diff --git a/app/src/main/indexer.ts b/app/src/main/indexer.ts
index fed07f1..d767952 100644
--- a/app/src/main/indexer.ts
+++ b/app/src/main/indexer.ts
@@ -35,6 +35,15 @@ interface FileInfo {
   source?: string;
 }
 
+// A rollback inside a catch must never throw over the real error. SQLite
+// auto-rolls back certain failures (SQLITE_BUSY, disk full, ...); a following
+// explicit ROLLBACK then throws "cannot rollback - no transaction is active",
+// which would both mask the true cause and turn a skippable per-file error into
+// a whole-build failure. Swallow only the rollback's own error.
+function safeRollback(db: { exec: (sql: string) => unknown }) {
+  try { db.exec('ROLLBACK'); } catch { /* no active transaction */ }
+}
+
 function resolveSchemaPath() {
   const candidates = [
     path.join(__dirname, 'schema.sql'),
@@ -576,7 +585,7 @@ function buildIndex({
         if (file.source !== 'codex') indexSubagentMeta(db, file);
         db.exec('COMMIT');
       } catch (error) {
-        db.exec('ROLLBACK');
+        safeRollback(db);
         console.warn(`Warning: failed to index ${file.path}: ${(error as Error).message}`);
       }
     }
@@ -596,7 +605,7 @@ function buildIndex({
       if (latestSourceMtime) writeIndexMarker(db, '__last_source_mtime__', latestSourceMtime);
       db.exec('COMMIT');
     } catch (error) {
-      db.exec('ROLLBACK');
+      safeRollback(db);
       throw error;
     }
     return { files: files.length, latestSourceMtime, affectedSessionIds: [...affectedSessionIds], ftsRebuilt };
diff --git a/docs/adr/0006-write-transaction-rollback-and-concurrency.md b/docs/adr/0006-write-transaction-rollback-and-concurrency.md
new file mode 100644
index 0000000..41bff9a
--- /dev/null
+++ b/docs/adr/0006-write-transaction-rollback-and-concurrency.md
@@ -0,0 +1,76 @@
+# Write-transaction rollback safety and SQLite concurrency
+
+**Context.** The app surfaced `Obelisk index build failed: cannot rollback - no
+transaction is active`. That message is a *secondary* error: SQLite auto-rolls
+back certain failures (notably `SQLITE_BUSY` / `SQLITE_BUSY_SNAPSHOT`, also disk
+full), after which the per-file build loop's unguarded `db.exec('ROLLBACK')` in
+its `catch` threw over the real error and aborted the whole build instead of
+skipping just the offending file. The underlying trigger is concurrency: the app
+runs a daemon indexer, manual rebuilds, and read queries against one WAL
+database, and the skill's passive-pull build can write the same database from a
+separate process.
+
+`busy_timeout` is **not** the root-cause fix and must not be treated as one.
+better-sqlite3's constructor already defaults `timeout` to 5000ms, so the app hit
+`SQLITE_BUSY` *despite* a 5s wait — which points at `SQLITE_BUSY_SNAPSHOT` from
+deferred (read-then-write) transactions, a snapshot conflict that `busy_timeout`
+does not wait on. Only `BEGIN IMMEDIATE` plus whole-transaction retry addresses
+that.
+
+**Decision.** Split the work into a cheap stopgap now and a correctness/
+concurrency fix later.
+
+*Stopgap (done).* Both indexers use a guarded rollback (`safeRollback`): a
+cleanup rollback swallows only its own error and never masks the primary one.
+Per-file failures are logged and the build continues; the finalize failure still
+propagates. The skill's connection (`node:sqlite`, which has **no** default busy
+timeout) gets an explicit `PRAGMA busy_timeout = 5000`; the app adds no such
+pragma because better-sqlite3 already defaults to 5000ms — adding it there was
+redundant and removed, precisely so nobody reads it as "the fix".
+
+*Planned full fix (deferred, two phases).*
+
+Phase 1 — transaction semantics:
+- A shared, binding-agnostic `runWriteTransaction(db, work)` (same injection
+  model as `persist`): BEGIN → work → COMMIT, guarded rollback on failure,
+  original exception always preserved. Both app and skill call it, so their
+  behaviour is identical.
+- Per-file callers catch and continue; finalize does **not** swallow — a finalize
+  failure fails the build (the skill currently only warns; that changes).
+- In-memory state such as `affectedSessionIds` is updated **only after** a
+  successful COMMIT (today the app adds the id before COMMIT, so a failed commit
+  can report a wrong affected set).
+- Structured diagnostics: `phase` (begin/file-write/commit/finalize/checkpoint),
+  SQLite `code`, file path, whether rollback succeeded, whether a txn is still
+  active; surface the skipped-file count in the build result rather than only
+  `console.warn` (no silent coverage gaps).
+
+Phase 2 — concurrency:
+- A stable concurrency test against real Electron `better-sqlite3` (daemon +
+  rebuild + skill writer), not an injected fake BUSY. Note this cannot run under
+  standalone `node --test` (better-sqlite3 is Electron-ABI); it needs an
+  Electron-hosted harness. The fast injected-BUSY unit test is kept as a
+  lower-level guard for `runWriteTransaction`, alongside it.
+- Serialize all index writes through a single writer: an in-process
+  `BuildCoordinator`, plus the existing cross-process daemon arbitration
+  (`__app_heartbeat__` markers) so the skill defers to a live daemon. Single
+  writer = both layers together.
+- `BEGIN IMMEDIATE` to take the write lock up front and avoid
+  `SQLITE_BUSY_SNAPSHOT` on read-then-write.
+- Bounded, short-backoff retry of the **whole** transaction (not the single
+  failed statement) on `SQLITE_BUSY` / `SQLITE_BUSY_SNAPSHOT`. This relies on the
+  per-file work being idempotent (upsert/replace + delete-session cascade), which
+  it is — an invariant the retry depends on.
+- Stop forcing `wal_checkpoint(TRUNCATE)` after every build; prefer a `PASSIVE`
+  checkpoint on idle, with `TRUNCATE` reserved for maintenance/exit.
+- Centralized connection configuration (explicit 5000ms for `node:sqlite` — a
+  real behaviour change; explicit for better-sqlite3 too, acknowledging its
+  default is already 5000ms).
+
+**Consequences.** With only the stopgap in place, contention no longer crashes a
+build, but a conflicting file is *skipped* (non-fatal) and its data is briefly
+missing until the next build — acceptable as a stopgap because the guard keeps
+the index consistent and self-healing. The true fix is tracked as the two-phase
+plan above. A future contributor should not "fix" the concurrency by bumping or
+re-adding `busy_timeout`; the direction is a shared transaction module, single
+writer, `BEGIN IMMEDIATE`, and whole-transaction retry.
diff --git a/scripts/db.mjs b/scripts/db.mjs
index 721232c..4d175cc 100644
--- a/scripts/db.mjs
+++ b/scripts/db.mjs
@@ -24,6 +24,9 @@ function openDb() {
   const db = new DatabaseSync(DB_PATH);
   db.exec('PRAGMA journal_mode=WAL');
   db.exec('PRAGMA synchronous=NORMAL');
+  // Wait for a contended lock instead of erroring with SQLITE_BUSY: a running
+  // app daemon may be writing the same database while the skill reads/indexes.
+  db.exec('PRAGMA busy_timeout=5000');
   migrateExistingColumns(db);
   db.exec(SCHEMA);
   migrateDb(db);
diff --git a/scripts/indexer.mjs b/scripts/indexer.mjs
index 56e60e3..b8f8d89 100644
--- a/scripts/indexer.mjs
+++ b/scripts/indexer.mjs
@@ -9,6 +9,15 @@ import { parse as codexParse } from './providers/codex.ts';
 
 const HISTORY_PATH = path.join(CLAUDE_DIR, 'history.jsonl');
 
+// A rollback inside a catch must never throw over the real error. SQLite
+// auto-rolls back certain failures (SQLITE_BUSY, disk full, ...); a following
+// explicit ROLLBACK then throws "cannot rollback - no transaction is active",
+// which would both mask the true cause and turn a skippable per-file error into
+// a whole-build failure. Swallow only the rollback's own error.
+function safeRollback(db) {
+  try { db.exec('ROLLBACK'); } catch { /* no active transaction */ }
+}
+
 
 function needsReindex(db, fp) {
   const mt = fs.statSync(fp).mtimeMs;
@@ -193,7 +202,7 @@ function buildIndex({ force = false } = {}) {
       }
       db.exec('COMMIT');
     } catch (e) {
-      db.exec('ROLLBACK');
+      safeRollback(db);
       process.stderr.write(`Warning: failed to index ${f.path}: ${e.message}\n`);
     }
   }
@@ -208,7 +217,7 @@ function buildIndex({ force = false } = {}) {
     db.prepare("INSERT OR REPLACE INTO index_state (jsonl_path, mtime, lines_processed) VALUES ('__last_build__', ?, 0)").run(Date.now());
     db.exec('COMMIT');
   } catch (e) {
-    db.exec('ROLLBACK');
+    safeRollback(db);
     process.stderr.write(`Warning: failed to finalize index: ${e.message}\n`);
   }
   db.close();
diff --git a/tests/app-rollback-guard.test.mjs b/tests/app-rollback-guard.test.mjs
new file mode 100644
index 0000000..5fa74cd
--- /dev/null
+++ b/tests/app-rollback-guard.test.mjs
@@ -0,0 +1,92 @@
+// Regression test for the "cannot rollback - no transaction is active" crash.
+// SQLite auto-rolls back certain failures (SQLITE_BUSY, disk full, ...). The
+// per-file build loop then ran an explicit ROLLBACK in its catch, which threw a
+// SECOND error over the real one and aborted the whole build instead of skipping
+// just the bad file. buildIndex now uses a guarded rollback; this test injects a
+// DB that faithfully reproduces the condition and asserts the build survives.
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { createRequire } from 'node:module';
+import { mkdtempSync, mkdirSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+
+const require = createRequire(import.meta.url);
+import { buildIndex } from '../app/src/main/indexer.ts';
+const { DatabaseSync } = require('node:sqlite');
+
+// Wraps node:sqlite and simulates SQLite's auto-rollback-on-error: the first
+// write inside a transaction throws a BUSY-like error AND ends the real
+// transaction, so a following explicit ROLLBACK errors with "no transaction".
+class AutoRollbackOnceDatabase {
+  constructor(dbPath) {
+    this.db = new DatabaseSync(dbPath);
+    this.inTxn = false;
+    this.fired = false;
+  }
+  pragma(statement) { this.db.exec(`PRAGMA ${statement}`); }
+  exec(sql) {
+    const head = sql.trim().slice(0, 8).toUpperCase();
+    if (head.startsWith('BEGIN')) { this.inTxn = true; return this.db.exec('BEGIN'); }
+    if (head.startsWith('COMMIT')) { this.inTxn = false; return this.db.exec('COMMIT'); }
+    if (head.startsWith('ROLLBACK')) {
+      if (!this.inTxn) throw new Error('cannot rollback - no transaction is active');
+      this.inTxn = false;
+      return this.db.exec('ROLLBACK');
+    }
+    return this.db.exec(sql);
+  }
+  prepare(sql) {
+    const stmt = this.db.prepare(sql);
+    const self = this;
+    return {
+      get: (...args) => stmt.get(...args),
+      all: (...args) => stmt.all(...args),
+      run: (...args) => {
+        if (!self.fired && self.inTxn) {
+          self.fired = true;
+          self.db.exec('ROLLBACK'); // SQLite auto-rolled the txn back on the error
+          self.inTxn = false;
+          throw new Error('SQLITE_BUSY: database is locked');
+        }
+        return stmt.run(...args);
+      },
+    };
+  }
+  close() { return this.db.close(); }
+}
+
+test('a per-file write that auto-rolls-back the transaction is skipped, not fatal', () => {
+  const home = mkdtempSync(join(tmpdir(), 'obelisk-rollback-guard-'));
+  const projectDir = join(home, '.claude', 'projects', '-tmp-proj');
+  mkdirSync(projectDir, { recursive: true });
+  const msg = (uuid) => JSON.stringify({
+    uuid, type: 'user', timestamp: '2026-06-10T10:00:00Z', cwd: '/tmp/proj',
+    message: { role: 'user', content: `hello ${uuid}` },
+  }) + '\n';
+  writeFileSync(join(projectDir, 'alpha.jsonl'), msg('a1'));
+  writeFileSync(join(projectDir, 'beta.jsonl'), msg('b1'));
+
+  const dbPath = join(home, '.obelisk', 'obelisk.sqlite');
+  mkdirSync(join(home, '.obelisk'), { recursive: true });
+
+  // The unguarded ROLLBACK used to make this throw the masking rollback error.
+  let result;
+  assert.doesNotThrow(() => {
+    result = buildIndex({
+      force: true,
+      claudeDir: join(home, '.claude'),
+      codexDir: join(home, '.codex'),
+      projectsDir: join(home, '.claude', 'projects'),
+      dbPath,
+      DatabaseImpl: AutoRollbackOnceDatabase,
+    });
+  }, 'build must not abort on a transaction-aborting per-file error');
+
+  // Exactly one file's write was poisoned; the other indexed cleanly.
+  const check = new DatabaseSync(dbPath);
+  const sessions = check.prepare('SELECT COUNT(*) AS c FROM sessions').get().c;
+  check.close();
+  assert.equal(sessions, 1, 'the surviving file is indexed; the poisoned one is skipped');
+  assert.ok(result.files >= 2, 'both files were discovered');
+});