diff --git a/packages/core/src/schema.sql b/packages/core/src/schema.sql
index 10d177e..1d5ef87 100644
--- a/packages/core/src/schema.sql
+++ b/packages/core/src/schema.sql
@@ -74,6 +74,8 @@ CREATE INDEX IF NOT EXISTS idx_tc_message ON tool_calls(message_uuid);
 CREATE INDEX IF NOT EXISTS idx_tc_file ON tool_calls(file_path);
 CREATE INDEX IF NOT EXISTS idx_tr_session ON tool_results(session_id);
 CREATE INDEX IF NOT EXISTS idx_tr_message ON tool_results(message_uuid);
+CREATE INDEX IF NOT EXISTS idx_tr_failure_session ON tool_results(session_id)
+  WHERE is_error = 1 OR content LIKE 'Exit code %';
 CREATE INDEX IF NOT EXISTS idx_sa_session ON subagents(session_id);
 CREATE INDEX IF NOT EXISTS idx_wf_session ON workflows(session_id);
 CREATE INDEX IF NOT EXISTS idx_wa_run ON workflow_agents(run_id);
diff --git a/tests/db-schema.test.mjs b/tests/db-schema.test.mjs
index 3f98cd8..9e25748 100644
--- a/tests/db-schema.test.mjs
+++ b/tests/db-schema.test.mjs
@@ -127,6 +127,35 @@ test('tool results schema indexes live session patch lookups', async () => {
   }
 });
 
+test('tool results schema limits failure scans to failure rows', async () => {
+  const db = new DatabaseSync(':memory:');
+  try {
+    db.exec(await readExecutableSchema());
+    const plan = db.prepare(`
+      EXPLAIN QUERY PLAN
+      SELECT tr.*
+      FROM tool_results tr
+      LEFT JOIN messages rm ON rm.uuid=tr.message_uuid
+      LEFT JOIN tool_calls tc ON tc.id=tr.tool_use_id
+      LEFT JOIN messages cm ON cm.uuid=tc.message_uuid
+      LEFT JOIN sessions s ON s.id=tr.session_id
+      WHERE (tr.is_error = 1 OR tr.content LIKE 'Exit code %')
+        AND COALESCE(rm.visibility,'visible')='visible'
+        AND COALESCE(cm.visibility,'visible')='visible'
+        AND COALESCE(s.source, 'claude') = ?
+      ORDER BY rm.timestamp DESC
+      LIMIT ?
+    `).all('codex', 50);
+
+    assert.ok(
+      plan.some(row => /USING INDEX idx_tr_failure_session/.test(String(row.detail))),
+      `expected partial failure index, got: ${plan.map(row => row.detail).join('; ')}`,
+    );
+  } finally {
+    db.close();
+  }
+});
+
 test('session detail queries use the visible main timeline index', async () => {
   const db = new DatabaseSync(':memory:');
   try {
diff --git a/tests/provider-schema-stability.test.mjs b/tests/provider-schema-stability.test.mjs
index 82514dd..5af87cd 100644
--- a/tests/provider-schema-stability.test.mjs
+++ b/tests/provider-schema-stability.test.mjs
@@ -12,8 +12,8 @@ test('canonical transcript persistence schema changes only by explicit decision'
   const schema = readFileSync(new URL('../packages/core/src/schema.sql', import.meta.url));
   assert.equal(
     createHash('sha256').update(schema).digest('hex'),
-    // 2026-08-24: indexed Activity queries and the visible main session timeline.
-    '0417d1a044a574a9adc8f347db5f46428a984e89900504bb082ce7366ebb1d4a',
+    // 2026-09-03: indexed the sparse failure-result subset by session.
+    '1375f420b93f62ad289bc9b2d10f7ba5fb30672195938eb17f54cd8206177e24',
   );
 });