diff --git a/graphify/cluster.py b/graphify/cluster.py
index 25b9db128..55af24f24 100644
--- a/graphify/cluster.py
+++ b/graphify/cluster.py
@@ -1,17 +1,44 @@
 """Community detection on NetworkX graphs. Uses Leiden (graspologic) if available, falls back to Louvain (networkx). Splits oversized communities. Returns cohesion scores."""
 from __future__ import annotations
+import contextlib
+import io
+import os
+import sys
 import networkx as nx
 
 
+def _suppress_output():
+    """Context manager to suppress stdout/stderr during library calls.
+
+    graspologic's leiden() emits ANSI escape sequences (progress bars,
+    colored warnings) that corrupt PowerShell 5.1's scroll buffer on
+    Windows (see issue #19). Redirecting stdout/stderr to devnull during
+    the call prevents this without losing any graphify output.
+    """
+    return contextlib.redirect_stdout(io.StringIO())
+
+
 def _partition(G: nx.Graph) -> dict[str, int]:
     """Run community detection. Returns {node_id: community_id}.
 
     Tries Leiden (graspologic) first — best quality.
     Falls back to Louvain (built into networkx) if graspologic is not installed.
+
+    Output from graspologic is suppressed to prevent ANSI escape codes
+    from corrupting terminal scroll buffers on Windows PowerShell 5.1.
     """
     try:
         from graspologic.partition import leiden
-        return leiden(G)
+        # Suppress graspologic output to prevent ANSI escape codes from
+        # corrupting PowerShell 5.1 scroll buffer (issue #19)
+        old_stderr = sys.stderr
+        try:
+            sys.stderr = io.StringIO()
+            with _suppress_output():
+                result = leiden(G)
+        finally:
+            sys.stderr = old_stderr
+        return result
     except ImportError:
         pass
 
diff --git a/graphify/skill-windows.md b/graphify/skill-windows.md
index 6675a7697..e8f9c9de1 100644
--- a/graphify/skill-windows.md
+++ b/graphify/skill-windows.md
@@ -1202,6 +1202,19 @@ graphify claude uninstall  # remove the section
 
 ---
 
+## Troubleshooting
+
+### PowerShell 5.1: Vertical scrolling stops working
+
+If vertical scrolling breaks in PowerShell after running graphify, this is caused by ANSI escape sequences from the `graspologic` library. Graphify v0.3.10+ suppresses this output, but if you still see the issue:
+
+1. **Upgrade graphify**: `pip install --upgrade graphifyy`
+2. **Use Windows Terminal** instead of the legacy PowerShell console — Windows Terminal handles ANSI codes correctly
+3. **Reset your terminal**: close and reopen PowerShell
+4. **Skip graspologic**: uninstall it (`pip uninstall graspologic`) and graphify will fall back to NetworkX's built-in Louvain algorithm, which produces no ANSI output
+
+---
+
 ## Honesty Rules
 
 - Never invent an edge. If unsure, use AMBIGUOUS.
diff --git a/tests/test_cluster.py b/tests/test_cluster.py
index de534016f..b5c16fad6 100644
--- a/tests/test_cluster.py
+++ b/tests/test_cluster.py
@@ -1,4 +1,5 @@
 import json
+import sys
 import networkx as nx
 from pathlib import Path
 from graphify.build import build_from_json
@@ -50,3 +51,26 @@ def test_score_all_keys_match_communities():
     communities = cluster(G)
     scores = score_all(G, communities)
     assert set(scores.keys()) == set(communities.keys())
+
+
+def test_cluster_does_not_write_to_stdout(capsys):
+    """Clustering should not emit ANSI escape codes or other output.
+
+    graspologic's leiden() can emit ANSI escape sequences that break
+    PowerShell 5.1's scroll buffer on Windows (issue #19). The output
+    suppression in _partition() should prevent any output from leaking.
+    """
+    G = make_graph()
+    cluster(G)
+    captured = capsys.readouterr()
+    assert captured.out == "", f"cluster() wrote to stdout: {captured.out!r}"
+
+
+def test_cluster_does_not_write_to_stderr(capsys):
+    """Same as above but for stderr — ANSI codes can go to either stream."""
+    G = make_graph()
+    cluster(G)
+    captured = capsys.readouterr()
+    # Allow logging output (starts with [graphify]) but no raw ANSI codes
+    for line in captured.err.splitlines():
+        assert "\x1b" not in line, f"cluster() wrote ANSI to stderr: {line!r}"