diff --git a/api/agent_sessions.py b/api/agent_sessions.py
index c0a5078f..3bccfd2d 100644
--- a/api/agent_sessions.py
+++ b/api/agent_sessions.py
@@ -6,6 +6,17 @@ from pathlib import Path
 
 logger = logging.getLogger(__name__)
 
+# state.db paths that already produced the "no 'source' column" warning below.
+# ``get_cli_sessions(all_profiles=True)`` re-reads every profile DB on every
+# sidebar poll (behind a 5 s cache), so a single pre-``source`` profile DB would
+# otherwise re-emit the identical WARNING line every ~15 s for the life of the
+# process. The condition is a property of the DB file, not of the poll, so it
+# is reported once per path. Process-lifetime only: a restart warns again,
+# which is the desired behaviour (the log line is the operator's cue that the
+# agent still needs upgrading). Plain ``set`` mutation under the GIL is
+# sufficient here; a duplicate line from two racing first calls is harmless.
+_SOURCE_COLUMN_WARNED_DB_PATHS: set[str] = set()
+
 
 def open_state_db_readonly(db_path: Path, log: logging.Logger | None = None) -> sqlite3.Connection:
     """Open the live agent ``state.db`` read-only for a pure-read projection.
@@ -562,12 +573,15 @@ def read_importable_agent_session_rows(
         cur.execute("PRAGMA table_info(messages)")
         message_cols = {row[1] for row in cur.fetchall()}
         if 'source' not in session_cols:
-            log.warning(
-                "agent session listing skipped: state.db at %s has no 'source' column "
-                "(older hermes-agent?). Agent sessions unavailable. "
-                "Upgrade hermes-agent to fix this.",
-                db_path,
-            )
+            warned_key = str(db_path.resolve())
+            if warned_key not in _SOURCE_COLUMN_WARNED_DB_PATHS:
+                _SOURCE_COLUMN_WARNED_DB_PATHS.add(warned_key)
+                log.warning(
+                    "agent session listing skipped: state.db at %s has no 'source' column "
+                    "(older hermes-agent?). Agent sessions unavailable. "
+                    "Upgrade hermes-agent to fix this.",
+                    db_path,
+                )
             return []
 
         parent_expr = _optional_col('parent_session_id', session_cols)
diff --git a/tests/test_issue634.py b/tests/test_issue634.py
index daa8c477..de9ebd32 100644
--- a/tests/test_issue634.py
+++ b/tests/test_issue634.py
@@ -77,3 +77,74 @@ class TestCliSessionsErrorSurface:
         assert select_pos != -1, "SELECT query not found"
         assert pragma_pos < select_pos, \
             "Schema introspection must run before the main SQL query"
+
+
+# ---------------------------------------------------------------------------
+# Follow-up: the warning must fire once per state.db, not once per poll.
+#
+# get_cli_sessions(all_profiles=True) calls read_importable_agent_session_rows()
+# for every profile on every sidebar poll (behind a 5 s cache). On an install
+# with one pre-``source`` profile DB that produced one identical WARNING line
+# every ~15 s in errors.log. The message is correct; its repetition is not.
+# ---------------------------------------------------------------------------
+import logging
+import sqlite3
+
+import pytest
+
+from api import agent_sessions
+
+
+def _make_pre_source_state_db(path):
+    """Build a state.db whose ``sessions`` table predates the ``source`` column."""
+    conn = sqlite3.connect(str(path))
+    conn.execute(
+        "CREATE TABLE sessions (id TEXT PRIMARY KEY, title TEXT, model TEXT, "
+        "message_count INTEGER, started_at REAL)"
+    )
+    conn.execute("CREATE TABLE messages (id INTEGER PRIMARY KEY, session_id TEXT, role TEXT, timestamp REAL)")
+    conn.commit()
+    conn.close()
+
+
+def _source_column_warnings(caplog):
+    return [
+        rec for rec in caplog.records
+        if rec.levelno == logging.WARNING and "has no 'source' column" in rec.getMessage()
+    ]
+
+
+class TestMissingSourceColumnWarnsOncePerDb:
+    @pytest.fixture(autouse=True)
+    def _fresh_dedupe_state(self, monkeypatch):
+        # The dedupe set is process-lifetime state; give each test its own.
+        monkeypatch.setattr(agent_sessions, "_SOURCE_COLUMN_WARNED_DB_PATHS", set(), raising=False)
+
+    def test_same_db_path_twice_logs_once(self, tmp_path, caplog):
+        db = tmp_path / "state.db"
+        _make_pre_source_state_db(db)
+        with caplog.at_level(logging.WARNING, logger="api.agent_sessions"):
+            first = agent_sessions.read_importable_agent_session_rows(db, exclude_sources=None)
+            second = agent_sessions.read_importable_agent_session_rows(db, exclude_sources=None)
+        assert first == [] and second == []
+        warnings = _source_column_warnings(caplog)
+        assert len(warnings) == 1, [w.getMessage() for w in warnings]
+        # Message text is unchanged: still names the path and the upgrade hint.
+        assert str(db) in warnings[0].getMessage()
+        assert "Upgrade hermes-agent" in warnings[0].getMessage()
+
+    def test_different_db_path_logs_again(self, tmp_path, caplog):
+        db_a = tmp_path / "planner" / "state.db"
+        db_b = tmp_path / "coder" / "state.db"
+        db_a.parent.mkdir()
+        db_b.parent.mkdir()
+        _make_pre_source_state_db(db_a)
+        _make_pre_source_state_db(db_b)
+        with caplog.at_level(logging.WARNING, logger="api.agent_sessions"):
+            agent_sessions.read_importable_agent_session_rows(db_a, exclude_sources=None)
+            agent_sessions.read_importable_agent_session_rows(db_b, exclude_sources=None)
+            agent_sessions.read_importable_agent_session_rows(db_a, exclude_sources=None)
+        warnings = _source_column_warnings(caplog)
+        assert len(warnings) == 2, [w.getMessage() for w in warnings]
+        assert str(db_a) in warnings[0].getMessage()
+        assert str(db_b) in warnings[1].getMessage()