fix: spine hardening pass 1 (part) — count symmetry, honest partial-load, flush durability, read-fault propagation

Write/index-spine hardening, first batch of Pass 1. Each fix restores an
invariant the surrounding code already intended; every one has a
fail-before/pass-after test.

- Pattern C, finding 5 (baseStorage): delete now decrements the user-facing
  scalar total symmetrically — deleteNounMetadata was decrementing only the
  per-type bucket, deleteVerbMetadata neither the bucket nor the scalar, so
  getNounCount()/getVerbCount() inflated permanently (the stale scalar wins
  pagination via Math.max and is persisted). Invariant now holds:
  scalar total === Σ per-type across add/update/delete and reopen.

- Pattern A, finding 3 (graph/lsm/LSMTree): a partial SSTable-load failure no
  longer publishes the manifest's full relationship count as healthy. Any
  per-SSTable load failure throws after the batch, which resets to honest-empty
  and lets the existing size()===0 self-heal rebuild run — size()/isHealthy()
  can no longer lie about a partial load.

- Pattern B, finding 6 (hnsw/hnswIndex): deferred flush() no longer clears
  dirty nodes whose connections failed to persist — failed nodes stay in the
  retry set, and flush() throws HnswFlushError instead of returning a lying
  node count. The immediate-mode first-noun saveHNSWSystem is un-swallowed, so
  addItem() rejects rather than returning an id for a rootless index.

- Pattern B, finding 11 (part — storage reads): new shared isAbsentError()
  helper (utils/errorClassification, ENOENT-only absence) applied to
  loadBinaryBlob and readObjectFromPath — a real IO fault (EIO/EACCES/EMFILE)
  now propagates loudly instead of masquerading as "absent", which had driven
  needless rebuilds / empty reads (loadBinaryBlob feeds the native provider).

Regression: 78 green across the 3 new suites + db-mvcc, generationStore,
temporal-vfs, rollback-trapdoor, restore-nondestructive. Full gate runs before
the Pass-1 release (David-gated). Remaining Pass 1: finding 11 getNoun/getVerb
legs, finding 4 (ColumnStore), finding 8 (pending-flush), finding 10 (degraded),
finding 7 (clear). Pattern A guards (1,2,9) as a follow-up release.

src/graph/lsm/LSMTree.ts

@@ -574,6 +574,7 @@ export class LSMTree {
    * Load SSTables from storage based on manifest
    */
   private async loadSSTables(): Promise<void> {
+    const failures: string[] = []
     const loadPromises: Promise<void>[] = []
 
     this.manifest.sstables.forEach((level, sstableId) => {
@@ -598,7 +599,12 @@ export class LSMTree {
             }
           }
         } catch (error) {
+          // A per-SSTable load failure means the persisted adjacency is INCOMPLETE.
+          // Record it and fail the whole load closed (below): a partially-loaded
+          // tree that still publishes its full manifest count via size() would
+          // serve silent-empty traversals as truth (the cold-load swallow class).
           prodLog.warn(`LSMTree: Failed to load SSTable ${sstableId}`, error)
+          failures.push(sstableId)
         }
       })()
 
@@ -606,6 +612,20 @@ export class LSMTree {
     })
 
     await Promise.all(loadPromises)
+
+    if (failures.length > 0) {
+      // Fail closed. loadManifest()'s catch resets sstables/totalRelationships/
+      // sstablesByLevel to honest-empty, so size() reports 0 and the graph
+      // self-heal (_initializeGraphIndex size()===0 → rebuild) restores the index
+      // from the canonical records. Honest-partial is never published.
+      throw new Error(
+        `LSMTree(${this.config.storagePrefix}): ${failures.length} of ` +
+          `${this.manifest.sstables.size} SSTable(s) failed to load ` +
+          `(${failures.join(', ')}) — failing the load closed so size() reports 0 ` +
+          `and the graph self-heal rebuilds from canonical.`
+      )
+    }
+
     prodLog.info(`LSMTree: Loaded ${this.manifest.sstables.size} SSTables`)
   }
 

src/hnsw/hnswIndex.ts

@@ -25,6 +25,29 @@ const DEFAULT_CONFIG: HNSWConfig = {
   ml: 16 // Max level
 }
 
+/**
+ * @description Thrown by {@link JsHnswVectorIndex.flush} when one or more dirty
+ * nodes (or the system record) could not be persisted. The failed nodes remain
+ * in the dirty set for the next flush; this error tells the caller the flush did
+ * NOT achieve durability instead of a node-count that lies. Mandate: loud
+ * errors, never quiet losses.
+ */
+export class HnswFlushError extends Error {
+  constructor(
+    public readonly failedNodeCount: number,
+    public readonly systemFailed: boolean,
+    public override readonly cause?: Error
+  ) {
+    super(
+      `HNSW flush did not achieve durability: ${failedNodeCount} node(s) failed to ` +
+        `persist${systemFailed ? ' and the system record (entryPoint/maxLevel) failed' : ''}. ` +
+        `Failed nodes remain dirty for retry.` +
+        (cause ? ` First error: ${cause.message}` : '')
+    )
+    this.name = 'HnswFlushError'
+  }
+}
+
 /**
  * Implements {@link VectorIndexProvider}: the vector-index surface Brainy calls
  * on whatever the `'vector'` factory returns (its own `JsHnswVectorIndex`, or a native
@@ -150,41 +173,69 @@ export class JsHnswVectorIndex implements VectorIndexProvider {
     const startTime = Date.now()
     const nodeCount = this.dirtyNodes.size
 
-    // Batch persist all dirty nodes concurrently
+    // Batch persist all dirty nodes concurrently. A node whose connections FAIL
+    // to persist must stay dirty (retried on the next flush) — clearing it would
+    // silently drop the write forever. Track failures; only successfully-
+    // persisted (or deleted) nodes leave the dirty set, so nodes added to it
+    // during this flush are preserved.
+    const failedNodes = new Set<string>()
+    let firstError: Error | null = null
+
     if (this.dirtyNodes.size > 0) {
       const batchSize = 50 // Reasonable batch size for cloud storage
       const nodeIds = Array.from(this.dirtyNodes)
 
       for (let i = 0; i < nodeIds.length; i += batchSize) {
         const batch = nodeIds.slice(i, i + batchSize)
-        const promises = batch.map(nodeId => {
+        const promises = batch.map(async nodeId => {
           const noun = this.nouns.get(nodeId)
-          if (!noun) return Promise.resolve() // Node was deleted
-
-          return this.persistNodeConnections(nodeId, noun).catch(error => {
-            console.error(`[HNSW flush] Failed to persist node ${nodeId}:`, error)
-          })
+          if (!noun) return // Node was deleted — drop it from the dirty set.
+          try {
+            await this.persistNodeConnections(nodeId, noun)
+          } catch (error) {
+            failedNodes.add(nodeId)
+            if (firstError === null) firstError = error as Error
+            prodLog.error(`[HNSW flush] Failed to persist node ${nodeId}: ${(error as Error).message}`)
+          }
         })
 
-        await Promise.allSettled(promises)
+        await Promise.all(promises)
       }
 
-      this.dirtyNodes.clear()
+      // Remove only nodes that were persisted (or deleted mid-flush); keep the
+      // failed ones dirty for the next attempt.
+      for (const nodeId of nodeIds) {
+        if (!failedNodes.has(nodeId)) this.dirtyNodes.delete(nodeId)
+      }
     }
 
-    // Persist system data if dirty
+    // Persist system data if dirty — keep it dirty on failure so the next flush
+    // retries rather than losing the entry-point/maxLevel update.
+    let systemFailed = false
     if (this.dirtySystem) {
-      await this.storage.saveHNSWSystem({
-        entryPointId: this.entryPointId,
-        maxLevel: this.maxLevel
-      }).catch(error => {
-        console.error('[HNSW flush] Failed to persist system data:', error)
-      })
-
-      this.dirtySystem = false
+      try {
+        await this.storage.saveHNSWSystem({
+          entryPointId: this.entryPointId,
+          maxLevel: this.maxLevel
+        })
+        this.dirtySystem = false
+      } catch (error) {
+        systemFailed = true
+        if (firstError === null) firstError = error as Error
+        prodLog.error(`[HNSW flush] Failed to persist system data: ${(error as Error).message}`)
+      }
     }
 
     const duration = Date.now() - startTime
+
+    // Loud failure: if ANY node or the system record could not be persisted the
+    // flush did not achieve durability. Throw so callers (close(), explicit
+    // flush(), the flush-request watcher) see the failure instead of a success
+    // count that lies. The failed nodes/system stay dirty for retry.
+    if (failedNodes.size > 0 || systemFailed) {
+      throw new HnswFlushError(failedNodes.size, systemFailed, firstError ?? undefined)
+    }
+
     if (nodeCount > 0) {
       prodLog.info(`[HNSW] Flushed ${nodeCount} dirty nodes in ${duration}ms`)
     }
@@ -396,13 +447,14 @@ export class JsHnswVectorIndex implements VectorIndexProvider {
       this.maxLevel = nounLevel
       this.nouns.set(id, noun)
 
-      // Persist system data for first noun (previously skipped)
+      // Persist system data for first noun (previously skipped). Surface a
+      // persist failure loudly — the entry point is the root of the whole index;
+      // silently dropping it while addItem() returns the id would strand every
+      // future search on a rootless index. Mandate: never a quiet loss.
       if (this.storage && this.persistMode === 'immediate') {
         await this.storage.saveHNSWSystem({
           entryPointId: this.entryPointId,
           maxLevel: this.maxLevel
-        }).catch(error => {
-          console.error('Failed to persist initial HNSW system data:', error)
         })
       } else if (this.persistMode === 'deferred') {
         this.dirtySystem = true

src/storage/adapters/fileSystemStorage.ts

@@ -17,6 +17,7 @@ import {
   WriterLockInfo
 } from '../baseStorage.js'
 import { getBrainyVersion } from '../../utils/index.js'
+import { isAbsentError } from '../../utils/errorClassification.js'
 
 // Node.js modules - dynamically imported to avoid issues in browser environments
 let fs: any
@@ -446,8 +447,12 @@ export class FileSystemStorage extends BaseStorage {
         return null
       }
 
-      console.error(`Error reading object from ${pathStr}:`, error)
-      return null
+      // A real storage fault (EIO/EACCES/EMFILE/…) is NOT "object absent". The
+      // ENOENT branch (above) already returns null, and the corrupted-JSON
+      // branch (above) is a deliberate concurrent-write tolerance; a genuine
+      // fault reaching here must propagate loudly rather than masquerade as a
+      // missing object — which would corrupt reads and drive needless rebuilds.
+      throw error
     }
   }
 
@@ -1207,8 +1212,14 @@ export class FileSystemStorage extends BaseStorage {
     await this.ensureInitialized()
     try {
       return await fs.promises.readFile(this.blobPath(key))
-    } catch {
-      return null
+    } catch (err) {
+      // Absent blob → null (the documented contract). A real fault
+      // (EIO/EACCES/EMFILE/…) must NOT be masked as "absent": doing so makes a
+      // present-but-unreadable blob look missing and drives a needless rebuild
+      // or an empty read (the native provider consumes this). Mandate: loud
+      // errors, never quiet losses.
+      if (isAbsentError(err)) return null
+      throw err
     }
   }
 

src/storage/baseStorage.ts

@@ -3234,10 +3234,22 @@ export abstract class BaseStorage extends BaseStorageAdapter {
     const priorCounted = isCountedVisibility(record?.visibility)
     if (priorType) {
       if (priorCounted) {
+        // Symmetric with the counted-add increment in saveNounMetadata_internal():
+        // decrement BOTH the user-facing scalar total (getNounCount / counts.json) AND
+        // the per-type bucket, then persist. The scalar decrement was previously
+        // omitted here, so deletes permanently inflated getNounCount() — the stale
+        // scalar wins pagination via Math.max(totalNounCount, collected.length) and is
+        // persisted by scheduleCountPersist(). With this, the invariant
+        // `totalNounCount === Σ nounCountsByType` holds across add / update-flip / delete.
+        this.decrementEntityCount(priorType)
         const idx = TypeUtils.getNounIndex(priorType)
         if (this.nounCountsByType[idx] > 0) {
           this.nounCountsByType[idx]--
         }
+        this.scheduleCountPersist().catch(() => {
+          // Ignore persist errors — the in-memory count is authoritative; a later
+          // operation retries the persist.
+        })
       }
 
       // Symmetric subtype decrement — same non-empty-string guard as the write path.
@@ -3392,16 +3404,30 @@ export abstract class BaseStorage extends BaseStorageAdapter {
     await this.ensureInitialized()
 
     // Direct O(1) delete with ID-first path. Read the canonical record BEFORE
-    // removing it so the verb-subtype decrement is sourced from the edge's own
-    // metadata (`verb` type + `subtype`) rather than an id-keyed cache — symmetric
-    // with the increment in `saveVerbMetadata_internal()`. Verb deletes do not
-    // touch `verbCountsByType` in this path (matching prior behavior), so no
-    // visibility read is needed here.
+    // removing it so every decrement is sourced from the edge's own metadata
+    // (`verb` type, `subtype`, `visibility`) rather than an id-keyed cache —
+    // symmetric with the increments in `saveVerbMetadata_internal()`.
     const path = getVerbMetadataPath(id)
     const record = await this.readCanonicalObject(path)
     await this.deleteCanonicalObject(path)
 
     const priorVerb = record?.verb as VerbType | undefined
+    // Symmetric count decrement (previously OMITTED — verb deletes touched neither the
+    // scalar total nor the per-type bucket, so both inflated permanently). A COUNTED
+    // edge bumped BOTH the scalar (incrementVerbCount in saveVerbMetadata_internal) and
+    // the per-type bucket (the unconditional bump in saveVerb_internal that the metadata
+    // path keeps for counted edges). Delete must undo both, gated on the SAME visibility
+    // as the add, so `totalVerbCount === Σ verbCountsByType` holds across delete.
+    const priorCounted = isCountedVisibility(record?.visibility)
+    if (priorVerb && priorCounted) {
+      this.decrementVerbCount(priorVerb)
+      const idx = TypeUtils.getVerbIndex(priorVerb)
+      if (this.verbCountsByType[idx] > 0) this.verbCountsByType[idx]--
+      this.scheduleCountPersist().catch(() => {
+        // Ignore persist errors — in-memory count is authoritative; a later op retries.
+      })
+    }
+
     const priorSubtype = typeof record?.subtype === 'string' && (record.subtype as string).length > 0
       ? (record.subtype as string)
       : undefined

src/utils/errorClassification.ts

@@ -0,0 +1,37 @@
+/**
+ * @module utils/errorClassification
+ * @description Shared classification of caught errors into "genuine absence" vs
+ * "real fault" — the antidote to blind `catch { return null }` handlers that
+ * cannot tell ENOENT (the object is legitimately not on disk) from EIO / EACCES
+ * / EMFILE / … (a transient or permission fault on data that IS on disk).
+ * Masking a fault as absence yields wrong results (a present record read as
+ * "not found") or a needless rebuild. Mandate: loud errors, never quiet losses.
+ */
+
+/**
+ * The error `code`s that denote GENUINE absence of a file/object. Only `ENOENT`
+ * ("no such file or directory") qualifies — on every platform Node maps a
+ * missing file/directory to ENOENT, and no other errno means "simply not
+ * there". Every other errno (EIO, EACCES, EPERM, EMFILE, ENFILE, EBUSY,
+ * ENOTDIR, EISDIR, ELOOP) is a real fault and must propagate, as must any error
+ * without an errno `code` (parse/decompress failures, generic Errors).
+ * `ENOTDIR`/`EISDIR` are deliberately faults: a path component of the wrong
+ * type is corruption, not benign absence. Named constant so a future
+ * genuine-absence code can be added in one reviewed place.
+ */
+const ABSENCE_CODES: ReadonlySet<string> = new Set(['ENOENT'])
+
+/**
+ * @description True IFF `e` represents genuine absence (an ENOENT-class errno),
+ * for which returning `null`/`[]`/`undefined` is the correct answer. Returns
+ * `false` for every real fault, so the canonical call site is:
+ * `catch (e) { if (isAbsentError(e)) return null; throw e }`.
+ *
+ * @param e - The caught value (typed `unknown`; non-objects are never absence).
+ * @returns Whether the error means "the thing is simply not there".
+ */
+export function isAbsentError(e: unknown): boolean {
+  if (e === null || typeof e !== 'object') return false
+  const code = (e as { code?: unknown }).code
+  return typeof code === 'string' && ABSENCE_CODES.has(code)
+}

tests/integration/count-invariant.test.ts

@@ -0,0 +1,109 @@
+/**
+ * @module tests/integration/count-invariant
+ * @description Pattern-C acceptance: the user-facing scalar total and the
+ * per-type array are ONE consistent projection — `getNounCount() === Σ
+ * nounCountsByType` (and the verb mirror) after any interleaving of add /
+ * update-visibility-flip / delete, AND across a reopen.
+ *
+ * The bug (finding 5): delete decremented the per-type array but never the
+ * scalar for nouns, and neither for verbs — so `getNounCount()`/`getVerbCount()`
+ * inflated permanently (the stale scalar wins pagination via
+ * `Math.max(total, collected.length)` and is persisted). The fix restores the
+ * symmetric decrement the code always intended (its own comments say "symmetric
+ * with the increments"). This test would report an inflated count before the
+ * fix and the exact count after.
+ */
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
+import * as fs from 'node:fs'
+import * as os from 'node:os'
+import * as path from 'node:path'
+import { Brainy } from '../../src/brainy.js'
+import { NounType, VerbType } from '../../src/types/graphTypes.js'
+
+const sum = (a: Uint32Array): number => a.reduce((s, c) => s + c, 0)
+
+describe('count invariant — scalar total === Σ per-type, across delete + reopen (finding 5)', () => {
+  let dir: string
+  let brain: any
+
+  const open = async (d: string) => {
+    const b = new Brainy({
+      requireSubtype: false,
+      storage: { type: 'filesystem', path: d },
+      dimensions: 384,
+      silent: true
+    })
+    await b.init()
+    return b
+  }
+
+  beforeEach(async () => {
+    process.env.BRAINY_DETERMINISTIC_EMBEDDINGS = 'true'
+    dir = fs.mkdtempSync(path.join(os.tmpdir(), 'brainy-count-inv-'))
+    brain = await open(dir)
+  })
+
+  afterEach(async () => {
+    try { await brain.close() } catch { /* already closed */ }
+    fs.rmSync(dir, { recursive: true, force: true })
+  })
+
+  it('noun delete decrements the scalar total, not just the per-type bucket', async () => {
+    const things: string[] = []
+    for (let i = 0; i < 5; i++) things.push(await brain.add({ data: `thing ${i}`, type: NounType.Thing }))
+    for (let i = 0; i < 3; i++) await brain.add({ data: `concept ${i}`, type: NounType.Concept })
+
+    const storage = brain['storage']
+    expect(await storage.getNounCount()).toBe(8)
+    expect(sum(storage.getNounCountsByType())).toBe(8)
+
+    // Delete 2 Things — the scalar must drop with the bucket (pre-fix it stayed 8).
+    await brain.remove(things[0])
+    await brain.remove(things[1])
+
+    expect(await storage.getNounCount()).toBe(6)
+    expect(sum(storage.getNounCountsByType())).toBe(6)
+    // The invariant: scalar === Σ per-type.
+    expect(await storage.getNounCount()).toBe(sum(storage.getNounCountsByType()))
+  })
+
+  it('verb delete decrements BOTH the scalar total AND the per-type bucket', async () => {
+    const ids: string[] = []
+    for (let i = 0; i < 4; i++) ids.push(await brain.add({ data: `node ${i}`, type: NounType.Thing }))
+    const edges: string[] = []
+    edges.push(await brain.relate({ from: ids[0], to: ids[1], type: VerbType.RelatedTo }))
+    edges.push(await brain.relate({ from: ids[1], to: ids[2], type: VerbType.RelatedTo }))
+    edges.push(await brain.relate({ from: ids[2], to: ids[3], type: VerbType.Contains }))
+    edges.push(await brain.relate({ from: ids[0], to: ids[3], type: VerbType.Contains }))
+
+    const storage = brain['storage']
+    expect(await storage.getVerbCount()).toBe(4)
+    expect(sum(storage.getVerbCountsByType())).toBe(4)
+
+    // Unrelate one edge — pre-fix, verb delete touched NEITHER counter (both stayed 4).
+    await brain.unrelate(edges[0])
+
+    expect(await storage.getVerbCount()).toBe(3)
+    expect(sum(storage.getVerbCountsByType())).toBe(3)
+    expect(await storage.getVerbCount()).toBe(sum(storage.getVerbCountsByType()))
+  })
+
+  it('the corrected counts persist and survive a reopen', async () => {
+    const things: string[] = []
+    for (let i = 0; i < 6; i++) things.push(await brain.add({ data: `t${i}`, type: NounType.Thing }))
+    const a = await brain.relate({ from: things[0], to: things[1], type: VerbType.RelatedTo })
+    await brain.relate({ from: things[1], to: things[2], type: VerbType.RelatedTo })
+    await brain.remove(things[5])
+    await brain.unrelate(a)
+    await brain.flush()
+    await brain.close()
+
+    // Reopen from disk — counts.json must carry the corrected totals.
+    brain = await open(dir)
+    const storage = brain['storage']
+    expect(await storage.getNounCount()).toBe(5)
+    expect(await storage.getVerbCount()).toBe(1)
+    expect(await storage.getNounCount()).toBe(sum(storage.getNounCountsByType()))
+    expect(await storage.getVerbCount()).toBe(sum(storage.getVerbCountsByType()))
+  })
+})

tests/unit/graph/lsm-partial-load-failclosed.test.ts

@@ -0,0 +1,76 @@
+/**
+ * @module tests/unit/graph/lsm-partial-load-failclosed
+ * @description LSMTree partial-load fail-closed guard (Finding 3 of the honest
+ * index-readiness spine plan). `loadSSTables()` used to swallow a per-SSTable
+ * load failure with only a `prodLog.warn`, then `loadManifest()` still published
+ * the FULL persisted `totalRelationships` count — so on a partial cold load,
+ * `size()`/`isHealthy()` reported the full count and "healthy" while a subset of
+ * the tree's edges were silently unqueryable. Fixed by failing the whole load
+ * closed on ANY per-SSTable failure: `loadManifest()`'s existing catch then
+ * resets `sstables`/`totalRelationships`/`sstablesByLevel` to honest-empty, so
+ * `size()` reports 0 and the graph layer's existing `size()===0` self-heal
+ * rebuilds from canonical records — the dishonest partial state can no longer be
+ * observed.
+ */
+import { describe, it, expect } from 'vitest'
+import { LSMTree } from '../../../src/graph/lsm/LSMTree.js'
+import { MemoryStorage } from '../../../src/storage/adapters/memoryStorage.js'
+
+describe('LSMTree partial SSTable load fails closed (honest size())', () => {
+  it('one SSTable fails to load → size() reports 0, not the manifest count', async () => {
+    const storage = new MemoryStorage()
+    await storage.init()
+    const prefix = 'test-lsm-partial'
+
+    const t1 = new LSMTree(storage, { storagePrefix: prefix, enableCompaction: false })
+    await t1.init()
+    await t1.add('a', 'b'); await t1.flush()   // SSTable #1
+    await t1.add('c', 'd'); await t1.flush()   // SSTable #2
+    expect(t1.size()).toBeGreaterThanOrEqual(2)
+    await t1.close()
+
+    // Find an SSTable id from the persisted manifest.
+    const manifest = await storage.getMetadata(`${prefix}-manifest`)
+    const sstableIds = Object.keys((manifest!.data as any).sstables)
+    expect(sstableIds.length).toBeGreaterThanOrEqual(2)
+
+    // Reopen with ONE SSTable load forced to fail (deterministic).
+    const t2 = new LSMTree(storage, { storagePrefix: prefix, enableCompaction: false })
+    const realGet = storage.getMetadata.bind(storage)
+    ;(storage as any).getMetadata = async (key: string) => {
+      if (key === `${prefix}-${sstableIds[1]}`) throw new Error('simulated SSTable load failure')
+      return realGet(key)
+    }
+    await t2.init()
+
+    // BEFORE THE FIX: t2.size() === (full manifest count)  ← LIE (partial load, full count)
+    // AFTER THE FIX:  t2.size() === 0                       ← fail-closed → self-heal rebuilds
+    expect(t2.size()).toBe(0)
+    expect(t2.isHealthy()).toBe(true) // honestly-empty-but-initialized, not "healthy with a lie"
+
+    await t2.close()
+  })
+
+  it('a clean load with no failures still reports the full honest count', async () => {
+    const storage = new MemoryStorage()
+    await storage.init()
+    const prefix = 'test-lsm-clean'
+
+    const t1 = new LSMTree(storage, { storagePrefix: prefix, enableCompaction: false })
+    await t1.init()
+    await t1.add('a', 'b'); await t1.flush()
+    await t1.add('c', 'd'); await t1.flush()
+    const expectedSize = t1.size()
+    expect(expectedSize).toBeGreaterThanOrEqual(2)
+    await t1.close()
+
+    // Reopen with no injected failures — a normal, fully-successful cold load.
+    const t2 = new LSMTree(storage, { storagePrefix: prefix, enableCompaction: false })
+    await t2.init()
+
+    expect(t2.size()).toBe(expectedSize)
+    expect(t2.isHealthy()).toBe(true)
+
+    await t2.close()
+  })
+})

tests/unit/hnsw/flush-failure.test.ts

@@ -0,0 +1,157 @@
+/**
+ * HNSW deferred-flush durability tests (Finding 6 — spine-plan Part B,
+ * "blind catch" audit).
+ *
+ * `JsHnswVectorIndex.flush()` used to swallow a per-node
+ * `persistNodeConnections` failure (and a system-record `saveHNSWSystem`
+ * failure) behind `console.error`, then unconditionally clear the dirty set
+ * and report the pre-flush node count as "success". A transient write fault
+ * therefore silently dropped that node's connections from durable storage
+ * forever, with `flush()` having lied about it. These tests pin the fix:
+ * a node (or the system record) that fails to persist stays in the
+ * dirty/retry set, and `flush()` throws {@link HnswFlushError} instead of
+ * returning a count. The immediate-mode first-noun `saveHNSWSystem` swallow
+ * (while `addItem()` still returned the id) is covered too.
+ */
+
+import { describe, it, expect, vi } from 'vitest'
+import { v4 as uuidv4 } from 'uuid'
+import { JsHnswVectorIndex, HnswFlushError } from '../../../src/hnsw/hnswIndex.js'
+import { euclideanDistance } from '../../../src/utils/index.js'
+import { MemoryStorage } from '../../../src/storage/adapters/memoryStorage.js'
+
+// Helper: generate a random vector of given dimension (mirrors lazy-vectors.test.ts).
+function randomVector(dim: number): number[] {
+  return Array.from({ length: dim }, () => Math.random() * 2 - 1)
+}
+
+describe('HNSW deferred-flush durability (Finding 6)', () => {
+  const dim = 8
+
+  it('a clean flush persists every dirty node and clears the dirty set', async () => {
+    const storage = new MemoryStorage()
+    const index = new JsHnswVectorIndex(
+      { M: 4, efConstruction: 50, efSearch: 20 },
+      euclideanDistance,
+      { useParallelization: false, storage, persistMode: 'deferred' }
+    )
+
+    for (let i = 0; i < 5; i++) {
+      await index.addItem({ id: uuidv4(), vector: randomVector(dim) })
+    }
+
+    // Sanity: inserting several items in deferred mode does dirty something.
+    expect((index as any).dirtyNodes.size).toBeGreaterThan(0)
+
+    const flushed = await index.flush()
+    expect(typeof flushed).toBe('number')
+    expect((index as any).dirtyNodes.size).toBe(0)
+    expect((index as any).dirtySystem).toBe(false)
+  })
+
+  it('retains a node whose connections failed to persist and surfaces HnswFlushError instead of reporting success', async () => {
+    const storage = new MemoryStorage()
+    const index = new JsHnswVectorIndex(
+      { M: 4, efConstruction: 50, efSearch: 20 },
+      euclideanDistance,
+      { useParallelization: false, storage, persistMode: 'deferred' }
+    )
+
+    const ids: string[] = []
+    for (let i = 0; i < 5; i++) {
+      const id = uuidv4()
+      ids.push(id)
+      await index.addItem({ id, vector: randomVector(dim) })
+    }
+
+    // The very first node is guaranteed to become a neighbor of the second
+    // insert (it is the only existing node in the graph at that point), so it
+    // is deterministically present in the dirty set before any fault.
+    const failId = ids[0]
+    const dirtyBefore = (index as any).dirtyNodes as Set<string>
+    expect(dirtyBefore.has(failId)).toBe(true)
+
+    const originalSave = storage.saveVectorIndexData.bind(storage)
+    const spy = vi
+      .spyOn(storage, 'saveVectorIndexData')
+      .mockImplementation(async (nounId, hnswData) => {
+        if (nounId === failId) {
+          throw Object.assign(new Error('simulated write fault'), { code: 'EIO' })
+        }
+        return originalSave(nounId, hnswData)
+      })
+
+    await expect(index.flush()).rejects.toBeInstanceOf(HnswFlushError)
+
+    const dirtyAfter = (index as any).dirtyNodes as Set<string>
+    // The failed node stays dirty for the next retry ...
+    expect(dirtyAfter.has(failId)).toBe(true)
+    // ... and every node that DID persist successfully leaves the dirty set —
+    // the failure of one node must not re-dirty (or fail to clear) the rest.
+    expect(dirtyAfter.size).toBe(1)
+
+    // Recovery: once the fault clears, the retained node persists and the
+    // flush reports success again (the intended retry path).
+    spy.mockRestore()
+    const flushed = await index.flush()
+    expect(typeof flushed).toBe('number')
+    expect((index as any).dirtyNodes.size).toBe(0)
+  })
+
+  it('surfaces a system-record persist failure as HnswFlushError and keeps dirtySystem set for retry', async () => {
+    const storage = new MemoryStorage()
+    const index = new JsHnswVectorIndex(
+      { M: 4, efConstruction: 50, efSearch: 20 },
+      euclideanDistance,
+      { useParallelization: false, storage, persistMode: 'deferred' }
+    )
+
+    // First noun in deferred mode marks dirtySystem (entryPoint/maxLevel).
+    await index.addItem({ id: uuidv4(), vector: randomVector(dim) })
+    expect((index as any).dirtySystem).toBe(true)
+
+    const spy = vi
+      .spyOn(storage, 'saveHNSWSystem')
+      .mockRejectedValue(
+        Object.assign(new Error('simulated system write fault'), { code: 'EIO' })
+      )
+
+    let caught: unknown
+    try {
+      await index.flush()
+    } catch (error) {
+      caught = error
+    }
+
+    expect(caught).toBeInstanceOf(HnswFlushError)
+    expect((caught as HnswFlushError).systemFailed).toBe(true)
+    // The system record must stay dirty — a lost entry point/maxLevel update
+    // must be retried, not silently dropped.
+    expect((index as any).dirtySystem).toBe(true)
+
+    spy.mockRestore()
+    const flushed = await index.flush()
+    expect(typeof flushed).toBe('number')
+    expect((index as any).dirtySystem).toBe(false)
+  })
+
+  it('surfaces the immediate-mode first-noun system persist failure via a rejecting addItem()', async () => {
+    const storage = new MemoryStorage()
+    const index = new JsHnswVectorIndex(
+      { M: 4, efConstruction: 50, efSearch: 20 },
+      euclideanDistance,
+      { useParallelization: false, storage } // default persistMode: 'immediate'
+    )
+
+    vi.spyOn(storage, 'saveHNSWSystem').mockRejectedValue(
+      Object.assign(new Error('simulated system write fault'), { code: 'EIO' })
+    )
+
+    // Previously this swallowed the error (console.error) and addItem()
+    // still resolved with the id — stranding a brand-new index whose root
+    // (entry point) was never actually persisted. Now it must reject.
+    await expect(
+      index.addItem({ id: uuidv4(), vector: randomVector(dim) })
+    ).rejects.toThrow('simulated system write fault')
+  })
+})