diff --git a/src/adapters/command-code-project-context.ts b/src/adapters/command-code-project-context.ts
index 50b2b5e9cf9..c79eff473c1 100644
--- a/src/adapters/command-code-project-context.ts
+++ b/src/adapters/command-code-project-context.ts
@@ -1,6 +1,6 @@
 import { constants } from "node:fs";
 import { lstat, open, opendir, realpath, stat } from "node:fs/promises";
-import { isAbsolute, join, relative, sep } from "node:path";
+import { join, sep } from "node:path";
 
 export type CommandCodeProjectContext = {
   memory: string;
@@ -140,14 +140,11 @@ async function canonicalPath(candidate: string, deadline: number, scope: ScanSco
   }
 }
 
-function normalizePathIdentity(path: string): string {
-  return process.platform === "win32" ? path.toLowerCase() : path;
-}
-
-/** Relative paths also work when cwd is a filesystem root; other volumes remain outside. */
-export function isContainedCanonicalPath(cwdCanonical: string, fileCanonical: string): boolean {
-  const rel = relative(normalizePathIdentity(cwdCanonical), normalizePathIdentity(fileCanonical));
-  return rel === "" || (rel !== ".." && !rel.startsWith(`..${sep}`) && !isAbsolute(rel));
+/** Exact canonical prefixes preserve case-sensitive Windows directory identities. */
+export function isContainedCanonicalPath(cwdCanonical: string, fileCanonical: string, separator = sep): boolean {
+  if (fileCanonical === cwdCanonical) return true;
+  const prefix = cwdCanonical.endsWith(separator) ? cwdCanonical : `${cwdCanonical}${separator}`;
+  return fileCanonical.startsWith(prefix);
 }
 
 async function confinedCanonicalPath(
@@ -199,8 +196,7 @@ async function openedFileIsConfined(
   const resolved = await withinDeadline(() => realpath(path), deadline, scope);
   // The input path was canonical before open. A changed intermediate symlink changes this
   // result even though O_NOFOLLOW protects only the final component on macOS and Linux.
-  if (!isContainedCanonicalPath(cwdCanonical, resolved)
-    || normalizePathIdentity(resolved) !== normalizePathIdentity(path)) return false;
+  if (!isContainedCanonicalPath(cwdCanonical, resolved) || resolved !== path) return false;
   const resolvedInfo = await withinDeadline(() => lstat(resolved), deadline, scope);
   return resolvedInfo.isFile() && opened.dev === resolvedInfo.dev && opened.ino === resolvedInfo.ino;
 }
diff --git a/structure/providers-and-adapters.md b/structure/providers-and-adapters.md
index 24d10b72046..53e94ff55e0 100644
--- a/structure/providers-and-adapters.md
+++ b/structure/providers-and-adapters.md
@@ -257,8 +257,9 @@ and `skills`, and leaves existing `config` metadata unchanged without invoking
 `src/adapters/command-code-project-context.ts`. The loader reads only the proxy process
 working directory's `AGENTS.md`, `.commandcode/taste/taste.md`, and immediate child
 `SKILL.md` files under `.commandcode/skills`, `.agents/skills`, and `.pi/skills`.
-Asynchronous path checks share one deadline and use relative-path containment even at
-filesystem roots. On macOS/Linux a nonblocking, no-follow open is followed by file-inode
+Asynchronous path checks share one deadline and require an exact, case-preserving canonical
+path prefix, including at filesystem roots. On macOS/Linux a nonblocking, no-follow open is
+followed by file-inode
 comparison and fresh canonical containment checks before and after reading; an intermediate
 directory replaced by an outside symlink cannot publish its file contents. Windows applies
 the path and identity checks as best effort. Every visited directory entry consumes the
diff --git a/tests/providers/command-code-project-context.test.ts b/tests/providers/command-code-project-context.test.ts
index 3774cee8c3b..cbb246aa114 100644
--- a/tests/providers/command-code-project-context.test.ts
+++ b/tests/providers/command-code-project-context.test.ts
@@ -85,7 +85,7 @@ describe("loadCommandCodeProjectContext", () => {
     expect(result).toEqual(EMPTY_COMMAND_CODE_PROJECT_CONTEXT);
   });
 
-  test("relative containment includes descendants of a filesystem root", () => {
+  test("canonical containment includes descendants of a filesystem root", () => {
     const fsRoot = parse(tmpdir()).root;
     expect(isContainedCanonicalPath(fsRoot, join(fsRoot, "AGENTS.md"))).toBe(true);
     const nested = join(fsRoot, "project");
@@ -93,6 +93,11 @@ describe("loadCommandCodeProjectContext", () => {
     expect(isContainedCanonicalPath(nested, join(fsRoot, "project-sibling", "SKILL.md"))).toBe(false);
   });
 
+  test("Windows containment preserves case-sensitive directory identities", () => {
+    expect(isContainedCanonicalPath("C:\\work\\project", "C:\\work\\project\\AGENTS.md", "\\")).toBe(true);
+    expect(isContainedCanonicalPath("C:\\work\\project", "C:\\work\\PROJECT\\secret.txt", "\\")).toBe(false);
+  });
+
   test("stalled asynchronous path metadata obeys the overall deadline", async () => {
     const root = makeTempDir("ocx-cc-ctx-metadata-timeout-");
     const agentsPath = join(root, "AGENTS.md");
@@ -699,6 +704,35 @@ describe("loadCommandCodeProjectContext", () => {
     }
   });
 
+  test("rejects a file whose post-open canonical path changes case", async () => {
+    // Windows-only delta: the post-open check compares the resolved path to the
+    // pre-open canonical path byte-for-byte now. Containment alone cannot catch a
+    // case-only rename because the parent prefix stays identical, so a filename
+    // whose realpath changes case between canonicalization and open is refused.
+    if (process.platform !== "win32") return;
+    const root = makeTempDir("ocx-cc-ctx-case-swap-");
+    const skillFile = join(root, ".commandcode", "skills", "case-skill", "SKILL.md");
+    try {
+      writeSkill(root, ".commandcode/skills", "case-skill", "case body");
+      setCommandCodeBeforeOpenForTests(path => {
+        if (path !== skillFile) return;
+        realpathMock.mockImplementation(async (p, opts) => {
+          const resolved = await realRealpath(p, opts);
+          return typeof resolved === "string" && resolved === skillFile
+            ? resolved.replace(/SKILL\.md$/, "skill.md")
+            : resolved;
+        });
+      });
+      const result = await loadCommandCodeProjectContext(root);
+      expect(result.skills).toBeNull();
+      expect(JSON.stringify(result)).not.toContain("case body");
+    } finally {
+      setCommandCodeBeforeOpenForTests(undefined);
+      realpathMock.mockImplementation(realRealpath);
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
+
   test("omits unreadable AGENTS.md when chmod is enforced", async () => {
     if (process.platform === "win32") return;
     const root = makeTempDir("ocx-cc-ctx-unreadable-");