diff --git a/src/config/kbIndexedStamp.sqlite.test.ts b/src/config/kbIndexedStamp.sqlite.test.ts
index 819ac810..5d4d1bb2 100644
--- a/src/config/kbIndexedStamp.sqlite.test.ts
+++ b/src/config/kbIndexedStamp.sqlite.test.ts
@@ -7,6 +7,7 @@
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
 import type { DocumentNode } from "workglow";
 import { Document, globalServiceRegistry, NodeKind } from "workglow";
+import { KB_DOCUMENT_TABLE, SEC_KB_TABLE_NAMES } from "../kb/secKbTables";
 import { getSecKnowledgeBase, resetSecKnowledgeBaseForTesting } from "../kb/secKnowledgeBase";
 import {
   FILING_DOCUMENT_REPOSITORY_TOKEN,
@@ -17,6 +18,7 @@ import { selectDocumentsToIndex } from "../task/kb/selectDocumentsToIndex";
 import { getDb } from "../util/db";
 import { syncKbIndexedStamp } from "./kbIndexedStamp";
 import { withSqliteDb } from "./testing/withSqliteDb";
+import { SEC_DRY_RUN } from "./tokens";
 
 const accession = (index: number) => `0000320193-26-${String(index).padStart(6, "0")}`;
 
@@ -40,10 +42,11 @@ const doc = (index: number, over: Partial<FilingDocument> = {}): FilingDocument
 });
 
 /**
- * The stamp is a cache of "is there a `kb_document` row for this", and the three
+ * The stamp is a cache of "is there a `kb_document` row for this", and the
  * properties below are what make it safe to read: it never claims more than the
- * anti-join, `db setup` never leaves it empty, and the steady state stops
- * touching the table.
+ * anti-join, `db setup` never leaves it empty, a stamp the knowledge base stops
+ * backing is cleared rather than left to hide the document, and the steady state
+ * stops touching the table.
  */
 describe("kb_indexed_at stamp (sqlite)", () => {
   withSqliteDb("kb_indexed_stamp", [FILING_DOCUMENT_REPOSITORY_TOKEN]);
@@ -77,6 +80,32 @@ describe("kb_indexed_at stamp (sqlite)", () => {
         .get(accession(index)) as { s: string | null } | undefined
     )?.s ?? null;
 
+  const selectionPlan = (): string[] =>
+    (
+      getDb()
+        .prepare(
+          `EXPLAIN QUERY PLAN
+           SELECT d.* FROM \`filing_document\` d
+            WHERE d.\`section_count\` > 0 AND d.\`kb_indexed_at\` IS NULL
+            ORDER BY d.\`filing_date\` DESC, d.\`accession_number\` DESC`
+        )
+        .all() as Array<{ detail?: string }>
+    ).map((row) => row.detail ?? "");
+
+  const selectionPlanDetail = (): string => selectionPlan().join(" | ");
+
+  /** A plan step that walks `filing_document` itself rather than an index. */
+  const scansTheTable = (): boolean =>
+    selectionPlan().some((step) => /^SCAN\b/.test(step) && !/\bUSING\b.*\bINDEX\b/.test(step));
+
+  const indexExists = (name: string): boolean =>
+    getDb().prepare("SELECT name FROM sqlite_master WHERE type = 'index' AND name = ?").all(name)
+      .length > 0;
+
+  const dropKbTables = (): void => {
+    for (const table of SEC_KB_TABLE_NAMES) getDb().exec(`DROP TABLE \`${table}\``);
+  };
+
   it("backfills a database whose documents were indexed before the column existed", async () => {
     // The state every existing deployment is in: rows in the knowledge base,
     // no stamp. Left un-backfilled the partial index would cover the whole
@@ -108,21 +137,33 @@ describe("kb_indexed_at stamp (sqlite)", () => {
     for (const i of [1, 2, 3]) await putInKb(i);
     syncKbIndexedStamp(getDb());
 
-    const plan = getDb()
-      .prepare(
-        `EXPLAIN QUERY PLAN
-         SELECT d.* FROM \`filing_document\` d
-          WHERE d.\`section_count\` > 0 AND d.\`kb_indexed_at\` IS NULL
-          ORDER BY d.\`filing_date\` DESC, d.\`accession_number\` DESC`
-      )
-      .all() as Array<{ detail?: string }>;
-    const detail = plan.map((row) => row.detail ?? "").join(" | ");
-
-    expect(detail).toContain("filing_document_kb_unindexed");
-    // The point of the partial index: no table scan, and no temp B-tree to
-    // sort what a scan would have produced.
-    expect(detail).not.toContain("SCAN d\n");
-    expect(detail).not.toContain("TEMP B-TREE");
+    // One access path and it is the partial index — which on a fully indexed
+    // corpus holds nothing, so the selection reads no rows at all. Spelled as
+    // the whole plan rather than a substring: SQLite reports a walk of the
+    // partial index as `SCAN d USING INDEX …` too, so a check for the absence
+    // of "SCAN" says nothing about which index is being walked.
+    const plan = selectionPlan();
+    expect(plan).toHaveLength(1);
+    expect(plan[0]).toMatch(/\bUSING\b.*\bINDEX filing_document_kb_unindexed\b/);
+    // No step reaches the table itself, and none sorts into a temp B-tree.
+    expect(scansTheTable()).toBe(false);
+    expect(selectionPlanDetail()).not.toContain("TEMP B-TREE");
+  });
+
+  it("falls back to an index over every row once the partial index is dropped", async () => {
+    // What gives the assertions above their teeth, and names what they buy.
+    // Without the partial index the planner still avoids a table scan — the
+    // schema carries a `filing_date, accession_number` index that serves the
+    // ORDER BY — so the cost of losing it is not a scan but an index that
+    // covers the whole corpus instead of only its unindexed tail.
+    await seed(3);
+    for (const i of [1, 2, 3]) await putInKb(i);
+    syncKbIndexedStamp(getDb());
+
+    getDb().exec("DROP INDEX `filing_document_kb_unindexed`");
+
+    expect(selectionPlanDetail()).not.toContain("filing_document_kb_unindexed");
+    expect(selectionPlanDetail()).toContain("filing_document_filing_date_accession_number");
   });
 
   it("never widens the selection: an unstamped document already in the kb is still skipped", async () => {
@@ -139,4 +180,95 @@ describe("kb_indexed_at stamp (sqlite)", () => {
 
     expect(picked.map((d) => d.accession_number)).toEqual([accession(2)]);
   });
+
+  it("clears a stamp the knowledge base no longer backs", async () => {
+    // The direction that costs a document rather than a traversal: the
+    // selection reads the stamp as a narrowing of its anti-join, so a stamp
+    // outliving its `kb_document` row hides a filing that is no longer indexed.
+    await seed(2);
+    await putInKb(1);
+    await putInKb(2);
+    syncKbIndexedStamp(getDb());
+    expect(stampOf(2)).not.toBeNull();
+
+    getDb()
+      .prepare(`DELETE FROM \`${KB_DOCUMENT_TABLE}\` WHERE \`doc_id\` = ?`)
+      .run(kbDocIdFor(accession(2), "primary.htm"));
+    syncKbIndexedStamp(getDb());
+
+    expect(stampOf(1)).not.toBeNull();
+    expect(stampOf(2)).toBeNull();
+
+    const picked = await selectDocumentsToIndex({ limit: 10 });
+    expect(picked.map((d) => d.accession_number)).toEqual([accession(2)]);
+  });
+
+  it("clears every stamp when the knowledge base tables are gone", async () => {
+    await seed(2);
+    await putInKb(1);
+    syncKbIndexedStamp(getDb());
+    expect(stampOf(1)).not.toBeNull();
+
+    await resetSecKnowledgeBaseForTesting();
+    dropKbTables();
+    syncKbIndexedStamp(getDb());
+
+    expect(stampOf(1)).toBeNull();
+  });
+
+  it("re-offers every document after the index is dropped and rebuilt empty", async () => {
+    // The documented way to change the embedding model: drop the three tables
+    // and re-run `sec index`. Opening the index re-creates `kb_document` empty,
+    // so the anti-join has nothing to exclude — and the stamps the previous
+    // index left behind must not exclude anything either, or the rebuild
+    // indexes nothing and `sec ask` answers from an empty index.
+    await seed(3);
+    for (const i of [1, 2, 3]) await putInKb(i);
+    syncKbIndexedStamp(getDb());
+    expect(stampOf(1)).not.toBeNull();
+
+    await resetSecKnowledgeBaseForTesting();
+    dropKbTables();
+    await getSecKnowledgeBase();
+
+    const picked = await selectDocumentsToIndex({ limit: 10 });
+
+    expect(picked.map((d) => d.accession_number)).toEqual([
+      accession(3),
+      accession(2),
+      accession(1),
+    ]);
+  });
+
+  it("leaves the stamps alone when the index still holds documents", async () => {
+    // The other half of the rule above: re-opening a populated index must not
+    // throw away the stamps, which would put the whole corpus back in front of
+    // every run.
+    await seed(2);
+    await putInKb(1);
+    syncKbIndexedStamp(getDb());
+    const stamp = stampOf(1);
+
+    await resetSecKnowledgeBaseForTesting();
+    await getSecKnowledgeBase();
+
+    expect(stampOf(1)).toBe(stamp);
+  });
+
+  it("writes nothing under a dry run", async () => {
+    // Raw SQL goes around the ReadOnlyTabularStorage wrapper `--dry-run`
+    // installs, so the bail lives in the function rather than at its callers.
+    await seed(1);
+    await putInKb(1);
+
+    globalServiceRegistry.registerInstance(SEC_DRY_RUN, true);
+    try {
+      syncKbIndexedStamp(getDb());
+    } finally {
+      globalServiceRegistry.registerInstance(SEC_DRY_RUN, false);
+    }
+
+    expect(stampOf(1)).toBeNull();
+    expect(indexExists("filing_document_kb_unindexed")).toBe(false);
+  });
 });
diff --git a/src/config/kbIndexedStamp.ts b/src/config/kbIndexedStamp.ts
index 15a222a6..bc5a6d73 100644
--- a/src/config/kbIndexedStamp.ts
+++ b/src/config/kbIndexedStamp.ts
@@ -5,16 +5,18 @@
  */
 
 import { Sqlite } from "workglow";
+import { isDryRun } from "../cli/isDryRun";
 import { KB_DOCUMENT_TABLE } from "../kb/secKbTables";
 
 /** The partial index the steady-state selection walks. */
 const PARTIAL_INDEX = "filing_document_kb_unindexed";
 
 /**
- * Brings `filing_document.kb_indexed_at` up to date and indexes the nulls.
+ * Brings `filing_document.kb_indexed_at` into step with the knowledge base and
+ * indexes the nulls.
  *
- * Two statements that have to travel together, which is why they are one
- * function rather than a registry entry. The column is a cache of "is there a
+ * Statements that have to travel together, which is why they are one function
+ * rather than a registry entry. The column is a cache of "is there a
  * `kb_document` row for this", and a cache that is added empty is worse than no
  * cache: every row reads as unindexed, so the partial index below covers the
  * whole table and the planner walks it in `filing_date` order, issuing a random
@@ -22,8 +24,17 @@ const PARTIAL_INDEX = "filing_document_kb_unindexed";
  * it replaced. Backfilling in the same pass that creates the index is what
  * keeps that state from existing.
  *
- * Both statements are idempotent and cheap on a database already in step: the
- * `UPDATE` matches nothing once every indexed document is stamped, and
+ * The cache moves in both directions, and the two directions cost differently.
+ * A missing stamp costs a traversal: the selection anti-joins `kb_document`
+ * anyway, so the document is found and skipped. A stamp the knowledge base no
+ * longer backs costs the document itself — the selection reads the stamp as a
+ * narrowing of that anti-join, so a filing whose `kb_document` row has gone is
+ * never offered again, and a knowledge base rebuilt from empty stays empty.
+ * So a row the knowledge base has gained is stamped, and one it no longer
+ * holds is cleared, in the same pass.
+ *
+ * Every statement is idempotent and cheap on a database already in step: both
+ * `UPDATE`s match nothing once the stamps and the knowledge base agree, and
  * `CREATE INDEX IF NOT EXISTS` is a catalog read.
  *
  * Callers must have applied the schema's columns first — the column is added
@@ -31,10 +42,16 @@ const PARTIAL_INDEX = "filing_document_kb_unindexed";
  * rather than assuming an order it cannot see.
  */
 export function syncKbIndexedStamp(db: Sqlite.Database): void {
+  // Raw SQL reaches around the repositories' ReadOnlyTabularStorage wrapper, so
+  // the bail lives here rather than at each call site, where a future caller
+  // would have to remember it.
+  if (isDryRun()) return;
   if (!hasColumn(db, "filing_document", "kb_indexed_at")) return;
   if (!tableExists(db, KB_DOCUMENT_TABLE)) {
-    // No knowledge base yet, so nothing is indexed and every row's null is
-    // already the truth. The index still pays off on the first `ask`.
+    // No knowledge base, so nothing is indexed: a null is already the truth,
+    // and any stamp left by one that used to exist is not. The index still
+    // pays off on the first `ask`.
+    clearStampsWithNoKbRow(db);
     createPartialIndex(db);
     return;
   }
@@ -52,10 +69,60 @@ export function syncKbIndexedStamp(db: Sqlite.Database): void {
                                  || ':' || \`filing_document\`.\`doc_file\`
             )`
   ).run();
+  clearStampsWithNoKbRow(db);
 
   createPartialIndex(db);
 }
 
+/**
+ * Clears every stamp when the knowledge base holds no documents at all.
+ *
+ * For the path that opens the index before filling it, which is where a rebuild
+ * starts: dropping the knowledge-base tables and re-running `sec index`
+ * re-creates them empty, and the stamps the previous index left in
+ * `filing_document` outlive it. Read as a narrowing of the anti-join, those
+ * stamps exclude the whole corpus from the rebuild, and the run reports nothing
+ * to do.
+ *
+ * Cheap enough to run on every open, which the full reconciliation
+ * {@link syncKbIndexedStamp} is not: this is one `LIMIT 1` probe wherever the
+ * index holds anything, which is the steady state, against a probe per stamped
+ * row. The scan it falls through to happens only where nothing is indexed, so
+ * it is charged against a run that is about to embed a corpus.
+ */
+export function clearKbIndexedStampIfIndexEmpty(db: Sqlite.Database): void {
+  if (isDryRun()) return;
+  if (!hasColumn(db, "filing_document", "kb_indexed_at")) return;
+  if (tableExists(db, KB_DOCUMENT_TABLE) && kbHoldsAnyDocument(db)) return;
+  db.prepare(
+    "UPDATE `filing_document` SET `kb_indexed_at` = NULL WHERE `kb_indexed_at` IS NOT NULL"
+  ).run();
+}
+
+/**
+ * Drops the stamp from every document the knowledge base does not hold, which
+ * is all of them when the table itself is gone.
+ */
+function clearStampsWithNoKbRow(db: Sqlite.Database): void {
+  const orphaned = tableExists(db, KB_DOCUMENT_TABLE)
+    ? `AND NOT EXISTS (
+              SELECT 1 FROM \`${KB_DOCUMENT_TABLE}\` k
+               WHERE k.\`doc_id\` = \`filing_document\`.\`accession_number\`
+                                 || ':' || \`filing_document\`.\`doc_file\`
+            )`
+    : "";
+  db.prepare(
+    `UPDATE \`filing_document\`
+        SET \`kb_indexed_at\` = NULL
+      WHERE \`kb_indexed_at\` IS NOT NULL
+        ${orphaned}`
+  ).run();
+}
+
+function kbHoldsAnyDocument(db: Sqlite.Database): boolean {
+  return db.prepare(`SELECT 1 AS present FROM \`${KB_DOCUMENT_TABLE}\` LIMIT 1`).all().length > 0;
+}
+
 /**
  * Indexed on the order the selection reads — newest first, ties by accession —
  * so the same walk serves both the filter and the `ORDER BY`. Partial, because
diff --git a/src/kb/secKnowledgeBase.ts b/src/kb/secKnowledgeBase.ts
index 4fc4ef03..e219bfed 100644
--- a/src/kb/secKnowledgeBase.ts
+++ b/src/kb/secKnowledgeBase.ts
@@ -20,6 +20,7 @@ import {
 } from "workglow";
 import { isDryRun } from "../cli/isDryRun";
 import { SecCliConfigurationError } from "../config/EnvToDI";
+import { clearKbIndexedStampIfIndexEmpty } from "../config/kbIndexedStamp";
 import { secEmbeddingDimensions, secEmbeddingModel } from "../config/models";
 import { SEC_DB_TYPE } from "../config/tokens";
 import { getDb } from "../util/db";
@@ -192,6 +193,12 @@ export async function getSecKnowledgeBase(): Promise<KnowledgeBase> {
     await documents.setupDatabase();
     await chunks.setupDatabase();
     await index.setupDatabase();
+    // An index holding no documents contradicts any `kb_indexed_at` stamp a
+    // previous one left behind — dropping these tables is how the embedding
+    // model is changed, and the DDL above re-creates them empty. The selection
+    // reads that stamp as a narrowing of its anti-join, so left standing it
+    // would keep every previously indexed filing out of the rebuild.
+    clearKbIndexedStampIfIndexEmpty(db);
   }
 
   const model = secEmbeddingModel();
diff --git a/src/task/kb/selectDocumentsToIndex.ts b/src/task/kb/selectDocumentsToIndex.ts
index 199bff9f..71c479fc 100644
--- a/src/task/kb/selectDocumentsToIndex.ts
+++ b/src/task/kb/selectDocumentsToIndex.ts
@@ -126,6 +126,12 @@ export async function selectDocumentsToIndex(
     // Conditional on the column existing because `db setup` backfills it in the
     // same pass that creates the index, so its presence is what says the values
     // can be trusted. Where it is absent the query is exactly what it was.
+    //
+    // What keeps it a narrowing and not a filter of its own is that the stamp
+    // is cleared wherever the knowledge base stops backing it — by `db setup`
+    // per row, and by opening an index that holds nothing. A stamp outliving
+    // its `kb_document` row would exclude a document the anti-join beside it
+    // selects, which is the one direction this clause must never take.
     if (kbIndexedStampAvailable(getDb())) clauses.push("d.`kb_indexed_at` IS NULL");
   }
   // SQLite numbers `?` by position, so the limit binds last because it is