FESADev/docs/superpowers/plans/2026-08-16-implementation-agent-terra-model.md

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.

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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:

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:

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:

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

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