diff --git a/src/hooks/prometheus-md-only/constants.ts b/src/hooks/prometheus-md-only/constants.ts
index b25db57f4..0c24b0498 100644
--- a/src/hooks/prometheus-md-only/constants.ts
+++ b/src/hooks/prometheus-md-only/constants.ts
@@ -4,7 +4,7 @@ export const PROMETHEUS_AGENTS = ["Prometheus (Planner)"]
 
 export const ALLOWED_EXTENSIONS = [".md"]
 
-export const ALLOWED_PATH_PREFIX = ".sisyphus/"
+export const ALLOWED_PATH_PREFIX = ".sisyphus"
 
 export const BLOCKED_TOOLS = ["Write", "Edit", "write", "edit"]
 
diff --git a/src/hooks/prometheus-md-only/index.test.ts b/src/hooks/prometheus-md-only/index.test.ts
index 28ae3261b..ac0c93c96 100644
--- a/src/hooks/prometheus-md-only/index.test.ts
+++ b/src/hooks/prometheus-md-only/index.test.ts
@@ -70,7 +70,7 @@ describe("prometheus-md-only", () => {
         callID: "call-1",
       }
       const output = {
-        args: { filePath: "/project/.sisyphus/plans/work-plan.md" },
+        args: { filePath: "/tmp/test/.sisyphus/plans/work-plan.md" },
       }
 
       // #when / #then
@@ -295,4 +295,136 @@ describe("prometheus-md-only", () => {
       ).resolves.toBeUndefined()
     })
   })
+
+  describe("cross-platform path validation", () => {
+    beforeEach(() => {
+      setupMessageStorage(TEST_SESSION_ID, "Prometheus (Planner)")
+    })
+
+    test("should allow Windows-style backslash paths under .sisyphus/", async () => {
+      // #given
+      const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+      const input = {
+        tool: "Write",
+        sessionID: TEST_SESSION_ID,
+        callID: "call-1",
+      }
+      const output = {
+        args: { filePath: ".sisyphus\\plans\\work-plan.md" },
+      }
+
+      // #when / #then
+      await expect(
+        hook["tool.execute.before"](input, output)
+      ).resolves.toBeUndefined()
+    })
+
+    test("should allow mixed separator paths under .sisyphus/", async () => {
+      // #given
+      const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+      const input = {
+        tool: "Write",
+        sessionID: TEST_SESSION_ID,
+        callID: "call-1",
+      }
+      const output = {
+        args: { filePath: ".sisyphus\\plans/work-plan.MD" },
+      }
+
+      // #when / #then
+      await expect(
+        hook["tool.execute.before"](input, output)
+      ).resolves.toBeUndefined()
+    })
+
+    test("should allow uppercase .MD extension", async () => {
+      // #given
+      const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+      const input = {
+        tool: "Write",
+        sessionID: TEST_SESSION_ID,
+        callID: "call-1",
+      }
+      const output = {
+        args: { filePath: ".sisyphus/plans/work-plan.MD" },
+      }
+
+      // #when / #then
+      await expect(
+        hook["tool.execute.before"](input, output)
+      ).resolves.toBeUndefined()
+    })
+
+    test("should block paths outside workspace root even if containing .sisyphus", async () => {
+      // #given
+      const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+      const input = {
+        tool: "Write",
+        sessionID: TEST_SESSION_ID,
+        callID: "call-1",
+      }
+      const output = {
+        args: { filePath: "/other/project/.sisyphus/plans/x.md" },
+      }
+
+      // #when / #then
+      await expect(
+        hook["tool.execute.before"](input, output)
+      ).rejects.toThrow("can only write/edit .md files inside .sisyphus/")
+    })
+
+    test("should block nested .sisyphus directories", async () => {
+      // #given
+      const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+      const input = {
+        tool: "Write",
+        sessionID: TEST_SESSION_ID,
+        callID: "call-1",
+      }
+      const output = {
+        args: { filePath: "src/.sisyphus/plans/x.md" },
+      }
+
+      // #when / #then
+      await expect(
+        hook["tool.execute.before"](input, output)
+      ).rejects.toThrow("can only write/edit .md files inside .sisyphus/")
+    })
+
+    test("should block path traversal attempts", async () => {
+      // #given
+      const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+      const input = {
+        tool: "Write",
+        sessionID: TEST_SESSION_ID,
+        callID: "call-1",
+      }
+      const output = {
+        args: { filePath: ".sisyphus/../secrets.md" },
+      }
+
+      // #when / #then
+      await expect(
+        hook["tool.execute.before"](input, output)
+      ).rejects.toThrow("can only write/edit .md files inside .sisyphus/")
+    })
+
+    test("should allow case-insensitive .SISYPHUS directory", async () => {
+      // #given
+      const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+      const input = {
+        tool: "Write",
+        sessionID: TEST_SESSION_ID,
+        callID: "call-1",
+      }
+      const output = {
+        args: { filePath: ".SISYPHUS/plans/work-plan.md" },
+      }
+
+      // #when / #then
+      await expect(
+        hook["tool.execute.before"](input, output)
+      ).resolves.toBeUndefined()
+    })
+  })
 })
diff --git a/src/hooks/prometheus-md-only/index.ts b/src/hooks/prometheus-md-only/index.ts
index b0d9c45cc..5a0d7f990 100644
--- a/src/hooks/prometheus-md-only/index.ts
+++ b/src/hooks/prometheus-md-only/index.ts
@@ -1,16 +1,48 @@
 import type { PluginInput } from "@opencode-ai/plugin"
 import { existsSync, readdirSync } from "node:fs"
-import { join } from "node:path"
+import { join, resolve, relative, isAbsolute } from "node:path"
 import { HOOK_NAME, PROMETHEUS_AGENTS, ALLOWED_EXTENSIONS, ALLOWED_PATH_PREFIX, BLOCKED_TOOLS, PLANNING_CONSULT_WARNING } from "./constants"
 import { findNearestMessageWithFields, MESSAGE_STORAGE } from "../../features/hook-message-injector"
 import { log } from "../../shared/logger"
 
 export * from "./constants"
 
-function isAllowedFile(filePath: string): boolean {
-  const hasAllowedExtension = ALLOWED_EXTENSIONS.some(ext => filePath.endsWith(ext))
-  const isInAllowedPath = filePath.includes(ALLOWED_PATH_PREFIX)
-  return hasAllowedExtension && isInAllowedPath
+/**
+ * Cross-platform path validator for Prometheus file writes.
+ * Uses path.resolve/relative instead of string matching to handle:
+ * - Windows backslashes (e.g., .sisyphus\\plans\\x.md)
+ * - Mixed separators (e.g., .sisyphus\\plans/x.md)
+ * - Case-insensitive directory/extension matching
+ * - Workspace confinement (blocks paths outside root or via traversal)
+ */
+function isAllowedFile(filePath: string, workspaceRoot: string): boolean {
+  // 1. Resolve to absolute path
+  const resolved = resolve(workspaceRoot, filePath)
+
+  // 2. Get relative path from workspace root
+  const rel = relative(workspaceRoot, resolved)
+
+  // 3. Reject if escapes root (starts with ".." or is absolute)
+  if (rel.startsWith("..") || isAbsolute(rel)) {
+    return false
+  }
+
+  // 4. Split by both separators and check first segment matches ALLOWED_PATH_PREFIX (case-insensitive)
+  // Guard: if rel is empty (filePath === workspaceRoot), segments[0] would be "" — reject
+  const segments = rel.split(/[/\\]/)
+  if (!segments[0] || segments[0].toLowerCase() !== ALLOWED_PATH_PREFIX.toLowerCase()) {
+    return false
+  }
+
+  // 5. Check extension matches one of ALLOWED_EXTENSIONS (case-insensitive)
+  const hasAllowedExtension = ALLOWED_EXTENSIONS.some(
+    ext => resolved.toLowerCase().endsWith(ext.toLowerCase())
+  )
+  if (!hasAllowedExtension) {
+    return false
+  }
+
+  return true
 }
 
 function getMessageDir(sessionID: string): string | null {
@@ -35,7 +67,7 @@ function getAgentFromSession(sessionID: string): string | undefined {
   return findNearestMessageWithFields(messageDir)?.agent
 }
 
-export function createPrometheusMdOnlyHook(_ctx: PluginInput) {
+export function createPrometheusMdOnlyHook(ctx: PluginInput) {
   return {
     "tool.execute.before": async (
       input: { tool: string; sessionID: string; callID: string },
@@ -72,7 +104,7 @@ export function createPrometheusMdOnlyHook(_ctx: PluginInput) {
         return
       }
 
-      if (!isAllowedFile(filePath)) {
+      if (!isAllowedFile(filePath, ctx.directory)) {
         log(`[${HOOK_NAME}] Blocked: Prometheus can only write to .sisyphus/*.md`, {
           sessionID: input.sessionID,
           tool: toolName,