diff --git a/src/Verify.Tests/Naming/NameForParameterTests.StringHash.verified.txt b/src/Verify.Tests/Naming/NameForParameterTests.StringHash.verified.txt
new file mode 100644
index 000000000..147b3b566
--- /dev/null
+++ b/src/Verify.Tests/Naming/NameForParameterTests.StringHash.verified.txt
@@ -0,0 +1 @@
+a-1
\ No newline at end of file
diff --git a/src/Verify.Tests/Naming/NameForParameterTests.cs b/src/Verify.Tests/Naming/NameForParameterTests.cs
index d006f67f7..bce0802e7 100644
--- a/src/Verify.Tests/Naming/NameForParameterTests.cs
+++ b/src/Verify.Tests/Naming/NameForParameterTests.cs
@@ -12,6 +12,12 @@ public Task StringEmpty() =>
     public Task StringInvalidPathChar() =>
         Verify(VerifierSettings.GetNameForParameter("a/a", counter: CounterBuilder.Empty()));
 
+    // `#` is reserved for the indexed-target namespace, so a value containing one
+    // would make this case's files look like the targets of the case without it
+    [Fact]
+    public Task StringHash() =>
+        Verify(VerifierSettings.GetNameForParameter("a#1", counter: CounterBuilder.Empty()));
+
     [Fact]
     public void CollectionItemPathFriendlyFalseNotCleaned()
     {
diff --git a/src/Verify.Tests/Naming/NamerTests.ParametersWithHash_value=x-1.verified.txt b/src/Verify.Tests/Naming/NamerTests.ParametersWithHash_value=x-1.verified.txt
new file mode 100644
index 000000000..1829444fc
--- /dev/null
+++ b/src/Verify.Tests/Naming/NamerTests.ParametersWithHash_value=x-1.verified.txt
@@ -0,0 +1 @@
+x#1
\ No newline at end of file
diff --git a/src/Verify.Tests/Naming/NamerTests.ParametersWithHash_value=x.verified.txt b/src/Verify.Tests/Naming/NamerTests.ParametersWithHash_value=x.verified.txt
new file mode 100644
index 000000000..b18d45413
--- /dev/null
+++ b/src/Verify.Tests/Naming/NamerTests.ParametersWithHash_value=x.verified.txt
@@ -0,0 +1 @@
+x
\ No newline at end of file
diff --git a/src/Verify.Tests/Naming/NamerTests.cs b/src/Verify.Tests/Naming/NamerTests.cs
index cb8702c19..725fc47c5 100644
--- a/src/Verify.Tests/Naming/NamerTests.cs
+++ b/src/Verify.Tests/Naming/NamerTests.cs
@@ -491,6 +491,15 @@ public Task ParametersWithBadPathChars(string value) =>
         Verify(value)
             .UseParameters(value);
 
+    // `{prefix}#` is the indexed-target namespace, so without cleaning the `x` case
+    // would claim the files of the `x#1` case
+    [Theory]
+    [InlineData("x")]
+    [InlineData("x#1")]
+    public Task ParametersWithHash(string value) =>
+        Verify(value)
+            .UseParameters(value);
+
     [Fact]
     public Task SingleTarget() =>
         Verify([new Target("txt", "data")]);
diff --git a/src/Verify/FileNameCleaner.cs b/src/Verify/FileNameCleaner.cs
index b7f987e00..2d34a19c7 100644
--- a/src/Verify/FileNameCleaner.cs
+++ b/src/Verify/FileNameCleaner.cs
@@ -42,7 +42,10 @@
         ':',
         '*',
         '?',
-        '/'
+        '/',
+        // Not invalid on disk, but reserved: `{prefix}#` is the indexed-target namespace,
+        // so a `#` in a value would make one case's files look like another case's targets
+        '#'
     ];
 
     static char[] invalidPathChars =
diff --git a/src/todo.md b/src/todo.md
index 03784f69b..b3b8b883a 100644
--- a/src/todo.md
+++ b/src/todo.md
@@ -35,7 +35,7 @@ All six resolved 2026-08-16 (five fixed here; the inline item resolved as by-des
 - [ ] **Sub-millisecond date parameters collide.**
   `Verify/Serialization/DateFormatter_DateTime.cs` (and the `DateTimeOffset` twin) use `Second == 0` / `Millisecond == 0` to omit the fraction, but sub-millisecond ticks leave those properties 0. `AddTicks(1)` and `AddTicks(2)` cases format identically → spurious "prefix has already been used" (or silent sharing of one verified file). Correct check is ticks-based.
 
-- [ ] **`#` in parameter values collides with the indexed-target namespace.**
+- [x] **`#` in parameter values collides with the indexed-target namespace.**
   `Verify/Naming/MatchingFileFinder.cs:9,20` — `indexedPattern: "{prefix}#"` matches by prefix, and `#` is not sanitized. Cases `"x"` and `"x#1"` on one method: running `"x"` deletes `C.M_p=x#1.received.txt` and sweeps `C.M_p=x#1.verified.txt` into the stale set (deleted under AutoVerify).
 
 - [ ] **Trimmed fraction format collapses into a standard format specifier.**