fix(tui): clarify shell tool availability errors (#2412)

Harvested from #2402 with thanks to @axobase001.

Keeps `allow_shell` guidance visible for gated shell tools even when missing-tool suggestions exist, removes the nonexistent task_shell_cancel matcher, and broadens regression coverage.

Partially addresses #2328.

crates/tui/src/core/engine/tests.rs

@@ -2359,6 +2359,44 @@ fn missing_tool_error_message_includes_discovery_guidance_when_no_match() {
     assert!(message.contains(TOOL_SEARCH_BM25_NAME));
 }
 
+#[test]
+fn missing_shell_tool_error_message_names_allow_shell_gate() {
+    let catalog = vec![api_tool("read_file")];
+
+    for tool_name in [
+        "exec_shell",
+        "exec_shell_wait",
+        "exec_shell_interact",
+        "task_shell_start",
+        "task_shell_wait",
+    ] {
+        let message = missing_tool_error_message(tool_name, &catalog);
+        assert!(message.contains("not available in the current tool catalog"));
+        assert!(message.contains("allow_shell"), "{tool_name}: {message}");
+        assert!(
+            message.contains("trusted workspaces"),
+            "{tool_name}: {message}"
+        );
+        assert!(
+            message.contains(TOOL_SEARCH_BM25_NAME),
+            "{tool_name}: {message}"
+        );
+    }
+}
+
+#[test]
+fn missing_shell_tool_error_message_keeps_allow_shell_hint_with_suggestions() {
+    let catalog = vec![api_tool("exec")];
+
+    let message = missing_tool_error_message("exec_shell", &catalog);
+
+    assert!(message.contains("Did you mean:"));
+    assert!(message.contains("exec"));
+    assert!(message.contains("allow_shell"));
+    assert!(message.contains("trusted workspaces"));
+    assert!(message.contains(TOOL_SEARCH_BM25_NAME));
+}
+
 #[test]
 fn filter_tool_call_delta_strips_bracket_marker() {
     let mut in_block = false;

crates/tui/src/core/engine/tool_catalog.rs

@@ -438,17 +438,52 @@ fn suggest_tool_names(catalog: &[Tool], requested: &str, limit: usize) -> Vec<St
 
 pub(super) fn missing_tool_error_message(tool_name: &str, catalog: &[Tool]) -> String {
     let suggestions = suggest_tool_names(catalog, tool_name, 3);
+    let shell_hint = if is_shell_tool_name(tool_name) {
+        Some(shell_tool_allow_shell_hint())
+    } else {
+        None
+    };
     if suggestions.is_empty() {
+        if let Some(shell_hint) = shell_hint {
+            return format!(
+                "Tool '{tool_name}' is not available in the current tool catalog. \
+                 {shell_hint}, or use {TOOL_SEARCH_BM25_NAME} with a short query."
+            );
+        }
         return format!(
             "Tool '{tool_name}' is not available in the current tool catalog. \
              Verify mode/feature flags, or use {TOOL_SEARCH_BM25_NAME} with a short query."
         );
     }
 
+    let suggestion_text = format!("Did you mean: {}?", suggestions.join(", "));
+    if let Some(shell_hint) = shell_hint {
+        return format!(
+            "Tool '{tool_name}' is not available in the current tool catalog. \
+             {suggestion_text} {shell_hint}. \
+             You can also use {TOOL_SEARCH_BM25_NAME} to discover tools."
+        );
+    }
+
     format!(
         "Tool '{tool_name}' is not available in the current tool catalog. \
-         Did you mean: {}? You can also use {TOOL_SEARCH_BM25_NAME} to discover tools.",
-        suggestions.join(", ")
+         {suggestion_text} You can also use {TOOL_SEARCH_BM25_NAME} to discover tools."
+    )
+}
+
+fn shell_tool_allow_shell_hint() -> &'static str {
+    "Shell tools are gated by `allow_shell`; enable `allow_shell = true` for trusted workspaces, \
+     or switch to an auto-approve mode that permits shell access"
+}
+
+fn is_shell_tool_name(tool_name: &str) -> bool {
+    matches!(
+        tool_name,
+        "exec_shell"
+            | "exec_shell_wait"
+            | "exec_shell_interact"
+            | "task_shell_start"
+            | "task_shell_wait"
     )
 }