diff --git a/src/hooks/write-existing-file-guard/hook.ts b/src/hooks/write-existing-file-guard/hook.ts
index bdaf5cad8..ab7bd9aef 100644
--- a/src/hooks/write-existing-file-guard/hook.ts
+++ b/src/hooks/write-existing-file-guard/hook.ts
@@ -76,7 +76,15 @@ export function isOverwriteEnabled(value: boolean | string | undefined): boolean
 export function createWriteExistingFileGuardHook(ctx: PluginInput): Hooks {
   const readPermissionsBySession = new Map<string, Set<string>>()
   const sessionLastAccess = new Map<string, number>()
-  const canonicalSessionRoot = toCanonicalPath(resolveInputPath(ctx, ctx.directory))
+  let canonicalSessionRoot: string | undefined
+
+  function getCanonicalSessionRoot(): string {
+    if (!canonicalSessionRoot) {
+      canonicalSessionRoot = toCanonicalPath(resolveInputPath(ctx, ctx.directory))
+    }
+
+    return canonicalSessionRoot
+  }
 
   return {
     "tool.execute.before": async (input, output) => {
@@ -86,7 +94,7 @@ export function createWriteExistingFileGuardHook(ctx: PluginInput): Hooks {
         output,
         readPermissionsBySession,
         sessionLastAccess,
-        canonicalSessionRoot,
+        getCanonicalSessionRoot,
         maxTrackedSessions: MAX_TRACKED_SESSIONS,
       })
     },
diff --git a/src/hooks/write-existing-file-guard/lazy-canonical-path-init.test.ts b/src/hooks/write-existing-file-guard/lazy-canonical-path-init.test.ts
new file mode 100644
index 000000000..0f1d4bb88
--- /dev/null
+++ b/src/hooks/write-existing-file-guard/lazy-canonical-path-init.test.ts
@@ -0,0 +1,65 @@
+import { afterEach, beforeEach, describe, expect, mock, test } from "bun:test"
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs"
+import { tmpdir } from "node:os"
+import { join } from "node:path"
+
+const realFs = await import("node:fs")
+
+const existsSyncMock = mock(realFs.existsSync)
+const realpathNativeMock = mock(realFs.realpathSync.native)
+
+mock.module("fs", () => ({
+  ...realFs,
+  existsSync: existsSyncMock,
+  realpathSync: {
+    ...realFs.realpathSync,
+    native: realpathNativeMock,
+  },
+}))
+
+const { createWriteExistingFileGuardHook } = await import("./index")
+
+describe("createWriteExistingFileGuardHook", () => {
+  let tempDir = ""
+
+  beforeEach(() => {
+    // given
+    tempDir = mkdtempSync(join(tmpdir(), "write-existing-file-guard-lazy-"))
+    mkdirSync(tempDir, { recursive: true })
+    existsSyncMock.mockClear()
+    realpathNativeMock.mockClear()
+  })
+
+  afterEach(() => {
+    rmSync(tempDir, { recursive: true, force: true })
+  })
+
+  test("#given hook factory #when created #then defers fs canonical path calls until first tool invocation", async () => {
+    // given
+    const existingFile = join(tempDir, "existing.txt")
+    writeFileSync(existingFile, "content")
+
+    // when
+    const hook = createWriteExistingFileGuardHook({ directory: tempDir } as never)
+
+    // then
+    expect(existsSyncMock).toHaveBeenCalledTimes(0)
+    expect(realpathNativeMock).toHaveBeenCalledTimes(0)
+
+    // when
+    await expect(
+      hook["tool.execute.before"]?.(
+        {
+          tool: "write",
+          sessionID: "ses_lazy",
+          callID: "call_lazy",
+        } as never,
+        { args: { filePath: existingFile, content: "updated" } } as never,
+      ),
+    ).rejects.toThrow("File already exists. Use edit tool instead.")
+
+    // then
+    expect(existsSyncMock).toHaveBeenCalledTimes(3)
+    expect(realpathNativeMock).toHaveBeenCalledTimes(2)
+  })
+})
diff --git a/src/hooks/write-existing-file-guard/tool-execute-before-handler.ts b/src/hooks/write-existing-file-guard/tool-execute-before-handler.ts
index 25eebbda3..848238a8a 100644
--- a/src/hooks/write-existing-file-guard/tool-execute-before-handler.ts
+++ b/src/hooks/write-existing-file-guard/tool-execute-before-handler.ts
@@ -90,10 +90,10 @@ export async function handleWriteExistingFileGuardToolExecuteBefore(params: {
   output: { args?: unknown }
   readPermissionsBySession: Map<string, Set<string>>
   sessionLastAccess: Map<string, number>
-  canonicalSessionRoot: string
+  getCanonicalSessionRoot: () => string
   maxTrackedSessions: number
 }): Promise<void> {
-  const { ctx, input, output, readPermissionsBySession, sessionLastAccess, canonicalSessionRoot, maxTrackedSessions } = params
+  const { ctx, input, output, readPermissionsBySession, sessionLastAccess, getCanonicalSessionRoot, maxTrackedSessions } = params
   const toolName = input.tool?.toLowerCase()
   if (toolName !== "write" && toolName !== "read") {
     return
@@ -107,6 +107,7 @@ export async function handleWriteExistingFileGuardToolExecuteBefore(params: {
   }
 
   const resolvedPath = resolveInputPath(ctx, filePath)
+  const canonicalSessionRoot = getCanonicalSessionRoot()
   const canonicalPath = toCanonicalPath(resolvedPath)
   if (!isPathInsideDirectory(canonicalPath, canonicalSessionRoot)) {
     return