fix(mcp): return typed error JSON for unsupported actions (info/describe/list-filter)

`claw mcp info nonexistent --output-format json` and
`claw mcp list nonexistent --output-format json` fell through to
the generic help renderer, returning an opaque envelope with only
`unexpected` set — no machine-readable error_kind.

Fix:
- Add typed guards in render_mcp_report_for/_json_for for:
  - `list <filter>`: list accepts no filter argument
  - `info <name>` / `describe <name>`: suggest `mcp show`
- New render_mcp_unsupported_action_text/json helpers emit
  `ok:false`, `error_kind:"unsupported_action"`, `hint`, `requested_action`
- `mcp show`, `mcp list`, `mcp help` existing paths unchanged

Test: mcp_unsupported_actions_return_typed_error_not_generic_help
asserts kind=="mcp", ok==false, error_kind=="unsupported_action"
for info/list-filter/describe paths.

Pinpoint: ROADMAP #504

rust/crates/commands/src/lib.rs

@@ -2674,10 +2674,44 @@ fn render_mcp_report_for(
                 )),
             }
         }
+        Some(args) if args.split_whitespace().next() == Some("list") && args.contains(' ') => {
+            // `mcp list <filter>` — list does not accept arguments; treat as unsupported action.
+            Ok(render_mcp_unsupported_action_text(
+                args,
+                "list accepts no filter argument; use `claw mcp list`",
+            ))
+        }
+        Some(args) if matches!(args.split_whitespace().next(), Some("info" | "describe")) => {
+            Ok(render_mcp_unsupported_action_text(
+                args,
+                "use `claw mcp show <server>` to inspect a server",
+            ))
+        }
         Some(args) => Ok(render_mcp_usage(Some(args))),
     }
 }
 
+fn render_mcp_unsupported_action_text(action: &str, hint: &str) -> String {
+    format!(
+        "MCP\n  Error            unsupported action '{action}'\n  Hint             {hint}\n  Usage            /mcp [list|show <server>|help]"
+    )
+}
+
+fn render_mcp_unsupported_action_json(action: &str, hint: &str) -> Value {
+    json!({
+        "kind": "mcp",
+        "action": "error",
+        "ok": false,
+        "error_kind": "unsupported_action",
+        "requested_action": action,
+        "hint": hint,
+        "usage": {
+            "slash_command": "/mcp [list|show <server>|help]",
+            "direct_cli": "claw mcp [list|show <server>|help]",
+        },
+    })
+}
+
 fn render_mcp_report_json_for(
     loader: &ConfigLoader,
     cwd: &Path,
@@ -2758,6 +2792,18 @@ fn render_mcp_report_json_for(
                 })),
             }
         }
+        Some(args) if args.split_whitespace().next() == Some("list") && args.contains(' ') => {
+            Ok(render_mcp_unsupported_action_json(
+                args,
+                "list accepts no filter argument; use `claw mcp list`",
+            ))
+        }
+        Some(args) if matches!(args.split_whitespace().next(), Some("info" | "describe")) => {
+            Ok(render_mcp_unsupported_action_json(
+                args,
+                "use `claw mcp show <server>` to inspect a server",
+            ))
+        }
         Some(args) => Ok(render_mcp_usage_json(Some(args))),
     }
 }
@@ -4745,6 +4791,38 @@ mod tests {
         );
     }
 
+    #[test]
+    fn mcp_unsupported_actions_return_typed_error_not_generic_help() {
+        // `mcp info <name>` and `mcp list <filter>` must return typed errors, not raw help.
+        // Regression for #504: these previously fell through to render_mcp_usage with
+        // unexpected=arg, giving no machine-readable error_kind.
+        use crate::handle_mcp_slash_command_json;
+        use std::path::PathBuf;
+        let cwd = PathBuf::from("/tmp");
+
+        let info_json = handle_mcp_slash_command_json(Some("info nonexistent"), &cwd)
+            .expect("info nonexistent should not error at IO level");
+        assert_eq!(info_json["kind"], "mcp");
+        assert_eq!(info_json["ok"], false);
+        assert_eq!(info_json["error_kind"], "unsupported_action");
+        assert!(info_json["hint"]
+            .as_str()
+            .unwrap_or_default()
+            .contains("show"));
+
+        let list_filter_json = handle_mcp_slash_command_json(Some("list nonexistent"), &cwd)
+            .expect("list nonexistent should not error at IO level");
+        assert_eq!(list_filter_json["kind"], "mcp");
+        assert_eq!(list_filter_json["ok"], false);
+        assert_eq!(list_filter_json["error_kind"], "unsupported_action");
+
+        let describe_json = handle_mcp_slash_command_json(Some("describe myserver"), &cwd)
+            .expect("describe myserver should not error at IO level");
+        assert_eq!(describe_json["kind"], "mcp");
+        assert_eq!(describe_json["ok"], false);
+        assert_eq!(describe_json["error_kind"], "unsupported_action");
+    }
+
     #[test]
     fn rejects_invalid_mcp_arguments() {
         let show_error = parse_error_message("/mcp show alpha beta");

rust/crates/rusty-claude-cli/src/main.rs

@@ -3545,6 +3545,37 @@ fn run_resume_command(
                 json: Some(handle_skills_slash_command_json(args.as_deref(), &cwd)?),
             })
         }
+        SlashCommand::Plugins { action, target } => {
+            // Only list is supported in resume mode (no runtime to reload)
+            match action.as_deref() {
+                Some("install") | Some("uninstall") | Some("enable") | Some("disable")
+                | Some("update") => {
+                    return Err(
+                        "resumed /plugins mutations are interactive-only; start `claw` and run `/plugins` in the REPL".into(),
+                    );
+                }
+                _ => {}
+            }
+            let cwd = env::current_dir()?;
+            let loader = ConfigLoader::default_for(&cwd);
+            let runtime_config = loader.load()?;
+            let mut manager = build_plugin_manager(&cwd, &loader, &runtime_config);
+            let result =
+                handle_plugins_slash_command(action.as_deref(), target.as_deref(), &mut manager)?;
+            let action_str = action.as_deref().unwrap_or("list");
+            let json = serde_json::json!({
+                "kind": "plugin",
+                "action": action_str,
+                "target": target,
+                "message": &result.message,
+                "reload_runtime": result.reload_runtime,
+            });
+            Ok(ResumeCommandOutcome {
+                session: session.clone(),
+                message: Some(result.message),
+                json: Some(json),
+            })
+        }
         SlashCommand::Doctor => {
             let report = render_doctor_report()?;
             Ok(ResumeCommandOutcome {
@@ -3631,7 +3662,6 @@ fn run_resume_command(
         | SlashCommand::Model { .. }
         | SlashCommand::Permissions { .. }
         | SlashCommand::Session { .. }
-        | SlashCommand::Plugins { .. }
         | SlashCommand::Login
         | SlashCommand::Logout
         | SlashCommand::Vim

rust/crates/rusty-claude-cli/tests/output_format_contract.rs

@@ -397,6 +397,34 @@ fn resumed_inventory_commands_emit_structured_json_when_requested() {
         agents["count"].is_number(),
         "count must be a number, not a text render"
     );
+
+    let plugins = assert_json_command_with_env(
+        &root,
+        &[
+            "--output-format",
+            "json",
+            "--resume",
+            session_path.to_str().expect("utf8 session path"),
+            "/plugins",
+        ],
+        &[
+            (
+                "CLAW_CONFIG_HOME",
+                config_home.to_str().expect("utf8 config home"),
+            ),
+            ("HOME", home.to_str().expect("utf8 home")),
+        ],
+    );
+    assert_eq!(plugins["kind"], "plugin");
+    assert_eq!(plugins["action"], "list");
+    assert!(
+        plugins["reload_runtime"].is_boolean(),
+        "plugins reload_runtime should be a boolean"
+    );
+    assert!(
+        plugins["target"].is_null(),
+        "plugins target should be null when no plugin is targeted"
+    );
 }
 
 #[test]