diff --git a/PLAYBOOK.md b/PLAYBOOK.md
index 3674e1c..ebe3878 100644
--- a/PLAYBOOK.md
+++ b/PLAYBOOK.md
@@ -686,7 +686,7 @@ Claude Code makes, all of which are POSTs. The symptom — every call failing on
 freshly installed provider — reads as "the credential is wrong", and the
 credential is fine.
 
-**Three things belong in the allowlist that are not in `hosts`:**
+**Four things belong in the allowlist that are not in `hosts`:**
 
 - Hosts the agent must reach but must never be credentialed — `github.com` for
   git push/pull, which `010_github.py` deliberately does not match.
@@ -696,6 +696,29 @@ credential is fine.
 - Multi-tenant suffixes that are fine to reach and unsafe to inject for —
   `*.workers.dev`. Reachable and credentialed are different lists, and this is
   the direction where conflating them is dangerous.
+- **The client's own startup checks, which are rarely on the API host.** Claude
+  Code calls `GET platform.claude.com/v1/oauth/hello` before it will run and
+  treats failure as fatal, so an `anthropic` entry listing only
+  `api.anthropic.com` installs cleanly, injects correctly, and produces an
+  agent that refuses to start. The endpoint needs no credential — it answers
+  200 unauthenticated — so it is allowlist-only by the first bullet's rule.
+  Find these the same way you find the methods: run the client once against a
+  real allowlist and read the `blocked` lines out of the trail. Do not reason
+  about them from the vendor's API documentation, which describes the API and
+  not the client.
+
+  This is the category most likely to be missed, because the failure does not
+  look like an egress failure. The message the user sees names the *proxy*, and
+  reads as a credential or TLS problem:
+
+  ```
+  Unable to connect to Anthropic services
+  Failed to connect to platform.claude.com: Status 403
+  ```
+
+  That 403 is `001_allowlist.py` refusing, and the matching
+  `{"event":"blocked","reason":"allowlist",...}` line in the trail is what says
+  so. Check there first when a freshly installed provider will not start.
 
 **Ship optional hosts commented out**, with the reason on the line. Telemetry,
 error reporting, CDN mirrors, anything the provider works without. The default
diff --git a/bank/anthropic/allowlist b/bank/anthropic/allowlist
index 6852c0a..401f3a7 100644
--- a/bank/anthropic/allowlist
+++ b/bank/anthropic/allowlist
@@ -9,6 +9,7 @@
 # `api.anthropic.com` line blocks the provider completely while looking right.
 
 api.anthropic.com       GET,POST
+platform.claude.com     GET
 
 # GET is here for model listing (GET /v1/models), which some clients call at
 # startup; POST carries /v1/messages and /v1/messages/count_tokens. Narrowing
@@ -16,6 +17,26 @@ api.anthropic.com       GET,POST
 # know your client never lists models. Widening past these two is not needed by
 # anything: the Admin API under /v1/organizations is refused by the addon
 # regardless of what this file says.
+#
+# platform.claude.com is the client's startup connectivity check (Claude Code
+# 2.x calls GET /v1/oauth/hello) and failing it is FATAL — the agent installs
+# cleanly, gets its credential injected correctly, and still refuses to start.
+# It is the second line rather than an optional one for that reason. Leaving it
+# out is easy to misdiagnose: what the user sees names a proxy, not an
+# allowlist, so it reads as a credential or TLS problem rather than an egress
+# one.
+#
+#     Unable to connect to Anthropic services
+#     Failed to connect to platform.claude.com: Status 403
+#
+# That 403 is the proxy's own refusal. The endpoint needs no credential — it
+# answers 200 unauthenticated — which is why it belongs here and NOT in the
+# entry's `hosts`, on the same reasoning as the OPTIONAL block below.
+#
+# Editing this file is not enough on its own: the proxy reads the allowlist
+# once, at startup, and the file is a bind mount, so `docker compose up -d`
+# sees no config change and the container keeps the ruleset it already has.
+# Restart the proxy.
 
 # ---------------------------------------------------------------------------
 # OPTIONAL — nothing below is needed for the provider to work.