diff --git a/skills/refactor/SKILL.md b/skills/refactor/SKILL.md
index ec1f105..147b0a6 100644
--- a/skills/refactor/SKILL.md
+++ b/skills/refactor/SKILL.md
@@ -28,12 +28,51 @@ Never `git stash` and never revert a working-tree file to compare - other agents
 and the operator share this tree. Read the committed version with
 `git show HEAD:<path>` into a temporary file instead.
 
-## Step 2: refactor each file
+## Step 2: get a safety net first
+
+A refactor is only provably behaviour-preserving if something failed before it
+and passes after it. Before editing, check what covers the change set:
+
+- **Coverage exists**: run it now and record the green baseline. A suite that
+  was already red gets reported, not refactored around.
+- **No coverage and the code is testable logic**: write the characterization
+  tests first, against the current behaviour - including the behaviour you
+  consider wrong, so the refactor cannot silently change it. Keep them small
+  and framework-free unless the project already has a framework. Fix the
+  captured wrongness in a separate change, never in this one.
+- **Not sensibly testable**: pure glue, declarative config, a thin wrapper over
+  a service the test would have to mock into meaninglessness, or a project with
+  no test setup at all. Say so in one line and refactor without a net; do not
+  invent a test harness the project never asked for.
+
+The new tests are part of the deliverable and must still pass unchanged after
+the refactor. A test that had to be edited to stay green means the behaviour
+moved: revert the refactor, not the test.
+
+## Step 3: refactor each file
 
 Read the whole file first, then apply the standards that the surrounding code
 and the project's own config already establish (formatter config, linter rules,
-existing idioms) over any generic preference of your own:
+existing idioms) over any generic preference of your own.
 
+Four principles govern the result:
+
+- **SRP**: one file, one responsibility; one function, one reason to change. A
+  unit that needs an "and" to describe it is two units.
+- **SPOT**: every fact - a constant, a path, a schema, a rule - lives in exactly
+  one place and everything else references it.
+- **KISS**: the simplest construct that works. No abstraction without a second
+  caller today, no configuration for a value that never varies.
+- **DRY**: the same logic is written once. A literal repeated three times is a
+  missing constant.
+
+Concretely:
+
+- **File size**: no file containing program code exceeds 250 lines. Split the
+  overflow along the SRP seam - it is the symptom, not the cause. Description
+  and markup languages are exempt (Markdown, YAML, JSON, HTML, translation and
+  data files), as is the rare file the language or framework genuinely forbids
+  splitting: name that reason in the report, "it was easier" does not qualify.
 - **Duplication**: fold repeated logic into the helper that already exists;
   create a new one only when two or more call sites need it now.
 - **Dead weight**: unused imports, variables, parameters, branches that cannot
@@ -50,8 +89,9 @@ Do not reformat regions you have no substantive fix for - cosmetic churn buries
 the real change in the diff. If a file's problem is architectural and cannot be
 fixed inside the change set, report it instead of half-doing it.
 
-## Step 3: keep behaviour provable
+## Step 4: keep behaviour provable
 
+- Re-run the step 2 net and require the same green result as the baseline.
 - Never fold a bug fix, a feature, or a dependency change into a refactor.
   Found a real bug? Name it in the report and leave it for a separate change.
 - Run the cheap checks the project already has for the touched files (formatter,
@@ -60,7 +100,7 @@ fixed inside the change set, report it instead of half-doing it.
 - State plainly what was verified and what was not. An unverified refactor is
   reported as unverified.
 
-## Step 4: report
+## Step 5: report
 
 One line per file: what changed and why it is safe. Then one line naming what
 was deliberately left alone (architectural findings, bugs, files skipped) and