diff --git a/src/lib/db/usageSummary.ts b/src/lib/db/usageSummary.ts
index 867794ef14..edef9b41b4 100644
--- a/src/lib/db/usageSummary.ts
+++ b/src/lib/db/usageSummary.ts
@@ -25,7 +25,8 @@ export function sumUsageTokensThisMonth(db: SqliteAdapter = getDbInstance()): nu
            COALESCE((
              SELECT SUM(tokens_input + tokens_output)
              FROM usage_history
-             WHERE substr(timestamp, 1, 7) = strftime('%Y-%m', 'now')
+             WHERE timestamp >= strftime('%Y-%m-01T00:00:00.000Z','now')
+               AND timestamp < strftime('%Y-%m-01T00:00:00.000Z','now','+1 month')
            ), 0) +
            COALESCE((
              SELECT SUM(total_input_tokens + total_output_tokens)
diff --git a/tests/unit/free-tier-used-this-month.test.ts b/tests/unit/free-tier-used-this-month.test.ts
index acde52c14c..d6d82c945f 100644
--- a/tests/unit/free-tier-used-this-month.test.ts
+++ b/tests/unit/free-tier-used-this-month.test.ts
@@ -60,3 +60,42 @@ test("sumUsageTokensThisMonth includes the current month's raw usage_history row
 
   assert.equal(sumUsageTokensThisMonth(), 400 + 50);
 });
+
+// #10509 sweep: the `substr(timestamp, 1, 7) = strftime('%Y-%m', 'now')` predicate was
+// fragile/non-indexable (SQLite cannot use a range index on a substr() expression, and a
+// non-ISO-shaped timestamp string silently mismatches). Replaced with an indexable UTC
+// month-range comparison (`timestamp >= <month start> AND timestamp < <next month start>`).
+// This test pins the exact boundary: the first instant of the current month is INCLUDED,
+// the last instant of the PREVIOUS month is EXCLUDED, and a NEXT-month row is EXCLUDED too
+// (guards the upper-bound half of the range, which substr() could never express directly).
+test("sumUsageTokensThisMonth uses an inclusive-start/exclusive-end UTC month range (#10509)", () => {
+  const db = getDbInstance();
+  db.exec(`CREATE TABLE IF NOT EXISTS usage_history (
+    id INTEGER PRIMARY KEY AUTOINCREMENT, provider TEXT, model TEXT, connection_id TEXT,
+    api_key_id TEXT, api_key_name TEXT, tokens_input INTEGER DEFAULT 0, tokens_output INTEGER DEFAULT 0,
+    tokens_cache_read INTEGER DEFAULT 0, tokens_cache_creation INTEGER DEFAULT 0, tokens_reasoning INTEGER DEFAULT 0,
+    service_tier TEXT DEFAULT 'standard', status TEXT, success INTEGER DEFAULT 1, latency_ms INTEGER DEFAULT 0,
+    ttft_ms INTEGER DEFAULT 0, error_code TEXT, timestamp TEXT NOT NULL);`);
+  db.exec(`CREATE TABLE IF NOT EXISTS daily_usage_summary (id INTEGER PRIMARY KEY AUTOINCREMENT, provider TEXT NOT NULL, model TEXT NOT NULL, date TEXT NOT NULL, total_requests INTEGER NOT NULL DEFAULT 0, total_input_tokens INTEGER NOT NULL DEFAULT 0, total_output_tokens INTEGER NOT NULL DEFAULT 0, total_cost REAL NOT NULL DEFAULT 0.0, created_at TEXT NOT NULL DEFAULT (datetime('now')));`);
+  db.exec("DELETE FROM usage_history");
+  db.exec("DELETE FROM daily_usage_summary");
+
+  const monthStart = db
+    .prepare("SELECT strftime('%Y-%m-01T00:00:00.000Z','now') AS s")
+    .get() as { s: string };
+  const nextMonthStart = db
+    .prepare("SELECT strftime('%Y-%m-01T00:00:00.000Z','now','+1 month') AS s")
+    .get() as { s: string };
+  const lastInstantOfPrevMonth = new Date(
+    new Date(monthStart.s).getTime() - 1
+  ).toISOString();
+
+  const insHistory = db.prepare(
+    "INSERT INTO usage_history (provider, model, tokens_input, tokens_output, timestamp) VALUES (?,?,?,?,?)"
+  );
+  insHistory.run("openai", "gpt-4.1", 10, 0, monthStart.s); // first instant of THIS month — included
+  insHistory.run("openai", "gpt-4.1", 9999, 0, lastInstantOfPrevMonth); // last ms of PREV month — excluded
+  insHistory.run("openai", "gpt-4.1", 9999, 0, nextMonthStart.s); // first instant of NEXT month — excluded
+
+  assert.equal(sumUsageTokensThisMonth(), 10);
+});