diff --git a/.changeset/windows-shared-memory-type.md b/.changeset/windows-shared-memory-type.md
new file mode 100644
index 0000000000..6b50888b59
--- /dev/null
+++ b/.changeset/windows-shared-memory-type.md
@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+summary: Fix embedded PostgreSQL failing to start on Windows after the mmap shared-memory default.
+category: fix
+dev: "shared_memory_type=mmap (default added 2026-07-16 for SysV shm exhaustion) is rejected on Windows, where the only valid value is windows — every Windows embedded start died with FATAL invalid value for parameter before the port opened, failing the v0.70.0/v0.70.1 Windows release smoke. Default flags are now platform-aware via defaultEmbeddedPostgresFlagsFor: empty on win32 (Windows needs no override; SysV exhaustion cannot occur there), mmap elsewhere."
diff --git a/packages/core/src/__tests__/postgres/embedded-lifecycle.test.ts b/packages/core/src/__tests__/postgres/embedded-lifecycle.test.ts
index 69583f020f..9412f1e938 100644
--- a/packages/core/src/__tests__/postgres/embedded-lifecycle.test.ts
+++ b/packages/core/src/__tests__/postgres/embedded-lifecycle.test.ts
@@ -34,6 +34,7 @@ import {
   EmbeddedStartTimeoutError,
   DEFAULT_START_TIMEOUT_MS,
   DEFAULT_EMBEDDED_POSTGRES_FLAGS,
+  defaultEmbeddedPostgresFlagsFor,
   isDataDirInitialized,
   isWindowsElevatedAdmin,
   normalizeMacosEmbeddedPostgresDylibSymlinks,
@@ -1124,6 +1125,24 @@ describe("embedded-lifecycle: shared-memory-safe postgres flags", () => {
     __setEmbeddedPostgresCtorForTests(RecordingEmbeddedPostgres as never);
   }
 
+  /*
+   * FNXC:PostgresEmbedded 2026-07-17-19:20:
+   * `shared_memory_type=mmap` is rejected by PostgreSQL on Windows (only value:
+   * `windows`) with a FATAL before the port opens — the mmap default broke every
+   * Windows embedded start and the v0.70.0/v0.70.1 Windows release smoke. The
+   * default flag set must stay platform-aware: no shared_memory_type override on
+   * win32, mmap everywhere else.
+   */
+  it.each(["win32", "darwin", "linux"] as const)("default flags are valid for %s", (platform) => {
+    const flags = defaultEmbeddedPostgresFlagsFor(platform);
+    if (platform === "win32") {
+      expect(flags).toEqual([]);
+    } else {
+      expect(flags).toEqual(["-c", "shared_memory_type=mmap"]);
+    }
+    expect(flags.join(" ")).not.toMatch(platform === "win32" ? /shared_memory_type/ : /shared_memory_type=(windows|sysv)/);
+  });
+
   it.each([
     ["omitted", undefined, [...DEFAULT_EMBEDDED_POSTGRES_FLAGS]],
     ["empty", [], [...DEFAULT_EMBEDDED_POSTGRES_FLAGS]],
diff --git a/packages/core/src/postgres/embedded-lifecycle.ts b/packages/core/src/postgres/embedded-lifecycle.ts
index f81083469a..40a2ee05a8 100644
--- a/packages/core/src/postgres/embedded-lifecycle.ts
+++ b/packages/core/src/postgres/embedded-lifecycle.ts
@@ -630,8 +630,21 @@ export const DEFAULT_EMBEDDED_DATABASE = "fusion";
  * so the zero-config cluster has been boot-smoke tested with a 64MB /dev/shm
  * lower bound. Defaults precede caller flags because PostgreSQL applies repeated
  * `-c key=value` settings last-wins, preserving an operator's explicit override.
+ *
+ * FNXC:PostgresEmbedded 2026-07-17-19:20:
+ * `mmap` is only valid on POSIX platforms. On Windows PostgreSQL accepts a single
+ * value, `windows`, and rejects `mmap` with FATAL `invalid value for parameter
+ * "shared_memory_type"` before opening the port — this broke every Windows
+ * embedded start (and the v0.70.0/v0.70.1 Windows release smoke) the day the
+ * mmap default landed. Windows needs no override at all, so the default flag set
+ * is empty there; the SysV exhaustion the mmap default fixes cannot occur on
+ * Windows anyway.
  */
-export const DEFAULT_EMBEDDED_POSTGRES_FLAGS = ["-c", "shared_memory_type=mmap"] as const;
+export function defaultEmbeddedPostgresFlagsFor(platform: NodeJS.Platform): readonly string[] {
+  return platform === "win32" ? [] : ["-c", "shared_memory_type=mmap"];
+}
+
+export const DEFAULT_EMBEDDED_POSTGRES_FLAGS = defaultEmbeddedPostgresFlagsFor(process.platform);
 
 /**
  * FNXC:PostgresEmbedded 2026-06-24-09:05: