diff --git a/tools/observer-retrofill-chain-ref.mjs b/tools/observer-retrofill-chain-ref.mjs
new file mode 100644
index 00000000..3d6df8d2
--- /dev/null
+++ b/tools/observer-retrofill-chain-ref.mjs
@@ -0,0 +1,58 @@
+#!/usr/bin/env node
+/**
+ * One-shot retrofill: add primary_rationale.chain_ref to existing v2 episodes
+ * in docs/observer/episodes-*.jsonl. Idempotent (skips lines that already have
+ * chain_ref), atomic per file (tmp + rename). Pure fs, no LLM.
+ *
+ * Usage: node tools/observer-retrofill-chain-ref.mjs [--dry-run]
+ */
+import { readFileSync, writeFileSync, renameSync, readdirSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import { dirname, join } from 'node:path';
+import { loadChainMap, chainsFor } from './observer-chain-detector.mjs';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const OBS_DIR = join(__dirname, '..', 'docs', 'observer');
+
+/** Add chain_ref to a single parsed episode object (pure). Idempotent. */
+export function retrofillLine(ep, map) {
+  if (!ep || ep.schema_version !== 2 || !ep.primary_rationale) return ep;
+  if ('chain_ref' in ep.primary_rationale) return ep; // idempotent
+  ep.primary_rationale.chain_ref = chainsFor(ep.primary_rationale.node_chosen, map);
+  return ep;
+}
+
+/** Process one JSONL file atomically (tmp + rename). Returns {changed, total}. */
+export function retrofillFile(path, map, { dryRun = false } = {}) {
+  const lines = readFileSync(path, 'utf8').split(/\r?\n/);
+  let changed = 0;
+  let total = 0;
+  const out = lines.map((line) => {
+    if (!line.trim()) return line;
+    total++;
+    const ep = JSON.parse(line);
+    const before = ep.primary_rationale && 'chain_ref' in ep.primary_rationale;
+    const next = retrofillLine(ep, map);
+    const after = next.primary_rationale && 'chain_ref' in next.primary_rationale;
+    if (!before && after) changed++;
+    return JSON.stringify(next);
+  });
+  if (!dryRun && changed > 0) {
+    const tmp = `${path}.tmp`;
+    writeFileSync(tmp, out.join('\n'), 'utf8');
+    renameSync(tmp, path);
+  }
+  return { changed, total };
+}
+
+function main() {
+  const dryRun = process.argv.includes('--dry-run');
+  const map = loadChainMap();
+  const files = readdirSync(OBS_DIR).filter((f) => /^episodes-\d{4}-\d{2}\.jsonl$/.test(f));
+  for (const f of files) {
+    const { changed, total } = retrofillFile(join(OBS_DIR, f), map, { dryRun });
+    console.log(`${dryRun ? '[dry-run] ' : ''}${f}: ${changed}/${total} lines get chain_ref`);
+  }
+}
+
+if (process.argv[1]?.endsWith('observer-retrofill-chain-ref.mjs')) main();
diff --git a/tools/observer-retrofill-chain-ref.test.mjs b/tools/observer-retrofill-chain-ref.test.mjs
new file mode 100644
index 00000000..9ba74107
--- /dev/null
+++ b/tools/observer-retrofill-chain-ref.test.mjs
@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'vitest';
+import { retrofillLine } from './observer-retrofill-chain-ref.mjs';
+import { loadChainMap } from './observer-chain-detector.mjs';
+
+const map = loadChainMap();
+
+describe('retrofillLine', () => {
+  it('adds chain_ref to a v2 episode with a known node', () => {
+    const ep = { schema_version: 2, primary_rationale: { node_chosen: 'billing-audit' } };
+    const out = retrofillLine(ep, map);
+    expect(out.primary_rationale.chain_ref).toEqual(['L13']);
+  });
+
+  it('sets chain_ref null for a direct v2 episode', () => {
+    const ep = { schema_version: 2, primary_rationale: { node_chosen: 'direct' } };
+    expect(retrofillLine(ep, map).primary_rationale.chain_ref).toBeNull();
+  });
+
+  it('is idempotent — does not overwrite existing chain_ref', () => {
+    const ep = { schema_version: 2, primary_rationale: { node_chosen: 'direct', chain_ref: ['L1'] } };
+    expect(retrofillLine(ep, map).primary_rationale.chain_ref).toEqual(['L1']);
+  });
+
+  it('skips v1 episodes (no schema_version 2)', () => {
+    const ep = { foo: 'bar' };
+    expect(retrofillLine(ep, map)).toEqual({ foo: 'bar' });
+  });
+});