# Implementation Agent Terra Model Implementation Plan

> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.

**Goal:** Configure the project-local `implementation-agent` to use `gpt-5.6-terra` while preserving its existing reasoning effort.

**Architecture:** Add one explicit model override to the existing Implementation Agent TOML profile. Do not introduce shared model policy, modify other profiles, or change the agent's instructions.

**Tech Stack:** TOML, Python 3 `tomllib`, Git

## Global Constraints

- Set `model` to exactly `gpt-5.6-terra` only in `.codex/agents/implementation-agent.toml`.
- Preserve `model_reasoning_effort = "extra high"`.
- Do not modify other custom agents, user-global Codex configuration, solver production code, or agent instructions.
- Do not add or modify contract tests or other test files.
- Verify the change only through TOML parsing, exact-value assertions, and Git diff inspection.

---

### Task 1: Add the Implementation Agent model override

**Files:**
- Modify: `.codex/agents/implementation-agent.toml`
- Test: none, per the approved design

**Interfaces:**
- Consumes: the existing project-local `implementation-agent` TOML profile
- Produces: `model = "gpt-5.6-terra"` with the existing `model_reasoning_effort = "extra high"`

- [ ] **Step 1: Add the model setting**

Insert the model key between `sandbox_mode` and `model_reasoning_effort` so the profile header is:

```toml
name = "implementation-agent"
description = "Implements FESA solver features in C++17/MSVC by following approved TDD-first implementation plans."
sandbox_mode = "workspace-write"
model = "gpt-5.6-terra"
model_reasoning_effort = "extra high"
```

- [ ] **Step 2: Parse the profile and verify the exact values**

Run:

```powershell
python -c "import pathlib, tomllib; p = tomllib.loads(pathlib.Path('.codex/agents/implementation-agent.toml').read_text(encoding='utf-8')); assert p['model'] == 'gpt-5.6-terra'; assert p['model_reasoning_effort'] == 'extra high'"
```

Expected: exit code `0` with no output.

- [ ] **Step 3: Verify the change is surgical**

Run:

```powershell
git diff --check
git diff -- .codex/agents/implementation-agent.toml
```

Expected: no whitespace errors, and the profile diff contains only the added
`model = "gpt-5.6-terra"` line.

- [ ] **Step 4: Commit the configuration change**

```powershell
git add -- .codex/agents/implementation-agent.toml
git commit -m "chore: use Terra for implementation agent"
```