diff --git a/src/hooks/sisyphus-orchestrator/index.test.ts b/src/hooks/sisyphus-orchestrator/index.test.ts
index c5e1f837..73e7d977 100644
--- a/src/hooks/sisyphus-orchestrator/index.test.ts
+++ b/src/hooks/sisyphus-orchestrator/index.test.ts
@@ -506,6 +506,90 @@ describe("sisyphus-orchestrator hook", () => {
         // #then
         expect(output.output).toBe(originalOutput)
       })
+
+      describe("cross-platform path validation (Windows support)", () => {
+        test("should NOT append reminder when orchestrator writes inside .sisyphus\\ (Windows backslash)", async () => {
+          // #given
+          const hook = createSisyphusOrchestratorHook(createMockPluginInput())
+          const originalOutput = "File written successfully"
+          const output = {
+            title: "Write",
+            output: originalOutput,
+            metadata: { filePath: ".sisyphus\\plans\\work-plan.md" },
+          }
+
+          // #when
+          await hook["tool.execute.after"](
+            { tool: "Write", sessionID: ORCHESTRATOR_SESSION },
+            output
+          )
+
+          // #then
+          expect(output.output).toBe(originalOutput)
+          expect(output.output).not.toContain("DELEGATION REQUIRED")
+        })
+
+        test("should NOT append reminder when orchestrator writes inside .sisyphus with mixed separators", async () => {
+          // #given
+          const hook = createSisyphusOrchestratorHook(createMockPluginInput())
+          const originalOutput = "File written successfully"
+          const output = {
+            title: "Write",
+            output: originalOutput,
+            metadata: { filePath: ".sisyphus\\plans/work-plan.md" },
+          }
+
+          // #when
+          await hook["tool.execute.after"](
+            { tool: "Write", sessionID: ORCHESTRATOR_SESSION },
+            output
+          )
+
+          // #then
+          expect(output.output).toBe(originalOutput)
+          expect(output.output).not.toContain("DELEGATION REQUIRED")
+        })
+
+        test("should NOT append reminder for absolute Windows path inside .sisyphus\\", async () => {
+          // #given
+          const hook = createSisyphusOrchestratorHook(createMockPluginInput())
+          const originalOutput = "File written successfully"
+          const output = {
+            title: "Write",
+            output: originalOutput,
+            metadata: { filePath: "C:\\Users\\test\\project\\.sisyphus\\plans\\x.md" },
+          }
+
+          // #when
+          await hook["tool.execute.after"](
+            { tool: "Write", sessionID: ORCHESTRATOR_SESSION },
+            output
+          )
+
+          // #then
+          expect(output.output).toBe(originalOutput)
+          expect(output.output).not.toContain("DELEGATION REQUIRED")
+        })
+
+        test("should append reminder for Windows path outside .sisyphus\\", async () => {
+          // #given
+          const hook = createSisyphusOrchestratorHook(createMockPluginInput())
+          const output = {
+            title: "Write",
+            output: "File written successfully",
+            metadata: { filePath: "C:\\Users\\test\\project\\src\\code.ts" },
+          }
+
+          // #when
+          await hook["tool.execute.after"](
+            { tool: "Write", sessionID: ORCHESTRATOR_SESSION },
+            output
+          )
+
+          // #then
+          expect(output.output).toContain("DELEGATION REQUIRED")
+        })
+      })
     })
   })
 
diff --git a/src/hooks/sisyphus-orchestrator/index.ts b/src/hooks/sisyphus-orchestrator/index.ts
index 570a6747..f50a0e2c 100644
--- a/src/hooks/sisyphus-orchestrator/index.ts
+++ b/src/hooks/sisyphus-orchestrator/index.ts
@@ -14,7 +14,14 @@ import type { BackgroundManager } from "../../features/background-agent"
 
 export const HOOK_NAME = "sisyphus-orchestrator"
 
-const ALLOWED_PATH_PREFIX = ".sisyphus/"
+/**
+ * Cross-platform check if a path is inside .sisyphus/ directory.
+ * Handles both forward slashes (Unix) and backslashes (Windows).
+ */
+function isSisyphusPath(filePath: string): boolean {
+  return /\.sisyphus[/\\]/.test(filePath)
+}
+
 const WRITE_EDIT_TOOLS = ["Write", "Edit", "write", "edit"]
 
 const DIRECT_WORK_REMINDER = `
@@ -549,7 +556,7 @@ export function createSisyphusOrchestratorHook(
       // Check Write/Edit tools for orchestrator - inject strong warning
       if (WRITE_EDIT_TOOLS.includes(input.tool)) {
         const filePath = (output.args.filePath ?? output.args.path ?? output.args.file) as string | undefined
-        if (filePath && !filePath.includes(ALLOWED_PATH_PREFIX)) {
+        if (filePath && !isSisyphusPath(filePath)) {
           // Store filePath for use in tool.execute.after
           if (input.callID) {
             pendingFilePaths.set(input.callID, filePath)
@@ -593,7 +600,7 @@ export function createSisyphusOrchestratorHook(
         if (!filePath) {
           filePath = output.metadata?.filePath as string | undefined
         }
-        if (filePath && !filePath.includes(ALLOWED_PATH_PREFIX)) {
+        if (filePath && !isSisyphusPath(filePath)) {
           output.output = (output.output || "") + DIRECT_WORK_REMINDER
           log(`[${HOOK_NAME}] Direct work reminder appended`, {
             sessionID: input.sessionID,