mason: add tool_call preview controls

Author: ualtinok

crates/aft/src/commands/tool_call.rs

@@ -51,6 +51,11 @@ fn handle_with_dispatch(req: &RawRequest, ctx: &AppContext, dispatch: &DispatchF
         .get("arguments")
         .cloned()
         .unwrap_or_else(|| json!({}));
+    let preview = req
+        .params
+        .get("preview")
+        .and_then(Value::as_bool)
+        .unwrap_or(false);
     let config = ctx.config();
     let project_root = config
         .project_root
@@ -61,6 +66,7 @@ fn handle_with_dispatch(req: &RawRequest, ctx: &AppContext, dispatch: &DispatchF
         session_id: Some(req.session().to_string()),
         request_id: req.id.clone(),
         diagnostics_on_edit: config.diagnostics_on_edit,
+        preview,
     };
 
     match run_tool_call(name, &arguments, &tool_ctx, ctx, dispatch) {

crates/aft/src/commands/write.rs

@@ -72,6 +72,19 @@ pub fn handle_write(req: &RawRequest, ctx: &AppContext) -> Response {
         String::new()
     };
 
+    if edit::wants_preview(&req.params) {
+        let mut result = serde_json::json!({
+            "file": file,
+            "created": !existed,
+            "formatted": false,
+        });
+        if existed && original == content {
+            result["no_op"] = serde_json::json!(true);
+        }
+        edit::attach_preview_diff(&mut result, &req.params, file, &original, content);
+        return Response::success(&req.id, result);
+    }
+
     // Auto-backup existing files before overwriting. For create-only writes,
     // record a tombstone so operation undo removes the created file.
     let backup_id = if existed {

crates/aft/src/run_tool_call.rs

@@ -24,13 +24,15 @@ pub enum ToolCallOutcome {
     Unary(ToolCallResult),
 }
 
-/// Single owner of per-call context (Oracle #4). diagnostics_on_edit has ONE source here.
+/// Server-owned settings for a single `tool_call` request.
+/// These fields cannot be supplied through the agent's arguments object.
 #[derive(Debug, Clone)]
 pub struct ToolCallContext {
     pub project_root: PathBuf,
     pub session_id: Option<String>,
     pub request_id: String,
     pub diagnostics_on_edit: bool,
+    pub preview: bool,
 }
 
 pub fn run_tool_call(
@@ -40,17 +42,19 @@ pub fn run_tool_call(
     app_ctx: &AppContext,
     dispatch: &DispatchFn<'_>,
 ) -> ToolCallOutcome {
+    let sanitized_args = strip_agent_preview_arg(args);
     let format_context = crate::subc_format::FormatContext::from_tool_call(
         bare_name,
-        args,
+        &sanitized_args,
         ctx.project_root.as_path(),
     );
     let translate_context = crate::subc_translate::TranslateContext {
         diagnostics_on_edit: ctx.diagnostics_on_edit,
+        preview: ctx.preview,
     };
     let (command, translated_args) = match crate::subc_translate::subc_translate_with_context(
         bare_name,
-        args,
+        &sanitized_args,
         ctx.project_root.as_path(),
         translate_context,
     ) {
@@ -64,7 +68,7 @@ pub fn run_tool_call(
             ));
         }
         Err(_) => {
-            let map = args.as_object().cloned().unwrap_or_default();
+            let map = sanitized_args.as_object().cloned().unwrap_or_default();
             (bare_name.to_string(), map)
         }
     };
@@ -98,6 +102,19 @@ pub fn run_tool_call(
     ))
 }
 
+fn strip_agent_preview_arg(args: &Value) -> Value {
+    let Some(map) = args.as_object() else {
+        return args.clone();
+    };
+    if !map.contains_key("preview") {
+        return args.clone();
+    }
+
+    let mut sanitized = map.clone();
+    sanitized.remove("preview");
+    Value::Object(sanitized)
+}
+
 fn tool_call_result_from_response(
     bare_name: &str,
     format_context: &crate::subc_format::FormatContext,

crates/aft/src/subc.rs

@@ -1937,6 +1937,7 @@ async fn handle_tool_call(
         session_id: Some(identity.session.clone()),
         request_id: request_id.clone(),
         diagnostics_on_edit,
+        preview: false,
     };
     let arguments_for_run = call.arguments.clone();
     let bare_name_for_run = bare_name.clone();

crates/aft/src/subc_translate.rs

@@ -13,6 +13,7 @@ pub struct Translated {
 #[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
 pub struct TranslateContext {
     pub diagnostics_on_edit: bool,
+    pub preview: bool,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -193,6 +194,7 @@ fn insert_common_mutation_flags(out: &mut Map<String, Value>, ctx: TranslateCont
         Value::Bool(ctx.diagnostics_on_edit),
     );
     out.insert("include_diff_content".to_string(), Value::Bool(true));
+    out.insert("preview".to_string(), Value::Bool(ctx.preview));
 }
 
 fn translate_read(args: &Value, project_root: &Path) -> Result<Translated, TranslateError> {

crates/aft/tests/fixtures/subc_parity/translate/edit_append/expected.json

@@ -5,7 +5,8 @@
     "diagnostics": true,
     "file": "<PROJECT_ROOT>/src/main.ts",
     "include_diff_content": true,
-    "op": "append"
+    "op": "append",
+    "preview": false
   },
   "command": "edit_match"
 }

crates/aft/tests/fixtures/subc_parity/translate/edit_batch/expected.json

@@ -10,7 +10,8 @@
       }
     ],
     "file": "<PROJECT_ROOT>/src/main.ts",
-    "include_diff_content": true
+    "include_diff_content": true,
+    "preview": false
   },
   "command": "batch"
 }

crates/aft/tests/fixtures/subc_parity/translate/edit_replace_occurrence_zero/expected.json

@@ -5,6 +5,7 @@
     "include_diff_content": true,
     "match": "value",
     "occurrence": 0,
+    "preview": false,
     "replacement": "answer"
   },
   "command": "edit_match"

crates/aft/tests/fixtures/subc_parity/translate/edit_symbol/expected.json

@@ -5,6 +5,7 @@
     "file": "<PROJECT_ROOT>/src/main.ts",
     "include_diff_content": true,
     "operation": "replace",
+    "preview": false,
     "symbol": "value"
   },
   "command": "edit_symbol"

crates/aft/tests/fixtures/subc_parity/translate/write_default_diagnostics/expected.json

@@ -4,7 +4,8 @@
     "create_dirs": true,
     "diagnostics": false,
     "file": "<PROJECT_ROOT>/src/new.ts",
-    "include_diff_content": true
+    "include_diff_content": true,
+    "preview": false
   },
   "command": "write"
 }