diff --git a/bin/gstack-config b/bin/gstack-config
index 6c23b5d8d..3521825f4 100755
--- a/bin/gstack-config
+++ b/bin/gstack-config
@@ -193,8 +193,16 @@ lookup_default() {
 # ──────────────────────────────────────────────────────────────────────
 
 # Compute sha8 of a string. Used for endpoint hashing.
+# shasum is macOS/perl; most Linux distros ship only coreutils sha256sum —
+# resolve whichever exists (same fallback chain as the codex-probe timeout
+# wrapper). Without this, any Linux user with a git email hit exit 127 in
+# resolve-user-slug's Layer-3 fallback.
 sha8_of() {
-  printf '%s' "$1" | shasum -a 256 | cut -c1-8
+  if command -v sha256sum >/dev/null 2>&1; then
+    printf '%s' "$1" | sha256sum | cut -c1-8
+  else
+    printf '%s' "$1" | shasum -a 256 | cut -c1-8
+  fi
 }
 
 # Detect the active brain endpoint hash. Reads ~/.claude.json for the gbrain
@@ -228,7 +236,11 @@ endpoint_hash_with_collision_check() {
   _claude_json="$HOME/.claude.json"
   if [ -n "$_matching" ] && [ -f "$_claude_json" ] && command -v jq >/dev/null 2>&1; then
     _url=$(jq -r '.mcpServers.gbrain.url // .mcpServers.gbrain.transport.url // empty' "$_claude_json" 2>/dev/null)
-    _sha16=$(printf '%s' "$_url" | shasum -a 256 | cut -c1-16)
+    if command -v sha256sum >/dev/null 2>&1; then
+      _sha16=$(printf '%s' "$_url" | sha256sum | cut -c1-16)
+    else
+      _sha16=$(printf '%s' "$_url" | shasum -a 256 | cut -c1-16)
+    fi
     # Look for any sha16-namespaced key that conflicts. If a stored sha16 exists
     # and differs from current sha16, that's the collision evidence; emit sha16.
     _stored16=$(grep -E "^(brain_trust_policy|user_slug_at)@${_sha16}" "$CONFIG_FILE" 2>/dev/null | head -1 || true)
diff --git a/browse/SKILL.md b/browse/SKILL.md
index 88f961de6..4d7f53465 100644
--- a/browse/SKILL.md
+++ b/browse/SKILL.md
@@ -192,7 +192,13 @@ If `NEEDS_SETUP`:
      BUN_INSTALL_SHA="bab8acfb046aac8c72407bdcce903957665d655d7acaa3e11c7c4616beae68dd"
      tmpfile=$(mktemp)
      curl -fsSL "https://bun.sh/install" -o "$tmpfile"
-     actual_sha=$(shasum -a 256 "$tmpfile" | awk '{print $1}')
+     # shasum is macOS/perl; coreutils-only Linux ships sha256sum instead —
+     # resolve whichever exists so the verify never fails on a missing tool.
+     if command -v sha256sum >/dev/null 2>&1; then
+       actual_sha=$(sha256sum "$tmpfile" | awk '{print $1}')
+     else
+       actual_sha=$(shasum -a 256 "$tmpfile" | awk '{print $1}')
+     fi
      if [ "$actual_sha" != "$BUN_INSTALL_SHA" ]; then
        echo "ERROR: bun install script checksum mismatch" >&2
        echo "  expected: $BUN_INSTALL_SHA" >&2
diff --git a/qa/SKILL.md b/qa/SKILL.md
index aa5a60782..9d456f1dd 100644
--- a/qa/SKILL.md
+++ b/qa/SKILL.md
@@ -543,7 +543,13 @@ If `NEEDS_SETUP`:
      BUN_INSTALL_SHA="bab8acfb046aac8c72407bdcce903957665d655d7acaa3e11c7c4616beae68dd"
      tmpfile=$(mktemp)
      curl -fsSL "https://bun.sh/install" -o "$tmpfile"
-     actual_sha=$(shasum -a 256 "$tmpfile" | awk '{print $1}')
+     # shasum is macOS/perl; coreutils-only Linux ships sha256sum instead —
+     # resolve whichever exists so the verify never fails on a missing tool.
+     if command -v sha256sum >/dev/null 2>&1; then
+       actual_sha=$(sha256sum "$tmpfile" | awk '{print $1}')
+     else
+       actual_sha=$(shasum -a 256 "$tmpfile" | awk '{print $1}')
+     fi
      if [ "$actual_sha" != "$BUN_INSTALL_SHA" ]; then
        echo "ERROR: bun install script checksum mismatch" >&2
        echo "  expected: $BUN_INSTALL_SHA" >&2
diff --git a/scripts/resolvers/browse.ts b/scripts/resolvers/browse.ts
index cff6db107..e04e3487e 100644
--- a/scripts/resolvers/browse.ts
+++ b/scripts/resolvers/browse.ts
@@ -138,7 +138,13 @@ If \`NEEDS_SETUP\`:
      BUN_INSTALL_SHA="bab8acfb046aac8c72407bdcce903957665d655d7acaa3e11c7c4616beae68dd"
      tmpfile=$(mktemp)
      curl -fsSL "https://bun.sh/install" -o "$tmpfile"
-     actual_sha=$(shasum -a 256 "$tmpfile" | awk '{print $1}')
+     # shasum is macOS/perl; coreutils-only Linux ships sha256sum instead —
+     # resolve whichever exists so the verify never fails on a missing tool.
+     if command -v sha256sum >/dev/null 2>&1; then
+       actual_sha=$(sha256sum "$tmpfile" | awk '{print $1}')
+     else
+       actual_sha=$(shasum -a 256 "$tmpfile" | awk '{print $1}')
+     fi
      if [ "$actual_sha" != "$BUN_INSTALL_SHA" ]; then
        echo "ERROR: bun install script checksum mismatch" >&2
        echo "  expected: $BUN_INSTALL_SHA" >&2
diff --git a/test/user-slug-fallback.test.ts b/test/user-slug-fallback.test.ts
index 82e733aa9..9021b9203 100644
--- a/test/user-slug-fallback.test.ts
+++ b/test/user-slug-fallback.test.ts
@@ -15,7 +15,8 @@
  */
 
 import { describe, test, expect, beforeEach, afterEach } from 'bun:test';
-import { mkdtempSync, existsSync, readFileSync, writeFileSync, rmSync, mkdirSync } from 'fs';
+import { mkdtempSync, existsSync, readFileSync, writeFileSync, rmSync, mkdirSync, symlinkSync } from 'fs';
+import { createHash } from 'crypto';
 import { join } from 'path';
 import { tmpdir } from 'os';
 import { spawnSync } from 'child_process';
@@ -112,6 +113,114 @@ describe('resolve-user-slug fallback chain', () => {
   });
 });
 
+describe('sha8_of portable hash (sha256sum → shasum fallback)', () => {
+  // sha8_of must work on coreutils-only Linux (no shasum: the exit-127
+  // regression) AND on stock macOS (no sha256sum). The ambient PATH decides
+  // which branch runs, so a plain subprocess call only ever covers one branch
+  // per platform. Pin BOTH deterministically: extract the real function text
+  // from bin/gstack-config (no drift-prone copy) and run it under /bin/sh
+  // with a shim-only PATH that makes exactly one hasher visible. The wrong
+  // branch exits 127 (its tool is absent from the shim dir), so branch
+  // selection is asserted structurally, not inferred.
+  const EXPECTED = '2cf24dba'; // sha256("hello") = 2cf24dba5fb0a30e2…
+
+  function sha8FnSource(): string {
+    const src = readFileSync(CONFIG_BIN, 'utf-8');
+    const m = src.match(/^sha8_of\(\) \{\n[\s\S]*?\n\}/m);
+    if (!m) throw new Error('sha8_of() not found in bin/gstack-config');
+    return m[0];
+  }
+
+  /** Absolute-path sha256 pipeline for shims (host has sha256sum OR shasum). */
+  function realHasherLine(): string {
+    const sha256sum = Bun.which('sha256sum');
+    if (sha256sum) return `exec ${sha256sum} "$@"`;
+    const shasum = Bun.which('shasum');
+    if (shasum) return `exec ${shasum} -a 256 "$@"`;
+    throw new Error('neither sha256sum nor shasum available on this host');
+  }
+
+  function runSha8(shimDir: string) {
+    const result = spawnSync('/bin/sh', ['-c', `${sha8FnSource()}\nsha8_of "hello"`], {
+      encoding: 'utf-8',
+      env: { PATH: shimDir }, // ONLY the shim dir: absent tools are really absent
+      timeout: 5000,
+    });
+    return { stdout: (result.stdout || '').trim(), status: result.status ?? -1, stderr: result.stderr || '' };
+  }
+
+  function makeShimDir(): string {
+    const dir = mkdtempSync(join(tmpdir(), 'gstack-sha8-shim-'));
+    const cut = Bun.which('cut');
+    if (!cut) throw new Error('cut not on PATH');
+    symlinkSync(cut, join(dir, 'cut'));
+    return dir;
+  }
+
+  test('coreutils-only PATH (sha256sum present, shasum absent) — the Linux exit-127 regression', () => {
+    const shim = makeShimDir();
+    try {
+      writeFileSync(join(shim, 'sha256sum'), `#!/bin/sh\n${realHasherLine()}\n`, { mode: 0o755 });
+      const result = runSha8(shim);
+      expect(result.stderr).toBe('');
+      expect(result.status).toBe(0);
+      expect(result.stdout).toBe(EXPECTED);
+    } finally {
+      rmSync(shim, { recursive: true, force: true });
+    }
+  });
+
+  test('sha256sum absent falls back to `shasum -a 256` with identical output (macOS branch)', () => {
+    const shim = makeShimDir();
+    try {
+      // Arg-validating shasum shim: wrong/missing `-a 256` exits 64, which
+      // would surface as a failed pipeline — pins the exact invocation.
+      writeFileSync(
+        join(shim, 'shasum'),
+        `#!/bin/sh\n[ "$1" = "-a" ] && [ "$2" = "256" ] || exit 64\nshift 2\n${realHasherLine()}\n`,
+        { mode: 0o755 },
+      );
+      const result = runSha8(shim);
+      expect(result.stderr).toBe('');
+      expect(result.status).toBe(0);
+      expect(result.stdout).toBe(EXPECTED); // same vector ⇒ branches are equivalent
+    } finally {
+      rmSync(shim, { recursive: true, force: true });
+    }
+  });
+});
+
+describe('endpoint-hash collision escalation (sha8 → sha16)', () => {
+  // endpoint_hash_with_collision_check's sha16 arm carries its own portable
+  // hash pipeline (sha256sum → shasum). This drives it end-to-end through the
+  // real binary: a gbrain MCP URL in $HOME/.claude.json plus config keys at
+  // BOTH the sha8 and sha16 namespaces is the recorded-collision evidence
+  // that makes `endpoint-hash` emit the 16-char hash. Skipped where jq is
+  // absent (the script itself degrades to 'local' there).
+  test('emits sha8 normally, sha16 when a stored sha16-namespaced key exists', () => {
+    if (!Bun.which('jq')) return; // endpoint_hash requires jq; degrades to 'local' without it
+    const url = 'https://gbrain.example.test/mcp';
+    const hex = createHash('sha256').update(url).digest('hex');
+    const sha8 = hex.slice(0, 8);
+    const sha16 = hex.slice(0, 16);
+    writeFileSync(join(TMP_HOME, '.claude.json'), JSON.stringify({ mcpServers: { gbrain: { url } } }));
+
+    // No collision evidence yet → plain sha8.
+    const plain = runConfig(['endpoint-hash'], { GSTACK_HOME: TMP_HOME });
+    expect(plain.status).toBe(0);
+    expect(plain.stdout.trim()).toBe(sha8);
+
+    // Keys stored at both namespaces → escalate to sha16.
+    writeFileSync(
+      join(TMP_HOME, 'config.yaml'),
+      `brain_trust_policy@${sha8}: personal\nbrain_trust_policy@${sha16}: shared\n`,
+    );
+    const escalated = runConfig(['endpoint-hash'], { GSTACK_HOME: TMP_HOME });
+    expect(escalated.status).toBe(0);
+    expect(escalated.stdout.trim()).toBe(sha16);
+  });
+});
+
 describe('brain_trust_policy@<endpoint-id> namespace', () => {
   test('default value is "unset"', () => {
     const result = runConfig(['get', 'brain_trust_policy@deadbeef'], { GSTACK_HOME: TMP_HOME });