diff --git a/src/TraceClient/TraceClient.cs b/src/TraceClient/TraceClient.cs
index 06241b6..4b48f7a 100644
--- a/src/TraceClient/TraceClient.cs
+++ b/src/TraceClient/TraceClient.cs
@@ -76,7 +76,8 @@ public sealed class TraceClient : IDisposable
         /// <summary>The most tags the server accepts on one report.</summary>
         public const int MaxTags = 32;
 
-        /// <summary>The longest application, name, tag key or tag value the server accepts, in UTF-16 chars.</summary>
+        /// <summary>The longest application, name, tag key or tag value the server accepts, in UTF-16 chars.
+        /// The program's version and installation ID are tag values too, so the constructor rejects either past it.</summary>
         public const int MaxLength = 255;
 
         /// <summary>Environment variable that turns reporting off: <c>off</c>, <c>false</c>, <c>0</c>, <c>no</c>.</summary>
@@ -122,9 +123,10 @@ public sealed class TraceClient : IDisposable
         /// <paramref name="version"/>, reporting to the trace server at
         /// <paramref name="baseUrl"/>. Throws <see cref="ArgumentException"/> only for a
         /// missing or malformed <paramref name="baseUrl"/>, a missing
-        /// <paramref name="application"/>, or a missing <paramref name="version"/> or
-        /// one longer than <see cref="MaxLength"/> characters -- programming errors,
-        /// not runtime ones.
+        /// <paramref name="application"/>, a missing <paramref name="version"/> or
+        /// one longer than <see cref="MaxLength"/> characters, or an
+        /// <paramref name="installId"/> longer than <see cref="MaxLength"/> characters --
+        /// programming errors, not runtime ones.
         /// </summary>
         /// <param name="baseUrl">The trace server, e.g. <c>https://trace.danielstephenson.dev</c>.</param>
         /// <param name="application">The program's name, exactly as its key was issued for.</param>
diff --git a/tests/TraceClient.Tests/TraceClientTest.cs b/tests/TraceClient.Tests/TraceClientTest.cs
index 6dbd83f..92b6300 100644
--- a/tests/TraceClient.Tests/TraceClientTest.cs
+++ b/tests/TraceClient.Tests/TraceClientTest.cs
@@ -453,6 +453,38 @@ public void Json_RejectsATagKeyLongerThanTheLimitAndNamesOnlyItsStart()
             Assert.Null(body);
         }
 
+        [Fact]
+        public void Json_RejectsANameLongerThanTheLimitButNotOneAtIt()
+        {
+            string longest = new string('n', TraceClient.MaxLength);
+            string tooLong = new string('n', TraceClient.MaxLength + 1);
+
+            string problem = TraceClient.Json("App", tooLong, null, null, out string rejected);
+
+            Assert.Equal("name longer than " + TraceClient.MaxLength + " characters", problem);
+            Assert.Null(rejected);
+            Assert.Null(TraceClient.Json("App", longest, null, null, out string accepted));
+            Assert.Contains(longest, accepted);
+        }
+
+        [Fact]
+        public void Json_RejectsATagValueLongerThanTheLimitAndNamesItsKey()
+        {
+            // The reason names the key, not the value: the value is what ran
+            // away, and the key is what the program can find in its source.
+            string tooLongValue = new string('v', TraceClient.MaxLength + 1);
+            string longestValue = new string('v', TraceClient.MaxLength);
+
+            string problem = TraceClient.Json("App", "n", null,
+                new Dictionary<string, string> { { "level", tooLongValue } }, out string body);
+
+            Assert.Equal("tag level longer than " + TraceClient.MaxLength + " characters", problem);
+            Assert.Null(body);
+            Assert.Null(TraceClient.Json("App", "n", null,
+                new Dictionary<string, string> { { "level", longestValue } }, out string accepted));
+            Assert.Contains("\"level\":\"" + longestValue + "\"", accepted);
+        }
+
         [Fact]
         public void Quote_EscapesCarriageReturnAndEveryOtherControlCharacter()
         {