diff --git a/examples/multi_platform_seller/src/app.py b/examples/multi_platform_seller/src/app.py
index 31ac5eded..d6e57d97a 100644
--- a/examples/multi_platform_seller/src/app.py
+++ b/examples/multi_platform_seller/src/app.py
@@ -114,23 +114,6 @@ def build_subdomain_middleware() -> tuple[type, dict[str, object]]:
     return SubdomainTenantMiddleware, {"router": subdomain_router}
 
 
-def _allowed_hosts() -> list[str]:
-    """The TransportSecurityMiddleware host allowlist.
-
-    FastMCP's DNS-rebinding-protection default only accepts loopback
-    patterns; the tenant subdomains have to be added explicitly. Both
-    bare hosts and ``host:*`` (any-port) wildcards work; using
-    ``host:*`` keeps the example port-agnostic for adopters who change
-    ``ADCP_PORT``.
-    """
-    return [
-        "tenant-a.localhost",
-        "tenant-a.localhost:*",
-        "tenant-b.localhost",
-        "tenant-b.localhost:*",
-    ]
-
-
 if __name__ == "__main__":
     router = build_router()
     middleware_class, middleware_kwargs = build_subdomain_middleware()
@@ -142,10 +125,8 @@ def _allowed_hosts() -> list[str]:
         auto_emit_completion_webhooks=False,
         # ``serve()`` forwards extra kwargs to ``adcp.server.serve``;
         # the underlying transport accepts a Starlette middleware list.
+        # serve() auto-synthesizes bare + :* allowed_hosts entries from
+        # the SubdomainTenantMiddleware router, so no separate host list
+        # is needed here.
         asgi_middleware=[(middleware_class, middleware_kwargs)],
-        # Extend FastMCP's host allowlist to include the tenant
-        # subdomains. Without this the transport returns 421 on every
-        # ``Host: tenant-x.localhost`` request before the subdomain
-        # router gets a chance to resolve.
-        allowed_hosts=_allowed_hosts(),
     )
diff --git a/src/adcp/server/serve.py b/src/adcp/server/serve.py
index 9fd90597a..1bb47c0c6 100644
--- a/src/adcp/server/serve.py
+++ b/src/adcp/server/serve.py
@@ -657,7 +657,16 @@ async def force_account_status(self, account_id, status):
         debug_traffic_source=debug_traffic_source,
     )
 
+    # Auto-synthesize bare+:* host pairs from any SubdomainTenantMiddleware
+    # in asgi_middleware so the router's host list is the single source of
+    # truth for both lookup and the FastMCP DNS-rebinding allowlist.
+    allowed_hosts = _synthesize_allowed_hosts(asgi_middleware, allowed_hosts)
+
     if transport == "a2a":
+        # allowed_hosts / allowed_origins are FastMCP / MCP-layer knobs and
+        # are not forwarded to the A2A path — the A2A transport doesn't use
+        # FastMCP's TransportSecuritySettings.  Host validation on the A2A
+        # path is handled by the outer ASGI stack (e.g. SubdomainTenantMiddleware).
         _serve_a2a(
             handler,
             name=name,
@@ -769,6 +778,76 @@ def _prepend_debug_endpoint(
     return [debug_entry, *asgi_middleware]
 
 
+def _synthesize_allowed_hosts(
+    asgi_middleware: Sequence[ASGIMiddlewareEntry] | None,
+    allowed_hosts: Sequence[str] | None,
+) -> Sequence[str] | None:
+    """Auto-expand ``allowed_hosts`` from :class:`SubdomainTenantMiddleware` routers.
+
+    When the middleware list contains a :class:`SubdomainTenantMiddleware`
+    entry (or a subclass) whose ``router`` exposes a ``hosts()`` method (as
+    :class:`InMemorySubdomainTenantRouter` does), this synthesizes both the
+    bare host and the ``:*`` port-wildcard variant for each registered host.
+    That makes the FastMCP DNS-rebinding allowlist symmetric with the
+    router's port-agnostic ``_normalize_host`` resolution — adopters no
+    longer have to maintain a separate ``_allowed_hosts()`` helper.
+
+    Synthesis is a one-time snapshot at ``serve()`` startup.  Production
+    adopters with dynamically provisioned tenants (SQL-backed routers) should
+    pass ``enable_dns_rebinding_protection=False`` and rely on
+    :class:`SubdomainTenantMiddleware` alone for host validation — it already
+    enforces the allowlist per-request.
+
+    Existing explicit ``allowed_hosts`` entries are preserved and
+    deduplicated against the synthesized set.  Custom router implementations
+    that want to benefit from auto-synthesis must expose a ``hosts() ->
+    list[str]`` method; see :class:`InMemorySubdomainTenantRouter` as the
+    reference.
+    """
+    import warnings
+
+    from adcp.server.tenant_router import SubdomainTenantMiddleware as _SubdomainTenantMw
+
+    synthesized: list[str] = []
+    for entry in asgi_middleware or []:
+        if not (isinstance(entry, tuple) and len(entry) == 2):
+            continue
+        cls, kwargs = entry
+        # Use issubclass (with a TypeError guard for non-class callables such
+        # as functools.partial) so adopters who subclass SubdomainTenantMiddleware
+        # to add logging or extra exclusion logic also trigger synthesis.
+        try:
+            is_subdomain_mw = isinstance(cls, type) and issubclass(cls, _SubdomainTenantMw)
+        except TypeError:
+            is_subdomain_mw = False
+        if not is_subdomain_mw:
+            continue
+        router = kwargs.get("router")
+        if router is None or not hasattr(router, "hosts"):
+            warnings.warn(
+                "SubdomainTenantMiddleware router does not expose hosts() — "
+                "auto-synthesis of allowed_hosts skipped. Multi-tenant subdomain "
+                "hosts may be rejected with 421 Misdirected Request unless listed "
+                "in allowed_hosts explicitly. Implement hosts() on your router or "
+                "pass enable_dns_rebinding_protection=False if the middleware "
+                "already handles all host validation.",
+                stacklevel=3,
+            )
+            continue
+        for bare_host in router.hosts():
+            if bare_host not in synthesized:
+                synthesized.append(bare_host)
+            port_wild = f"{bare_host}:*"
+            if port_wild not in synthesized:
+                synthesized.append(port_wild)
+
+    if not synthesized:
+        return allowed_hosts
+
+    existing = list(allowed_hosts or [])
+    return existing + [h for h in synthesized if h not in existing]
+
+
 def _apply_asgi_middleware(
     app: Any,
     asgi_middleware: Sequence[ASGIMiddlewareEntry] | None,
diff --git a/src/adcp/server/tenant_router.py b/src/adcp/server/tenant_router.py
index beb8cce6c..610d693a3 100644
--- a/src/adcp/server/tenant_router.py
+++ b/src/adcp/server/tenant_router.py
@@ -138,6 +138,17 @@ async def resolve(self, host: str) -> Tenant | None:
         """
         ...
 
+    # Optional convention (not part of the formal Protocol contract):
+    # Implementing ``hosts(self) -> list[str]`` on a custom router lets
+    # :func:`adcp.server.serve.serve` auto-synthesize the FastMCP
+    # ``allowed_hosts`` allowlist (bare + ``:*`` variants) from the
+    # router's registered host list, eliminating the need for a separate
+    # ``_allowed_hosts()`` helper in adopter code.  See
+    # :class:`InMemorySubdomainTenantRouter.hosts` for the reference
+    # implementation.  Routers without ``hosts()`` are skipped with a
+    # startup warning — the adopter must pass ``allowed_hosts`` explicitly
+    # or set ``enable_dns_rebinding_protection=False``.
+
 
 class InMemorySubdomainTenantRouter:
     """Reference :class:`SubdomainTenantRouter` for dev / test.
@@ -159,6 +170,17 @@ def __init__(self, tenants: Mapping[str, Tenant]) -> None:
     async def resolve(self, host: str) -> Tenant | None:
         return self._tenants.get(_normalize_host(host))
 
+    def hosts(self) -> list[str]:
+        """Return registered host names (normalized: lower-cased, port-stripped).
+
+        Called by :func:`adcp.server.serve.serve` to auto-synthesize the
+        FastMCP ``allowed_hosts`` allowlist — so the host list passed to
+        the router is the single source of truth for both lookup and
+        DNS-rebinding-protection.  Custom :class:`SubdomainTenantRouter`
+        implementations can expose the same method to get the same benefit.
+        """
+        return list(self._tenants.keys())
+
 
 # Module-level contextvar — request-scoped via the ASGI middleware's
 # per-call `set()`. ASGI guarantees per-task context isolation, so
diff --git a/tests/test_serve_transport_security.py b/tests/test_serve_transport_security.py
index 2d77f4a73..89ac51c21 100644
--- a/tests/test_serve_transport_security.py
+++ b/tests/test_serve_transport_security.py
@@ -17,10 +17,16 @@
 
 from __future__ import annotations
 
+import warnings
 from typing import Any
 
 from adcp.server.base import ADCPHandler
-from adcp.server.serve import create_mcp_server
+from adcp.server.serve import _synthesize_allowed_hosts, create_mcp_server
+from adcp.server.tenant_router import (
+    InMemorySubdomainTenantRouter,
+    SubdomainTenantMiddleware,
+    Tenant,
+)
 
 
 class _StubHandler(ADCPHandler[Any]):
@@ -69,6 +75,109 @@ def test_allowed_origins_extends_default_list() -> None:
     assert "http://acme.localhost:*" in ts.allowed_origins
 
 
+# ----- _synthesize_allowed_hosts -----------------------------------------
+
+
+def test_synthesize_produces_bare_and_port_wildcard() -> None:
+    """Each registered host gets both the bare form and ``:*`` so the
+    FastMCP allowlist is port-agnostic, matching the router's
+    ``_normalize_host`` port-stripping at lookup time."""
+    router = InMemorySubdomainTenantRouter(
+        tenants={
+            "acme.localhost": Tenant(id="acme", display_name="Acme"),
+            "beta.localhost": Tenant(id="beta", display_name="Beta"),
+        }
+    )
+    result = _synthesize_allowed_hosts(
+        [(SubdomainTenantMiddleware, {"router": router})],
+        allowed_hosts=None,
+    )
+    assert result is not None
+    assert "acme.localhost" in result
+    assert "acme.localhost:*" in result
+    assert "beta.localhost" in result
+    assert "beta.localhost:*" in result
+
+
+def test_synthesize_merges_with_explicit_allowed_hosts() -> None:
+    """Explicit ``allowed_hosts`` are preserved; synthesized entries are
+    appended without duplicates."""
+    router = InMemorySubdomainTenantRouter(
+        tenants={"acme.localhost": Tenant(id="acme", display_name="Acme")}
+    )
+    result = _synthesize_allowed_hosts(
+        [(SubdomainTenantMiddleware, {"router": router})],
+        allowed_hosts=["extra.example.com"],
+    )
+    assert result is not None
+    assert "extra.example.com" in result
+    assert "acme.localhost" in result
+    assert "acme.localhost:*" in result
+    # No duplicate bare host
+    assert list(result).count("acme.localhost") == 1
+
+
+def test_synthesize_noop_when_no_subdomain_middleware() -> None:
+    """No SubdomainTenantMiddleware in the list → returned value is
+    unchanged (None stays None, explicit list stays unchanged)."""
+    assert _synthesize_allowed_hosts(None, None) is None
+    assert _synthesize_allowed_hosts([], None) is None
+    explicit: list[str] = ["host.example.com"]
+    result = _synthesize_allowed_hosts([], explicit)
+    assert result is explicit
+
+
+def test_synthesize_noop_for_router_without_hosts_method() -> None:
+    """Custom routers that don't expose ``hosts()`` are skipped and emit a
+    startup warning — no AttributeError, no silent 421."""
+
+    class _CustomRouter:
+        async def resolve(self, host: str) -> None:
+            return None
+
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        result = _synthesize_allowed_hosts(
+            [(SubdomainTenantMiddleware, {"router": _CustomRouter()})],
+            allowed_hosts=None,
+        )
+
+    assert result is None
+    assert len(caught) == 1
+    assert "hosts()" in str(caught[0].message)
+    assert "421" in str(caught[0].message)
+
+
+def test_synthesize_matches_subclass_of_subdomain_middleware() -> None:
+    """Adopters who subclass SubdomainTenantMiddleware (e.g. to add request
+    logging) still trigger synthesis — the check uses issubclass, not identity."""
+
+    class _LoggingTenantMiddleware(SubdomainTenantMiddleware):
+        pass
+
+    router = InMemorySubdomainTenantRouter(
+        tenants={"acme.localhost": Tenant(id="acme", display_name="Acme")}
+    )
+    result = _synthesize_allowed_hosts(
+        [(_LoggingTenantMiddleware, {"router": router})],
+        allowed_hosts=None,
+    )
+    assert result is not None
+    assert "acme.localhost" in result
+    assert "acme.localhost:*" in result
+
+
+def test_synthesize_skips_callable_factory_entries() -> None:
+    """Callable-factory middleware entries (not tuples) are ignored."""
+    router = InMemorySubdomainTenantRouter(tenants={})
+
+    def _factory(app: Any) -> Any:
+        return app
+
+    result = _synthesize_allowed_hosts([_factory], allowed_hosts=None)
+    assert result is None
+
+
 def test_enable_dns_rebinding_protection_false_disables_check() -> None:
     """Adopters with their own tenant-aware host validation pass
     ``enable_dns_rebinding_protection=False`` so the MCP-layer check
diff --git a/tests/test_subdomain_tenant_router.py b/tests/test_subdomain_tenant_router.py
index c0d1d2501..3b966e981 100644
--- a/tests/test_subdomain_tenant_router.py
+++ b/tests/test_subdomain_tenant_router.py
@@ -114,6 +114,24 @@ def test_in_memory_router_satisfies_protocol() -> None:
     assert isinstance(router, SubdomainTenantRouter)
 
 
+def test_in_memory_router_hosts_returns_normalized_keys() -> None:
+    """hosts() returns the normalized (lower-cased, port-stripped) keys so
+    serve() can synthesize the FastMCP allowlist from the same source."""
+    router = InMemorySubdomainTenantRouter(
+        tenants={
+            "Acme.Localhost": Tenant(id="acme", display_name="Acme"),
+            "beta.localhost:8080": Tenant(id="beta", display_name="Beta"),
+        }
+    )
+    result = sorted(router.hosts())
+    assert result == ["acme.localhost", "beta.localhost"]
+
+
+def test_in_memory_router_hosts_empty() -> None:
+    router = InMemorySubdomainTenantRouter(tenants={})
+    assert router.hosts() == []
+
+
 # ----- middleware: known host happy path ------------------------------