diff --git a/src/edge0/moe/spec.py b/src/edge0/moe/spec.py
index 176c43c..6d0d92c 100644
--- a/src/edge0/moe/spec.py
+++ b/src/edge0/moe/spec.py
@@ -110,6 +110,35 @@ def block_of(self, model, layer: int):
                 obj = getattr(obj, part)
         return obj
 
+    def layer_exists(self, model, layer: int) -> bool:
+        """True if ``layer`` resolves at block_path's layer-index segment.
+
+        Used by layer-count discovery (``install_streaming_experts`` with
+        ``num_layers=None``) to find where the layer list ends. Only an
+        ``AttributeError``/``IndexError`` raised while resolving the
+        segment templated by ``{layer}`` itself means "past the last
+        layer"; the same errors raised by a *different* segment further
+        down ``block_path`` (e.g. a fixed expert-slot index) indicate a
+        bug in the spec or model and are re-raised rather than read as
+        end-of-list.
+        """
+        template_parts = self.block_path.split(".")
+        layer_pos = next(
+            i for i, p in enumerate(template_parts) if "{layer}" in p)
+        obj = model
+        for i, raw_part in enumerate(template_parts):
+            part = raw_part.format(layer=layer)
+            try:
+                if part.isdigit():
+                    obj = obj[int(part)]
+                else:
+                    obj = getattr(obj, part)
+            except (AttributeError, IndexError):
+                if i == layer_pos:
+                    return False
+                raise
+        return True
+
     def layer_of(self, model, layer: int):
         """Resolve the decoder layer object (the block's owner).
 
diff --git a/src/edge0/streaming/install.py b/src/edge0/streaming/install.py
index 37b0e7e..c0c84de 100644
--- a/src/edge0/streaming/install.py
+++ b/src/edge0/streaming/install.py
@@ -38,11 +38,7 @@ def install_streaming_experts(
     """
     if num_layers is None:
         n = 0
-        while True:
-            try:
-                spec.block_of(model, n)
-            except AttributeError:
-                break
+        while spec.layer_exists(model, n):
             n += 1
         if n == 0:
             raise ValueError(
diff --git a/tests/test_streaming_math.py b/tests/test_streaming_math.py
index e77154e..0d2bcbd 100644
--- a/tests/test_streaming_math.py
+++ b/tests/test_streaming_math.py
@@ -299,3 +299,71 @@ def test_double_buffered_swap(layer):
         ref2 = lay(x, mx.array([second], dtype=mx.int32))
         assert mx.allclose(out1, ref1).item()
         assert mx.allclose(out2, ref2).item()
+
+
+def test_install_discovers_layer_count(tmp_path):
+    """install_streaming_experts(num_layers=None) probes block_path with
+    increasing layer indices until it stops resolving. Layers live in a
+    list, so running off the end raises IndexError, not AttributeError."""
+    from types import SimpleNamespace
+
+    from edge0.streaming.install import install_streaming_experts
+
+    path = tmp_path / "w.safetensors"
+    _write_shard(path, fuse_gu=False)
+    spec = MoESpec(
+        num_experts=N_EXPERTS, top_k=4, intermediate_size=INTER,
+        quant=QuantSpec(bits=4, group_size=64),
+        layout=WeightLayout.SEPARATE,
+        key_template="layers.{layer}.mlp.switch_mlp",
+        block_path="layers.{layer}.mlp",
+    )
+    moe = SimpleNamespace(switch_mlp=object())
+    model = SimpleNamespace(layers=[SimpleNamespace(mlp=moe),
+                                    SimpleNamespace(mlp=SimpleNamespace()),
+                                    SimpleNamespace(mlp=SimpleNamespace())])
+    twins = install_streaming_experts(
+        model, [SafetensorsMmap(str(path))], spec, options=_options())
+    assert len(twins) == 3
+    assert isinstance(twins[0], StreamingSwitchGLU)
+    assert twins[1] is None and twins[2] is None      # dense layers
+    assert moe.switch_mlp is twins[0]
+    twins[0].close()
+
+
+def test_layer_exists_stops_on_attribute_error():
+    """The AttributeError half of ``layer_exists``'s except clause is
+    reachable independently of IndexError: a family whose layer container
+    is attribute-based (no list, no ``__getitem__``) runs off the end via
+    a plain missing attribute, not an out-of-range index."""
+    from types import SimpleNamespace
+
+    spec = MoESpec(
+        num_experts=N_EXPERTS, top_k=4, intermediate_size=INTER,
+        key_template="layer_{layer}.mlp.switch_mlp",
+        block_path="layer_{layer}",
+    )
+    model = SimpleNamespace(layer_0=object(), layer_1=object())
+    assert spec.layer_exists(model, 0) is True
+    assert spec.layer_exists(model, 1) is True
+    assert spec.layer_exists(model, 2) is False  # no `layer_2` attribute
+
+
+def test_layer_exists_reraises_unrelated_index_error():
+    """An IndexError from a segment *other* than the ``{layer}`` slot is a
+    real bug (a malformed block_path or a broken model), not end-of-list,
+    and must propagate instead of being read as "past the last layer" --
+    otherwise install_streaming_experts(num_layers=None) would silently
+    under-count layers instead of surfacing the break."""
+    from types import SimpleNamespace
+
+    spec = MoESpec(
+        num_experts=N_EXPERTS, top_k=4, intermediate_size=INTER,
+        key_template="layers.{layer}.experts.9.switch_mlp",
+        block_path="layers.{layer}.experts.9",
+    )
+    # `layer=0` is in range for `layers`, but the fixed trailing index `9`
+    # is out of range for `experts` -- unrelated to layer-count discovery.
+    model = SimpleNamespace(layers=[SimpleNamespace(experts=[object()])])
+    with pytest.raises(IndexError):
+        spec.layer_exists(model, 0)