fix(moe/deepseek): create the output directory before writing config.json (#697)

[Anai-Guo]

Fixes #697.

Problem

DeepseekMoE.write_model() writes the output config with a plain open():

with open(os.path.join(out_path, "config.json"), "w", encoding="utf-8") as f:
    json.dump(out_cfg, f, indent=4)

Nothing creates out_path before that point:

• mergekit.scripts.moe.build() calls out_arch.write_model(out_path, ...) directly — it never creates the directory itself.
• initialize_io() (which constructs the TensorWriter that does create it) runs after the config.json write.

So a Deepseek MoE merge whose out_path does not already exist dies with FileNotFoundError before a single weight is written.

The other output architectures don't hit this because they persist their config through transformers:

writer	config persistence
mergekit/moe/mixtral.py:118	out_cfg.save_pretrained(out_path)
mergekit/moe/qwen.py:113	out_cfg.save_pretrained(out_path)
mergekit/moe/qwen3.py:93	out_cfg.save_pretrained(out_path)
mergekit/moe/deepseek.py:121	raw open(...)

PretrainedConfig.save_pretrained() starts with os.makedirs(save_directory, exist_ok=True), so Mixtral/Qwen/Qwen3 get the directory for free. Deepseek builds a plain dict (it needs the auto_map remote-code keys), so it can't use save_pretrained and has to do this explicitly.

Fix

One line, immediately before the write:

os.makedirs(out_path, exist_ok=True)

exist_ok=True makes it a no-op for the already-common case where the directory exists, so behaviour is unchanged for every currently-working merge.

Notes

• No test added: write_model() needs real source checkpoints to run end to end, and tests/ has no MoE coverage to hang a unit test off. The change is a strict superset of the previous behaviour.
• Lint checked against the repo's pinned hooks — black 25.1.0 and isort 6.0.0 (with pyproject.toml settings and mergekit as first-party) both report the file clean.

🤖 Generated with Claude Code

---

Note

Low Risk
Single-line filesystem guard before config write; no change to merge logic or weight handling.

Overview
DeepSeek MoE merges no longer fail when the output path does not exist yet.

DeepseekMoE.write_model() writes config.json with a direct open() before initialize_io() runs, unlike Mixtral/Qwen paths that use save_pretrained() (which creates the directory). The change adds os.makedirs(out_path, exist_ok=True) immediately before that write so behavior matches the other MoE writers without affecting merges that already use an existing directory.

^{Reviewed by Cursor Bugbot for commit 8764ddd. Bugbot is set up for automated code reviews on this repo. Configure here.}

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[Anai-Guo]

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