diff --git a/emrg/server/open_source_prompt.md b/emrg/server/open_source_prompt.md
index dfc1422d..fa5b46e8 100644
--- a/emrg/server/open_source_prompt.md
+++ b/emrg/server/open_source_prompt.md
@@ -246,6 +246,7 @@ cd "$DEV" && git checkout -b <branch name per project convention> "upstream/$DEF
 - **Work in the clone from §3.1.4** (`cd "$DEV"`): every edit lands there, and `{{ source_dir }}` stays a read-only reference.
 - **One problem per change** — no opportunistic refactoring; the change should follow the conventions read in §3.1.3 and the design intent from §3.1.3.
 - **Read the surrounding context before editing**: understand the responsibilities of the code you touch.
+- **A claim you write into the project's documents says who decided it.** Before a line goes into a decisions file, a design document, an issue reply or a pull request body as the project's rule or as the maintainer's, name the message it rests on — its instant and its text. A rule you inferred, or proposed yourself, is written as a proposal, in wording that does not borrow a decision's authority: once your own proposal sits in your own context, the marker "I wrote this" is gone from it, and a premise you have forgotten you invented is the one you defend as a requirement. Memory is not the boundary; the project's documents are, because they are what the next reader acts on.
 - **A document with a translated twin is edited in one change.** Where the file you are changing has a counterpart in another language, both are changed in the same change and read against each other before it is committed: a twin left behind states the old rule to the next reader, and the reader who opens that side first is told the opposite of what the project now does. Where the project states which copy governs a disagreement, that statement sets the order — the other copy follows it, never the reverse.
 
 ##### 3.1.6 Test
diff --git a/tests/test_prompt_templates.py b/tests/test_prompt_templates.py
index 124fc718..fea2ebd0 100644
--- a/tests/test_prompt_templates.py
+++ b/tests/test_prompt_templates.py
@@ -3204,3 +3204,37 @@ def test_the_not_run_rule_has_one_home_and_the_zone_list_points_at_it() -> None:
         "the zone list names the owner but not where its lines are defined, so a reader who "
         "follows it still has to search"
     )
+
+
+def test_the_claim_provenance_rule_lives_in_the_section_that_writes_into_a_project() -> None:
+    """A claim written into a project's documents says who decided it (issue #2687).
+
+    Three facts are asserted together because each can go missing on its own: the rule is
+    *present*, it sits in the section that owns writing into the project rather than appended
+    at the end of the file, and it states the test a claim passes — a message that can be
+    pointed at, with its instant and its text. A rule that says "distinguish the host's
+    rulings from your proposals" without naming that test leaves the reader exactly where it
+    found them, because a proposal in the agent's own context reads like a ruling.
+    """
+    text = (PROMPTS_DIR / "open_source_prompt.md").read_text(encoding="utf-8")
+    section = _bounded_section(text, "##### 3.1.5 ", "##### 3.1.6 ")
+
+    assert "says who decided it" in section, (
+        "§3.1.5 no longer carries the provenance rule, so a task writes its own proposal into "
+        "the project's decisions file as the maintainer's and the next reader acts on it"
+    )
+    assert "its instant and its text" in section, (
+        "the rule no longer names the test a claim passes — a message that can be pointed at, "
+        "named by instant and text; without it the distinction is advice rather than a check"
+    )
+    assert "pull request body" in section, (
+        "the rule no longer names where it binds; memory is not the boundary, the project's "
+        "documents are, and a reader given no surface reads it as being about memory only"
+    )
+
+    # One home: the phrase that opens the rule must appear once in the whole template, or the
+    # file states the rule twice and a reader holds two answers with no way to choose.
+    assert text.count("says who decided it") == 1, (
+        f"the provenance rule is stated {text.count('says who decided it')} times in "
+        "open_source_prompt.md; this template's own rule is that a fact lives in exactly one place"
+    )