diff --git a/src/content/changelog/trust-rules-refuse-pull-request-target.md b/src/content/changelog/trust-rules-refuse-pull-request-target.md
new file mode 100644
index 00000000..c6a32aaa
--- /dev/null
+++ b/src/content/changelog/trust-rules-refuse-pull-request-target.md
@@ -0,0 +1,18 @@
+---
+title: Trust rules refuse pull_request_target runs
+date: 2026-10-23
+area: API
+type: Changed
+docLink:
+  label: What a rule matches
+  href: /account/trust-rules/#what-a-rule-matches
+---
+
+GitHub Actions jobs started by `pull_request_target` can no longer sign in with
+a trust rule, because anyone who opens a pull request can start one, and it runs
+with your default branch's ref.
+
+- The sign-in is refused before any rule is checked, with the message
+  `pull_request_target runs cannot exchange tokens`.
+- Run Shorebird from a workflow started by `push`, `release`, or
+  `workflow_dispatch` instead.
diff --git a/src/content/docs/account/trust-rules.mdx b/src/content/docs/account/trust-rules.mdx
index 13756fae..3dd15804 100644
--- a/src/content/docs/account/trust-rules.mdx
+++ b/src/content/docs/account/trust-rules.mdx
@@ -61,9 +61,14 @@ A job signs in only when everything the rule names matches it:
   ending in a single `*`, such as `refs/tags/*` or `refs/heads/release/*`.
   Without one, the rule matches any ref, including pull request refs.
 
-A rule doesn't limit which events can start the job. To keep a workflow from
-signing in from any branch or pull request, set an environment that has required
-reviewers or branch restrictions in GitHub, or a ref pattern.
+Jobs started by `pull_request_target` can't sign in, whatever the rule says:
+such a run carries the default branch's ref but can be started by anyone who
+opens a pull request. A rule doesn't otherwise limit which events can start the
+job, and a ref pattern limits which branch the job runs on, not who started it.
+Some other events, such as `issue_comment` and `workflow_run`, also run on the
+default branch and can be started by people outside your repository. To keep
+those from signing in, set an environment that has required reviewers in GitHub,
+or don't start the workflow from them.
 
 Only the rules in the project named by `SHOREBIRD_PROJECT_ID` are considered. If
 rules on two different service accounts in that project match the same job, the
@@ -116,6 +121,10 @@ The CLI prints the reason. The common ones:
 - `The job runs a workflow from another repository`: the job is defined in a
   reusable workflow from another repository. Move the steps that run Shorebird
   into a workflow in the repository itself.
+- `pull_request_target runs cannot exchange tokens`: the job was started by
+  `pull_request_target`. Run Shorebird from a workflow started by an event such
+  as `push`, `release`, or `workflow_dispatch`, or use a
+  [service account key](/account/service-accounts/).
 - `Signing in with GitHub Actions OIDC requires SHOREBIRD_PROJECT_ID`: set
   `SHOREBIRD_PROJECT_ID` in the job's `env`.
 - `You must be logged in to run this command`, in GitHub Actions: the job has
diff --git a/src/content/docs/code-push/ci/github.mdx b/src/content/docs/code-push/ci/github.mdx
index b25f6c67..fcb2d046 100644
--- a/src/content/docs/code-push/ci/github.mdx
+++ b/src/content/docs/code-push/ci/github.mdx
@@ -194,7 +194,9 @@ repository secret, then
 
 If the sign-in is refused, the CLI prints the reason, such as
 `No trust rule matches this workflow`. Check that the repository, workflow file,
-environment and ref of the job match the trust rule.
+environment and ref of the job match the trust rule. Jobs started by
+`pull_request_target` can't sign in; see
+[when a sign-in is refused](/account/trust-rules/#when-a-sign-in-is-refused).
 
 ## Android code signing