diff --git a/lib/bin-context.ts b/lib/bin-context.ts
index 28021b56a..3260951ac 100644
--- a/lib/bin-context.ts
+++ b/lib/bin-context.ts
@@ -6,9 +6,9 @@
  */
 
 import { spawnSync } from "child_process";
-import { existsSync, mkdirSync, readFileSync, renameSync, writeFileSync } from "fs";
+import { existsSync, mkdirSync, readFileSync, renameSync, statSync, writeFileSync } from "fs";
 import { homedir } from "os";
-import { basename, join } from "path";
+import { basename, dirname, join } from "path";
 
 /** Keep the slug inside the [a-zA-Z0-9._-] alphabet gstack-slug promises (`tr -cd`). */
 function sanitizeSlug(s: string): string {
@@ -28,42 +28,125 @@ export function toMsysPath(p: string): string {
   return drive ? `/${drive[1].toLowerCase()}${body}` : body;
 }
 
+/** `-f` in bash terms: a regular file (following symlinks), never a directory. */
+function isFile(p: string): boolean {
+  try {
+    return statSync(p).isFile();
+  } catch {
+    return false;
+  }
+}
+
+// Marker tiers mirror bin/gstack-slug's `_outermost_project_root` exactly.
+// STRONG = canonical version-control / language project files ("this directory
+// is a real project of its own"); .git is checked separately because it can be
+// a directory (normal repo) or a file (worktree / submodule pointer).
+// WEAK = content-only project signals (markdown bundles, asset collections).
+const STRONG_FILE_MARKERS = [".project.yaml", "package.json", "pyproject.toml", "Cargo.toml", "Gemfile", "go.mod"];
+const WEAK_FILE_MARKERS = ["README.md", "README", "README.rst", "LICENSE", "LICENSE.md"];
+
+/**
+ * Native port of bin/gstack-slug's `_outermost_project_root` (:77-113): walk UP
+ * from `startDir` tracking the OUTERMOST ancestor holding a strong marker and
+ * the outermost holding a weak marker. Outermost STRONG wins; else outermost
+ * WEAK; else "". Build/deploy artifacts (.vercel, node_modules, dist, ...) are
+ * deliberately NOT markers, so they can't establish a phantom project root.
+ *
+ * Termination mirrors the bash fix for windows-free-tests: break on dirname's
+ * FIXED POINT (drive roots `C:\`, relative `.`, UNC `//srv` never reach the
+ * literal "/"), with a 64-depth belt-and-braces cap. Exported for the
+ * hostile-path termination tests.
+ */
+export function outermostProjectRoot(startDir: string): string {
+  let dir = startDir;
+  let outermostStrong = "";
+  let outermostWeak = "";
+  let depth = 0;
+  while (dir && dir !== "/" && depth < 64) {
+    if (existsSync(join(dir, ".git")) || STRONG_FILE_MARKERS.some((m) => isFile(join(dir, m)))) {
+      outermostStrong = dir;
+    } else if (WEAK_FILE_MARKERS.some((m) => isFile(join(dir, m)))) {
+      outermostWeak = dir;
+    }
+    const parent = dirname(dir);
+    if (parent === dir) break; // dirname fixed point (C:\, ., //srv)
+    dir = parent;
+    depth += 1;
+  }
+  // Strong markers win over weak; either wins over nothing.
+  return outermostStrong || outermostWeak;
+}
+
 /**
  * Native port of bin/gstack-slug's resolution order, used when that script cannot be
- * spawned (see resolveSlug). Same three steps, same alphabet, same cache file — so
- * this and the shell path always agree. They must: the bins WRITE using this, while
- * the Context Recovery preamble READS using the script.
+ * spawned (see resolveSlug). Same steps, same alphabet, same cache file — so this and
+ * the shell path always agree. They must: the bins WRITE using this, while the
+ * Context Recovery preamble READS using the script.
+ *
+ * Resolution order (parity with the bash script, pinned by
+ * test/bin-context-windows-slug.test.ts against test/gstack-slug-cwd-walk-up.test.ts):
+ *   0. $GSTACK_PROJECT_SLUG env override — wins over everything, never cached.
+ *   1. Walk UP to the OUTERMOST project root (see outermostProjectRoot). Without
+ *      the walk, a nested/vendored repo derived its slug from the INNERMOST
+ *      `git remote get-url origin`, splitting the store the bash side keeps whole.
+ *   2. Cached slug is sticky — EXCEPT the provable old-bug shape (#1125): cached
+ *      value equals basename(cwd) while the walk-up says cwd is NOT the project
+ *      root; that cache came from the pre-walk-up resolver, so recompute and heal.
+ *   3. Git remote AT THE PROJECT ROOT: [:/]<owner>/<repo>[.git] → owner-repo.
+ *   4. Project root's basename; else basename(cwd) for plain non-project folders.
  */
 export function slugFromEnvironment(gstackHome?: string, cwd: string = process.cwd()): string {
   const home = gstackHome || process.env.GSTACK_HOME || join(homedir(), ".gstack");
   const cacheDir = join(home, "slug-cache");
   const cacheFile = join(cacheDir, toMsysPath(cwd).replace(/\//g, "_"));
 
+  // 0. explicit env override — per-invocation escape hatch, never persisted
+  //    (caching it would rebind THIS cwd's slug for every later env-less run).
+  const envSlug = sanitizeSlug((process.env.GSTACK_PROJECT_SLUG || "").trim());
+  if (envSlug) return envSlug;
+
+  // 1. outermost project root along the cwd ancestor chain (may be "").
+  const projectRoot = outermostProjectRoot(cwd);
+
   let slug = "";
-  // 1. cached slug wins (guarantees consistency across sessions)
+  // 2. cached slug is sticky (#2212), except the old-bug shape (#1125).
   if (existsSync(cacheFile)) {
     try {
-      slug = sanitizeSlug(readFileSync(cacheFile, "utf-8").trim());
+      const cached = sanitizeSlug(readFileSync(cacheFile, "utf-8").trim());
+      const pwdBase = sanitizeSlug(basename(cwd));
+      const oldBugShape = cached === pwdBase && projectRoot !== "" && projectRoot !== cwd;
+      if (cached && !oldBugShape) slug = cached;
     } catch {
       slug = "";
     }
   }
-  // 2. else derive from the git remote: [:/]<owner>/<repo>[.git] → owner-repo
-  if (!slug) {
-    const r = spawnSync("git", ["remote", "get-url", "origin"], { encoding: "utf-8", cwd });
+  // 3. derive from the project root's git remote (a subdir without its own
+  //    remote inherits the parent's — same as `git -C "$PROJECT_ROOT"`).
+  if (!slug && projectRoot) {
+    const r = spawnSync("git", ["-C", projectRoot, "remote", "get-url", "origin"], { encoding: "utf-8" });
     const m = (r.stdout || "").trim().match(/[:/]([^/]+\/[^/]+?)(?:\.git)?$/);
     if (m) slug = sanitizeSlug(m[1].replace(/\//g, "-"));
   }
-  // 3. else the directory name
+  // 4. project root's basename, else pwd basename for plain folders.
+  if (!slug && projectRoot) slug = sanitizeSlug(basename(projectRoot));
   if (!slug) slug = sanitizeSlug(basename(cwd));
   if (!slug) return "unknown";
 
-  // 4. cache it, as gstack-slug does — atomic, and failures stay silent (`|| true`)
+  // 5. cache it, as gstack-slug does — atomic, self-healing (only rewrites when
+  //    the value changed — single-shot, key-local), and failures stay silent.
   try {
-    mkdirSync(cacheDir, { recursive: true });
-    const tmp = `${cacheFile}.tmp.${process.pid}`;
-    writeFileSync(tmp, slug, "utf-8");
-    renameSync(tmp, cacheFile);
+    let current = "";
+    try {
+      current = readFileSync(cacheFile, "utf-8");
+    } catch {
+      // no cache yet — write below
+    }
+    if (current !== slug) {
+      mkdirSync(cacheDir, { recursive: true });
+      const tmp = `${cacheFile}.tmp.${process.pid}`;
+      writeFileSync(tmp, slug, "utf-8");
+      renameSync(tmp, cacheFile);
+    }
   } catch {
     // best-effort cache; a miss only costs a re-derive on the next call
   }
diff --git a/test/bin-context-windows-slug.test.ts b/test/bin-context-windows-slug.test.ts
index 47f05b8da..be7291d8d 100644
--- a/test/bin-context-windows-slug.test.ts
+++ b/test/bin-context-windows-slug.test.ts
@@ -6,6 +6,7 @@ import * as path from "path";
 import {
   toMsysPath,
   slugFromEnvironment,
+  outermostProjectRoot,
   resolveSlug,
   NEEDS_NATIVE_SLUG_ON_WINDOWS,
 } from "../lib/bin-context";
@@ -14,8 +15,21 @@ const ROOT = path.resolve(import.meta.dir, "..");
 const read = (rel: string) => fs.readFileSync(path.join(ROOT, rel), "utf-8");
 
 let tmp: string;
-beforeEach(() => { tmp = fs.mkdtempSync(path.join(os.tmpdir(), "gstack-slug-")); });
-afterEach(() => { try { fs.rmSync(tmp, { recursive: true, force: true }); } catch {} });
+let savedEnvSlug: string | undefined;
+beforeEach(() => {
+  // realpathSync so the native cwd matches what the bash script's `pwd` reports
+  // (macOS: /var/folders/... is a symlink to /private/var/folders/...).
+  tmp = fs.realpathSync(fs.mkdtempSync(path.join(os.tmpdir(), "gstack-slug-")));
+  // An ambient GSTACK_PROJECT_SLUG (leaked from an operator shell or a sibling
+  // test in a shared-process shard) would override every derivation under test.
+  savedEnvSlug = process.env.GSTACK_PROJECT_SLUG;
+  delete process.env.GSTACK_PROJECT_SLUG;
+});
+afterEach(() => {
+  try { fs.rmSync(tmp, { recursive: true, force: true }); } catch {}
+  if (savedEnvSlug === undefined) delete process.env.GSTACK_PROJECT_SLUG;
+  else process.env.GSTACK_PROJECT_SLUG = savedEnvSlug;
+});
 
 /**
  * Windows cannot exec bin/gstack-slug -- a `#!/usr/bin/env bash` script with no file
@@ -111,3 +125,183 @@ describe("the fallback stays win32-gated", () => {
     }
   });
 });
+
+/**
+ * Walk-up parity: the native fallback must resolve the same OUTERMOST project
+ * root as bin/gstack-slug's `_outermost_project_root` (see
+ * test/gstack-slug-cwd-walk-up.test.ts for the bash-side pins). Before this
+ * port, the native path derived the slug from `git remote get-url origin` in
+ * cwd — the INNERMOST repo — so a Windows session inside a nested/vendored
+ * repo or an artifact-only subdir filed its state under a different slug than
+ * every bash-side consumer.
+ *
+ * Each scenario is run through BOTH implementations on the same fixture
+ * (separate GSTACK_HOMEs so neither reads the other's cache) and pinned to the
+ * same expected slug. The bash leg is skipped on win32, where bash isn't
+ * reliably spawnable — the native leg still pins the ported semantics there.
+ */
+describe("walk-up parity with bin/gstack-slug (outermost project root)", () => {
+  const SCRIPT = path.join(ROOT, "bin", "gstack-slug");
+  const HAS_BASH = process.platform !== "win32";
+
+  function bashSlug(cwd: string, extraEnv: Record<string, string> = {}): string {
+    const env: Record<string, string | undefined> = {
+      ...process.env,
+      HOME: path.join(tmp, "bash-home"),
+      GSTACK_HOME: path.join(tmp, "bash-home", ".gstack"),
+    };
+    delete env.GSTACK_PROJECT_SLUG; // only set when a scenario passes it explicitly
+    Object.assign(env, extraEnv);
+    const r = spawnSync("bash", [SCRIPT], { cwd, env, encoding: "utf-8", timeout: 10_000 });
+    const m = (r.stdout || "").match(/^SLUG=([^\n]*)$/m);
+    return m ? m[1] : "";
+  }
+
+  const nativeHome = () => path.join(tmp, "native-home");
+
+  /** Assert native === expected, and bash === expected where bash is available. */
+  function expectBoth(cwd: string, expected: string) {
+    expect(slugFromEnvironment(nativeHome(), cwd)).toBe(expected);
+    if (HAS_BASH) expect(bashSlug(cwd)).toBe(expected);
+  }
+
+  test("AC-1: .git at root, artifact-only subdir — slug is the ROOT basename", () => {
+    const projectRoot = path.join(tmp, "loadout");
+    const siteSubdir = path.join(projectRoot, "site");
+    fs.mkdirSync(path.join(projectRoot, ".git"), { recursive: true });
+    fs.mkdirSync(path.join(siteSubdir, ".vercel"), { recursive: true });
+    fs.writeFileSync(path.join(siteSubdir, ".vercel", "project.json"), "{}\n");
+    expectBoth(siteSubdir, "loadout");
+  });
+
+  test("AC-1 variant: package.json at root, node_modules-only subdir — ROOT basename", () => {
+    const projectRoot = path.join(tmp, "monorepo");
+    const subdir = path.join(projectRoot, "packages", "web");
+    fs.mkdirSync(subdir, { recursive: true });
+    fs.writeFileSync(path.join(projectRoot, "package.json"), "{}\n");
+    fs.mkdirSync(path.join(subdir, "node_modules"), { recursive: true });
+    expectBoth(subdir, "monorepo");
+  });
+
+  test("AC-2: stale cache (old-bug shape) self-heals to the outermost-root slug", () => {
+    const projectRoot = path.join(tmp, "loadout");
+    const siteSubdir = path.join(projectRoot, "site");
+    fs.mkdirSync(path.join(projectRoot, ".git"), { recursive: true });
+    fs.mkdirSync(path.join(siteSubdir, ".vercel"), { recursive: true });
+    // Pre-seed the native cache with the WRONG value (pre-walk-up poisoning:
+    // cached == basename(pwd) while pwd is NOT the project root).
+    const cacheDir = path.join(nativeHome(), "slug-cache");
+    fs.mkdirSync(cacheDir, { recursive: true });
+    const cacheFile = path.join(cacheDir, toMsysPath(siteSubdir).replace(/\//g, "_"));
+    fs.writeFileSync(cacheFile, "site");
+
+    expect(slugFromEnvironment(nativeHome(), siteSubdir)).toBe("loadout");
+    // The cache file itself must have been overwritten (self-healing).
+    expect(fs.readFileSync(cacheFile, "utf-8")).toBe("loadout");
+  });
+
+  test("sticky cache (#2212): a cached identity that is NOT the old-bug shape survives", () => {
+    const projectRoot = path.join(tmp, "renamed-project");
+    fs.mkdirSync(path.join(projectRoot, ".git"), { recursive: true });
+    const cacheDir = path.join(nativeHome(), "slug-cache");
+    fs.mkdirSync(cacheDir, { recursive: true });
+    const cacheFile = path.join(cacheDir, toMsysPath(projectRoot).replace(/\//g, "_"));
+    fs.writeFileSync(cacheFile, "legacy-name");
+    // cached != basename(pwd), so the sticky rule holds — no recompute.
+    expect(slugFromEnvironment(nativeHome(), projectRoot)).toBe("legacy-name");
+  });
+
+  test("AC-3: cwd IS the project root with .git — slug = basename", () => {
+    const projectRoot = path.join(tmp, "myproject");
+    fs.mkdirSync(path.join(projectRoot, ".git"), { recursive: true });
+    expectBoth(projectRoot, "myproject");
+  });
+
+  test("AC-4: no markers anywhere on the chain — slug = pwd basename (fallback)", () => {
+    const deep = path.join(tmp, "just", "a", "plain", "folder");
+    fs.mkdirSync(deep, { recursive: true });
+    expectBoth(deep, "folder");
+  });
+
+  test("AC-5: subdir of a repo with a remote — slug derived from the ROOT's remote", () => {
+    // Pins the `git -C "$PROJECT_ROOT"` port: the subdir has no repo of its
+    // own, so the old native path (git in cwd) also reached the parent repo —
+    // but only the walk-up guarantees BOTH implementations root the remote
+    // lookup at the same directory.
+    const projectRoot = path.join(tmp, "realgit");
+    const subdir = path.join(projectRoot, "src", "deep");
+    fs.mkdirSync(subdir, { recursive: true });
+    spawnSync("git", ["init", "-q", projectRoot]);
+    spawnSync("git", ["-C", projectRoot, "remote", "add", "origin", "https://github.com/foo/bar.git"]);
+    expectBoth(subdir, "foo-bar");
+  });
+
+  test("weak marker: README.md at root, artifact-only subdir — ROOT basename", () => {
+    const projectRoot = path.join(tmp, "loadout");
+    const siteSubdir = path.join(projectRoot, "site");
+    fs.mkdirSync(siteSubdir, { recursive: true });
+    fs.writeFileSync(path.join(projectRoot, "README.md"), "# loadout\n");
+    fs.mkdirSync(path.join(siteSubdir, ".vercel"), { recursive: true });
+    expectBoth(siteSubdir, "loadout");
+  });
+
+  test("two-tier: vendored sub-repo with .git wins over parent README (strong > weak)", () => {
+    const projectRoot = path.join(tmp, "loadout");
+    const subRepo = path.join(projectRoot, "starter-pack");
+    fs.mkdirSync(subRepo, { recursive: true });
+    fs.writeFileSync(path.join(projectRoot, "README.md"), "# loadout\n");
+    fs.mkdirSync(path.join(subRepo, ".git"), { recursive: true });
+    expectBoth(subRepo, "starter-pack");
+  });
+
+  test("two-tier: outermost weak wins when no strong marker exists on the chain", () => {
+    const projectRoot = path.join(tmp, "loadout");
+    const subdir = path.join(projectRoot, "docs");
+    fs.mkdirSync(subdir, { recursive: true });
+    fs.writeFileSync(path.join(projectRoot, "README.md"), "# loadout\n");
+    fs.writeFileSync(path.join(subdir, "README.md"), "# docs\n");
+    expectBoth(subdir, "loadout");
+  });
+
+  test("nested repo: outermost .git wins — nested/vendored repos don't split stores", () => {
+    // THE bug this port fixes: the old native path asked the INNERMOST repo's
+    // remote. bin/gstack-slug resolves the OUTERMOST strong marker instead.
+    const outer = path.join(tmp, "outer-project");
+    const inner = path.join(outer, "vendor", "inner-lib");
+    fs.mkdirSync(inner, { recursive: true });
+    spawnSync("git", ["init", "-q", outer]);
+    spawnSync("git", ["-C", outer, "remote", "add", "origin", "git@github.com:acme/outer.git"]);
+    spawnSync("git", ["init", "-q", inner]);
+    spawnSync("git", ["-C", inner, "remote", "add", "origin", "git@github.com:vendor/inner.git"]);
+    expectBoth(inner, "acme-outer");
+  });
+
+  test("GSTACK_PROJECT_SLUG env override beats every other resolution path, never cached", () => {
+    const projectRoot = path.join(tmp, "loadout");
+    const siteSubdir = path.join(projectRoot, "site");
+    fs.mkdirSync(path.join(projectRoot, ".git"), { recursive: true });
+    fs.mkdirSync(siteSubdir, { recursive: true });
+
+    process.env.GSTACK_PROJECT_SLUG = "custom-override";
+    try {
+      expect(slugFromEnvironment(nativeHome(), siteSubdir)).toBe("custom-override");
+      if (HAS_BASH) {
+        expect(bashSlug(siteSubdir, { GSTACK_PROJECT_SLUG: "custom-override" })).toBe("custom-override");
+      }
+    } finally {
+      delete process.env.GSTACK_PROJECT_SLUG;
+    }
+    // Per-invocation escape hatch, never a durable identity: no cache written.
+    const cacheFile = path.join(nativeHome(), "slug-cache", toMsysPath(siteSubdir).replace(/\//g, "_"));
+    expect(fs.existsSync(cacheFile)).toBe(false);
+  });
+
+  test("outermostProjectRoot terminates on hostile path forms (dirname fixed points)", () => {
+    // Mirrors the windows-free-tests regression on the bash side: mixed-form
+    // paths must hit the dirname fixed point, not loop. A hang here would trip
+    // the suite timeout; reaching the assertions IS the pass.
+    for (const hostile of ["C:/Users/nobody/project", ".", "//server/share/dir"]) {
+      expect(typeof outermostProjectRoot(hostile)).toBe("string");
+    }
+  });
+});