diff --git a/conductor/edit_workflow.md b/conductor/edit_workflow.md
index bc22dc81..e19b7b44 100644
--- a/conductor/edit_workflow.md
+++ b/conductor/edit_workflow.md
@@ -16,16 +16,19 @@ The `manual-slop_edit_file` tool requires **exact string matches** (character-fo
 Before ANY edit to a function you haven't touched recently:
 
 ```
-1. Run: git checkout -- src/gui_2.py
-2. Run: py_check_syntax on src/gui_2.py
-3. Get current state with get_file_slice
+1. Run: py_check_syntax on src/<file>.py
+2. Get current state with get_file_slice (the exact lines you're about to touch)
+3. Read the contract: does this function/field/method's signature, yield shape, or return type have callers I need to update?
 ```
 
+DO NOT use `git checkout` or `git restore` to "revert" your way to a clean state. That destroys in-progress work. If a previous edit left the file in a broken state, ask the user.
+
 ### 3. Reading Before Editing (CRITICAL)
 
-- Use `get_file_slice` to get the EXACT text including all whitespace
+- Use `get_file_slice` to get the EXACT text including all whitespace and EOL
 - Copy text directly from the tool output - do NOT reformat
-- If using get_definition, verify the text matches before editing
+- If using `get_definition`, verify the text matches before editing
+- For `set_file_slice`: confirm the exact `start_line` and `end_line` (1-indexed, inclusive) by reading the file first. Off-by-one is a common silent failure.
 
 ### 4. The Edit Tool Parameters (snake_case)
 
@@ -72,9 +75,47 @@ This keeps the `@property` attached to its original method.
 2. Instantiate the class
 3. Call the new method in the way it's expected to be called (`ctrl.foo_ts` for a property, `ctrl.foo_ts()` for a method)
 
-### 8. Do Not Use `set_file_slice` For Multi-Line Content (Added 2026-06-07)
+### 8. `set_file_slice` IS Valid for Multi-Line Content (Revised 2026-06-09)
 
-`set_file_slice` does literal line replacement by design. It does not reindent, does not normalize EOL, does not parse decorators. Use it for surgical line-level edits (3-10 lines). If you need to insert or replace a multi-method block, use `manual-slop_edit_file` with verified exact-text old_string/new_string, or use `py_add_def` / `py_update_definition` for class/method-level work.
+The previous rule ("Do not use set_file_slice for multi-line content") was wrong. `set_file_slice` does literal line replacement by design and is the right tool for 3-10 line surgical edits.
+
+**When to use which tool:**
+
+- **`set_file_slice`** for surgical 3-10 line edits where you know the exact line range. Verify the line range with `get_file_slice` first. The `start_line` and `end_line` are 1-indexed and inclusive. The new content must reproduce the line count exactly (or be a precise replacement of the same N lines).
+- **`manual-slop_edit_file`** for exact-string replacement when you don't know the line range, or when the edit has a unique anchor string.
+- **`py_update_definition`** for whole-function replacement (AST-detected).
+- **`py_add_def`** for adding a new method/class to a class.
+- **`py_remove_def`** for removing a method/class.
+
+**The contract-change check (mandatory for any edit that changes a public interface):**
+
+Before any edit, search the codebase for callers of the function/symbol/yield shape you're changing. If your edit changes:
+- A function signature (add/remove/rename a parameter)
+- A return type or yield shape (e.g. `yield process, gui_script` → `yield process, gui_script, workspace_path`)
+- A class hierarchy (add/remove a base class, change a method's name)
+- A module-level function name (rename)
+- A public attribute name
+
+...you MUST update ALL callers in the same atomic commit. Use `py_find_usages` to locate them. If you change a contract and don't update callers, you have broken the codebase.
+
+**The whitespace-and-EOL rule (mandatory for set_file_slice):**
+
+The `new_content` must preserve:
+- The file's line ending convention (CRLF on Windows, LF on Linux — pick from the surrounding file, not from your text editor's default)
+- The indentation of the surrounding code (1 space per level, per `conductor/code_styleguides/python.md` §1)
+- The number of lines replaced (`start_line`..`end_line` must equal `len(new_content.splitlines())`)
+
+If you mismatch any of these, the file will fail to parse. Run `py_check_syntax` and a real `import` after every `set_file_slice`.
+
+### 9. No Diagnostic Noise in Production Code (Added 2026-06-09)
+
+`sys.stderr.write(f"[XYZ_DIAG] ...")` lines added to `src/*.py` for debugging are technical debt the moment they ship. If you need to instrument for a one-time investigation:
+
+- Write the diag output to a log file: `tests/artifacts/<test_name>.diag.log`
+- Or to a standalone diagnostic script under `/tmp/diag_<name>.py` that imports the production module and exercises it
+- Or read the production source with `get_file_slice` and reason about it directly
+
+Do NOT add diag lines to `src/*.py` "temporarily." If you must add them for a single test run, they are part of the same atomic commit as the fix — they do not live uncommitted in the working tree. If you "revert everything," that means the diag lines are also reverted.
 
 ## Step-by-Step Workflow for gui_2.py
 
@@ -96,9 +137,16 @@ get_file_slice path=src/gui_2.py start_line=X end_line=Y
 
 - The text you're trying to replace doesn't EXACTLY match
 - Use `get_file_slice` to get the exact text
-- Copy it character-for-character including whitespace
+- Copy it character-for-character including whitespace and EOL
 - Try again with exact match
 
+### If `set_file_slice` produces wrong indentation:
+
+- You wrote the wrong indent in `new_content`. The tool did what you asked.
+- Re-read the file with `get_file_slice` to confirm the surrounding indent
+- Rewrite the `new_content` with the correct indent
+- Do NOT use `git checkout` to "revert"
+
 ## Alternative: Update Definition Approach
 
 For large function rewrites, use `py_update_definition`: