diff --git a/crates/apr-cli/src/commands_enum.rs b/crates/apr-cli/src/commands_enum.rs
index 5da86438e9..90903cc498 100644
--- a/crates/apr-cli/src/commands_enum.rs
+++ b/crates/apr-cli/src/commands_enum.rs
@@ -1067,3 +1067,7 @@ pub enum DebugCommands {
 #[cfg(all(test, feature = "training"))]
 #[path = "code_output_format_help_tests.rs"]
 mod code_output_format_help_tests;
+
+#[cfg(test)]
+#[path = "ptx_help_marker_tests.rs"]
+mod ptx_help_marker_tests;
diff --git a/crates/apr-cli/src/extended_commands.rs b/crates/apr-cli/src/extended_commands.rs
index f89c669ed2..1c245cd941 100644
--- a/crates/apr-cli/src/extended_commands.rs
+++ b/crates/apr-cli/src/extended_commands.rs
@@ -400,7 +400,8 @@ pub enum ExtendedCommands {
         not(feature = "trueno-explain"),
         command(
             name = "ptx",
-            about = "PTX analysis and bug detection [unavailable in this build: cargo install aprender --features ptx; `apr ptx-debug` is always built]"
+            about = "PTX analysis and bug detection [unavailable in this build: cargo install aprender --features ptx; `apr ptx-debug` is always built]",
+            long_about = "PTX analysis and bug detection [unavailable in this build: cargo install aprender --features ptx; `apr ptx-debug` is always built]"
         )
     )]
     Ptx {
diff --git a/crates/apr-cli/src/ptx_help_marker_tests.rs b/crates/apr-cli/src/ptx_help_marker_tests.rs
new file mode 100644
index 0000000000..7f3ec12b90
--- /dev/null
+++ b/crates/apr-cli/src/ptx_help_marker_tests.rs
@@ -0,0 +1,40 @@
+//! `apr ptx --help` must carry the "unavailable in this build" marker (#4474, #2399 finding 1).
+//!
+//! Clap derives `long_about` from the doc comment unless it is set explicitly, so
+//! overriding only `about` left `--help` (the long form) without the marker.
+
+const MARKER: &str = "unavailable in this build";
+
+/// Clap's recursive build of the full `Commands` enum overflows the default
+/// test-thread stack in debug builds, as in code_output_format_help_tests.rs.
+#[test]
+fn ptx_help_says_when_the_analyzer_is_not_built_in() {
+    std::thread::Builder::new()
+        .stack_size(16 * 1024 * 1024)
+        .spawn(ptx_help_body)
+        .expect("spawn")
+        .join()
+        .expect("ptx help row panicked");
+}
+
+fn ptx_help_body() {
+    use clap::CommandFactory;
+    let cmd = crate::Cli::command();
+    let ptx = cmd.find_subcommand("ptx").expect("ptx subcommand");
+    let short = ptx.get_about().map(ToString::to_string).unwrap_or_default();
+    let long = ptx
+        .get_long_about()
+        .or_else(|| ptx.get_about())
+        .map(ToString::to_string)
+        .unwrap_or_default();
+    let built = cfg!(feature = "trueno-explain");
+    for (what, text) in [("about", &short), ("long_about", &long)] {
+        assert_eq!(
+            text.contains(MARKER),
+            !built,
+            "apr ptx {what} marker presence must be {} when trueno-explain is {}: {text}",
+            !built,
+            if built { "on" } else { "off" }
+        );
+    }
+}
diff --git a/docs/audits/surface_audit.csv b/docs/audits/surface_audit.csv
index f59b14d503..050400c383 100644
--- a/docs/audits/surface_audit.csv
+++ b/docs/audits/surface_audit.csv
@@ -251,7 +251,7 @@ apr,apr pretrain,6,x86_64-linux,pytorch,yes,1,apr-lint-diag,crates/apr-cli/src/e
 apr,apr profile,3,x86_64-linux;nvidia-cuda,llama.cpp,yes,9,profiling,crates/apr-cli/src/extended_commands.rs:186,high
 apr,apr prometheus-lint,1,x86_64-linux,NONE,yes,1,apr-lint-diag,crates/apr-cli/src/extended_commands.rs:1138,high
 apr,apr prune,4,x86_64-linux,unsloth,yes,10,apr-core-commands,crates/apr-cli/src/model_ops_commands.rs:113,high
-apr,apr ptx,4,x86_64-linux,NONE,yes,1,apr-lint-diag,crates/apr-cli/src/extended_commands.rs:387,high
+apr,apr ptx,4,x86_64-linux,NONE,yes,1,apr-lint-diag,crates/apr-cli/src/extended_commands.rs:407,high
 apr,apr ptx-map,6,x86_64-linux,NONE,yes,1,apr-lint-diag,crates/apr-cli/src/extended_commands.rs:355,high
 apr,apr publish,6,x86_64-linux,llama.cpp,yes,10,apr-core-commands,crates/apr-cli/src/tool_commands.rs:46,high
 apr,apr pull,4,x86_64-linux,llama.cpp,yes,10,apr-core-commands,crates/apr-cli/src/commands_enum.rs:552,high
@@ -453,7 +453,7 @@ apr,apr train sweep,6,x86_64-linux,pytorch,no,10,apr-core-commands,crates/apr-cl
 apr,apr train watch,6,x86_64-linux,pytorch,no,10,apr-core-commands,crates/apr-cli/src/train_commands.rs:172,low
 apr,apr tree,4,x86_64-linux,NONE,yes,1,apr-lint-diag,crates/apr-cli/src/extended_commands.rs:691,high
 apr,apr tui,9,x86_64-linux,NONE,yes,10,apr-core-commands,crates/apr-cli/src/commands_enum.rs:820,medium
-apr,apr tune,4,x86_64-linux,pytorch,yes,1,apr-lint-diag,crates/apr-cli/src/extended_commands.rs:409,high
+apr,apr tune,4,x86_64-linux,pytorch,yes,1,apr-lint-diag,crates/apr-cli/src/extended_commands.rs:429,high
 apr,apr typical-p-lint,1,x86_64-linux,NONE,yes,1,apr-lint-diag,crates/apr-cli/src/extended_commands.rs:1162,high
 apr,apr unified-search-lint,1,x86_64-linux,NONE,yes,1,apr-lint-diag,crates/apr-cli/src/extended_commands.rs:1215,high
 apr,apr unshard,9,x86_64-linux,llama.cpp,yes,1,apr-lint-diag,crates/apr-cli/src/extended_commands.rs:1254,medium