diff --git a/scripts/indexer.mjs b/scripts/indexer.mjs
index cead875..e959002 100644
--- a/scripts/indexer.mjs
+++ b/scripts/indexer.mjs
@@ -108,7 +108,27 @@ function indexJsonl(db, fi) {
 
   const ins = {
     ses: db.prepare('INSERT OR REPLACE INTO sessions (id,title,project,project_path,started_at,ended_at,git_branch,version,message_count,jsonl_path,source) VALUES (?,?,?,?,?,?,?,?,?,?,?)'),
-    msg: db.prepare('INSERT OR REPLACE INTO messages (uuid,session_id,type,parent_uuid,timestamp,role,text,content_type,is_meta,model,is_sidechain,agent_id,input_tokens,output_tokens,cwd,skill,source) VALUES (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)'),
+    msg: db.prepare(`
+      INSERT INTO messages (uuid,session_id,type,parent_uuid,timestamp,role,text,content_type,is_meta,model,is_sidechain,agent_id,input_tokens,output_tokens,cwd,skill,source)
+      VALUES (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)
+      ON CONFLICT(uuid) DO UPDATE SET
+        session_id=excluded.session_id,
+        type=excluded.type,
+        parent_uuid=excluded.parent_uuid,
+        timestamp=excluded.timestamp,
+        role=excluded.role,
+        text=excluded.text,
+        content_type=excluded.content_type,
+        is_meta=excluded.is_meta,
+        model=excluded.model,
+        is_sidechain=excluded.is_sidechain,
+        agent_id=excluded.agent_id,
+        input_tokens=excluded.input_tokens,
+        output_tokens=excluded.output_tokens,
+        cwd=excluded.cwd,
+        skill=excluded.skill,
+        source=excluded.source
+    `),
     tc:  db.prepare('INSERT OR REPLACE INTO tool_calls (id,message_uuid,session_id,name,input_json,file_path) VALUES (?,?,?,?,?,?)'),
     tr:  db.prepare('INSERT OR REPLACE INTO tool_results (tool_use_id,message_uuid,session_id,content,file_path,is_error) VALUES (?,?,?,?,?,?)'),
     sum: db.prepare('INSERT OR REPLACE INTO summaries (id,session_id,timestamp,source,content) VALUES (?,?,?,?,?)'),
@@ -122,7 +142,7 @@ function indexJsonl(db, fi) {
     git_branch: existing?.git_branch || null,
     version: existing?.version || null,
     title: existing?.title || null,
-    n: existing?.message_count || 0,
+    n: skip > 0 ? (existing?.message_count || 0) : 0,
     cwds: [],
   };
 
@@ -788,4 +808,4 @@ function buildIndex({ force = false } = {}) {
   db.close();
 }
 
-export { buildIndex, inferProjectPath, refreshSessionProjectPaths, shouldSkipBuild };
+export { buildIndex, indexJsonl, inferProjectPath, refreshSessionProjectPaths, shouldSkipBuild };
diff --git a/tests/indexer-upsert-drift.test.mjs b/tests/indexer-upsert-drift.test.mjs
new file mode 100644
index 0000000..a214314
--- /dev/null
+++ b/tests/indexer-upsert-drift.test.mjs
@@ -0,0 +1,64 @@
+// Regression test for the indexer silent-drift fix.
+//
+// scripts/indexer.mjs and app/indexer.js had diverged in indexJsonl's message
+// write: scripts used INSERT OR REPLACE (churns rowid → FTS churn) and always
+// carried the previous message_count forward (inflating it on a full re-scan),
+// while app used ON CONFLICT DO UPDATE and reset the count when skip===0. app's
+// semantics are canonical; this pins them so the two cannot drift again and so
+// the Phase 5 provider-adapter merge inherits one known-correct behavior.
+
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { createRequire } from 'node:module';
+import { mkdtempSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+
+import { indexJsonl } from '../scripts/indexer.mjs';
+
+const require = createRequire(import.meta.url);
+const { DatabaseSync } = require('node:sqlite');
+const SCHEMA = require('node:fs').readFileSync(new URL('../scripts/schema.sql', import.meta.url), 'utf8');
+
+function writeSessionJsonl() {
+  const dir = mkdtempSync(join(tmpdir(), 'obelisk-drift-'));
+  const jsonlPath = join(dir, 'sid-drift.jsonl');
+  const lines = [
+    { uuid: 'u-1', type: 'user', timestamp: '2026-06-10T10:00:00Z', cwd: '/tmp/proj', message: { role: 'user', content: 'first question' } },
+    { uuid: 'a-1', type: 'assistant', timestamp: '2026-06-10T10:00:05Z', message: { role: 'assistant', model: 'claude-opus', content: 'first answer' } },
+    { uuid: 'u-2', type: 'user', timestamp: '2026-06-10T10:00:10Z', cwd: '/tmp/proj', message: { role: 'user', content: 'second question' } },
+  ];
+  writeFileSync(jsonlPath, lines.map(l => JSON.stringify(l)).join('\n') + '\n');
+  return jsonlPath;
+}
+
+test('re-indexing a session upserts messages (stable rowid) and does not inflate message_count', () => {
+  const db = new DatabaseSync(':memory:');
+  db.exec(SCHEMA);
+  const fi = { path: writeSessionJsonl(), sessionId: 'sid-drift', project: 'quiet-zero' };
+
+  indexJsonl(db, fi);
+
+  const countAfterFirst = db.prepare('SELECT message_count FROM sessions WHERE id=?').get('sid-drift').message_count;
+  const rowidAfterFirst = db.prepare('SELECT rowid FROM messages WHERE uuid=?').get('u-1').rowid;
+  const totalMessages = db.prepare('SELECT COUNT(*) AS c FROM messages').get().c;
+  assert.equal(countAfterFirst, 3, 'three user/assistant messages counted');
+  assert.equal(totalMessages, 3);
+
+  // Simulate a fresh full re-scan (force / lost index_state): skip resets to 0.
+  db.prepare('DELETE FROM index_state').run();
+  indexJsonl(db, fi);
+
+  const countAfterSecond = db.prepare('SELECT message_count FROM sessions WHERE id=?').get('sid-drift').message_count;
+  const rowidAfterSecond = db.prepare('SELECT rowid FROM messages WHERE uuid=?').get('u-1').rowid;
+  const totalAfterSecond = db.prepare('SELECT COUNT(*) AS c FROM messages').get().c;
+
+  // message_count is reset+recounted, not accumulated (would be 6 under the old bug).
+  assert.equal(countAfterSecond, 3, 'message_count must not inflate on re-scan');
+  // No duplicate rows.
+  assert.equal(totalAfterSecond, 3);
+  // Upsert preserves rowid; INSERT OR REPLACE would have churned it.
+  assert.equal(rowidAfterSecond, rowidAfterFirst, 'upsert must preserve message rowid (no REPLACE churn)');
+
+  db.close();
+});