diff --git a/tests/tui_gateway/test_readiness_singleflight.py b/tests/tui_gateway/test_readiness_singleflight.py
index 76d38d563e..6dee859c30 100644
--- a/tests/tui_gateway/test_readiness_singleflight.py
+++ b/tests/tui_gateway/test_readiness_singleflight.py
@@ -192,3 +192,69 @@ def test_probe_outliving_its_budget_answers_retryable_unknown_then_reprobes(monk
     assert second["result"]["ok"] is True
     assert second["result"]["provider"] == "custom"
     assert len(calls) == 2
+
+
+def test_fast_probe_cannot_deadlock_the_share_lock(monkeypatch):
+    """A probe that settles before ``add_done_callback`` is registered must not
+    self-deadlock: the done callback acquires the same non-reentrant lock the
+    owner still held when registering it (a probe settling in that window runs
+    the callback inline on the RPC worker thread, freezing every later
+    readiness call and hanging the desktop at "Gateway checking")."""
+    def instant_resolve(requested=None, **kwargs):
+        return {"provider": "custom", "api_key": "no-key-required", "source": "config"}
+
+    _patch_fast_probe_env(monkeypatch, instant_resolve)
+    transport = _RecordingTransport()
+
+    # Real dispatch through the shared pool; the probe resolves immediately, so
+    # the future is routinely already done when the callback is registered.
+    for index in range(20):
+        _dispatch(transport, f"fast-{index}", "setup.runtime_check")
+
+    for index in range(20):
+        response = transport.wait_for(f"fast-{index}", timeout=5)
+        # An overlapping poll may legitimately answer the retryable error while
+        # the probe is in flight; it must NEVER hang the worker that owns it.
+        assert "result" in response or response["error"]["code"] == server._READINESS_IN_PROGRESS_ERR
+
+    # The lock must be free: the inflight entry was cleared, and a further
+    # readiness call neither blocks nor answers the retryable error.
+    deadline = time.monotonic() + 2
+    while server._readiness_inflight and time.monotonic() < deadline:
+        time.sleep(0.01)
+    assert not server._readiness_inflight
+    _dispatch(transport, "after", "setup.runtime_check")
+    after = transport.wait_for("after", timeout=5)
+    assert after["result"]["ok"] is True
+
+
+def test_overlapping_poll_joins_a_fast_probe_instead_of_erroring(monkeypatch):
+    """Two consumers poll at the same seam (boot, the post-assignment
+    ``setup.ready`` broadcast): the joiner must read the shared result of a
+    fast probe, not answer the retryable error at once — the onboarding gate
+    treats an unknown pair (setup.status + setup.runtime_check both errored)
+    as not-ready and the blocking overlay never closes (the E2E hang)."""
+    release = threading.Event()
+
+    def brief_resolve(requested=None, **kwargs):
+        release.wait(timeout=10)
+        return {"provider": "custom", "api_key": "no-key-required", "source": "config"}
+
+    _patch_fast_probe_env(monkeypatch, brief_resolve)
+    transport = _RecordingTransport()
+
+    _dispatch(transport, "owner", "setup.runtime_check")
+
+    # Give the owner's probe a head start, then fire the overlapping poll.
+    time.sleep(0.05)
+    _dispatch(transport, "joiner", "setup.runtime_check")
+    time.sleep(0.05)
+
+    release.set()
+
+    owner = transport.wait_for("owner", timeout=3)
+    joiner = transport.wait_for("joiner", timeout=3)
+    assert owner["result"]["ok"] is True
+    # Inside the join grace the joiner reads the shared result — the same
+    # authoritative answer, not the retryable-in-progress error.
+    assert joiner["result"]["ok"] is True
diff --git a/tui_gateway/methods_config.py b/tui_gateway/methods_config.py
index 9605fa27a3..a7597e0354 100644
--- a/tui_gateway/methods_config.py
+++ b/tui_gateway/methods_config.py
@@ -27,10 +27,17 @@ _profile_scoped = _registry.profile_scoped
 # ``(kind, profile, requested provider)``:
 #
 # * the first caller submits the probe and waits a bounded budget;
-# * an overlapping poll for a still-running probe answers a retryable error
-#   immediately — a JSON-RPC error, never a fabricated ``ok`` (the result
-#   contract requires the real shape, and the desktop already treats an errored
-#   runtime_check as unknown, keeping setup.status authoritative);
+# * an overlapping poll for a still-running probe waits a short join grace for
+#   it — the Desktop fires setup.status + setup.runtime_check from independent
+#   consumers at the same seam (boot, the post-assignment ``setup.ready``
+#   broadcast), and answering the retryable error AT ONCE made both legs of
+#   one consumer transiently unknown, which the onboarding gate read as
+#   not-ready and the overlay never closed. Fast (config-read) probes settle
+#   well inside the grace, so an overlapping poll reads the shared result; a
+#   probe still running after it answers a retryable error instead —
+#   a JSON-RPC error, never a fabricated ``ok`` (the result contract requires
+#   the real shape, and the desktop already treats an errored runtime_check as
+#   unknown, keeping setup.status authoritative);
 # * a probe that outlives the budget answers the same retryable error while it
 #   keeps running in the background; the in-flight entry is cleared when it
 #   settles, so the next poll starts a fresh probe and never reads a stale one.
@@ -41,6 +48,14 @@ atexit.register(lambda: _readiness_pool.shutdown(wait=False, cancel_futures=True
 _readiness_lock = threading.Lock()
 _readiness_inflight: dict[tuple, concurrent.futures.Future] = {}
 _READINESS_SHARE_WAIT_SECONDS = 4.0
+# Join grace for an overlapping poll on a still-running probe: fast (config-read)
+# probes settle in milliseconds, so an overlapping consumer at the same seam
+# (statusbar + onboarding, both polling setup.status / setup.runtime_check) reads
+# the shared result instead of an unknown-readiness error pair — which the
+# onboarding gate read as not-ready and the blocking overlay never closed.
+# Kept well under a second: a joiner occupies its shared RPC worker for at most
+# the grace, so overlapping polls still cannot starve the pool (#65151).
+_READINESS_JOIN_GRACE_SECONDS = 0.5
 # setup.status's probe legitimately blocks on the boot bootstrap's record
 # (free_tier_bootstrap.SETUP_READY_WAIT_SECONDS = 8s): its join budget must
 # cover that wait or every boot poll would answer the retryable error.
@@ -301,28 +316,36 @@ def _readiness_cleared(key):
 def _readiness_share(rid, key, run_probe, wait_seconds):
     """Run ``run_probe`` single-flighted under ``key`` on the dedicated readiness pool.
 
-    The first caller submits the probe and waits up to ``wait_seconds``; a caller that
-    finds a still-running probe answers the retryable error immediately (its shared RPC
-    worker is freed at once — the probe keeps running for the first caller), and one that
-    finds it settled reads the shared result. A probe that outlives the budget answers
-    the same retryable error while it continues in the background."""
+    The first caller submits the probe and waits up to ``wait_seconds``; a caller that finds a
+    still-running probe waits the short join grace for it first — the Desktop fires
+    setup.status + setup.runtime_check from independent consumers at the same seam (boot, the
+    post-assignment ``setup.ready`` broadcast), and answering the retryable error at once made
+    both legs of one consumer transiently unknown, which its onboarding gate read as not-ready.
+    Fast (config-read) probes settle well inside the grace, so an overlapping poll reads the
+    shared result; one that outlives the grace answers a retryable error, freeing its shared RPC
+    worker (the probe keeps running for the first caller). A probe that outlives ``wait_seconds``
+    answers the same retryable error while it continues in the background."""
     with _readiness_lock:
         future = _readiness_inflight.get(key)
         owner = future is None
         if owner:
             future = _readiness_pool.submit(run_probe)
             _readiness_inflight[key] = future
-            future.add_done_callback(_readiness_cleared(key))
-    if not owner and not future.done():
-        return _err(rid, _READINESS_IN_PROGRESS_ERR,
-                    "readiness check still in progress; retrying next tick")
+    if owner:
+        # Registered OUTSIDE the lock: a probe that already settled runs the
+        # callback inline on this thread, and the callback acquires the same
+        # non-reentrant lock — under the lock that self-deadlocks the RPC
+        # worker and every later readiness call blocks on it (the gateway
+        # hangs answering setup.status / setup.runtime_check at all).
+        future.add_done_callback(_readiness_cleared(key))
     try:
-        return _ok(rid, future.result(timeout=wait_seconds))
+        return _ok(rid, future.result(timeout=wait_seconds if owner else
+                                      min(wait_seconds, _READINESS_JOIN_GRACE_SECONDS)))
     except concurrent.futures.TimeoutError:
-        logger.warning("readiness probe %s exceeded %.1fs; it continues in the background",
+        logger.warning("readiness probe %s exceeded its budget (%.1fs); it continues in the background",
                        key, wait_seconds)
         return _err(rid, _READINESS_IN_PROGRESS_ERR,
-                    f"readiness check timed out after {wait_seconds:.0f}s; retrying next tick")
+                    "readiness check still in progress; retrying next tick")
 
 
 def _readiness_check(rid, params, probe, *, probe_key, wait_seconds):