diff --git a/.changeset/fix-chat-mailbox-nul-byte-sanitize.md b/.changeset/fix-chat-mailbox-nul-byte-sanitize.md
new file mode 100644
index 0000000000..6173b88d56
--- /dev/null
+++ b/.changeset/fix-chat-mailbox-nul-byte-sanitize.md
@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+summary: Fix a crash where chat messages and mailbox sends containing a raw NUL byte would abort mid-conversation.
+category: fix
+dev: PostgreSQL text/jsonb columns reject U+0000 outright ("unsupported Unicode escape sequence" / "\u0000 cannot be converted to text"). Tool output piped into a chat message or agent mailbox send could carry a literal NUL byte and crash addChatMessage/addChatRoomMessage/sendMessage with an uncaught PostgresError. Extracted the existing stripNulChars/deepStripNulChars sanitizer (previously only used by the one-time SQLite migration) into a shared packages/core/src/postgres/nul-sanitize.ts module and wired it into all three live write paths. Also fixes a related embedded-Postgres startup race (JoinedInstanceUnreachableError) where a joiner could hit ECONNREFUSED before the owning process's listener was ready; now retried once, mirroring the existing NonUtf8EmbeddedClusterError retry pattern.
diff --git a/packages/core/src/__tests__/nul-sanitize.test.ts b/packages/core/src/__tests__/nul-sanitize.test.ts
new file mode 100644
index 0000000000..2853593277
--- /dev/null
+++ b/packages/core/src/__tests__/nul-sanitize.test.ts
@@ -0,0 +1,86 @@
+/**
+ * FNXC:PostgresMigrationNulSanitize 2026-07-20:
+ * Unit coverage for the shared NUL-byte sanitizer (packages/core/src/postgres/nul-sanitize.ts),
+ * extracted from sqlite-migrator.ts so live write paths (async-chat-store.ts,
+ * async-message-store.ts) can reuse the same tested behavior. No PostgreSQL
+ * connection required — pure function coverage.
+ */
+
+import { describe, it, expect } from "vitest";
+import {
+  stripNulChars,
+  deepStripNulChars,
+  sanitizeTextValue,
+  sanitizeJsonbValue,
+} from "../postgres/nul-sanitize.js";
+
+describe("stripNulChars", () => {
+  it("removes U+0000 from a string", () => {
+    expect(stripNulChars("hello\u0000world")).toBe("helloworld");
+  });
+
+  it("is a no-op when no NUL is present", () => {
+    expect(stripNulChars("clean string")).toBe("clean string");
+  });
+
+  it("strips multiple NUL bytes", () => {
+    expect(stripNulChars("a\u0000b\u0000c\u0000")).toBe("abc");
+  });
+});
+
+describe("deepStripNulChars", () => {
+  it("strips NUL from nested string values and object keys", () => {
+    const input = {
+      ["key\u0000nul"]: "value\u0000nul",
+      nested: ["a\u0000b", { deeper: "c\u0000d" }],
+    };
+    expect(deepStripNulChars(input)).toEqual({
+      keynul: "valuenul",
+      nested: ["ab", { deeper: "cd" }],
+    });
+  });
+
+  it("passes through non-string primitives and null unchanged", () => {
+    expect(deepStripNulChars(42)).toBe(42);
+    expect(deepStripNulChars(true)).toBe(true);
+    expect(deepStripNulChars(null)).toBe(null);
+  });
+});
+
+describe("sanitizeTextValue", () => {
+  it("strips NUL from a string value", () => {
+    expect(sanitizeTextValue("diag\u0000nostic dump")).toBe("diagnostic dump");
+  });
+
+  it("passes through null/undefined unchanged", () => {
+    expect(sanitizeTextValue(null)).toBe(null);
+    expect(sanitizeTextValue(undefined)).toBe(undefined);
+  });
+});
+
+describe("sanitizeJsonbValue", () => {
+  it("deep-strips NUL from an object destined for a jsonb column", () => {
+    expect(sanitizeJsonbValue({ note: "tail\u0000end" })).toEqual({ note: "tailend" });
+  });
+
+  it("passes through null/undefined unchanged", () => {
+    expect(sanitizeJsonbValue(null)).toBe(null);
+    expect(sanitizeJsonbValue(undefined)).toBe(undefined);
+  });
+
+  it("handles the exact failure signature observed in production: a raw NUL embedded in a diagnostics dump", () => {
+    // Real-world trigger: an agent piped a Windows CLI diagnostics dump
+    // (tasklist/netstat output) directly into a chat message body. The dump
+    // contained a raw NUL byte, which Postgres's json_ereport_error rejected
+    // with "unsupported Unicode escape sequence" / "\u0000 cannot be
+    // converted to text", aborting the chat write mid-conversation.
+    const diagnosticDump =
+      "===FUSION DB AGENTS===\n===NODES===\n\u0000===RUNNING PROCESSES===\n";
+    expect(sanitizeTextValue(diagnosticDump)).toBe(
+      "===FUSION DB AGENTS===\n===NODES===\n===RUNNING PROCESSES===\n",
+    );
+    expect(sanitizeJsonbValue({ toolOutput: diagnosticDump })).toEqual({
+      toolOutput: "===FUSION DB AGENTS===\n===NODES===\n===RUNNING PROCESSES===\n",
+    });
+  });
+});
diff --git a/packages/core/src/__tests__/postgres/chat-store-content-search-edit.pg.test.ts b/packages/core/src/__tests__/postgres/chat-store-content-search-edit.pg.test.ts
index 0967ec37b7..d8544f98f9 100644
--- a/packages/core/src/__tests__/postgres/chat-store-content-search-edit.pg.test.ts
+++ b/packages/core/src/__tests__/postgres/chat-store-content-search-edit.pg.test.ts
@@ -234,4 +234,37 @@ pgDescribe("async chat store content search + edit primitives (PostgreSQL)", ()
       updateChatMessageMetadata(ctx.layer.db, "msg-missing", { x: 1 }),
     ).rejects.toThrow(/not found/);
   });
+
+  /*
+  FNXC:PostgresMigrationNulSanitize 2026-07-20:
+  Production incident: the CEO agent's chat turn crashed mid-conversation
+  because a tool call piped raw Windows CLI diagnostics output (containing a
+  literal U+0000 byte) straight into the chat message body. Postgres rejected
+  the INSERT with PostgresError code 22P05 ("unsupported Unicode escape
+  sequence" / "\u0000 cannot be converted to text"), which aborted the write
+  and killed the turn. addChatMessage now sanitizes content/thinkingOutput
+  (text) and metadata/attachments (jsonb) before insert — verify a message
+  containing a raw NUL byte round-trips instead of throwing.
+  */
+  it("addChatMessage strips a raw U+0000 byte instead of throwing (regression for the CEO-agent chat crash)", async () => {
+    ctx = await setupCtx();
+    const session = await makeSession(ctx);
+
+    const diagnosticDump =
+      "===FUSION DB AGENTS===\n===NODES===\n\u0000===RUNNING PROCESSES===\n";
+    const sent = await addMessage(ctx, session.id, "assistant", diagnosticDump, {
+      toolOutput: `payload\u0000tail`,
+    });
+
+    expect(sent.content).toBe(
+      "===FUSION DB AGENTS===\n===NODES===\n===RUNNING PROCESSES===\n",
+    );
+    expect(sent.metadata).toEqual({ toolOutput: "payloadtail" });
+
+    const [persisted] = await getChatMessages(ctx.layer.db, session.id);
+    expect(persisted.content).toBe(
+      "===FUSION DB AGENTS===\n===NODES===\n===RUNNING PROCESSES===\n",
+    );
+    expect(persisted.metadata).toEqual({ toolOutput: "payloadtail" });
+  });
 });
diff --git a/packages/core/src/__tests__/postgres/message-store.pg.test.ts b/packages/core/src/__tests__/postgres/message-store.pg.test.ts
index 3a53ce0ab6..9d98bcfbd6 100644
--- a/packages/core/src/__tests__/postgres/message-store.pg.test.ts
+++ b/packages/core/src/__tests__/postgres/message-store.pg.test.ts
@@ -122,4 +122,41 @@ pgTest("MessageStore send (PostgreSQL backend mode)", () => {
       .filter((message) => message.metadata?.kind === "postgres-migration-complete");
     expect(completionMessages).toHaveLength(1);
   });
+
+  /*
+  FNXC:PostgresMigrationNulSanitize 2026-07-20:
+  Same NUL-byte hazard as the chat-store regression (chat-store-content-search-edit.pg.test.ts):
+  agent-to-agent/agent-to-user mailbox content can carry raw tool output
+  containing a literal U+0000 byte, which Postgres's text/jsonb columns
+  reject outright ("unsupported Unicode escape sequence" / "\u0000 cannot be
+  converted to text"). sendMessage now sanitizes content/metadata before
+  insert — verify a NUL-laden send round-trips instead of throwing.
+  */
+  it("sendMessage strips a raw U+0000 byte instead of throwing", async () => {
+    const { MessageStore } = await import("../../message-store.js");
+    const store = new MessageStore(null, { asyncLayer: h.layer() });
+
+    const diagnosticDump =
+      "===FUSION DB AGENTS===\n===NODES===\n\u0000===RUNNING PROCESSES===\n";
+    const sent = await store.sendMessage({
+      fromId: "agent-ceo",
+      fromType: "agent",
+      toId: "user-x",
+      toType: "user",
+      content: diagnosticDump,
+      type: "agent-to-user",
+      metadata: { toolOutput: "payload\u0000tail" },
+    });
+
+    expect(sent.content).toBe(
+      "===FUSION DB AGENTS===\n===NODES===\n===RUNNING PROCESSES===\n",
+    );
+    expect(sent.metadata?.toolOutput).toBe("payloadtail");
+
+    const fetched = await store.getMessage(sent.id);
+    expect(fetched?.content).toBe(
+      "===FUSION DB AGENTS===\n===NODES===\n===RUNNING PROCESSES===\n",
+    );
+    expect(fetched?.metadata?.toolOutput).toBe("payloadtail");
+  });
 });
diff --git a/packages/core/src/async-chat-store.ts b/packages/core/src/async-chat-store.ts
index 8b6ee7216e..4182df6a4a 100644
--- a/packages/core/src/async-chat-store.ts
+++ b/packages/core/src/async-chat-store.ts
@@ -19,6 +19,7 @@
 import { and, asc, desc, eq, gt, ilike, inArray, isNull, lte, ne, or as orFn, sql as drizzleSql } from "drizzle-orm";
 import * as schema from "./postgres/schema/index.js";
 import type { AsyncDataLayer, DbTransaction } from "./postgres/data-layer.js";
+import { sanitizeTextValue, sanitizeJsonbValue } from "./postgres/nul-sanitize.js";
 import type {
   ChatAttachment,
   ChatInFlightGenerationState,
@@ -190,21 +191,37 @@ export async function addChatMessage(
   handle: QueryHandle,
   message: ChatMessage,
 ): Promise<ChatMessage> {
+  // FNXC:PostgresMigrationNulSanitize 2026-07-20: agent/tool output persisted
+  // here can contain a raw NUL byte (e.g. piped-through Windows CLI dumps),
+  // which Postgres text/jsonb columns reject outright. Sanitize before
+  // insert instead of letting the write throw mid-conversation, and return
+  // the sanitized value so the in-memory result matches what was persisted
+  // (the original unsanitized `message` object must not be handed back).
+  const sanitizedAttachments = message.attachments === undefined
+    ? undefined
+    : sanitizeJsonbValue(message.attachments);
+  const sanitized: ChatMessage = {
+    ...message,
+    content: sanitizeTextValue(message.content),
+    thinkingOutput: sanitizeTextValue(message.thinkingOutput),
+    metadata: sanitizeJsonbValue(message.metadata),
+    attachments: sanitizedAttachments,
+  };
   await handle.insert(schema.project.chatMessages).values({
-    id: message.id,
-    sessionId: message.sessionId,
-    role: message.role,
-    content: message.content,
-    thinkingOutput: message.thinkingOutput,
-    metadata: message.metadata,
-    attachments: message.attachments ?? null,
-    createdAt: message.createdAt,
+    id: sanitized.id,
+    sessionId: sanitized.sessionId,
+    role: sanitized.role,
+    content: sanitized.content,
+    thinkingOutput: sanitized.thinkingOutput,
+    metadata: sanitized.metadata,
+    attachments: sanitizedAttachments ?? null,
+    createdAt: sanitized.createdAt,
   });
   await handle
     .update(schema.project.chatSessions)
-    .set({ updatedAt: message.createdAt })
-    .where(eq(schema.project.chatSessions.id, message.sessionId));
-  return message;
+    .set({ updatedAt: sanitized.createdAt })
+    .where(eq(schema.project.chatSessions.id, sanitized.sessionId));
+  return sanitized;
 }
 
 /**
@@ -432,23 +449,36 @@ export async function addChatRoomMessage(
   handle: QueryHandle,
   message: ChatRoomMessage,
 ): Promise<ChatRoomMessage> {
+  // FNXC:PostgresMigrationNulSanitize 2026-07-20: same NUL-byte hazard as
+  // addChatMessage above — sanitize before insert, and return the sanitized
+  // value so the in-memory result matches what was persisted.
+  const sanitizedAttachments = message.attachments === undefined
+    ? undefined
+    : sanitizeJsonbValue(message.attachments);
+  const sanitized: ChatRoomMessage = {
+    ...message,
+    content: sanitizeTextValue(message.content),
+    thinkingOutput: sanitizeTextValue(message.thinkingOutput),
+    metadata: sanitizeJsonbValue(message.metadata),
+    attachments: sanitizedAttachments,
+  };
   await handle.insert(schema.project.chatRoomMessages).values({
-    id: message.id,
-    roomId: message.roomId,
-    role: message.role,
-    content: message.content,
-    thinkingOutput: message.thinkingOutput,
-    metadata: message.metadata,
-    attachments: message.attachments ?? null,
-    senderAgentId: message.senderAgentId,
-    mentions: message.mentions,
-    createdAt: message.createdAt,
+    id: sanitized.id,
+    roomId: sanitized.roomId,
+    role: sanitized.role,
+    content: sanitized.content,
+    thinkingOutput: sanitized.thinkingOutput,
+    metadata: sanitized.metadata,
+    attachments: sanitizedAttachments ?? null,
+    senderAgentId: sanitized.senderAgentId,
+    mentions: sanitized.mentions,
+    createdAt: sanitized.createdAt,
   });
   await handle
     .update(schema.project.chatRooms)
-    .set({ updatedAt: message.createdAt })
-    .where(eq(schema.project.chatRooms.id, message.roomId));
-  return message;
+    .set({ updatedAt: sanitized.createdAt })
+    .where(eq(schema.project.chatRooms.id, sanitized.roomId));
+  return sanitized;
 }
 
 /**
diff --git a/packages/core/src/async-message-store.ts b/packages/core/src/async-message-store.ts
index 69affaf878..bdd0f25835 100644
--- a/packages/core/src/async-message-store.ts
+++ b/packages/core/src/async-message-store.ts
@@ -22,6 +22,7 @@ import { and, desc, eq, inArray, lte, or, sql } from "drizzle-orm";
 import { randomUUID } from "node:crypto";
 import * as schema from "./postgres/schema/index.js";
 import type { AsyncDataLayer, DbTransaction } from "./postgres/data-layer.js";
+import { sanitizeTextValue, sanitizeJsonbValue } from "./postgres/nul-sanitize.js";
 import {
   DASHBOARD_USER_ID,
   type Message,
@@ -96,21 +97,32 @@ export async function sendMessage(
   handle: QueryHandle,
   message: PersistedMessage,
 ): Promise<Message> {
+  // FNXC:PostgresMigrationNulSanitize 2026-07-20: agent-to-agent/agent-to-user
+  // mailbox content can carry raw tool output (including a stray NUL byte
+  // from piped Windows CLI dumps), which Postgres text/jsonb columns reject
+  // outright. Sanitize before insert instead of letting the send throw, and
+  // return the sanitized value so the in-memory result matches what was
+  // persisted (the original unsanitized `message` object must not be handed
+  // back to the caller).
+  const sanitizedContent = sanitizeTextValue(message.content);
+  const sanitizedMetadata = sanitizeJsonbValue(message.metadata);
   await handle.insert(schema.project.messages).values({
     id: message.id,
     fromId: message.fromId,
     fromType: message.fromType,
     toId: message.toId,
     toType: message.toType,
-    content: message.content,
+    content: sanitizedContent,
     type: message.type,
     read: message.read ? 1 : 0,
-    metadata: message.metadata,
+    metadata: sanitizedMetadata,
     createdAt: message.createdAt,
     updatedAt: message.updatedAt,
   });
   return rowToMessage({
     ...message,
+    content: sanitizedContent,
+    metadata: sanitizedMetadata,
     read: message.read ? 1 : 0,
   });
 }
diff --git a/packages/core/src/message-store.ts b/packages/core/src/message-store.ts
index 56826a797a..bb1817d0da 100644
--- a/packages/core/src/message-store.ts
+++ b/packages/core/src/message-store.ts
@@ -18,6 +18,7 @@ import { createLogger } from "./logger.js";
 import { DASHBOARD_USER_ID, normalizeMessageParticipant, validateMessageMetadata, type Message, type MessageCreateInput, type MessageFilter, type MessageType, type Mailbox, type ParticipantType } from "./types.js";
 import type { AsyncDataLayer } from "./postgres/data-layer.js";
 import * as asyncMessageStore from "./async-message-store.js";
+import { sanitizeTextValue, sanitizeJsonbValue } from "./postgres/nul-sanitize.js";
 
 const messageStoreLog = createLogger("message-store");
 
@@ -176,10 +177,17 @@ export class MessageStore extends EventEmitter<MessageStoreEvents> {
       fromType: from.type,
       toId: to.id,
       toType: to.type,
-      content: input.content,
+      // FNXC:PostgresMigrationNulSanitize 2026-07-21: sanitize here so the
+      // exact same object is persisted, emitted (message:sent/message:received,
+      // consumed by the agent wake hook), and returned to the caller. The
+      // async-message-store.ts sendMessage() layer also sanitizes before its
+      // own insert, but this class discarded that function's return value and
+      // used its own locally-built (unsanitized) `message` object for
+      // everything else — this closes that gap.
+      content: sanitizeTextValue(input.content),
       type: input.type,
       read: false,
-      metadata: input.metadata,
+      metadata: sanitizeJsonbValue(input.metadata),
       createdAt: now,
       updatedAt: now,
     };
@@ -263,10 +271,15 @@ export class MessageStore extends EventEmitter<MessageStoreEvents> {
       fromType: from.type,
       toId: to.id,
       toType: to.type,
-      content: input.content,
+      // FNXC:PostgresMigrationNulSanitize 2026-07-21: see sendMessage() above
+      // for why this must sanitize here rather than rely solely on
+      // asyncMessageStore.sendMessageOnce's own internal sanitization —
+      // that function only returns a boolean, so this class's locally-built
+      // `message` object is what actually gets emitted/returned.
+      content: sanitizeTextValue(input.content),
       type: input.type,
       read: false,
-      metadata: input.metadata,
+      metadata: sanitizeJsonbValue(input.metadata),
       createdAt: now,
       updatedAt: now,
     };
diff --git a/packages/core/src/postgres/nul-sanitize.ts b/packages/core/src/postgres/nul-sanitize.ts
new file mode 100644
index 0000000000..02e8e9634d
--- /dev/null
+++ b/packages/core/src/postgres/nul-sanitize.ts
@@ -0,0 +1,67 @@
+/*
+FNXC:PostgresMigrationNulSanitize 2026-07-17-10:05 (extracted 2026-07-20):
+PostgreSQL rejects U+0000 in text/varchar ("invalid byte sequence" / "\u0000
+cannot be converted to text.") and in json/jsonb ("unsupported Unicode escape
+sequence"). Any write path that persists free-form agent/tool output as
+Postgres text or jsonb — chat messages, chat room messages, and the
+agent/user mailbox — can receive a raw NUL byte from unsanitized upstream
+tool output (e.g. Windows CLI dumps piped straight into a message body) and
+crash the write with an uncaught PostgresError.
+
+This module was originally written for the one-time SQLite -> PostgreSQL
+first-boot migration (see sqlite-migrator.ts) and is promoted here so live
+write paths (async-chat-store.ts, async-message-store.ts) can reuse the same
+tested sanitization instead of duplicating it or, worse, not sanitizing at
+all.
+*/
+
+// eslint-disable-next-line no-control-regex -- matching the NUL control character is the point
+const NUL_CHAR_RE = /\u0000/g;
+
+/** Strip U+0000 (NUL) characters from a plain string. Cheap no-op when absent. */
+export function stripNulChars(text: string): string {
+  return text.includes("\u0000") ? text.replace(NUL_CHAR_RE, "") : text;
+}
+
+/**
+ * Recursively strip U+0000 from all string values and object keys in a
+ * parsed JSON-like value (object/array/string/number/boolean/null).
+ */
+export function deepStripNulChars(value: unknown): unknown {
+  if (typeof value === "string") {
+    return stripNulChars(value);
+  }
+  if (Array.isArray(value)) {
+    return value.map(deepStripNulChars);
+  }
+  if (value !== null && typeof value === "object") {
+    return Object.fromEntries(
+      Object.entries(value as Record<string, unknown>).map(
+        ([key, entry]) => [stripNulChars(key), deepStripNulChars(entry)],
+      ),
+    );
+  }
+  return value;
+}
+
+/**
+ * Sanitize a value that Drizzle will bind as a `text`/`varchar` column.
+ * Strings are NUL-stripped; everything else (including null/undefined)
+ * passes through unchanged.
+ */
+export function sanitizeTextValue<T>(value: T): T {
+  return (typeof value === "string" ? (stripNulChars(value) as unknown as T) : value);
+}
+
+/**
+ * Sanitize a value that Drizzle will bind as a `jsonb` column. Accepts
+ * either a JS value (object/array/etc, including null/undefined) that will
+ * be strip-cleaned recursively, since Drizzle's jsonb columns take JS values
+ * directly (not JSON strings) at the call sites this is used from.
+ */
+export function sanitizeJsonbValue<T>(value: T): T {
+  if (value === null || value === undefined) {
+    return value;
+  }
+  return deepStripNulChars(value) as T;
+}
\ No newline at end of file
diff --git a/packages/core/src/postgres/sqlite-migrator.ts b/packages/core/src/postgres/sqlite-migrator.ts
index 671f9f9949..5a1735e6e4 100644
--- a/packages/core/src/postgres/sqlite-migrator.ts
+++ b/packages/core/src/postgres/sqlite-migrator.ts
@@ -1412,33 +1412,15 @@ function classifyColumnType(pgCol: {
 }
 
 /*
-FNXC:PostgresMigrationNulSanitize 2026-07-17-10:05:
+FNXC:PostgresMigrationNulSanitize 2026-07-17-10:05 (moved to nul-sanitize.ts 2026-07-20):
 PostgreSQL rejects U+0000 in text/varchar ("invalid byte sequence" / "\u0000 cannot be converted to text") and in json/jsonb ("unsupported Unicode escape sequence"), but SQLite TEXT stores it freely, so legacy databases can contain NUL bytes that abort the first-boot auto-migration. Strip U+0000 from every migrated string — plain text cells, string values and object keys inside JSON documents, and opaque legacy-preservation text cells — rather than failing the cutover. Sanitization happens inside convertValue/tagLegacyCell so the content-checksum verification (computeSourceCanonicalRows reuses convertValue) compares the sanitized source against the sanitized target and still passes.
+
+stripNulChars/deepStripNulChars now live in ./nul-sanitize.js and are also
+reused by the live chat/mailbox write paths (async-chat-store.ts,
+async-message-store.ts), which can receive the same U+0000 byte from
+unsanitized tool output at runtime, not just from legacy SQLite migration.
 */
-// eslint-disable-next-line no-control-regex -- matching the NUL control character is the point
-const NUL_CHAR_RE = /\u0000/g;
-
-function stripNulChars(text: string): string {
-  return text.includes("\u0000") ? text.replace(NUL_CHAR_RE, "") : text;
-}
-
-/** Recursively strip U+0000 from all string values and object keys in a parsed JSON document. */
-function deepStripNulChars(value: unknown): unknown {
-  if (typeof value === "string") {
-    return stripNulChars(value);
-  }
-  if (Array.isArray(value)) {
-    return value.map(deepStripNulChars);
-  }
-  if (value !== null && typeof value === "object") {
-    return Object.fromEntries(
-      Object.entries(value as Record<string, unknown>).map(
-        ([key, entry]) => [stripNulChars(key), deepStripNulChars(entry)],
-      ),
-    );
-  }
-  return value;
-}
+import { stripNulChars, deepStripNulChars } from "./nul-sanitize.js";
 
 /**
  * Convert a SQLite value to its PostgreSQL representation based on the column
diff --git a/packages/core/src/postgres/startup-factory.ts b/packages/core/src/postgres/startup-factory.ts
index a45e203360..8dc5bad7eb 100644
--- a/packages/core/src/postgres/startup-factory.ts
+++ b/packages/core/src/postgres/startup-factory.ts
@@ -281,6 +281,35 @@ class NonUtf8EmbeddedClusterError extends Error {
   }
 }
 
+/*
+FNXC:PostgresStartupRace 2026-07-20-22:10:
+Companion to embedded-lifecycle.ts's FNXC:PostgresStartupRace 2026-07-15-20:45
+note: isAlreadyRunning() joins a data dir optimistically off postmaster.pid
+without probing liveness (a stale pid file from a crash must still resolve to
+a port, by design), and ensureJoinedDatabase() is explicitly best-effort —
+"never convert an optimistic join into a hard startup failure". But nothing
+downstream actually retried: a joiner that raced the true owner's TCP bind
+(process started, postmaster.pid written and "database system is ready"
+logged, but the listener not yet accepting) got exactly one shot at
+createConnectionSetFromUrl(), which throws ECONNREFUSED, which
+bootSchemaBackendOnce turns into a hard `startup-factory: failed to
+initialize PostgreSQL schema backend` error — observed reproducibly via
+`pnpm smoke:boot`'s isolated embedded-instance boot. This class marks that
+one specific case (joined instance, i.e. embeddedOwnsProcess === false,
+connection-refused) as retryable, mirroring the existing
+NonUtf8EmbeddedClusterError one-retry pattern below.
+*/
+class JoinedInstanceUnreachableError extends Error {
+  constructor(override readonly cause: unknown) {
+    super("embedded postgres: joined instance was not yet accepting connections");
+  }
+}
+
+/** Matches a TCP-level connection-refused failure (ECONNREFUSED / "connect ECONNREFUSED"). */
+function isConnectionRefusedError(chainText: string): boolean {
+  return /ECONNREFUSED/i.test(chainText);
+}
+
 /** Matches PostgreSQL's encoding-conversion failure raised by a non-UTF-8 cluster. */
 export function isEncodingConversionError(chainText: string): boolean {
   return /has no equivalent in encoding/i.test(chainText);
@@ -328,6 +357,15 @@ async function bootSchemaBackend(
   try {
     return await bootSchemaBackendOnce(options, bypassProjectIsolation);
   } catch (error) {
+    if (error instanceof JoinedInstanceUnreachableError) {
+      log.warn(
+        "startup-factory: joined embedded postgres instance was not yet accepting connections " +
+          "(startup race with the true owner's TCP bind — see FNXC:PostgresStartupRace 2026-07-20-22:10). " +
+          "Retrying once after a short delay.",
+      );
+      await new Promise((resolve) => setTimeout(resolve, 500));
+      return await bootSchemaBackendOnce(options, bypassProjectIsolation);
+    }
     if (!(error instanceof NonUtf8EmbeddedClusterError)) throw error;
     log.warn(
       `startup-factory: embedded cluster at ${error.dataDir} was created with a non-UTF-8 OS-locale ` +
@@ -443,6 +481,18 @@ async function bootSchemaBackendOnce(
     ) {
       recoverable = await isEmptyNonUtf8Cluster(connections.migration).catch(() => false);
     }
+    /*
+    FNXC:PostgresStartupRace 2026-07-20-22:10:
+    A joined (not owned) embedded instance that fails with a connection-refused
+    is the documented startup race (embedded-lifecycle.ts FNXC:PostgresStartupRace
+    2026-07-15-20:45): we joined off postmaster.pid before the true owner's
+    listener was accepting connections. This is retryable — see bootSchemaBackend's
+    JoinedInstanceUnreachableError handling — rather than a hard failure.
+    */
+    const joinedConnectionRefused =
+      embeddedLifecycle !== null &&
+      !embeddedOwnsProcess &&
+      isConnectionRefusedError(error instanceof Error ? `${error.message} ${String(error.cause ?? "")}` : String(error));
     await connections?.close().catch(() => undefined);
     await stopEmbeddedRuntime(
       embeddedLifecycle,
@@ -453,6 +503,9 @@ async function bootSchemaBackendOnce(
     if (recoverable && embeddedDataDir !== null) {
       throw new NonUtf8EmbeddedClusterError(embeddedDataDir, error);
     }
+    if (joinedConnectionRefused) {
+      throw new JoinedInstanceUnreachableError(error);
+    }
     throw error;
   }
 }