test: snapshot_idf vs corpus_idf + colluding-third — 23/23 (20 invariants + 3 known-defeats)

test_swarmmetrics.py

@@ -408,6 +408,55 @@ def test_genuine_novel_concept_zero_echo():
     # for concepts that have had time to propagate.
 
 
+def test_snapshot_idf_vs_corpus_idf():
+    """snapshot_idf=True should compute IDF at emission time, not on full corpus.
+
+    Setup: alice introduces 'novel-x' at t=-5d, nobody else uses it until
+    charlie uses it at t=-1d. With corpus IDF, novel-x has IDF=log(N/2)
+    at scoring time because charlie used it too. With snapshot IDF,
+    novel-x has IDF=log(N/1) at emission time because charlie hadn't
+    used it yet.
+
+    Snapshot IDF should give HIGHER weight to novel-x because at the time
+    alice introduced it, it was used by only 1 agent (alice herself).
+    Corpus IDF gives lower weight because by the end, 2 agents used it.
+
+    This is ColonistOne's IDF endogeneity fix: the concept became common
+    BECAUSE it diffused, so post-hoc IDF penalizes successful diffusion."""
+    msgs = [
+        # alice introduces novel-x
+        {"from_id": "alice", "to_id": "bob",
+         "timestamp": now - 5*day, "concepts": ["novel-x"]},
+        # bob uses it back (echo)
+        {"from_id": "bob", "to_id": "charlie",
+         "timestamp": now - 4*day, "concepts": ["novel-x"]},
+        # charlie picks it up too (more echo for alice)
+        {"from_id": "charlie", "to_id": "dave",
+         "timestamp": now - 1*day, "concepts": ["novel-x"]},
+        # background noise
+        {"from_id": "dave", "to_id": "eve",
+         "timestamp": now - 0.5*day, "concepts": ["common-stuff"]},
+        {"from_id": "eve", "to_id": "frank",
+         "timestamp": now - 0.3*day, "concepts": ["common-stuff"]},
+    ]
+
+    echo_corpus = score_echo(msgs, use_idf=True, snapshot_idf=False)
+    echo_snapshot = score_echo(msgs, use_idf=True, snapshot_idf=True)
+
+    alice_corpus = echo_corpus.get("alice", 0)
+    alice_snapshot = echo_snapshot.get("alice", 0)
+
+    # Snapshot should give alice higher or equal echo because at emission
+    # time, novel-x was rarer (only alice had used it)
+    assert alice_snapshot >= alice_corpus, \
+        f"Snapshot IDF should give >= corpus IDF for novel concepts: " \
+        f"snapshot={alice_snapshot:.4f}, corpus={alice_corpus:.4f}"
+
+    # Both should be positive (concept did propagate)
+    assert alice_corpus > 0, f"Alice should have positive echo (corpus): {alice_corpus}"
+    assert alice_snapshot > 0, f"Alice should have positive echo (snapshot): {alice_snapshot}"
+
+
 def test_colluding_third_party_echo():
     """Adversarial case #3 (Atomic Raven, Colony 2026-07-28): three agents
     in a trench coat manufacturing echo.