diff --git a/agent/insights.py b/agent/insights.py
index 2e45b222d4152..1f5b9493046f3 100644
--- a/agent/insights.py
+++ b/agent/insights.py
@@ -99,6 +99,29 @@ class InsightsEngine:
         """
         self.db = db
         self._conn = db._conn
+        # INDEXED BY is a hard dependency (SQLite errors on a missing index).
+        # A read-only open of a state.db written by an older version skips
+        # schema init and lacks the partial index — probe once and fall back
+        # to the unpinned variants (identical rows, optimizer-chosen plan).
+        try:
+            self._has_assistant_calls_index = bool(
+                self._conn.execute(
+                    "SELECT 1 FROM sqlite_master WHERE type='index' AND name=?",
+                    (self._MESSAGES_ASSISTANT_CALLS_INDEX,),
+                ).fetchone()
+            )
+        except sqlite3.Error:
+            self._has_assistant_calls_index = False
+        if not self._has_assistant_calls_index:
+            _strip = f" INDEXED BY {self._MESSAGES_ASSISTANT_CALLS_INDEX}"
+            self._GET_TOOL_CALLS_WITH_SOURCE = (
+                self._GET_TOOL_CALLS_WITH_SOURCE.replace(_strip, "")
+            )
+            self._GET_TOOL_CALLS_ALL = self._GET_TOOL_CALLS_ALL.replace(_strip, "")
+            self._GET_SKILL_CALLS_WITH_SOURCE = (
+                self._GET_SKILL_CALLS_WITH_SOURCE.replace(_strip, "")
+            )
+            self._GET_SKILL_CALLS_ALL = self._GET_SKILL_CALLS_ALL.replace(_strip, "")
 
     def generate(self, days: int = 30, source: str = None) -> Dict[str, Any]:
         """
@@ -200,11 +223,14 @@ class InsightsEngine:
     # is deterministic on a freshly initialized state.db (before ANALYZE has
     # run) for BOTH the unfiltered and source-filtered branches — without the
     # hint the optimizer falls back to ``idx_messages_session_active`` for the
-    # source-filtered probe and scans each session's non-tool-call rows.  Safe
-    # because the index is declared in ``SCHEMA_SQL`` (created by every
-    # read-write ``SessionDB._init_schema``); every ``InsightsEngine`` caller
-    # opens a read-write ``SessionDB`` — read-only attachments (which skip
-    # schema init) are never used for insights.
+    # source-filtered probe and scans each session's non-tool-call rows.
+    #
+    # The pin is a HARD dependency: SQLite raises ``no such index`` when the
+    # named index is absent. That happens in practice — the web dashboard's
+    # usage analytics open the DB ``read_only=True`` (skipping
+    # ``_init_schema``), so a state.db created by an older writer has no
+    # partial index yet. ``__init__`` probes for the index once and falls
+    # back to the unpinned (still-correct, just optimizer-chosen) variants.
     _MESSAGES_ASSISTANT_CALLS_INDEX = "idx_messages_assistant_calls_by_session"
     _GET_TOOL_CALLS_WITH_SOURCE = (
         "SELECT m.tool_calls"
diff --git a/tests/agent/test_insights.py b/tests/agent/test_insights.py
index b638be70df4b5..73bce06f4fb41 100644
--- a/tests/agent/test_insights.py
+++ b/tests/agent/test_insights.py
@@ -452,16 +452,26 @@ class TestInsightsPopulated:
         assert any(t["tool_name"] == "search_files" for t in tools_cli)
         assert isinstance(skills, list) and isinstance(skills_cli, list)
 
-    def test_indexed_by_requires_the_index_to_exist(self, populated_db):
-        """INDEXED BY is a hard dependency: if the index were ever missing the
-        query fails loudly rather than silently degrading. This documents why
-        every InsightsEngine caller must open a read-write SessionDB."""
+    def test_missing_index_falls_back_to_unpinned_queries(self, populated_db):
+        """INDEXED BY would be a hard error if the index is missing — which
+        happens on read-only opens of a state.db written by an older version
+        (web dashboard analytics). The engine must probe and fall back to the
+        unpinned variants instead of crashing, returning identical rows."""
+        engine_pinned = InsightsEngine(populated_db)
+        tools_before = engine_pinned._get_tool_usage(0.0)
+
         populated_db._conn.execute(f"DROP INDEX IF EXISTS {self._INDEX}")
         populated_db._conn.commit()
-        with pytest.raises(sqlite3.OperationalError, match="no such index"):
-            populated_db._conn.execute(
-                InsightsEngine._GET_TOOL_CALLS_ALL, (0.0,)
-            ).fetchall()
+
+        engine = InsightsEngine(populated_db)
+        assert engine._has_assistant_calls_index is False
+        assert "INDEXED BY" not in engine._GET_TOOL_CALLS_ALL
+        tools_after = engine._get_tool_usage(0.0)
+        assert sorted(t["tool_name"] for t in tools_after) == sorted(
+            t["tool_name"] for t in tools_before
+        )
+        # And with the index present, the pin stays.
+        assert "INDEXED BY" in InsightsEngine._GET_TOOL_CALLS_ALL
 
 
 # =========================================================================