diff --git a/.changeset/fn-9158-windows-av-blocked-embedded-pg-payload.md b/.changeset/fn-9158-windows-av-blocked-embedded-pg-payload.md
new file mode 100644
index 0000000000..1350e2b9d7
--- /dev/null
+++ b/.changeset/fn-9158-windows-av-blocked-embedded-pg-payload.md
@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+summary: Recover automatically when Windows antivirus blocks a bundled PostgreSQL library.
+category: fix
+dev: Marker v3 verifies cached payload inventory, failed verification leaves no marker, and reports EmbeddedPostgresPayloadBlockedError.
diff --git a/docs/solutions/database-issues/windows-antivirus-blocks-embedded-postgres-dll.md b/docs/solutions/database-issues/windows-antivirus-blocks-embedded-postgres-dll.md
new file mode 100644
index 0000000000..79cda73184
--- /dev/null
+++ b/docs/solutions/database-issues/windows-antivirus-blocks-embedded-postgres-dll.md
@@ -0,0 +1,27 @@
+---
+category: database-issues
+module: embedded-postgres
+tags: [windows, antivirus, postgresql, embedded-postgres]
+problem_type: installation
+applies_when: Windows startup reports `could not load library` with `unknown error 4551` for an embedded PostgreSQL DLL.
+---
+
+# Windows antivirus blocks an embedded PostgreSQL DLL
+
+## Symptom
+
+Fusion fails during embedded PostgreSQL initialization with a message such as `could not load library .../lib/dict_snowball.dll: unknown error 4551`.
+
+## Cause
+
+Windows error 4551 is `ERROR_VIRUS_DELETED`: Windows Defender or another antivirus product quarantined a DLL from Fusion's host-local embedded PostgreSQL runtime payload. See [issue #3489](https://github.com/Runfusion/Fusion/issues/3489).
+
+Earlier versions trusted a marker that identified the source payload but did not verify the copied destination. A quarantined destination DLL could therefore remain cached across every restart.
+
+## Remedy
+
+1. In **Windows Security**, open **Virus & threat protection** → **Manage settings** → **Exclusions** and add `%USERPROFILE%\.fusion\embedded-postgres`.
+2. Restore the quarantined DLL from **Protection history**.
+3. Restart Fusion.
+
+Fusion verifies the runtime-bin payload on startup. Once the exclusion permits the copy, Fusion automatically re-materializes missing or truncated files and clears the recovery diagnosis.
diff --git a/docs/storage.md b/docs/storage.md
index aedc836724..7a01c79573 100644
--- a/docs/storage.md
+++ b/docs/storage.md
@@ -847,6 +847,10 @@ Project-scoped structured recall records for durable decisions, preferences, and
 
 - Knowledge-graph artifact: `<rootDir>/.fusion-knowledge/graph/` (`nodes.json`, `edges.json`, and `manifest.json`). This is deliberately outside ignored `.fusion` and may be committed at the operator's discretion.
 
+### Embedded PostgreSQL on Windows — antivirus and the runtime-bin payload
+
+If Windows startup reports `unknown error 4551` while loading an embedded PostgreSQL DLL such as `dict_snowball.dll`, antivirus quarantined part of `%USERPROFILE%\\.fusion\\embedded-postgres`. Add that directory as a Windows Security exclusion, restore the file from Protection history, and restart Fusion. Fusion verifies and automatically re-copies the runtime payload; see [Windows antivirus blocks an embedded PostgreSQL DLL](solutions/database-issues/windows-antivirus-blocks-embedded-postgres-dll.md).
+
 ### Bounded task-intake lookups
 
 Recommendation proposal claims use the indexed `findTaskByProposalClaimId` read (`uqTasksProjectProposalClaimId`), and same-agent intake reads only matching source lineage (`idxTasksProjectSourceAgentId` and `idxTasksSourceParentTaskId`). Do not replace either read with a `listTasks()` scan. Workflow terminal flags for intake duplicate checks are derived from workflow definitions, not board rows. Guarded-intake near-duplicate checks must remain bounded to their candidates (the fallback is `limit: 50`) and must not hydrate the full board.
diff --git a/packages/core/src/__tests__/postgres/embedded-lifecycle.test.ts b/packages/core/src/__tests__/postgres/embedded-lifecycle.test.ts
index f2d5f41be3..ef6916a68f 100644
--- a/packages/core/src/__tests__/postgres/embedded-lifecycle.test.ts
+++ b/packages/core/src/__tests__/postgres/embedded-lifecycle.test.ts
@@ -56,6 +56,14 @@ import {
   materializeEmbeddedPostgresRuntimeBinaries,
   installElectronAsarNativePathPatch,
   uninstallElectronAsarNativePathPatchForTests,
+  isWindowsBlockedNativeLibraryError,
+  describeWindowsBlockedNativeLibraryError,
+  EmbeddedPostgresPayloadBlockedError,
+  recordEmbeddedPayloadIntegrityFailure,
+  getEmbeddedPayloadIntegrityFailure,
+  clearEmbeddedPayloadIntegrityFailure,
+  decorateWindowsBlockedNativeLibraryError,
+  embeddedPostgresRuntimeBinRoot,
   type EmbeddedLifecycleOptions,
 } from "../../postgres/embedded-lifecycle.js";
 
@@ -75,6 +83,7 @@ afterEach(async () => {
   __setWindowsElevatedAdminForTests(null);
   __setWindowsEmbeddedPostgresNativeRootForTests(null);
   __setWindowsLauncherForTests(null);
+  clearEmbeddedPayloadIntegrityFailure();
   vi.useRealTimers();
   while (tracked.length > 0) {
     const { lifecycle, dataDir } = tracked.pop()!;
@@ -124,6 +133,27 @@ describe("embedded-lifecycle: isDataDirInitialized (PG_VERSION marker)", () => {
   });
 });
 
+describe("embedded-lifecycle: Windows blocked native library classifier", () => {
+  const issueOutput = `ERROR OUTPUT: 2026-08-19 09:20:48.546 CEST [23152] FATAL: could not load library "C:/Users/ppp/.fusion/embedded-postgres/runtime-bin/win32-x64/lib/dict_snowball.dll": unknown error 4551`;
+
+  it("classifies the Defender ERROR_VIRUS_DELETED failure from issue #3489", () => {
+    expect(isWindowsBlockedNativeLibraryError(issueOutput)).toBe(true);
+    expect(describeWindowsBlockedNativeLibraryError(issueOutput)).toContain("dict_snowball.dll");
+    expect(describeWindowsBlockedNativeLibraryError(issueOutput)).toContain(
+      "%USERPROFILE%\\.fusion\\embedded-postgres",
+    );
+  });
+
+  it("does not misclassify unrelated library or encoding errors", () => {
+    expect(isWindowsBlockedNativeLibraryError(
+      "could not load library: The specified module could not be found",
+    )).toBe(false);
+    expect(isWindowsBlockedNativeLibraryError(
+      "invalid byte sequence for encoding UTF8",
+    )).toBe(false);
+  });
+});
+
 describe("embedded-lifecycle: Windows elevation probe (no process)", () => {
   it("isWindowsElevatedAdmin is false on non-Windows platforms", () => {
     // FNXC:WindowsDesktopPackaging 2026-07-15-04:55:
@@ -303,6 +333,33 @@ describe("embedded-lifecycle: Electron asar unpacked path rewrite", () => {
   });
 });
 
+describe("embedded-lifecycle: payload integrity diagnosis lifecycle", () => {
+  const verification = { acceptable: false, mismatches: ["lib/dict_snowball.dll"], mismatchCount: 1 };
+
+  it("is root-scoped, latest-wins, and resettable", () => {
+    const first = new EmbeddedPostgresPayloadBlockedError("/native-a", "/runtime-a", verification);
+    const second = new EmbeddedPostgresPayloadBlockedError("/native-b", "/runtime-b", verification);
+    recordEmbeddedPayloadIntegrityFailure(first);
+    recordEmbeddedPayloadIntegrityFailure(second);
+    expect(getEmbeddedPayloadIntegrityFailure("/runtime-a")).toBeNull();
+    expect(getEmbeddedPayloadIntegrityFailure("/runtime-b")).toBe(second);
+    clearEmbeddedPayloadIntegrityFailure();
+    expect(getEmbeddedPayloadIntegrityFailure()).toBeNull();
+  });
+
+  it("decorates only matching-root start failures and preserves unrelated failures", () => {
+    const root = embeddedPostgresRuntimeBinRoot();
+    const failure = new EmbeddedPostgresPayloadBlockedError("/native", root, verification);
+    recordEmbeddedPayloadIntegrityFailure(failure);
+    expect(decorateWindowsBlockedNativeLibraryError(new Error("initdb failed"), root).message).toContain(
+      "Windows antivirus blocked a bundled PostgreSQL library",
+    );
+    expect(decorateWindowsBlockedNativeLibraryError(new Error("initdb failed"), "/other-root").message).toBe(
+      "initdb failed",
+    );
+  });
+});
+
 describe("embedded-lifecycle: materialize runtime binaries (update-safe marker)", () => {
   /*
    * FNXC:DesktopEmbeddedPostgres 2026-07-15-02:55:
@@ -327,6 +384,7 @@ describe("embedded-lifecycle: materialize runtime binaries (update-safe marker)"
     writeFileSync(join(root, "bin", initdbBin), "initdb-stub");
     writeFileSync(join(root, "bin", pgCtlBin), "pg_ctl-stub");
     writeFileSync(join(root, "lib", "postgresql", "plpgsql.so"), "ext-v1");
+    writeFileSync(join(root, "lib", "dict_snowball.dll"), "snowball-v1");
     writeFileSync(join(root, "share", "postgresql", "postgres.bki"), "share-v1");
   }
 
@@ -400,7 +458,7 @@ describe("embedded-lifecycle: materialize runtime binaries (update-safe marker)"
       seedNativeRoot(nativeRoot, "postgres-body");
       const marker = buildEmbeddedPostgresMaterializationMarker(nativeRoot);
       const fingerprint = fingerprintEmbeddedPostgresNativeRoot(nativeRoot);
-      expect(marker.startsWith("v2\n")).toBe(true);
+      expect(marker.startsWith("v3\n")).toBe(true);
       expect(marker).toContain(nativeRoot);
       expect(marker).toContain(fingerprint);
       // Path alone must not equal the full marker (legacy path-only markers rematerialize).
@@ -437,6 +495,62 @@ describe("embedded-lifecycle: materialize runtime binaries (update-safe marker)"
     }
   });
 
+  it("clears a recorded integrity failure after a verified repair", () => {
+    const nativeRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-repair-src-"));
+    const destRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-repair-dst-"));
+    try {
+      seedNativeRoot(nativeRoot, "postgres-stable");
+      recordEmbeddedPayloadIntegrityFailure(
+        new EmbeddedPostgresPayloadBlockedError(nativeRoot, destRoot, {
+          acceptable: false,
+          mismatches: ["lib/dict_snowball.dll"],
+          mismatchCount: 1,
+        }),
+      );
+      materializeEmbeddedPostgresRuntimeBinaries(nativeRoot, { destRoot });
+      expect(existsSync(join(destRoot, ".materialized-from"))).toBe(true);
+      expect(getEmbeddedPayloadIntegrityFailure(destRoot)).toBeNull();
+      expect(decorateWindowsBlockedNativeLibraryError(new Error("unrelated failure"), destRoot).message).toBe(
+        "unrelated failure",
+      );
+    } finally {
+      rmSync(nativeRoot, { recursive: true, force: true });
+      rmSync(destRoot, { recursive: true, force: true });
+    }
+  });
+
+  it("restores a Defender-quarantined dict_snowball.dll on the next launch", () => {
+    const nativeRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-av-src-"));
+    const destRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-av-dst-"));
+    try {
+      seedNativeRoot(nativeRoot, "postgres-stable");
+      materializeEmbeddedPostgresRuntimeBinaries(nativeRoot, { destRoot });
+      rmSync(join(destRoot, "lib", "dict_snowball.dll"));
+
+      materializeEmbeddedPostgresRuntimeBinaries(nativeRoot, { destRoot });
+      expect(readFileSync(join(destRoot, "lib", "dict_snowball.dll"), "utf8")).toBe("snowball-v1");
+    } finally {
+      rmSync(nativeRoot, { recursive: true, force: true });
+      rmSync(destRoot, { recursive: true, force: true });
+    }
+  });
+
+  it("restores a zero-byte truncated postgres executable", () => {
+    const nativeRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-av-bin-src-"));
+    const destRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-av-bin-dst-"));
+    try {
+      seedNativeRoot(nativeRoot, "postgres-stable");
+      materializeEmbeddedPostgresRuntimeBinaries(nativeRoot, { destRoot });
+      writeFileSync(join(destRoot, "bin", postgresBin), "");
+
+      materializeEmbeddedPostgresRuntimeBinaries(nativeRoot, { destRoot });
+      expect(readFileSync(join(destRoot, "bin", postgresBin), "utf8")).toBe("postgres-stable");
+    } finally {
+      rmSync(nativeRoot, { recursive: true, force: true });
+      rmSync(destRoot, { recursive: true, force: true });
+    }
+  });
+
   it("re-copies when payload changes even though nativeRoot path is unchanged", () => {
     const nativeRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-mat-update-"));
     const destRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-mat-update-dst-"));
@@ -463,6 +577,29 @@ describe("embedded-lifecycle: materialize runtime binaries (update-safe marker)"
     }
   });
 
+  it("treats v2 markers as stale and rematerializes", () => {
+    const nativeRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-mat-v2-"));
+    const destRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-mat-v2-dst-"));
+    try {
+      seedNativeRoot(nativeRoot, "postgres-current");
+      materializeEmbeddedPostgresRuntimeBinaries(nativeRoot, { destRoot });
+      writeFileSync(
+        join(destRoot, ".materialized-from"),
+        buildEmbeddedPostgresMaterializationMarker(nativeRoot).replace("v3\n", "v2\n"),
+      );
+      writeFileSync(join(destRoot, ".reuse-sentinel"), "must-be-removed");
+
+      materializeEmbeddedPostgresRuntimeBinaries(nativeRoot, { destRoot });
+      expect(existsSync(join(destRoot, ".reuse-sentinel"))).toBe(false);
+      expect(readFileSync(join(destRoot, ".materialized-from"), "utf8")).toBe(
+        buildEmbeddedPostgresMaterializationMarker(nativeRoot),
+      );
+    } finally {
+      rmSync(nativeRoot, { recursive: true, force: true });
+      rmSync(destRoot, { recursive: true, force: true });
+    }
+  });
+
   it("treats legacy path-only markers as stale and rematerializes", () => {
     const nativeRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-mat-legacy-"));
     const destRoot = mkdtempSync(join(tmpdir(), "fusion-embedded-mat-legacy-dst-"));
diff --git a/packages/core/src/__tests__/postgres/startup-factory.test.ts b/packages/core/src/__tests__/postgres/startup-factory.test.ts
index 35a36e1e6c..9c0d3589f8 100644
--- a/packages/core/src/__tests__/postgres/startup-factory.test.ts
+++ b/packages/core/src/__tests__/postgres/startup-factory.test.ts
@@ -32,6 +32,7 @@ import {
   EMBEDDED_PG_ENV,
   NO_EMBEDDED_PG_ENV,
   resolveStartupDatabaseOptions,
+  formatPostgresSchemaBackendBootError,
 } from "../../postgres/startup-factory.js";
 import { resolveBackend } from "../../postgres/backend-resolver.js";
 
@@ -265,6 +266,24 @@ encoding-conversion failure raised when a non-UTF-8 cluster (WIN1252/WIN1254
 from a pre-fix initdb on a non-UTF-8 OS locale) receives the UTF-8 schema
 SQL — and nothing else, so ordinary schema errors never delete a data dir.
 */
+describe("startup-factory Windows antivirus boot hint (#3489)", () => {
+  const issueOutput = 'ERROR OUTPUT: FATAL: could not load library "C:/Users/ppp/.fusion/embedded-postgres/runtime-bin/win32-x64/lib/dict_snowball.dll": unknown error 4551';
+
+  it("adds exactly one actionable hint to the wrapped initdb failure", async () => {
+    const message = await formatPostgresSchemaBackendBootError(
+      new Error(`startup-factory: failed to start embedded PostgreSQL: ${issueOutput}`),
+    );
+    expect(message).toMatch(/^startup-factory: failed to initialize PostgreSQL schema backend:/);
+    expect(message).toContain("dict_snowball.dll");
+    expect(message.match(/Windows antivirus blocked a bundled PostgreSQL library/g)).toHaveLength(1);
+  });
+
+  it("does not add the antivirus hint to unrelated boot failures", async () => {
+    const message = await formatPostgresSchemaBackendBootError(new Error("connection refused"));
+    expect(message).not.toContain("Windows antivirus blocked a bundled PostgreSQL library");
+  });
+});
+
 describe("isEncodingConversionError (#2286 recovery trigger)", () => {
   it("matches the encoding-conversion failure from a non-UTF-8 cluster", () => {
     expect(
diff --git a/packages/core/src/postgres/embedded-lifecycle.ts b/packages/core/src/postgres/embedded-lifecycle.ts
index 4c73e3725d..b1e9ee5053 100644
--- a/packages/core/src/postgres/embedded-lifecycle.ts
+++ b/packages/core/src/postgres/embedded-lifecycle.ts
@@ -62,7 +62,7 @@ import {
 import { createHash } from "node:crypto";
 import { homedir } from "node:os";
 import { createServer, type Server } from "node:net";
-import { dirname, join, basename, sep } from "node:path";
+import { dirname, join, basename, sep, resolve } from "node:path";
 import { createRequire, syncBuiltinESMExports } from "node:module";
 import { createLogger } from "../process/logger.js";
 import { redactConnectionString } from "./credential-redact.js";
@@ -202,7 +202,7 @@ const EMBEDDED_PG_BIN_NAMES = new Set([
  * host-local caches always rematerialize after a desktop update that ships a
  * new fingerprinting strategy (e.g. content-hashing lib/share, not path+size).
  */
-const MATERIALIZATION_MARKER_VERSION = 2;
+const MATERIALIZATION_MARKER_VERSION = 3;
 
 /**
  * FNXC:DesktopEmbeddedPostgres 2026-07-14-18:30:
@@ -361,6 +361,40 @@ function hashPayloadTreeContents(
  * binaries instead of reusing the previous release's host-local cache.
  * Legacy path-only markers fail equality and force rematerialization.
  */
+/*
+ * FNXC:PostgresEmbedded 2026-08-20-01:11:
+ * Issue #3489 reports Windows Defender removing dict_snowball.dll while PostgreSQL
+ * initializes. Win32 4550/4551 are the virus-infected/deleted codes, so match both
+ * signatures narrowly; generic library-load errors have unrelated remediation.
+ */
+export function isWindowsBlockedNativeLibraryError(errorOrText: unknown): boolean {
+  const text = errorOrText instanceof Error ? errorOrText.message : String(errorOrText ?? "");
+  return (/could not load library/i.test(text) && /unknown error 455[01]/i.test(text))
+    || /ERROR_VIRUS_(?:INFECTED|DELETED)/i.test(text);
+}
+
+/** Creates the shared, actionable Windows antivirus recovery instruction. */
+export function describeWindowsBlockedNativeLibraryError(errorOrText: unknown): string {
+  const text = errorOrText instanceof Error ? errorOrText.message : String(errorOrText ?? "");
+  const blockedPath = text.match(/could not load library\s+["']([^"']+)["']/i)?.[1];
+  const pathDetail = blockedPath ? ` Blocked file: ${blockedPath}.` : "";
+  return `${pathDetail} HINT: Windows antivirus blocked a bundled PostgreSQL library. Add an exclusion for %USERPROFILE%\\.fusion\\embedded-postgres in Windows Security → Virus & threat protection → Manage settings → Exclusions, restore the quarantined file from Protection history, then restart Fusion. Fusion re-copies the runtime automatically.`;
+}
+
+export function decorateWindowsBlockedNativeLibraryError(error: unknown, destRoot?: string): Error {
+  const recorded = getEmbeddedPayloadIntegrityFailure(destRoot);
+  if (!isWindowsBlockedNativeLibraryError(error) && !recorded) {
+    return error instanceof Error ? error : new Error(String(error));
+  }
+  const message = error instanceof Error ? error.message : String(error);
+  if (message.includes("Windows antivirus blocked a bundled PostgreSQL library")) {
+    return error instanceof Error ? error : new Error(message);
+  }
+  return new Error(`${message}${recorded ? ` ${recorded.message}` : describeWindowsBlockedNativeLibraryError(error)}`, {
+    cause: error,
+  });
+}
+
 export function buildEmbeddedPostgresMaterializationMarker(nativeRoot: string): string {
   const fingerprint = fingerprintEmbeddedPostgresNativeRoot(nativeRoot);
   return `v${MATERIALIZATION_MARKER_VERSION}\n${nativeRoot}\n${fingerprint}\n`;
@@ -374,6 +408,134 @@ function resolveMaterializedEmbeddedPostgresBinary(filePath: string): string | n
   return existsSync(candidate) ? candidate : null;
 }
 
+type PayloadInventoryResult = {
+  readonly acceptable: boolean;
+  readonly mismatches: readonly string[];
+  readonly mismatchCount: number;
+};
+
+/**
+ * FNXC:PostgresEmbedded 2026-08-20-01:11:
+ * A marker fingerprints only the source. Defender can remove a copied DLL after
+ * materialization, leaving that marker truthful but the destination unusable.
+ * Compare the bounded source inventory on reuse; exhaustion and unreadable source
+ * fail open so a healthy host never incurs an unbounded copy or boot refusal.
+ */
+function verifyEmbeddedPostgresPayloadInventory(
+  nativeRoot: string,
+  destRoot: string,
+): PayloadInventoryResult {
+  const budget = { remaining: 4096 };
+  const mismatches: string[] = [];
+  let mismatchCount = 0;
+  let sourceUnreadable = false;
+  const report = (relativePath: string) => {
+    mismatchCount += 1;
+    if (mismatches.length < 10) mismatches.push(relativePath);
+  };
+  const visit = (sourcePath: string, destPath: string, relativePath: string): void => {
+    if (budget.remaining <= 0 || sourceUnreadable) return;
+    let sourceEntries: string[];
+    try {
+      sourceEntries = readdirSync(sourcePath).sort();
+    } catch {
+      sourceUnreadable = true;
+      return;
+    }
+    for (const entry of sourceEntries) {
+      if (budget.remaining <= 0 || sourceUnreadable) return;
+      const sourceEntry = join(sourcePath, entry);
+      const destEntry = join(destPath, entry);
+      const relativeEntry = relativePath ? `${relativePath}/${entry}` : entry;
+      try {
+        const sourceStat = lstatSync(sourceEntry);
+        if (sourceStat.isDirectory()) {
+          visit(sourceEntry, destEntry, relativeEntry);
+        } else if (sourceStat.isFile() || sourceStat.isSymbolicLink()) {
+          budget.remaining -= 1;
+          let destStat: ReturnType<typeof lstatSync>;
+          try {
+            destStat = lstatSync(destEntry);
+          } catch {
+            report(relativeEntry);
+            continue;
+          }
+          if (sourceStat.isFile()) {
+            if (!destStat.isFile() || destStat.size !== sourceStat.size) report(relativeEntry);
+          } else {
+            if (!destStat.isSymbolicLink()) report(relativeEntry);
+          }
+        }
+      } catch {
+        sourceUnreadable = true;
+      }
+    }
+  };
+  for (const tree of ["bin", "lib", "share"] as const) {
+    const sourceTree = join(nativeRoot, tree);
+    try {
+      if (lstatSync(sourceTree).isDirectory()) visit(sourceTree, join(destRoot, tree), tree);
+    } catch {
+      sourceUnreadable = true;
+    }
+  }
+  return {
+    acceptable: sourceUnreadable || budget.remaining <= 0 || mismatchCount === 0,
+    mismatches,
+    mismatchCount,
+  };
+}
+
+export class EmbeddedPostgresPayloadBlockedError extends Error {
+  readonly destRoot: string;
+  readonly nativeRoot: string;
+  readonly affectedPaths: readonly string[];
+  readonly affectedPathCount: number;
+
+  constructor(nativeRoot: string, destRoot: string, verification: PayloadInventoryResult) {
+    const affected = verification.mismatches.join(", ") || "unknown payload entry";
+    super(
+      `Embedded PostgreSQL runtime payload is incomplete (${affected}${verification.mismatchCount > verification.mismatches.length ? ", …" : ""}).${describeWindowsBlockedNativeLibraryError(affected)}`,
+    );
+    this.name = "EmbeddedPostgresPayloadBlockedError";
+    this.nativeRoot = nativeRoot;
+    this.destRoot = destRoot;
+    this.affectedPaths = verification.mismatches;
+    this.affectedPathCount = verification.mismatchCount;
+  }
+}
+
+type EmbeddedPayloadIntegrityFailure = {
+  readonly destRoot: string;
+  readonly error: EmbeddedPostgresPayloadBlockedError;
+  readonly recordedAt: number;
+};
+
+let embeddedPayloadIntegrityFailure: EmbeddedPayloadIntegrityFailure | null = null;
+
+/** Records only the newest root-scoped incomplete-payload diagnosis. */
+export function recordEmbeddedPayloadIntegrityFailure(error: EmbeddedPostgresPayloadBlockedError): void {
+  embeddedPayloadIntegrityFailure = { destRoot: resolve(error.destRoot), error, recordedAt: Date.now() };
+}
+
+/** Returns the diagnostic only for its originating materialized runtime root. */
+export function getEmbeddedPayloadIntegrityFailure(destRoot?: string): EmbeddedPostgresPayloadBlockedError | null {
+  if (!embeddedPayloadIntegrityFailure) return null;
+  if (destRoot && resolve(destRoot) !== embeddedPayloadIntegrityFailure.destRoot) return null;
+  return embeddedPayloadIntegrityFailure.error;
+}
+
+/** Clears the test-resettable, in-memory payload diagnostic. */
+export function clearEmbeddedPayloadIntegrityFailure(): void {
+  embeddedPayloadIntegrityFailure = null;
+}
+
+function clearEmbeddedPayloadIntegrityFailureForRoot(destRoot: string): void {
+  if (embeddedPayloadIntegrityFailure?.destRoot === resolve(destRoot)) {
+    clearEmbeddedPayloadIntegrityFailure();
+  }
+}
+
 export interface MaterializeEmbeddedPostgresOptions {
   /**
    * Override the host-local dest root (tests). Defaults to
@@ -413,8 +575,10 @@ export function materializeEmbeddedPostgresRuntimeBinaries(
     existsSync(marker) &&
     readFileSync(marker, "utf8") === sourceMarker &&
     existsSync(join(destBin, process.platform === "win32" ? "postgres.exe" : "postgres")) &&
-    existsSync(join(destRoot, "lib", "postgresql"))
+    existsSync(join(destRoot, "lib", "postgresql")) &&
+    verifyEmbeddedPostgresPayloadInventory(nativeRoot, destRoot).acceptable
   ) {
+    clearEmbeddedPayloadIntegrityFailureForRoot(destRoot);
     return destRoot;
   }
 
@@ -424,6 +588,12 @@ export function materializeEmbeddedPostgresRuntimeBinaries(
    * payload cannot linger beside the updated binaries (force-copy alone does not
    * delete orphans).
    */
+  // Never let a prior marker certify a partially copied payload after a crash or AV action.
+  try {
+    unlinkSync(marker);
+  } catch {
+    // best-effort; recursive destination removal below is the normal cleanup path
+  }
   if (existsSync(destRoot)) {
     rmSync(destRoot, { recursive: true, force: true });
   }
@@ -446,7 +616,19 @@ export function materializeEmbeddedPostgresRuntimeBinaries(
   }
   // Re-apply macOS ABI compatibility links against the materialized lib dir.
   normalizeMacosEmbeddedPostgresDylibSymlinks(destRoot);
+  const verification = verifyEmbeddedPostgresPayloadInventory(nativeRoot, destRoot);
+  if (!verification.acceptable) {
+    try {
+      unlinkSync(marker);
+    } catch {
+      // No marker is the retry contract; ignore an already-absent marker.
+    }
+    const error = new EmbeddedPostgresPayloadBlockedError(nativeRoot, destRoot, verification);
+    recordEmbeddedPayloadIntegrityFailure(error);
+    throw error;
+  }
   writeFileSync(marker, sourceMarker, "utf8");
+  clearEmbeddedPayloadIntegrityFailureForRoot(destRoot);
   return destRoot;
 }
 
@@ -525,7 +707,10 @@ export function installElectronAsarNativePathPatch(): void {
         materializeEmbeddedPostgresRuntimeBinaries(sourceRoot);
       }
     }
-  } catch {
+  } catch (error) {
+    if (error instanceof EmbeddedPostgresPayloadBlockedError) {
+      recordEmbeddedPayloadIntegrityFailure(error);
+    }
     // Materialization is best-effort; path rewrite still helps when possible.
   }
 
@@ -970,7 +1155,10 @@ function resolveWindowsEmbeddedPostgresNativeRoot(): string | null {
       return materializeEmbeddedPostgresRuntimeBinaries(
         resolveElectronAsarUnpackedPath(nativeRoot),
       );
-    } catch {
+    } catch (error) {
+      if (error instanceof EmbeddedPostgresPayloadBlockedError) {
+        recordEmbeddedPayloadIntegrityFailure(error);
+      }
       return resolveElectronAsarUnpackedPath(nativeRoot);
     }
   }
@@ -1531,7 +1719,11 @@ export class EmbeddedPostgresLifecycle {
       this.options.onLog(
         `embedded postgres: initializing new data directory at ${this.options.dataDir} (initdb)`,
       );
-      await pg.initialise();
+      try {
+        await pg.initialise();
+      } catch (error) {
+        throw decorateWindowsBlockedNativeLibraryError(error, embeddedPostgresRuntimeBinRoot());
+      }
     }
 
     if (signal?.aborted) {
@@ -1603,7 +1795,9 @@ export class EmbeddedPostgresLifecycle {
       then failed later read back its OWN postmaster.pid, "join itself" with ownsProcess=false,
       and orphan a live postmaster nothing would ever stop. See isPostgresLockCollisionError.
       */
-      if (!isPostgresLockCollisionError(error)) throw error;
+      if (!isPostgresLockCollisionError(error)) {
+        throw decorateWindowsBlockedNativeLibraryError(error, embeddedPostgresRuntimeBinRoot());
+      }
       const existing = await isAlreadyRunning(this.options.dataDir, this.options.onLog);
       if (!existing) throw error;
 
diff --git a/packages/core/src/postgres/startup-factory.ts b/packages/core/src/postgres/startup-factory.ts
index 1332739295..ea263c2a12 100644
--- a/packages/core/src/postgres/startup-factory.ts
+++ b/packages/core/src/postgres/startup-factory.ts
@@ -130,6 +130,19 @@ function fallbackProjectIdForRoot(rootDir: string): string {
  * kept for backward compatibility with scripts/docs that still set it. It
  * cannot force embedded mode when DATABASE_URL is set (external always wins).
  */
+export async function formatPostgresSchemaBackendBootError(err: unknown): Promise<string> {
+  const chain = describeErrorChain(err);
+  const encodingHint = /has no equivalent in encoding/i.test(chain)
+    ? " HINT: this embedded PostgreSQL cluster was created with a non-UTF-8 encoding inherited from the OS locale by an earlier Fusion version. It cannot be converted in place — stop Fusion, delete the embedded data directory (default: ~/.fusion/embedded-postgres/default), and start again so the cluster is recreated as UTF-8."
+    : "";
+  const { isWindowsBlockedNativeLibraryError, describeWindowsBlockedNativeLibraryError } =
+    await import("./embedded-lifecycle.js");
+  const blockedLibraryHint = isWindowsBlockedNativeLibraryError(chain)
+    ? describeWindowsBlockedNativeLibraryError(chain)
+    : "";
+  return `startup-factory: failed to initialize PostgreSQL schema backend: ${chain}${encodingHint}${blockedLibraryHint}`;
+}
+
 export const EMBEDDED_PG_ENV = "FUSION_EMBEDDED_PG";
 
 /**
@@ -869,20 +882,13 @@ export async function createTaskStoreForBackend(
     boot = await bootSchemaBackend(effectiveOptions);
     log.log(`startup phase backend.schemaBackend: ${Date.now() - schemaT0}ms`);
   } catch (err) {
-    const chain = describeErrorChain(err);
     /*
-    FNXC:PostgresEmbedded 2026-07-18-00:20:
-    Issue #2286: a cluster initdb'd by an earlier version on a non-UTF-8 OS
-    locale cannot store the UTF-8 schema SQL and cannot be converted in place.
-    Newly created clusters are forced to UTF-8 (DEFAULT_EMBEDDED_INITDB_FLAGS);
-    existing ones need a manual re-init, so say exactly that.
+    FNXC:PostgresEmbedded 2026-08-20-01:11:
+    Issue #3489 uses the same outer boot mapper as #2286's encoding guidance.
+    initdb wraps PostgreSQL's FATAL before this boundary, so retain the chain
+    while adding one actionable antivirus hint only for 4550/4551 signatures.
     */
-    const encodingHint = /has no equivalent in encoding/i.test(chain)
-      ? " HINT: this embedded PostgreSQL cluster was created with a non-UTF-8 encoding inherited from the OS locale by an earlier Fusion version. It cannot be converted in place — stop Fusion, delete the embedded data directory (default: ~/.fusion/embedded-postgres/default), and start again so the cluster is recreated as UTF-8."
-      : "";
-    throw new Error(
-      `startup-factory: failed to initialize PostgreSQL schema backend: ${chain}${encodingHint}`,
-    );
+    throw new Error(await formatPostgresSchemaBackendBootError(err));
   }
   let { connections } = boot;
   const {