRODUCTION_CLIENT_SECRET \
+PRODUCTION_GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS=YOUR_PRODUCTION_ANDROID_CLIENT_ID \
 PRODUCTION_CODEX_SESSION_ID=YOUR_MAIN_CODEX_SESSION_ID \
   ./scripts/init-production-env.sh whoneedhelp.com
 ./scripts/validate-production-env.sh .env whoneedhelp.com
diff --git a/e2e/tests/mutual-aid.spec.ts b/e2e/tests/mutual-aid.spec.ts
index 103e142..f88bc72 100644
--- a/e2e/tests/mutual-aid.spec.ts
+++ b/e2e/tests/mutual-aid.spec.ts
@@ -211,7 +211,9 @@ test("two users complete medicine tracking, handover, realtime chat, and blind r
   await expect(
     replacement.page.getByRole("heading", { name: "Private match chat" }),
   ).toBeVisible();
-  await expect(requester.page.getByText("Helper: E2E Replacement Helper")).toBeVisible();
+  await expect(
+    requester.page.getByRole("link", { name: "Staging E2E Replacement Helper" }),
+  ).toBeVisible();
   await expect(helper.page.getByRole("heading", { name: "Private match chat" })).toHaveCount(0);
 
   assertRequesterClean();
diff --git a/lib/mix/tasks/wnh.staging_e2e.ex b/lib/mix/tasks/wnh.staging_e2e.ex
index 8773e2f..2246a4a 100644
--- a/lib/mix/tasks/wnh.staging_e2e.ex
+++ b/lib/mix/tasks/wnh.staging_e2e.ex
@@ -80,7 +80,14 @@ defmodule Mix.Tasks.Wnh.StagingE2e do
       Repo.transaction(fn ->
         %{
           requester: insert_user!(emails.requester, "Staging E2E Requester", password_hash, now),
-          helper: insert_user!(emails.helper, "Staging E2E Helper", password_hash, now)
+          helper: insert_user!(emails.helper, "Staging E2E Helper", password_hash, now),
+          replacement_helper:
+            insert_user!(
+              emails.replacement_helper,
+              "Staging E2E Replacement Helper",
+              password_hash,
+              now
+            )
         }
       end)
 
@@ -91,7 +98,9 @@ defmodule Mix.Tasks.Wnh.StagingE2e do
       "requester_id" => users.requester.id,
       "requester_email" => users.requester.email,
       "helper_id" => users.helper.id,
-      "helper_email" => users.helper.email
+      "helper_email" => users.helper.email,
+      "replacement_helper_id" => users.replacement_helper.id,
+      "replacement_helper_email" => users.replacement_helper.email
     }
 
     context.manifest_path |> Path.dirname() |> File.mkdir_p!()
@@ -107,7 +116,8 @@ defmodule Mix.Tasks.Wnh.StagingE2e do
 
     requester_id = manifest["requester_id"]
     helper_id = manifest["helper_id"]
-    user_ids = [requester_id, helper_id]
+    replacement_helper_id = manifest["replacement_helper_id"]
+    user_ids = [requester_id, helper_id, replacement_helper_id]
 
     request_ids =
       HelpRequest
@@ -121,7 +131,12 @@ defmodule Mix.Tasks.Wnh.StagingE2e do
       |> select([assignment], assignment.id)
       |> Repo.all()
 
-    validate_owned_domain!(user_ids, request_ids, assignment_ids, helper_id)
+    validate_owned_domain!(
+      user_ids,
+      request_ids,
+      assignment_ids,
+      [helper_id, replacement_helper_id]
+    )
 
     {:ok, deleted} =
       Repo.transaction(fn ->
@@ -227,8 +242,8 @@ defmodule Mix.Tasks.Wnh.StagingE2e do
         }
       end)
 
-    unless deleted.users == 2 do
-      Mix.raise("cleanup removed #{deleted.users} users instead of exactly 2")
+    unless deleted.users == 3 do
+      Mix.raise("cleanup removed #{deleted.users} users instead of exactly 3")
     end
 
     Mix.shell().info("removed exact staging E2E fixture: #{inspect(deleted)}")
@@ -241,19 +256,29 @@ defmodule Mix.Tasks.Wnh.StagingE2e do
              manifest["database"] == context.database and
              manifest["requester_email"] == expected_emails.requester and
              manifest["helper_email"] == expected_emails.helper and
-             uuid?(manifest["requester_id"]) and uuid?(manifest["helper_id"]) do
+             manifest["replacement_helper_email"] == expected_emails.replacement_helper and
+             uuid?(manifest["requester_id"]) and uuid?(manifest["helper_id"]) and
+             uuid?(manifest["replacement_helper_id"]) do
       Mix.raise("staging E2E manifest does not match the requested run and database")
     end
 
     expected =
       MapSet.new([
         {manifest["requester_id"], manifest["requester_email"]},
-        {manifest["helper_id"], manifest["helper_email"]}
+        {manifest["helper_id"], manifest["helper_email"]},
+        {manifest["replacement_helper_id"], manifest["replacement_helper_email"]}
       ])
 
     observed =
       User
-      |> where([user], user.id in ^[manifest["requester_id"], manifest["helper_id"]])
+      |> where(
+        [user],
+        user.id in ^[
+          manifest["requester_id"],
+          manifest["helper_id"],
+          manifest["replacement_helper_id"]
+        ]
+      )
       |> select([user], {user.id, user.email})
       |> Repo.all()
       |> MapSet.new()
@@ -263,7 +288,7 @@ defmodule Mix.Tasks.Wnh.StagingE2e do
     end
   end
 
-  defp validate_owned_domain!(user_ids, request_ids, assignment_ids, helper_id) do
+  defp validate_owned_domain!(user_ids, request_ids, assignment_ids, helper_ids) do
     unexpected_request? =
       Repo.exists?(
         from(request in HelpRequest,
@@ -276,7 +301,7 @@ defmodule Mix.Tasks.Wnh.StagingE2e do
         from(assignment in Assignment,
           where:
             assignment.helper_id in ^user_ids and
-              (assignment.id not in ^assignment_ids or assignment.helper_id != ^helper_id)
+              (assignment.id not in ^assignment_ids or assignment.helper_id not in ^helper_ids)
         )
       )
 
@@ -286,7 +311,7 @@ defmodule Mix.Tasks.Wnh.StagingE2e do
     unexpected_proposal? =
       Repo.exists?(from(proposal in CategoryProposal, where: proposal.proposer_id in ^user_ids))
 
-    unless length(request_ids) <= 2 and length(assignment_ids) <= 2 and
+    unless length(request_ids) <= 2 and length(assignment_ids) <= 3 and
              not unexpected_request? and not unexpected_assignment? and
              not unexpected_activity? and not unexpected_proposal? do
       Mix.raise("staging E2E users own records outside the exact medicine-flow scope")
@@ -309,7 +334,8 @@ defmodule Mix.Tasks.Wnh.StagingE2e do
   defp emails(run_id) do
     %{
       requester: "wnh-staging-e2e-#{run_id}-requester@example.invalid",
-      helper: "wnh-staging-e2e-#{run_id}-helper@example.invalid"
+      helper: "wnh-staging-e2e-#{run_id}-helper@example.invalid",
+      replacement_helper: "wnh-staging-e2e-#{run_id}-replacement-helper@example.invalid"
     }
   end
 
diff --git a/lib/who_need_help/google_auth/assent_adapter.ex b/lib/who_need_help/google_auth/assent_adapter.ex
index 05b7690..bc4e411 100644
--- a/lib/who_need_help/google_auth/assent_adapter.ex
+++ b/lib/who_need_help/google_auth/assent_adapter.ex
@@ -4,6 +4,7 @@ defmodule WhoNeedHelp.GoogleAuth.AssentAdapter do
   @behaviour WhoNeedHelp.GoogleAuth
 
   alias Assent.Strategy.{Google, OIDC}
+  alias WhoNeedHelp.GoogleAuth.CrossClientIdTokenVerifier
 
   @impl true
   def enabled?, do: not is_nil(provider_config())
@@ -44,21 +45,40 @@ defmodule WhoNeedHelp.GoogleAuth.AssentAdapter do
   @impl true
   def verify_id_token(id_token, nonce)
       when is_binary(id_token) and id_token != "" and is_binary(nonce) and nonce != "" do
-    with config when is_list(config) <- provider_config(),
-         {:ok, jwt} <-
-           config
-           |> Keyword.put(:session_params, %{nonce: nonce})
-           |> OIDC.validate_id_token(id_token) do
-      normalize_identity(jwt.claims)
-    else
-      nil -> {:error, :provider_disabled}
-      {:error, _reason} = error -> error
-      _invalid -> {:error, :invalid_id_token}
+    case provider_config() do
+      config when is_list(config) ->
+        config = Keyword.put(config, :session_params, %{nonce: nonce})
+
+        case OIDC.validate_id_token(config, id_token) do
+          {:ok, jwt} ->
+            normalize_identity(jwt.claims)
+
+          {:error, reason} = error ->
+            verify_cross_client_id_token(config, id_token, nonce, reason, error)
+        end
+
+      nil ->
+        {:error, :provider_disabled}
     end
   end
 
   def verify_id_token(_id_token, _nonce), do: {:error, :invalid_id_token}
 
+  defp verify_cross_client_id_token(config, id_token, nonce, reason, original_error) do
+    if authorization_party_mismatch?(reason) do
+      with {:ok, jwt} <- CrossClientIdTokenVerifier.verify(config, id_token, nonce) do
+        normalize_identity(jwt.claims)
+      end
+    else
+      original_error
+    end
+  end
+
+  defp authorization_party_mismatch?("Invalid authorized party \"" <> authorized_party_error),
+    do: String.ends_with?(authorized_party_error, "\" in ID Token")
+
+  defp authorization_party_mismatch?(_reason), do: false
+
   def normalize_identity(
         %{
           "sub" => provider_uid,
diff --git a/lib/who_need_help/google_auth/cross_client_id_token_verifier.ex b/lib/who_need_help/google_auth/cross_client_id_token_verifier.ex
new file mode 100644
index 0000000..c82f76a
--- /dev/null
+++ b/lib/who_need_help/google_auth/cross_client_id_token_verifier.ex
@@ -0,0 +1,237 @@
+defmodule WhoNeedHelp.GoogleAuth.CrossClientIdTokenVerifier do
+  @moduledoc false
+
+  alias Assent.HTTPAdapter.HTTPResponse
+  alias Assent.Strategy
+
+  @required_claims ~w(iss sub aud exp iat)
+
+  def verify(config, id_token, nonce)
+      when is_list(config) and is_binary(id_token) and is_binary(nonce) do
+    with {:ok, authorized_party_ids} <- fetch_authorized_party_ids(config),
+         {:ok, openid_configuration} <- fetch_openid_configuration(config),
+         {:ok, issuer} <- fetch_binary(openid_configuration, "issuer"),
+         {:ok, client_id} <- fetch_binary_config(config, :client_id),
+         {:ok, jwt} <- verify_jwt(id_token, openid_configuration, config),
+         :ok <- validate_required_claims(jwt),
+         :ok <- validate_issuer(jwt, issuer),
+         :ok <- validate_audience(jwt, client_id, config),
+         :ok <- validate_authorized_party(jwt, authorized_party_ids),
+         :ok <- validate_algorithm(jwt, config),
+         :ok <- validate_signature(jwt),
+         :ok <- validate_expiration(jwt),
+         :ok <- validate_issued_at(jwt, config),
+         :ok <- validate_nonce(jwt, nonce) do
+      {:ok, jwt}
+    end
+  end
+
+  def verify(_config, _id_token, _nonce), do: {:error, :invalid_id_token}
+
+  defp fetch_authorized_party_ids(config) do
+    case Keyword.get(config, :authorized_party_ids, []) do
+      ids when is_list(ids) and ids != [] ->
+        if Enum.all?(ids, &valid_identifier?/1),
+          do: {:ok, ids},
+          else: {:error, :unauthorized_party}
+
+      _missing_or_invalid ->
+        {:error, :unauthorized_party}
+    end
+  end
+
+  defp valid_identifier?(identifier),
+    do: is_binary(identifier) and identifier != "" and String.trim(identifier) == identifier
+
+  defp fetch_openid_configuration(config) do
+    case Keyword.get(config, :openid_configuration) do
+      configuration when is_map(configuration) ->
+        {:ok, configuration}
+
+      nil ->
+        with {:ok, base_url} <- fetch_binary_config(config, :base_url) do
+          path =
+            Keyword.get(
+              config,
+              :openid_configuration_uri,
+              "/.well-known/openid-configuration"
+            )
+
+          base_url
+          |> Strategy.to_url(path)
+          |> fetch_json(config, :openid_configuration_unavailable)
+        end
+
+      _invalid ->
+        {:error, :invalid_openid_configuration}
+    end
+  end
+
+  defp fetch_json(url, config, error_name) do
+    case Strategy.http_request(:get, url, nil, [], config) do
+      {:ok, %HTTPResponse{status: 200, body: body}} when is_map(body) ->
+        {:ok, body}
+
+      _unavailable_or_invalid ->
+        {:error, error_name}
+    end
+  end
+
+  defp verify_jwt(id_token, openid_configuration, config) do
+    with {:ok, header} <- peek_header(id_token, config),
+         {:ok, verification_key} <-
+           fetch_verification_key(header, openid_configuration, config) do
+      Strategy.verify_jwt(id_token, verification_key, config)
+    end
+  end
+
+  defp peek_header(id_token, config) do
+    with [encoded_header, _claims, _signature] <- String.split(id_token, "."),
+         {:ok, json} <- Base.url_decode64(encoded_header, padding: false),
+         {:ok, header} when is_map(header) <- Assent.json_library(config).decode(json) do
+      {:ok, header}
+    else
+      _invalid -> {:error, :invalid_id_token}
+    end
+  end
+
+  defp fetch_verification_key(%{"alg" => "HS" <> _rest}, _openid_configuration, config),
+    do: fetch_binary_config(config, :client_secret)
+
+  defp fetch_verification_key(header, openid_configuration, config) do
+    with {:ok, jwks_uri} <- fetch_binary(openid_configuration, "jwks_uri"),
+         {:ok, %{"keys" => keys}} <-
+           fetch_json(jwks_uri, config, :signing_keys_unavailable),
+         true <- is_list(keys),
+         {:ok, key} <- find_verification_key(header, keys) do
+      {:ok, key}
+    else
+      false -> {:error, :invalid_signing_keys}
+      {:error, _reason} = error -> error
+      _invalid -> {:error, :invalid_signing_keys}
+    end
+  end
+
+  defp find_verification_key(%{"kid" => kid}, keys) when is_binary(kid) do
+    case Enum.find(keys, &(is_map(&1) and Map.get(&1, "kid") == kid)) do
+      nil -> {:error, :signing_key_not_found}
+      key -> {:ok, key}
+    end
+  end
+
+  defp find_verification_key(_header, [key]) when is_map(key), do: {:ok, key}
+  defp find_verification_key(_header, _keys), do: {:error, :signing_key_not_found}
+
+  defp validate_required_claims(%{claims: claims}) when is_map(claims) do
+    valid? =
+      Enum.all?(@required_claims, &Map.has_key?(claims, &1)) and
+        non_empty_binary?(claims["iss"]) and
+        non_empty_binary?(claims["sub"]) and
+        valid_audience_claim?(claims["aud"]) and
+        is_integer(claims["exp"]) and
+        is_integer(claims["iat"])
+
+    if valid?, do: :ok, else: {:error, :invalid_required_claims}
+  end
+
+  defp validate_required_claims(_jwt), do: {:error, :invalid_required_claims}
+
+  defp validate_issuer(%{claims: %{"iss" => issuer}}, issuer), do: :ok
+  defp validate_issuer(_jwt, _issuer), do: {:error, :invalid_issuer}
+
+  defp validate_audience(%{claims: %{"aud" => audience}}, client_id, config) do
+    audiences = List.wrap(audience)
+    trusted = [client_id | Keyword.get(config, :trusted_audiences, [])]
+
+    if client_id in audiences and Enum.all?(audiences, &(&1 in trusted)),
+      do: :ok,
+      else: {:error, :invalid_audience}
+  end
+
+  defp validate_audience(_jwt, _client_id, _config), do: {:error, :invalid_audience}
+
+  defp validate_authorized_party(
+         %{claims: %{"azp" => authorized_party}},
+         authorized_party_ids
+       )
+       when is_binary(authorized_party) do
+    if authorized_party in authorized_party_ids,
+      do: :ok,
+      else: {:error, :unauthorized_party}
+  end
+
+  defp validate_authorized_party(_jwt, _authorized_party_ids),
+    do: {:error, :unauthorized_party}
+
+  defp validate_algorithm(%{header: %{"alg" => algorithm}}, config) do
+    if algorithm == Keyword.get(config, :id_token_signed_response_alg, "RS256"),
+      do: :ok,
+      else: {:error, :invalid_algorithm}
+  end
+
+  defp validate_algorithm(_jwt, _config), do: {:error, :invalid_algorithm}
+
+  defp validate_signature(%{verified?: true}), do: :ok
+  defp validate_signature(_jwt), do: {:error, :invalid_signature}
+
+  defp validate_expiration(%{claims: %{"exp" => expires_at}}) when is_integer(expires_at) do
+    if expires_at > System.system_time(:second),
+      do: :ok,
+      else: {:error, :expired_id_token}
+  end
+
+  defp validate_expiration(_jwt), do: {:error, :invalid_expiration}
+
+  defp validate_issued_at(%{claims: %{"iat" => issued_at}}, config)
+       when is_integer(issued_at) do
+    now = System.system_time(:second)
+
+    case Keyword.get(config, :id_token_ttl_seconds) do
+      nil when issued_at <= now ->
+        :ok
+
+      ttl when is_integer(ttl) and ttl > 0 and issued_at <= now and issued_at + ttl > now ->
+        :ok
+
+      nil ->
+        {:error, :invalid_issued_at}
+
+      _invalid_or_expired ->
+        {:error, :invalid_issued_at}
+    end
+  end
+
+  defp validate_issued_at(_jwt, _config), do: {:error, :invalid_issued_at}
+
+  defp validate_nonce(%{claims: %{"nonce" => provided_nonce}}, stored_nonce)
+       when is_binary(provided_nonce) do
+    if Assent.constant_time_compare(stored_nonce, provided_nonce),
+      do: :ok,
+      else: {:error, :invalid_nonce}
+  end
+
+  defp validate_nonce(_jwt, _stored_nonce), do: {:error, :invalid_nonce}
+
+  defp fetch_binary(map, key) do
+    case Map.get(map, key) do
+      value when is_binary(value) and value != "" -> {:ok, value}
+      _missing_or_invalid -> {:error, :invalid_openid_configuration}
+    end
+  end
+
+  defp fetch_binary_config(config, key) do
+    case Keyword.get(config, key) do
+      value when is_binary(value) and value != "" -> {:ok, value}
+      _missing_or_invalid -> {:error, :invalid_provider_configuration}
+    end
+  end
+
+  defp non_empty_binary?(value), do: is_binary(value) and value != ""
+
+  defp valid_audience_claim?(audience) when is_binary(audience), do: audience != ""
+
+  defp valid_audience_claim?(audiences) when is_list(audiences),
+    do: audiences != [] and Enum.all?(audiences, &non_empty_binary?/1)
+
+  defp valid_audience_claim?(_audience), do: false
+end
diff --git a/scripts/check-environment-readiness.sh b/scripts/check-environment-readiness.sh
index fdd4040..d2d5324 100755
--- a/scripts/check-environment-readiness.sh
+++ b/scripts/check-environment-readiness.sh
@@ -117,6 +117,19 @@ valid_sha256_fingerprint_list() {
   done
 }
 
+valid_nonempty_csv() {
+  local item compact
+  local -a items
+
+  IFS=',' read -r -a items <<<"$1"
+  [[ ${#items[@]} -gt 0 ]] || return 1
+
+  for item in "${items[@]}"; do
+    compact=${item//[[:space:]]/}
+    [[ -n "$compact" ]] || return 1
+  done
+}
+
 failures=0
 warnings=0
 
@@ -196,6 +209,18 @@ else
   partial "Google sign-in" "client ID and secret must be configured together"
 fi
 
+if is_set GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS; then
+  if ! all_set GOOGLE_OAUTH_CLIENT_ID GOOGLE_OAUTH_CLIENT_SECRET; then
+    invalid "Android Google sign-in" "authorized parties require the Google OAuth client"
+  elif valid_nonempty_csv "$(value GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS)"; then
+    ready "Android Google sign-in" "one or more authorized Android OAuth clients"
+  else
+    invalid "Android Google sign-in" "authorized party IDs must be a non-empty CSV list"
+  fi
+else
+  missing "Android Google sign-in" "GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS"
+fi
+
 if all_empty WEB_PUSH_VAPID_PUBLIC_KEY WEB_PUSH_VAPID_PRIVATE_KEY WEB_PUSH_VAPID_SUBJECT; then
   missing "browser Web Push" "VAPID public/private keys and subject"
 elif all_set WEB_PUSH_VAPID_PUBLIC_KEY WEB_PUSH_VAPID_PRIVATE_KEY WEB_PUSH_VAPID_SUBJECT; then
diff --git a/scripts/init-production-env.sh b/scripts/init-production-env.sh
index c47a36f..5b6dca5 100755
--- a/scripts/init-production-env.sh
+++ b/scripts/init-production-env.sh
@@ -78,6 +78,7 @@ public_upstream_name=${PRODUCTION_PUBLIC_UPSTREAM_NAME:-who-need-help-production
 codex_session_id=${PRODUCTION_CODEX_SESSION_ID:-}
 google_oauth_client_id=${PRODUCTION_GOOGLE_OAUTH_CLIENT_ID:-}
 google_oauth_client_secret=${PRODUCTION_GOOGLE_OAUTH_CLIENT_SECRET:-}
+google_oauth_authorized_party_ids=${PRODUCTION_GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS:-}
 web_push_vapid_public_key=${PRODUCTION_WEB_PUSH_VAPID_PUBLIC_KEY:-}
 web_push_vapid_private_key=${PRODUCTION_WEB_PUSH_VAPID_PRIVATE_KEY:-}
 web_push_vapid_subject=${PRODUCTION_WEB_PUSH_VAPID_SUBJECT:-}
@@ -104,6 +105,8 @@ require_single_line_env_value PRODUCTION_PUBLIC_UPSTREAM_NAME "$public_upstream_
 require_single_line_env_value PRODUCTION_CODEX_SESSION_ID "$codex_session_id"
 require_single_line_env_value PRODUCTION_GOOGLE_OAUTH_CLIENT_ID "$google_oauth_client_id"
 require_single_line_env_value PRODUCTION_GOOGLE_OAUTH_CLIENT_SECRET "$google_oauth_client_secret"
+require_single_line_env_value \
+  PRODUCTION_GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS "$google_oauth_authorized_party_ids"
 require_single_line_env_value PRODUCTION_WEB_PUSH_VAPID_PUBLIC_KEY "$web_push_vapid_public_key"
 require_single_line_env_value PRODUCTION_WEB_PUSH_VAPID_PRIVATE_KEY "$web_push_vapid_private_key"
 require_single_line_env_value PRODUCTION_WEB_PUSH_VAPID_SUBJECT "$web_push_vapid_subject"
@@ -130,6 +133,17 @@ if { [ -n "$google_oauth_client_id" ] || [ -n "$google_oauth_client_secret" ]; }
   echo "Production Google OAuth client ID and secret must either both be set or both be empty." >&2
   exit 1
 fi
+if [ -n "$google_oauth_authorized_party_ids" ] &&
+  { [ -z "$google_oauth_client_id" ] || [ -z "$google_oauth_client_secret" ]; }; then
+  echo "Production Android Google authorized parties require the Google OAuth client." >&2
+  exit 1
+fi
+case "$google_oauth_authorized_party_ids" in
+  ,* | *,,* | *,)
+    echo "PRODUCTION_GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS contains an empty value." >&2
+    exit 1
+    ;;
+esac
 
 if {
   [ -n "$android_app_links_package_name" ] ||
@@ -395,6 +409,7 @@ CODEX_SESSION_ID_VALUE=$codex_session_id \
 GIT_SHA_VALUE=$git_sha \
 GOOGLE_OAUTH_CLIENT_ID_VALUE=$google_oauth_client_id \
 GOOGLE_OAUTH_CLIENT_SECRET_VALUE=$google_oauth_client_secret \
+GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS_VALUE=$google_oauth_authorized_party_ids \
 WEB_PUSH_VAPID_PUBLIC_KEY_VALUE=$web_push_vapid_public_key \
 WEB_PUSH_VAPID_PRIVATE_KEY_VALUE=$web_push_vapid_private_key \
 WEB_PUSH_VAPID_SUBJECT_VALUE=$web_push_vapid_subject \
@@ -462,6 +477,7 @@ TEST_UPSTREAM_VALUE=$test_upstream \
       replacement["SUPPORT_INBOX_ADDRESS"] = ENVIRON["SUPPORT_INBOX_ADDRESS_VALUE"]
       replacement["GOOGLE_OAUTH_CLIENT_ID"] = ENVIRON["GOOGLE_OAUTH_CLIENT_ID_VALUE"]
       replacement["GOOGLE_OAUTH_CLIENT_SECRET"] = ENVIRON["GOOGLE_OAUTH_CLIENT_SECRET_VALUE"]
+      replacement["GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS"] = ENVIRON["GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS_VALUE"]
       replacement["WEB_PUSH_VAPID_PUBLIC_KEY"] = ENVIRON["WEB_PUSH_VAPID_PUBLIC_KEY_VALUE"]
       replacement["WEB_PUSH_VAPID_PRIVATE_KEY"] = ENVIRON["WEB_PUSH_VAPID_PRIVATE_KEY_VALUE"]
       replacement["WEB_PUSH_VAPID_SUBJECT"] = ENVIRON["WEB_PUSH_VAPID_SUBJECT_VALUE"]
diff --git a/scripts/init-test-env.sh b/scripts/init-test-env.sh
index f798130..bedfc76 100755
--- a/scripts/init-test-env.sh
+++ b/scripts/init-test-env.sh
@@ -74,6 +74,7 @@ mailpit_port=${TEST_MAILPIT_PORT:-8027}
 codex_session_id=${TEST_CODEX_SESSION_ID:-}
 google_oauth_client_id=${TEST_GOOGLE_OAUTH_CLIENT_ID:-}
 google_oauth_client_secret=${TEST_GOOGLE_OAUTH_CLIENT_SECRET:-}
+google_oauth_authorized_party_ids=${TEST_GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS:-}
 web_push_vapid_public_key=${TEST_WEB_PUSH_VAPID_PUBLIC_KEY:-}
 web_push_vapid_private_key=${TEST_WEB_PUSH_VAPID_PRIVATE_KEY:-}
 web_push_vapid_subject=${TEST_WEB_PUSH_VAPID_SUBJECT:-}
@@ -98,6 +99,8 @@ require_single_line_env_value TEST_MAILPIT_PORT "$mailpit_port"
 require_single_line_env_value TEST_CODEX_SESSION_ID "$codex_session_id"
 require_single_line_env_value TEST_GOOGLE_OAUTH_CLIENT_ID "$google_oauth_client_id"
 require_single_line_env_value TEST_GOOGLE_OAUTH_CLIENT_SECRET "$google_oauth_client_secret"
+require_single_line_env_value \
+  TEST_GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS "$google_oauth_authorized_party_ids"
 require_single_line_env_value TEST_WEB_PUSH_VAPID_PUBLIC_KEY "$web_push_vapid_public_key"
 require_single_line_env_value TEST_WEB_PUSH_VAPID_PRIVATE_KEY "$web_push_vapid_private_key"
 require_single_line_env_value TEST_WEB_PUSH_VAPID_SUBJECT "$web_push_vapid_subject"
@@ -133,6 +136,17 @@ if [[ -n "$google_oauth_client_id" || -n "$google_oauth_client_secret" ]]; then
     exit 1
   }
 fi
+if [[ -n "$google_oauth_authorized_party_ids" &&
+  (-z "$google_oauth_client_id" || -z "$google_oauth_client_secret") ]]; then
+  echo "Test Android Google authorized parties require the Google OAuth client." >&2
+  exit 1
+fi
+if [[ "$google_oauth_authorized_party_ids" == ,* ||
+  "$google_oauth_authorized_party_ids" == *,,* ||
+  "$google_oauth_authorized_party_ids" == *, ]]; then
+  echo "TEST_GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS contains an empty value." >&2
+  exit 1
+fi
 
 if [[ -n "$android_app_links_package_name" || -n "$android_app_links_fingerprints" ]]; then
   [[ -n "$android_app_links_package_name" && -n "$android_app_links_fingerprints" ]] || {
@@ -270,6 +284,7 @@ RELEASE_COOKIE_VALUE=$release_cookie \
 METRICS_TOKEN_VALUE=$metrics_token \
 GOOGLE_OAUTH_CLIENT_ID_VALUE=$google_oauth_client_id \
 GOOGLE_OAUTH_CLIENT_SECRET_VALUE=$google_oauth_client_secret \
+GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS_VALUE=$google_oauth_authorized_party_ids \
 WEB_PUSH_VAPID_PUBLIC_KEY_VALUE=$web_push_vapid_public_key \
 WEB_PUSH_VAPID_PRIVATE_KEY_VALUE=$web_push_vapid_private_key \
 WEB_PUSH_VAPID_SUBJECT_VALUE=$web_push_vapid_subject \
@@ -332,6 +347,7 @@ CODEX_SESSION_ID_VALUE=$codex_session_id \
       replacement["SUPPORT_INBOX_ADDRESS"] = ENVIRON["SUPPORT_INBOX_ADDRESS_VALUE"]
       replacement["GOOGLE_OAUTH_CLIENT_ID"] = ENVIRON["GOOGLE_OAUTH_CLIENT_ID_VALUE"]
       replacement["GOOGLE_OAUTH_CLIENT_SECRET"] = ENVIRON["GOOGLE_OAUTH_CLIENT_SECRET_VALUE"]
+      replacement["GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS"] = ENVIRON["GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS_VALUE"]
       replacement["WEB_PUSH_VAPID_PUBLIC_KEY"] = ENVIRON["WEB_PUSH_VAPID_PUBLIC_KEY_VALUE"]
       replacement["WEB_PUSH_VAPID_PRIVATE_KEY"] = ENVIRON["WEB_PUSH_VAPID_PRIVATE_KEY_VALUE"]
       replacement["WEB_PUSH_VAPID_SUBJECT"] = ENVIRON["WEB_PUSH_VAPID_SUBJECT_VALUE"]
diff --git a/scripts/quality.sh b/scripts/quality.sh
index ddf96b2..cfc50ef 100755
--- a/scripts/quality.sh
+++ b/scripts/quality.sh
@@ -332,6 +332,8 @@ printf '%s\n' \
   'PHX_SCHEME=https' \
   'PHX_URL_PORT=443' \
   'WNH_BASE_URL=https://dev.help.test' \
+  'GOOGLE_OAUTH_CLIENT_ID=quality-web-client' \
+  'GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS=quality-android-client' \
   'ANDROID_APP_LINKS_PACKAGE_NAME=org.whoneedhelp.mobile.development' \
   "ANDROID_APP_LINKS_SHA256_CERT_FINGERPRINTS=$android_fingerprint" \
   'ANDROID_PLAY_APP_SIGNING_SHA256_CERT_FINGERPRINTS=' \
@@ -340,6 +342,16 @@ chmod 600 "$android_env"
 ./scripts/validate-android-environment.sh \
   "$android_env" development >/dev/null
 
+android_missing_authorized_party_env="$scan_dir/android-missing-authorized-party.env"
+grep -v '^GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS=' \
+  "$android_env" >"$android_missing_authorized_party_env"
+chmod 600 "$android_missing_authorized_party_env"
+if ./scripts/validate-android-environment.sh \
+  "$android_missing_authorized_party_env" development >/dev/null 2>&1; then
+  echo "Android validation accepted an empty Google authorized-party allowlist." >&2
+  exit 1
+fi
+
 sed -i \
   's|^ANDROID_APP_LINKS_PACKAGE_NAME=.*|ANDROID_APP_LINKS_PACKAGE_NAME=org.whoneedhelp.mobile.staging|' \
   "$android_env"
@@ -457,6 +469,7 @@ fi
 TEST_CODEX_SESSION_ID=00000000-0000-0000-0000-000000000001 \
 TEST_GOOGLE_OAUTH_CLIENT_ID=quality-test-client \
 TEST_GOOGLE_OAUTH_CLIENT_SECRET="quality-test\$secret" \
+TEST_GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS=quality-test-android-client \
 TEST_WEB_PUSH_VAPID_PUBLIC_KEY=quality-test-vapid-public \
 TEST_WEB_PUSH_VAPID_PRIVATE_KEY=quality-test-vapid-private \
 TEST_WEB_PUSH_VAPID_SUBJECT=mailto:test@help.test \
@@ -486,6 +499,8 @@ grep -Fx 'SMTP_RELAY=mailpit' "$test_env" >/dev/null
 grep -Fx 'EMAIL_FROM_NAME="Who Need Help Test"' "$test_env" >/dev/null
 grep -Fx 'GOOGLE_OAUTH_CLIENT_ID=quality-test-client' "$test_env" >/dev/null
 grep -Fx "GOOGLE_OAUTH_CLIENT_SECRET=quality-test\$\$secret" "$test_env" >/dev/null
+grep -Fx 'GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS=quality-test-android-client' \
+  "$test_env" >/dev/null
 grep -Fx 'WEB_PUSH_VAPID_SUBJECT=mailto:test@help.test' "$test_env" >/dev/null
 grep -Fx 'WNH_FIREBASE_APPLICATION_ID=1:123456789:android:quality-test' "$test_env" >/dev/null
 grep -Fx 'WNH_FIREBASE_PROJECT_ID=quality-test' "$test_env" >/dev/null
@@ -592,6 +607,7 @@ PRODUCTION_EMAIL_FROM_ADDRESS=contact@help.test \
 PRODUCTION_CODEX_SESSION_ID=00000000-0000-0000-0000-000000000001 \
 PRODUCTION_GOOGLE_OAUTH_CLIENT_ID=quality-production-client \
 PRODUCTION_GOOGLE_OAUTH_CLIENT_SECRET=quality-production-secret \
+PRODUCTION_GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS=quality-production-android-client \
 PRODUCTION_WEB_PUSH_VAPID_PUBLIC_KEY=quality-vapid-public \
 PRODUCTION_WEB_PUSH_VAPID_PRIVATE_KEY=quality-vapid-private \
 PRODUCTION_WEB_PUSH_VAPID_SUBJECT=mailto:contact@help.test \
@@ -608,6 +624,8 @@ PRODUCTION_SUPPORT_INBOX_ADDRESS=support@help.test \
   ./scripts/init-production-env.sh help.test "$production_env" >/dev/null
 test "$(stat -c '%a' "$production_env")" = 600
 grep -Fx "SMTP_PASSWORD=quality\$\$password" "$production_env" >/dev/null
+grep -Fx 'GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS=quality-production-android-client' \
+  "$production_env" >/dev/null
 ./scripts/validate-production-env.sh "$production_env" help.test >/dev/null
 ./scripts/check-environment-readiness.sh "$production_env" --require-release >/dev/null
 if PRODUCTION_CODEX_SESSION_ID=00000000-0000-0000-0000-000000000001 \
diff --git a/scripts/validate-android-environment.sh b/scripts/validate-android-environment.sh
index 3e540fb..dd1bfcb 100755
--- a/scripts/validate-android-environment.sh
+++ b/scripts/validate-android-environment.sh
@@ -63,6 +63,8 @@ phx_host=$(read_unique PHX_HOST)
 phx_scheme=$(read_unique PHX_SCHEME)
 phx_port=$(read_unique PHX_URL_PORT)
 base_url=$(read_unique WNH_BASE_URL)
+google_oauth_client_id=$(read_unique GOOGLE_OAUTH_CLIENT_ID)
+google_oauth_authorized_party_ids=$(read_unique GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS)
 app_links_package=$(read_unique ANDROID_APP_LINKS_PACKAGE_NAME)
 app_links_fingerprints=$(read_unique ANDROID_APP_LINKS_SHA256_CERT_FINGERPRINTS)
 play_app_signing_fingerprints=
@@ -102,6 +104,22 @@ fi
   echo "WNH_BASE_URL must equal the canonical PHX origin: $expected_origin" >&2
   exit 1
 }
+[[ -n "$google_oauth_client_id" ]] || {
+  echo "Android builds require the environment's Google Web OAuth client ID." >&2
+  exit 1
+}
+[[ -n "$google_oauth_authorized_party_ids" ]] || {
+  echo "Android builds require at least one authorized Android Google OAuth client ID." >&2
+  exit 1
+}
+IFS=',' read -r -a google_authorized_parties <<<"$google_oauth_authorized_party_ids"
+for authorized_party in "${google_authorized_parties[@]}"; do
+  compact_party=${authorized_party//[[:space:]]/}
+  [[ -n "$compact_party" ]] || {
+    echo "GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS contains an empty value." >&2
+    exit 1
+  }
+done
 
 IFS=',' read -r -a fingerprints <<<"$app_links_fingerprints"
 for fingerprint in "${fingerprints[@]}"; do
diff --git a/scripts/validate-deployment-isolation.sh b/scripts/validate-deployment-isolation.sh
index 92d12c4..8907218 100755
--- a/scripts/validate-deployment-isolation.sh
+++ b/scripts/validate-deployment-isolation.sh
@@ -94,6 +94,21 @@ if [[ -n "$test_google_id" || -n "$production_google_id" ]]; then
   }
 fi
 
+test_google_authorized_parties=$(
+  read_env "$test_env" GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS 2>/dev/null || true
+)
+production_google_authorized_parties=$(
+  read_env "$production_env" GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS 2>/dev/null || true
+)
+if [[ -n "$test_google_authorized_parties" || -n "$production_google_authorized_parties" ]]; then
+  [[ -n "$test_google_authorized_parties" &&
+    -n "$production_google_authorized_parties" &&
+    "$test_google_authorized_parties" != "$production_google_authorized_parties" ]] || {
+    echo "Configured test and production Android Google clients must be different." >&2
+    exit 1
+  }
+fi
+
 test_email_provider=$(read_env "$test_env" EMAIL_DELIVERY_PROVIDER)
 production_email_provider=$(read_env "$production_env" EMAIL_DELIVERY_PROVIDER)
 
diff --git a/scripts/validate-production-env.sh b/scripts/validate-production-env.sh
index 83d16f4..936d537 100755
--- a/scripts/validate-production-env.sh
+++ b/scripts/validate-production-env.sh
@@ -68,6 +68,19 @@ valid_fcm_service_account_json() {
   ' >/dev/null 2>&1
 }
 
+valid_nonempty_csv() {
+  local item compact
+  local -a items
+
+  IFS=',' read -r -a items <<<"$1"
+  [[ ${#items[@]} -gt 0 ]] || return 1
+
+  for item in "${items[@]}"; do
+    compact=${item//[[:space:]]/}
+    [[ -n "$compact" ]] || return 1
+  done
+}
+
 reject_marker() {
   local key=$1
   local value=$2
@@ -119,6 +132,7 @@ email_from_address=$(require_value EMAIL_FROM_ADDRESS)
 support_inbox_address=$(optional_value SUPPORT_INBOX_ADDRESS)
 google_oauth_client_id=$(optional_value GOOGLE_OAUTH_CLIENT_ID)
 google_oauth_client_secret=$(optional_value GOOGLE_OAUTH_CLIENT_SECRET)
+google_oauth_authorized_party_ids=$(optional_value GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS)
 web_push_vapid_public_key=$(optional_value WEB_PUSH_VAPID_PUBLIC_KEY)
 web_push_vapid_private_key=$(optional_value WEB_PUSH_VAPID_PRIVATE_KEY)
 web_push_vapid_subject=$(optional_value WEB_PUSH_VAPID_SUBJECT)
@@ -337,6 +351,19 @@ if [[ -n "$google_oauth_client_id" || -n "$google_oauth_client_secret" ]]; then
   reject_marker GOOGLE_OAUTH_CLIENT_ID "$google_oauth_client_id"
   reject_marker GOOGLE_OAUTH_CLIENT_SECRET "$google_oauth_client_secret"
 fi
+if [[ -n "$google_oauth_authorized_party_ids" ]]; then
+  [[ -n "$google_oauth_client_id" && -n "$google_oauth_client_secret" ]] || {
+    echo "Android Google authorized parties require the Google OAuth client." >&2
+    exit 1
+  }
+
+  reject_marker GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS "$google_oauth_authorized_party_ids"
+
+  valid_nonempty_csv "$google_oauth_authorized_party_ids" || {
+    echo "GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS must be a non-empty CSV list." >&2
+    exit 1
+  }
+fi
 
 firebase_values=(
   "$firebase_application_id"
diff --git a/scripts/validate-test-env.sh b/scripts/validate-test-env.sh
index 4271b96..ce12ba2 100755
--- a/scripts/validate-test-env.sh
+++ b/scripts/validate-test-env.sh
@@ -50,6 +50,19 @@ valid_fcm_service_account_json() {
   ' >/dev/null 2>&1
 }
 
+valid_nonempty_csv() {
+  local item compact
+  local -a items
+
+  IFS=',' read -r -a items <<<"$1"
+  [[ ${#items[@]} -gt 0 ]] || return 1
+
+  for item in "${items[@]}"; do
+    compact=${item//[[:space:]]/}
+    [[ -n "$compact" ]] || return 1
+  done
+}
+
 [[ "$(require_value DEPLOYMENT_ENV)" == test ]] || {
   echo "Test validation requires DEPLOYMENT_ENV=test." >&2
   exit 1
@@ -126,12 +139,24 @@ require_value EMAIL_FROM_ADDRESS >/dev/null
 
 google_id=$(read_value GOOGLE_OAUTH_CLIENT_ID 2>/dev/null || true)
 google_secret=$(read_value GOOGLE_OAUTH_CLIENT_SECRET 2>/dev/null || true)
+google_authorized_party_ids=$(
+  read_value GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS 2>/dev/null || true
+)
 if [[ -n "$google_id" || -n "$google_secret" ]]; then
   [[ -n "$google_id" && -n "$google_secret" ]] || {
     echo "Test Google OAuth ID and secret must be configured together." >&2
     exit 1
   }
 fi
+if [[ -n "$google_authorized_party_ids" && (-z "$google_id" || -z "$google_secret") ]]; then
+  echo "Test Android Google authorized parties require the Google OAuth client." >&2
+  exit 1
+fi
+if [[ -n "$google_authorized_party_ids" ]] &&
+  ! valid_nonempty_csv "$google_authorized_party_ids"; then
+  echo "Test GOOGLE_OAUTH_AUTHORIZED_PARTY_IDS must be a non-empty CSV list." >&2
+  exit 1
+fi
 
 firebase_values=(
   "$(read_value WNH_FIREBASE_APPLICATION_ID 2>/dev/null || true)"
diff --git a/test/who_need_help/google_auth_test.exs b/test/who_need_help/google_auth_test.exs
index 872ed69..d5372f6 100644
--- a/test/who_need_help/google_auth_test.exs
+++ b/test/who_need_help/google_auth_test.exs
@@ -3,6 +3,25 @@ defmodule WhoNeedHelp.GoogleAuthTest do
 
   alias WhoNeedHelp.GoogleAuth.AssentAdapter
 
+  defmodule GoogleJwksHTTPAdapter do
+    @behaviour Assent.HTTPAdapter
+
+    alias Assent.HTTPAdapter.HTTPResponse
+
+    @impl true
+    def request(:get, "https://accounts.google.test/keys", nil, _headers, options) do
+      {:ok,
+       %HTTPResponse{
+         status: 200,
+         headers: [{"content-type", "application/json"}],
+         body: Jason.encode!(%{"keys" => Keyword.fetch!(options, :jwks)})
+       }}
+    end
+
+    def request(_method, _url, _body, _headers, _options),
+      do: {:error, :unexpected_google_test_request}
+  end
+
   test "Google authorization uses OIDC state, nonce, PKCE, and identity-only scopes" do
     nonce = "session-bound-nonce"
 
@@ -145,10 +164,217 @@ defmodule WhoNeedHelp.GoogleAuthTest do
     assert {:error, _reason} = AssentAdapter.verify_id_token(expired, nonce)
   end
 
+  test "native ID token accepts only an allowlisted Android authorized party" do
+    web_client_id = "web-client.apps.googleusercontent.com"
+    android_client_id = "android-client.apps.googleusercontent.com"
+    client_secret = "native-test-signing-secret-with-sufficient-length"
+    nonce = "one-time-cross-client-nonce"
+    now = System.system_time(:second)
+
+    restore_google_auth_config()
+
+    Application.put_env(
+      :who_need_help,
+      :google_auth,
+      client_id: web_client_id,
+      client_secret: client_secret,
+      authorized_party_ids: [android_client_id],
+      id_token_signed_response_alg: "HS256",
+      openid_configuration: %{"issuer" => "https://accounts.google.com"}
+    )
+
+    claims = %{
+      "iss" => "https://accounts.google.com",
+      "sub" => "cross-client-subject",
+      "aud" => web_client_id,
+      "azp" => android_client_id,
+      "iat" => now,
+      "exp" => now + 300,
+      "nonce" => nonce,
+      "email" => "CrossClient@Example.COM",
+      "email_verified" => true,
+      "name" => "Cross Client"
+    }
+
+    token = signed_id_token(client_secret, claims)
+
+    assert {:ok,
+            %{
+              provider_uid: "cross-client-subject",
+              email: "crossclient@example.com",
+              email_verified: true,
+              display_name: "Cross Client"
+            }} = AssentAdapter.verify_id_token(token, nonce)
+
+    unauthorized_token =
+      signed_id_token(client_secret, %{claims | "azp" => "other.apps.googleusercontent.com"})
+
+    assert {:error, _reason} = AssentAdapter.verify_id_token(unauthorized_token, nonce)
+
+    assert {:error, _reason} =
+             AssentAdapter.verify_id_token(token, "different-cross-client-nonce")
+
+    expired_token =
+      signed_id_token(client_secret, %{claims | "iat" => now - 600, "exp" => now - 300})
+
+    assert {:error, _reason} = AssentAdapter.verify_id_token(expired_token, nonce)
+
+    future_token = signed_id_token(client_secret, %{claims | "iat" => now + 300})
+
+    assert {:error, _reason} = AssentAdapter.verify_id_token(future_token, nonce)
+
+    invalid_signature =
+      signed_id_token("a-different-signing-secret-with-sufficient-length", claims)
+
+    assert {:error, _reason} = AssentAdapter.verify_id_token(invalid_signature, nonce)
+  end
+
+  test "native ID token does not bypass ordinary issuer, audience, or algorithm checks" do
+    web_client_id = "web-client.apps.googleusercontent.com"
+    android_client_id = "android-client.apps.googleusercontent.com"
+    client_secret = "native-test-signing-secret-with-sufficient-length"
+    nonce = "cross-client-negative-nonce"
+    now = System.system_time(:second)
+
+    restore_google_auth_config()
+
+    Application.put_env(
+      :who_need_help,
+      :google_auth,
+      client_id: web_client_id,
+      client_secret: client_secret,
+      authorized_party_ids: [android_client_id],
+      id_token_signed_response_alg: "HS256",
+      openid_configuration: %{"issuer" => "https://accounts.google.com"}
+    )
+
+    valid_claims = %{
+      "iss" => "https://accounts.google.com",
+      "sub" => "cross-client-subject",
+      "aud" => web_client_id,
+      "azp" => android_client_id,
+      "iat" => now,
+      "exp" => now + 300,
+      "nonce" => nonce,
+      "email" => "crossclient@example.com",
+      "email_verified" => true
+    }
+
+    wrong_issuer =
+      signed_id_token(client_secret, %{valid_claims | "iss" => "https://issuer.invalid"})
+
+    assert {:error, _reason} = AssentAdapter.verify_id_token(wrong_issuer, nonce)
+
+    wrong_audience =
+      signed_id_token(client_secret, %{
+        valid_claims
+        | "aud" => "other-web.apps.googleusercontent.com"
+      })
+
+    assert {:error, _reason} = AssentAdapter.verify_id_token(wrong_audience, nonce)
+
+    wrong_algorithm =
+      "a-different-signing-secret-with-sufficient-length"
+      |> signed_id_token_with_algorithm("HS384", valid_claims)
+
+    assert {:error, _reason} = AssentAdapter.verify_id_token(wrong_algorithm, nonce)
+  end
+
+  test "native cross-client verification validates an RS256 signature with the selected JWK" do
+    web_client_id = "web-client.apps.googleusercontent.com"
+    android_client_id = "android-client.apps.googleusercontent.com"
+    nonce = "cross-client-rs256-nonce"
+    now = System.system_time(:second)
+    private_jwk = JOSE.JWK.generate_key({:rsa, 2048})
+
+    public_jwk =
+      private_jwk
+      |> JOSE.JWK.to_public()
+      |> JOSE.JWK.to_map()
+      |> elem(1)
+      |> Map.put("kid", "google-test-key")
+
+    restore_google_auth_config()
+
+    Application.put_env(
+      :who_need_help,
+      :google_auth,
+      client_id: web_client_id,
+      client_secret: "unused-by-rs256-verification",
+      authorized_party_ids: [android_client_id],
+      http_adapter: {GoogleJwksHTTPAdapter, jwks: [public_jwk]},
+      openid_configuration: %{
+        "issuer" => "https://accounts.google.com",
+        "jwks_uri" => "https://accounts.google.test/keys"
+      }
+    )
+
+    token =
+      private_jwk
+      |> JOSE.JWT.sign(
+        %{"alg" => "RS256", "kid" => "google-test-key"},
+        %{
+          "iss" => "https://accounts.google.com",
+          "sub" => "cross-client-rs256-subject",
+          "aud" => web_client_id,
+          "azp" => android_client_id,
+          "iat" => now,
+          "exp" => now + 300,
+          "nonce" => nonce,
+          "email" => "rs256@example.com",
+          "email_verified" => true
+        }
+      )
+      |> JOSE.JWS.compact()
+      |> elem(1)
+
+    assert {:ok, %{provider_uid: "cross-client-rs256-subject"}} =
+             AssentAdapter.verify_id_token(token, nonce)
+
+    wrong_private_jwk = JOSE.JWK.generate_key({:rsa, 2048})
+
+    invalid_signature =
+      wrong_private_jwk
+      |> JOSE.JWT.sign(
+        %{"alg" => "RS256", "kid" => "google-test-key"},
+        %{
+          "iss" => "https://accounts.google.com",
+          "sub" => "cross-client-rs256-subject",
+          "aud" => web_client_id,
+          "azp" => android_client_id,
+          "iat" => now,
+          "exp" => now + 300,
+          "nonce" => nonce,
+          "email" => "rs256@example.com",
+          "email_verified" => true
+        }
+      )
+      |> JOSE.JWS.compact()
+      |> elem(1)
+
+    assert {:error, _reason} = AssentAdapter.verify_id_token(invalid_signature, nonce)
+  end
+
+  defp restore_google_auth_config do
+    original = Application.get_env(:who_need_help, :google_auth)
+
+    on_exit(fn ->
+      if is_nil(original) do
+        Application.delete_env(:who_need_help, :google_auth)
+      else
+        Application.put_env(:who_need_help, :google_auth, original)
+      end
+    end)
+  end
+
   defp signed_id_token(secret, claims) do
+    signed_id_token_with_algorithm(secret, "HS256", claims)
+  end
+
+  defp signed_id_token_with_algorithm(secret, algorithm, claims) do
     secret
     |> JOSE.JWK.from_oct()
-    |> JOSE.JWT.sign(%{"alg" => "HS256"}, claims)
+    |> JOSE.JWT.sign(%{"alg" => algorithm}, claims)
     |> JOSE.JWS.compact()
     |> elem(1)
   end