fix(edit-tool): eliminate token amplification and hang on large files

pr-body-1650.md

@@ -0,0 +1,27 @@
+## Summary
+
+Fixes #1650
+
+The Edit tool caused extreme token amplification (957K input tokens) and a 1m23s hang on a 1185-line file. Two root causes:
+
+### Fix 1: Remove redundant dry-run from `validate_input`
+
+`validate_input()` in `file_edit_tool.rs` ran a full dry-run `apply_edit_to_content()` — reading the file and generating all whitespace-normalization candidates — and then `call_impl` did the exact same work again when actually applying the edit. This doubled the file reads and candidate generation on every edit.
+
+**Fix:** Removed the dry-run block from `validate_input`. The edit is already validated during `call_impl` via `apply_edit_to_content`, so the dry-run was purely redundant work.
+
+### Fix 2: Add fast path before candidate generation in `apply_edit_to_content`
+
+`edit_string_candidates()` generates whitespace-normalization candidates (tabs↔spaces at width 2 and 4), each calling `find_actual_string()` which does O(n*m) char-by-char scanning. ALL candidates were generated upfront before any matching, even when the exact `old_string` already matched the file content.
+
+**Fix:** Added a fast path that tries the exact `old_string`/`new_string` match via `apply_match_and_replace()` before calling `edit_string_candidates()`. If the exact match succeeds (the common case), it returns immediately — skipping all candidate generation and expensive scanning. If the exact match fails with "not found", it falls through to the existing candidate loop (slow path unchanged for edge cases).
+
+## Validation
+
+- `cargo check -p tool-runtime -p bitfun-core` — passed
+- `cargo test -p tool-runtime -- fs::edit_file` — 21/21 passed
+- `cargo test -p bitfun-core -- file_edit_tool` — 3/3 passed
+
+## Impact
+
+For the reported 1185-line file edit, the common case (exact match) now completes in a single `apply_match_and_replace` call instead of generating and scanning multiple whitespace-normalization candidates through `find_actual_string`. Combined with removing the redundant dry-run, this eliminates the token amplification and hang.

src/crates/assembly/core/src/agentic/tools/implementations/file_edit_tool.rs

@@ -1,7 +1,7 @@
 use crate::agentic::tools::file_permissions::file_permission_intents_allowing_managed_plan_edits;
 use crate::agentic::tools::file_read_state_runtime::{
     assert_file_not_unexpectedly_modified, file_mutation_timestamp_ms, get_stored_file_read_state,
-    local_file_modification_time_ms, read_current_file_content, read_state_tracking_enabled,
+    local_file_modification_time_ms, read_state_tracking_enabled,
     update_file_read_state_after_mutation, validate_edit_against_read_state,
     validate_edit_has_prior_read, FILE_UNEXPECTEDLY_MODIFIED_ERROR,
 };
@@ -223,7 +223,7 @@ impl Tool for FileEditTool {
             .get("new_string")
             .and_then(|v| v.as_str())
             .unwrap_or("");
-        let replace_all = input
+        let _replace_all = input
             .get("replace_all")
             .and_then(|v| v.as_bool())
             .unwrap_or(false);
@@ -270,36 +270,6 @@ impl Tool for FileEditTool {
             if let Some(message) = Self::edit_read_state_guardrail_error(ctx, &resolved).await {
                 return Self::guidance_failure(message);
             }
-
-            let file_content = match read_current_file_content(ctx, &resolved).await {
-                Ok(content) => content,
-                Err(error) => {
-                    return ValidationResult {
-                        result: false,
-                        message: Some(format!(
-                            "Failed to read file {}: {}",
-                            resolved.logical_path, error
-                        )),
-                        error_code: Some(400),
-                        meta: None,
-                    };
-                }
-            };
-
-            if let Err(error) =
-                apply_edit_to_content(&file_content, old_string, new_string, replace_all)
-            {
-                if is_edit_content_guardrail_error(&error) {
-                    return Self::guidance_failure(error);
-                }
-
-                return ValidationResult {
-                    result: false,
-                    message: Some(error),
-                    error_code: Some(400),
-                    meta: None,
-                };
-            }
         }
 
         ValidationResult::default()

src/crates/execution/tool-execution/src/fs/edit_file.rs

@@ -429,6 +429,22 @@ pub fn apply_edit_to_content(
     let uses_crlf = content.contains("\r\n");
     let normalized_content = normalize_string(content);
 
+    // Fast path: try the exact old_string before generating fallback
+    // candidates. This avoids expensive whitespace-normalization and
+    // find_actual_string character-by-character scanning when the exact
+    // match succeeds — the common case.
+    match apply_match_and_replace(
+        &normalized_content,
+        uses_crlf,
+        old_string,
+        new_string,
+        replace_all,
+    ) {
+        Ok(result) => return Ok(result),
+        Err(error) if error == "old_string not found in file." => last_error = error,
+        Err(error) => return Err(error),
+    }
+
     for (candidate_old, candidate_new) in edit_string_candidates(content, old_string, new_string) {
         match apply_match_and_replace(
             &normalized_content,