fix(ce-work-beta): add explicit stdin piping guidance for Codex delegation

Codex CLI rejects shell redirects (`< file`) with "stdin is not a
terminal". The delegation step said "pipe via stdin" but gave no
concrete example, so the agent used a redirect instead.

Add a concrete `cat file | codex exec ...` example and an explicit
warning against `< file`.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: huntharo

docs/plans/2026-03-24-001-fix-codex-stdin-pipe-guidance-plan.md

@@ -0,0 +1,43 @@
+---
+title: "fix: add explicit stdin piping guidance for Codex delegation"
+type: fix
+status: active
+date: 2026-03-24
+---
+
+# fix: add explicit stdin piping guidance for Codex delegation
+
+## Problem
+
+When ce:work-beta delegates to Codex CLI, the skill says "piping the prompt file via stdin" but provides no concrete command example. The agent interprets this as a shell redirect:
+
+```bash
+codex --dangerously-bypass-approvals-and-sandbox < /tmp/codex_task_20980.txt 2>&1
+```
+
+This fails with:
+
+```
+Error: Exit code 1
+Error: stdin is not a terminal
+```
+
+Codex CLI checks whether stdin is a terminal (likely `isatty()`) and rejects shell redirects (`< file`). A pipe (`cat file | codex`) works because it presents data differently to the process.
+
+## Approach
+
+In step 5 ("Delegate") of the External Delegate Mode section in `ce-work-beta/SKILL.md`, add:
+
+1. A concrete command example using `cat file | codex ...`
+2. An explicit warning that `< file` (shell redirect) does NOT work -- Codex rejects it with "stdin is not a terminal"
+3. Keep the existing note about avoiding argv expansion for large prompts
+
+## Files
+
+- `plugins/compound-engineering/skills/ce-work-beta/SKILL.md` -- step 5 of External Delegate Mode
+
+## Acceptance Criteria
+
+- [ ] Step 5 contains a concrete `cat ... | codex exec ...` command example with placeholder flags from step 2
+- [ ] Explicit warning: shell redirects (`< file`) fail with "stdin is not a terminal"
+- [ ] Existing guidance about avoiding argv expansion is preserved

plugins/compound-engineering/skills/ce-work-beta/SKILL.md

@@ -472,7 +472,17 @@ When external delegation is active, follow this workflow for each tagged task. D
 
 3. **Write prompt to file** — Save the assembled prompt to a unique temporary file to avoid shell quoting issues and cross-task races. Use a unique filename per task.
 
-4. **Delegate** — Run the delegate CLI, piping the prompt file via stdin (not argv expansion, which hits `ARG_MAX` on large prompts). Omit the model flag to use the delegate's default model, which stays current without manual updates.
+4. **Delegate** — Run the delegate CLI, piping the prompt file via stdin. Omit the model flag to use the delegate's default model, which stays current without manual updates.
+
+   **Use `cat ... | codex`, not shell redirects.** Codex CLI rejects `< file` with "stdin is not a terminal". Always pipe:
+
+   ```bash
+   cat /tmp/codex-task-XXXXX.txt | codex exec --approval-mode full-auto --quiet 2>&1
+   ```
+
+   Do not use: `codex exec ... < /tmp/codex-task-XXXXX.txt` (fails immediately).
+
+   Piping also avoids `ARG_MAX` limits that would occur if passing the prompt as a CLI argument.
 
 5. **Review diff** — After the delegate finishes, verify the diff is non-empty and in-scope. Run the project's test/lint commands. If the diff is empty or out-of-scope, fall back to standard mode for that task.