diff --git a/templates/commands/constitution.md b/templates/commands/constitution.md
index 7b2f3684fb..431045ecc0 100644
--- a/templates/commands/constitution.md
+++ b/templates/commands/constitution.md
@@ -109,7 +109,12 @@ Follow this execution flow:
    - Ensure each Principle section: succinct name line, paragraph (or bullet list) capturing non‑negotiable rules, explicit rationale if not obvious.
    - Ensure Governance section lists amendment procedure, versioning policy, and compliance review expectations.
 
-4. Produce a Sync Impact Report (prepend as an HTML comment at top of the constitution file after update):
+4. Produce a Sync Impact Report as an HTML comment at the top of the constitution file after update.
+   This report is temporary scratch material for human review of the amendment, not governance
+   content; it is expected to be removed before the amended constitution file is committed.
+   - If the file already starts with an HTML comment (e.g. a Sync Impact Report left over because
+     it was not removed before a prior commit), remove it entirely before adding the new one. The
+     file must never carry more than one Sync Impact Report; replace, never stack.
    - Version change: old → new
    - List of modified principles (old title → new title if renamed)
    - Added sections
diff --git a/tests/test_constitution_template_sync_report.py b/tests/test_constitution_template_sync_report.py
new file mode 100644
index 0000000000..42310eb8c8
--- /dev/null
+++ b/tests/test_constitution_template_sync_report.py
@@ -0,0 +1,24 @@
+"""Covers #4431: /constitution must not stack Sync Impact Report comments.
+
+The Outline step that produces the Sync Impact Report only said to "prepend"
+it as an HTML comment, with no instruction to remove a previous one. Every
+run of /constitution therefore added another comment block on top of the
+last, growing the raw constitution file (and the token cost of reading it)
+without bound.
+"""
+
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).parent.parent
+CONSTITUTION_TEMPLATE = REPO_ROOT / "templates" / "commands" / "constitution.md"
+
+
+def test_sync_impact_report_step_instructs_removing_prior_report():
+    content = CONSTITUTION_TEMPLATE.read_text(encoding="utf-8")
+    step = content.split("Produce a Sync Impact Report", 1)[1].split("\n\n", 1)[0]
+    assert "remove" in step.lower(), (
+        "Step 4 must instruct removing/replacing any existing Sync Impact "
+        "Report comment before adding the new one, otherwise reports stack "
+        "on every /constitution run"
+    )
+    assert "never stack" in step.lower() or "not stack" in step.lower()