diff --git a/src/Changes b/src/Changes
index de5c529b76..df3a576af0 100644
--- a/src/Changes
+++ b/src/Changes
@@ -1,5 +1,16 @@
 Version 7.1
 
+10/15/26  [GH #305]
+Attribute namespaces that only start like a known one are not recognized
+
+An attribute namespace was treated as known if it was merely the first part
+of the name of a known namespace, so a namespace such as "cl", "g" or "ms"
+was accepted as if it were "clang", "gnu" or "msvc".  The whole name must
+now match.  For example:
+
+  [[cl::x]] int a;    // Warning: attribute namespace "cl" is unrecognized
+  [[g::y]] int b;     // Previously, the namespace was accepted.
+
 10/9/26  [GH #27]
 Clearer diagnostic when auto is deduced from an incomplete type
 
diff --git a/src/attribute.c b/src/attribute.c
index 86c8b0a627..630f4a949c 100644
--- a/src/attribute.c
+++ b/src/attribute.c
@@ -1096,7 +1096,9 @@ namespace.  If non-zero, length represents the number of characters in name
     length = strlen(name);
   }  /* if */
   for (np = valid_attribute_namespaces; *np != NULL; np++) {
-    if (strncmp(*np, name, length) == 0) {
+    /* The whole name must match: a name that is only a prefix of a known
+       namespace (e.g., "cl" or "g") is not recognized. */
+    if (strncmp(*np, name, length) == 0 && (*np)[length] == '\0') {
       result = TRUE;
       break;
     }  /* if */
diff --git a/tests/tests/changes/.GH-ns-exact.rto/default.1.1.txt b/tests/tests/changes/.GH-ns-exact.rto/default.1.1.txt
new file mode 100644
index 0000000000..f4be6205c7
--- /dev/null
+++ b/tests/tests/changes/.GH-ns-exact.rto/default.1.1.txt
@@ -0,0 +1,41 @@
+fe_only -DTEST_NUMBER=1 --c++17 Test_name.c
+"Test_name.c", line 19: warning: attribute namespace "cl" is unrecognized
+  [[cl::x]] int a;
+    ^
+
+"Test_name.c", line 20: warning: attribute namespace "gn" is unrecognized
+  [[gn::y]] int b;
+    ^
+
+"Test_name.c", line 21: warning: attribute namespace "g" is unrecognized
+  [[g::z]] int c;
+    ^
+
+"Test_name.c", line 22: warning: attribute namespace "ms" is unrecognized
+  [[ms::w]] int d;
+    ^
+
+"Test_name.c", line 23: warning: attribute namespace "_Cl" is unrecognized
+  [[_Cl::v]] int e;
+    ^
+
+"Test_name.c", line 25: warning: unknown attribute "gnu::x"
+  [[gnu::x]] int f;
+    ^
+
+"Test_name.c", line 26: warning: unknown attribute "__gnu__::x"
+  [[__gnu__::x]] int g;
+    ^
+
+"Test_name.c", line 27: warning: unknown attribute "clang::x"
+  [[clang::x]] int h;
+    ^
+
+"Test_name.c", line 28: warning: unknown attribute "msvc::x"
+  [[msvc::x]] int i;
+    ^
+
+"Test_name.c", line 29: warning: unknown attribute "_Clang::x"
+  [[_Clang::x]] int j;
+    ^
+
diff --git a/tests/tests/changes/.GH-ns-exact.rto/default.1.matches.txt b/tests/tests/changes/.GH-ns-exact.rto/default.1.matches.txt
new file mode 100644
index 0000000000..9aa901a538
--- /dev/null
+++ b/tests/tests/changes/.GH-ns-exact.rto/default.1.matches.txt
@@ -0,0 +1,24 @@
+Matches for "attribute namespace "cl" is unrecognized":
+  <-----------------------------------------------------------------------------
+  attribute namespace "cl" is unrecognized
+  ----------------------------------------------------------------------------->
+Matches for "attribute namespace "gn" is unrecognized":
+  <-----------------------------------------------------------------------------
+  attribute namespace "gn" is unrecognized
+  ----------------------------------------------------------------------------->
+Matches for "attribute namespace "g" is unrecognized":
+  <-----------------------------------------------------------------------------
+  attribute namespace "g" is unrecognized
+  ----------------------------------------------------------------------------->
+Matches for "attribute namespace "ms" is unrecognized":
+  <-----------------------------------------------------------------------------
+  attribute namespace "ms" is unrecognized
+  ----------------------------------------------------------------------------->
+Matches for "attribute namespace "_Cl" is unrecognized":
+  <-----------------------------------------------------------------------------
+  attribute namespace "_Cl" is unrecognized
+  ----------------------------------------------------------------------------->
+Matches for "\A(?![\s\S]*attribute namespace "(gnu|__gnu__|clang|_Clang|msvc)" is unrecognized)":
+  <-----------------------------------------------------------------------------
+  
+  ----------------------------------------------------------------------------->
diff --git a/tests/tests/changes/.GH-ns-exact.rto/edg_x86_32_cp.1.1.txt b/tests/tests/changes/.GH-ns-exact.rto/edg_x86_32_cp.1.1.txt
new file mode 100644
index 0000000000..c601710a51
--- /dev/null
+++ b/tests/tests/changes/.GH-ns-exact.rto/edg_x86_32_cp.1.1.txt
@@ -0,0 +1,21 @@
+fe_only -DTEST_NUMBER=1 --c++17 Test_name.c
+"Test_name.c", line 19: warning: attribute namespace "cl" is unrecognized
+  [[cl::x]] int a;
+    ^
+
+"Test_name.c", line 20: warning: attribute namespace "gn" is unrecognized
+  [[gn::y]] int b;
+    ^
+
+"Test_name.c", line 21: warning: attribute namespace "g" is unrecognized
+  [[g::z]] int c;
+    ^
+
+"Test_name.c", line 22: warning: attribute namespace "ms" is unrecognized
+  [[ms::w]] int d;
+    ^
+
+"Test_name.c", line 23: warning: attribute namespace "_Cl" is unrecognized
+  [[_Cl::v]] int e;
+    ^
+
diff --git a/tests/tests/changes/.GH-ns-exact.rto/edg_x86_64_cp.1.1.txt b/tests/tests/changes/.GH-ns-exact.rto/edg_x86_64_cp.1.1.txt
new file mode 100644
index 0000000000..c601710a51
--- /dev/null
+++ b/tests/tests/changes/.GH-ns-exact.rto/edg_x86_64_cp.1.1.txt
@@ -0,0 +1,21 @@
+fe_only -DTEST_NUMBER=1 --c++17 Test_name.c
+"Test_name.c", line 19: warning: attribute namespace "cl" is unrecognized
+  [[cl::x]] int a;
+    ^
+
+"Test_name.c", line 20: warning: attribute namespace "gn" is unrecognized
+  [[gn::y]] int b;
+    ^
+
+"Test_name.c", line 21: warning: attribute namespace "g" is unrecognized
+  [[g::z]] int c;
+    ^
+
+"Test_name.c", line 22: warning: attribute namespace "ms" is unrecognized
+  [[ms::w]] int d;
+    ^
+
+"Test_name.c", line 23: warning: attribute namespace "_Cl" is unrecognized
+  [[_Cl::v]] int e;
+    ^
+
diff --git a/tests/tests/changes/GH-ns-exact.sft.cpp b/tests/tests/changes/GH-ns-exact.sft.cpp
new file mode 100644
index 0000000000..da5409fb5b
--- /dev/null
+++ b/tests/tests/changes/GH-ns-exact.sft.cpp
@@ -0,0 +1,29 @@
+//type:fp
+//options:--c++17
+//remark:[GH #305] Attribute namespaces that only start like a known one
+//match_regex:attribute namespace "cl" is unrecognized
+//match_regex:attribute namespace "gn" is unrecognized
+//match_regex:attribute namespace "g" is unrecognized
+//match_regex:attribute namespace "ms" is unrecognized
+//match_regex:attribute namespace "_Cl" is unrecognized
+//match_regex:\A(?![\s\S]*attribute namespace "(gnu|__gnu__|clang|_Clang|msvc)" is unrecognized)
+// 10/14/26  [GH #305]
+//
+// A namespace that is only a prefix of a namespace known to the front end
+// ("clang", "gnu", "msvc", "_Clang") is not recognized: it gets the
+// "attribute namespace is unrecognized" warning like any other unknown one.
+// The known namespaces themselves are still recognized: they get the
+// "unknown attribute" warning for the unknown attribute name only (the last
+// regular expression above fails if one of them is reported as unrecognized).
+
+[[cl::x]] int a;
+[[gn::y]] int b;
+[[g::z]] int c;
+[[ms::w]] int d;
+[[_Cl::v]] int e;
+
+[[gnu::x]] int f;
+[[__gnu__::x]] int g;
+[[clang::x]] int h;
+[[msvc::x]] int i;
+[[_Clang::x]] int j;