diff --git a/openspec/changes/agent-claude-harden-guardex-compress-cmd-shell-quote-2026-06-18-11-43/.openspec.yaml b/openspec/changes/agent-claude-harden-guardex-compress-cmd-shell-quote-2026-06-18-11-43/.openspec.yaml
new file mode 100644
index 0000000..95ae5a2
--- /dev/null
+++ b/openspec/changes/agent-claude-harden-guardex-compress-cmd-shell-quote-2026-06-18-11-43/.openspec.yaml
@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-06-18
diff --git a/openspec/changes/agent-claude-harden-guardex-compress-cmd-shell-quote-2026-06-18-11-43/notes.md b/openspec/changes/agent-claude-harden-guardex-compress-cmd-shell-quote-2026-06-18-11-43/notes.md
new file mode 100644
index 0000000..188c278
--- /dev/null
+++ b/openspec/changes/agent-claude-harden-guardex-compress-cmd-shell-quote-2026-06-18-11-43/notes.md
@@ -0,0 +1,19 @@
+# agent-claude-harden-guardex-compress-cmd-shell-quote-2026-06-18-11-43 (minimal / T1)
+
+Branch: `agent/claude/harden-guardex-compress-cmd-shell-quote-2026-06-18-11-43`
+
+Replace the naive whitespace split in `resolveCompressCommand` (GUARDEX_COMPRESS_CMD,
+src/output/index.js) with a shell-quote-aware `tokenizeCommand` so values like
+`sh -c "tr a-z A-Z"` parse correctly; malformed input (unterminated quote / dangling
+backslash) returns null so the caller falls back to plain output. Follow-up to PR #649.
+
+## Handoff
+
+- Handoff: change=`agent-claude-harden-guardex-compress-cmd-shell-quote-2026-06-18-11-43`; branch=`agent/<your-name>/<branch-slug>`; scope=`TODO`; action=`continue this sandbox or finish cleanup after a usage-limit/manual takeover`.
+- Copy prompt: Continue `agent-claude-harden-guardex-compress-cmd-shell-quote-2026-06-18-11-43` on branch `agent/<your-name>/<branch-slug>`. Work inside the existing sandbox, review `openspec/changes/agent-claude-harden-guardex-compress-cmd-shell-quote-2026-06-18-11-43/notes.md`, continue from the current state instead of creating a new sandbox, and when the work is done run `gx branch finish --branch agent/<your-name>/<branch-slug> --base dev --via-pr --wait-for-merge --cleanup`.
+
+## Cleanup
+
+- [ ] Run: `gx branch finish --branch agent/<your-name>/<branch-slug> --base dev --via-pr --wait-for-merge --cleanup`
+- [ ] Record PR URL + `MERGED` state in the completion handoff.
+- [ ] Confirm sandbox worktree is gone (`git worktree list`, `git branch -a`).
diff --git a/src/output/index.js b/src/output/index.js
index f35226f..dc1994f 100644
--- a/src/output/index.js
+++ b/src/output/index.js
@@ -659,17 +659,77 @@ function printAutoFinishSummary(summary, options = {}) {
 
 const COMPRESS_MIN_CHARS = 400;
 
-// resolveCompressCommand parses GUARDEX_COMPRESS_CMD into an argv array (split
-// on whitespace, run with shell:false so no shell interpolation). Returns null
-// when unset/empty so callers fall straight through to plain output. Commands
-// needing args with embedded spaces should be wrapped in a small script.
+// tokenizeCommand splits a command string into an argv array, honoring single
+// quotes, double quotes (with \" and \\ escapes), and backslash escaping, so a
+// value like  sh -c "tr a-z A-Z"  parses to ['sh','-c','tr a-z A-Z'] instead of
+// being naively split on whitespace. Returns null on an unterminated quote
+// (malformed) so callers fall back to plain output rather than run a half-parsed
+// command. No shell-metacharacter handling (globs, $vars, pipes): tokens are
+// passed to spawnSync with shell:false as literal argv.
+function tokenizeCommand(raw) {
+  const text = String(raw);
+  const tokens = [];
+  let current = '';
+  let started = false;
+  let quote = null; // "'" or '"' while inside a quoted run
+  for (let i = 0; i < text.length; i += 1) {
+    const ch = text[i];
+    if (quote === "'") {
+      if (ch === "'") quote = null;
+      else current += ch;
+      continue;
+    }
+    if (quote === '"') {
+      if (ch === '"') quote = null;
+      else if (ch === '\\' && (text[i + 1] === '"' || text[i + 1] === '\\')) {
+        current += text[i + 1];
+        i += 1;
+        started = true;
+      } else current += ch;
+      continue;
+    }
+    if (ch === "'" || ch === '"') {
+      quote = ch;
+      started = true;
+      continue;
+    }
+    if (ch === '\\') {
+      if (i + 1 >= text.length) return null; // dangling backslash -> malformed
+      current += text[i + 1];
+      i += 1;
+      started = true;
+      continue;
+    }
+    if (/\s/.test(ch)) {
+      if (started) {
+        tokens.push(current);
+        current = '';
+        started = false;
+      }
+      continue;
+    }
+    current += ch;
+    started = true;
+  }
+  if (quote) return null; // unterminated quote -> malformed
+  if (started) tokens.push(current);
+  return tokens;
+}
+
+// resolveCompressCommand parses GUARDEX_COMPRESS_CMD into an argv array via
+// tokenizeCommand (shell-quote aware), run with shell:false so no shell
+// interpolation. Returns null when unset/empty/malformed so callers fall
+// straight through to plain output.
 function resolveCompressCommand(env = process.env) {
   const raw = String(env.GUARDEX_COMPRESS_CMD || '').trim();
   if (!raw) {
     return null;
   }
-  const argv = raw.split(/\s+/).filter(Boolean);
-  return argv.length > 0 ? argv : null;
+  const argv = tokenizeCommand(raw);
+  if (!argv || argv.length === 0) {
+    return null;
+  }
+  return argv;
 }
 
 // looksMachineReadable keeps JSON / structured payloads (status --json, lock
@@ -758,6 +818,7 @@ module.exports = {
   detectRecoverableAutoFinishConflict,
   summarizeAutoFinishDetail,
   printAutoFinishSummary,
+  tokenizeCommand,
   resolveCompressCommand,
   compressBlock,
   printCompressible,
diff --git a/test/output.test.js b/test/output.test.js
index 9168133..b641364 100644
--- a/test/output.test.js
+++ b/test/output.test.js
@@ -5,6 +5,7 @@ const {
   printAutoFinishSummary,
   compressBlock,
   resolveCompressCommand,
+  tokenizeCommand,
 } = require('../src/output');
 
 // A large lowercase block so the `tr a-z A-Z` stub visibly transforms it and it
@@ -134,3 +135,48 @@ test('compressBlock respects the terse-mode gate (skips when verbose)', () => {
     }
   }
 });
+
+test('tokenizeCommand honors double and single quotes with embedded spaces', () => {
+  assert.deepEqual(tokenizeCommand('sh -c "tr a-z A-Z"'), ['sh', '-c', 'tr a-z A-Z']);
+  assert.deepEqual(tokenizeCommand("sh -c 'a b c'"), ['sh', '-c', 'a b c']);
+  assert.deepEqual(tokenizeCommand('  tr   a-z   A-Z  '), ['tr', 'a-z', 'A-Z']);
+  assert.deepEqual(tokenizeCommand('a "b c" d'), ['a', 'b c', 'd']);
+});
+
+test('tokenizeCommand returns null on malformed input (unterminated quote or dangling backslash)', () => {
+  assert.equal(tokenizeCommand('sh -c "oops no close'), null);
+  assert.equal(tokenizeCommand("it's"), null);
+  assert.equal(tokenizeCommand('foo\\'), null);
+});
+
+test('resolveCompressCommand parses a shell-quoted command into argv', () => {
+  assert.deepEqual(
+    resolveCompressCommand({ GUARDEX_COMPRESS_CMD: 'sh -c "tr a-z A-Z"' }),
+    ['sh', '-c', 'tr a-z A-Z'],
+  );
+  // malformed (unterminated quote) -> null so the caller falls back to plain output
+  assert.equal(resolveCompressCommand({ GUARDEX_COMPRESS_CMD: 'sh -c "broken' }), null);
+});
+
+test('compressBlock runs a shell-quoted compressor command end to end', () => {
+  const out = compressBlock(BIG_BLOCK, {
+    env: { GUARDEX_COMPRESS_CMD: 'sh -c "tr a-z A-Z"' },
+    force: true,
+  });
+  assert.equal(out, BIG_BLOCK.toUpperCase());
+});
+
+test('compressBlock falls back to the original when the compressor times out (discards partial stdout)', () => {
+  const out = compressBlock(BIG_BLOCK, {
+    env: {
+      // emits partial stdout, then sleeps well past the timeout so it is always
+      // killed mid-run (long sleep removes any chance the child exits early).
+      GUARDEX_COMPRESS_CMD: 'sh -c "printf PARTIAL; sleep 30"',
+      GUARDEX_COMPRESS_TIMEOUT_MS: '200',
+    },
+    force: true,
+  });
+  // Whether killed before or after the printf, the partial/empty stdout is
+  // discarded and the original block is returned.
+  assert.equal(out, BIG_BLOCK);
+});