Introduction

DeepSeek Harness (DSH) allows different models, such as DeepSeek and GPT, to be used together in the same environment. However, models like GPT-5.6 perform poorly in DSH’s default Standard mode, because their post-training expects the prompts, tools, architecture, and output formats used by Codex. The dsh-codex plugin creates a familiar programming environment for GPT models by injecting Codex-style tool interfaces and system prompts, while retaining DSH’s native support for DeepSeek models.

Core Capabilities

The plugin injects the following capabilities into selected GPT models through a plug-and-play extension package:

  1. Structured Patch Tool: Provides syntax-based OpenAI custom tool calls specifically for apply_patch operations, ensuring the patch format matches expectations.
  2. Unified Execution Environment: Unifies exec_command and write_stdin into a Codex-style execution interface, supporting generated background sessions and interactive PTYs (pseudo-terminals).
  3. Terminal View Customization: Uses the native Web Bash line to replace generic views, and displays a Codex-style “processes only” background terminal list.
  4. Collaboration and Planning Tools: Introduces tools such as update_plan, request_user_input, and view_image, supporting /plan collaboration mode and /goal long-running workflows.
  5. System Prompt Injection: Loads the GPT-5.6 Sol Codex system prompt from a user-selected prompt library.
  6. Preserved Underlying Behavior: Beneath the Codex-style wire format, it continues to use DSH’s native sandbox, approval, to-do list, question, attachment, and tool display logic.

Installation and Enablement

Installing the plugin requires adding the package to the DSH profile. After installation, restart the process and select the corresponding preset.

dsh plugin --profile web add @ryantzhou/dsh-codex

After installation, restart the corresponding DSH process. When creating a session, select Codex-style mode to enable this environment.

To uninstall, run the following command:

dsh plugin --profile web remove @ryantzhou/dsh-codex

Compatibility and Notes

Before deployment, pay attention to the following technical details:

  • Version Dependency: Version 0.2.0 is designed specifically for DSH 0.1.0-rc.6 and pi-ai 0.82.1.
  • Model Routing Requirements: Syntax-constrained tools (such as apply_patch) require GPT-5+ pi-ai routing, and supportsOpenAIGrammarTools must be enabled in the routing’s model metadata. If the route is unsupported, the plugin falls back to pi-ai standard function tools.
  • Dependency Locking: Dependencies are locked. If an DSH upgrade causes dependency incompatibility, installation will fail, avoiding silent behavioral changes caused by version conflicts.
  • License: The original code is under the MIT license. Codex-derived grammar and behavior references retain OpenAI’s Apache-2.0 notice.

Conclusion

dsh-codex enables GPT models to leverage their programming capabilities optimized for Codex interfaces within DSH, while allowing developers to continue using DSH’s native state management and approval mechanisms. This makes it possible to use DeepSeek and GPT models together in the same DSH instance.