Codex vs Mellum by JetBrains: How They Actually Differ
Pick Codex if you want one agent that moves between terminal, IDE, and cloud and reviews GitHub pull requests on GPT models. Pick Mellum by JetBrains if you already live in a JetBrains IDE and want a fast, open-source completion model.
Compiled 2026-09-02 from our catalogue entries for Codex and Mellum by JetBrains.
The short answer
- Pick Codex if
- You want a coding agent that runs across CLI, IDE, desktop, and cloud, with built-in GitHub pull request code review, AGENTS.md and MCP support, and you are fine with OpenAI's GPT models.
- Pick Mellum by JetBrains if
- You already work in a JetBrains IDE and want a low-latency, open-source code completion model that plugs in through the JetBrains AI Assistant plugin.
- Cheapest way in
- Codex Free at $0 /month is the only listed $0 tier. Mellum by JetBrains has no recorded pricing, plans, or platforms on file.
- What we could not compare
- Mellum's pricing, platforms, plans, public repo, and docs are all 'not recorded'. Codex's Business and Enterprise pricing is 'Not published'. We do not have those numbers to compare.
Codex is OpenAI's agentic coding tool. It runs the same agent across a terminal CLI, an IDE extension, a desktop app, and a cloud environment, so work started in one surface can continue in another. It reads the shared AGENTS.md instructions file, speaks the Model Context Protocol, and can spin up a cloud code review on a GitHub pull request that comments inline and proposes fixes. Codex runs on OpenAI's GPT models and is included with ChatGPT plans from the free tier up through Pro.
Mellum by JetBrains is a different shape of product: a code language model, open-sourced by JetBrains on Hugging Face, built for low-latency code completion rather than broader agentic work. It is meant to live inside a JetBrains IDE through the JetBrains AI Assistant plugin, and the fact sheet describes it as purpose-built for completion, with narrower capability breadth than larger frontier code models.
Side by side
Every cell is a field on our catalogue entry for that tool, copied as recorded. Rows where the two differ come first; blank fields say so rather than being filled in.
| Codex | Mellum by JetBrains | |
|---|---|---|
| Pricing model | Freemium | Not recorded |
| Starts at | Free — $0 /month | Not recorded |
| Runs on | web, mac, win, linux, cli, vscode-ext | Not recorded |
| Category | AI Coding Agents | AI Coding Agents |
| Public source code | No public repo listed | No public repo listed |
| Public docs | Not listed | Not listed |
- Public source code: From the repository link on each catalogue entry, not a licence audit.
Where Codex is the better fit
- One agent across CLI, IDE, desktop, and cloud, so work can move between surfaces.
- Cloud code review on a GitHub pull request that comments inline and proposes fixes.
- Reads the shared AGENTS.md standard, supports MCP, and ships included from the Free ChatGPT tier up through Pro.
Where Mellum by JetBrains is the better fit
- Open-source code language model released by JetBrains on Hugging Face.
- Purpose-built for low-latency code completion in IDE workflows.
- Integrates with JetBrains IDEs through the JetBrains AI Assistant plugin.
Which one should you pick?
Same agent works in CLI, IDE, desktop, and cloud, and the Free $0 /month tier is on file as a starting point.
It targets low-latency inline completion and plugs in through the JetBrains AI Assistant plugin.
The fact sheet lists a cloud code review on a pull request that catches real bugs and proposes fixes.
Frequently asked questions
- Is Mellum by JetBrains an agent like Codex?
- No. The fact sheet says Mellum is focused on code completion rather than broader agentic coding tasks, and its capability breadth is narrower than larger frontier code models.
- How much does Mellum by JetBrains cost?
- Pricing is not recorded in the fact sheet, and no platforms, plans, public repo, or docs are on file.
- Can Codex use models other than GPT?
- Per the fact sheet, Codex is locked to OpenAI's GPT models, with GPT-5.5 listed as the recommended frontier model. No Claude or Gemini support is mentioned.
What we compared
This page states nothing that is not in the two catalogue entries below. The exact text used to write it is reproduced here so you can check our reading, and both tools link through to their own pages and to the vendor.
- /tools/codex — checked 2026-07-02 · official site
- /tools/mellum-by-jetbrains — checked 2026-06-21 · official site
Show the source text
## Codex (/tools/codex) Tagline: OpenAI's coding agent — works in the terminal, IDE, and cloud, reviews GitHub pull requests, and runs on GPT-5 models. Category: AI Coding Agents Pricing model: freemium Starts at: Free — $0 /month Platforms: web, mac, win, linux, cli, vscode-ext Public repo: none listed Docs: none listed Verified plans (read 2026-08-21): Free $0 /month | Go $8 /month | Plus $20 /month | Pro 5x $100 /month | Pro 20x $200 /month | Business / Enterprise Not published | API Key Pay only for the tokens Codex uses, based on API pricing Strengths on file: Runs one agent across CLI, IDE, desktop, and cloud — work moves between surfaces.; Strong GitHub integration: a cloud code review on a pull request that catches real bugs and proposes fixes.; Reads the shared AGENTS.md standard and supports MCP tools.; Included with ChatGPT plans, from the free tier up through Pro. Limits on file: Locked to OpenAI's GPT models — no Claude or Gemini out of the box.; Fewer built-in customization primitives (hooks, sub-agents) than some rivals. Codex is OpenAI's agentic coding tool. It runs the same agent across a terminal CLI, an IDE extension, a desktop app, and a cloud environment, so work started in one surface can continue in another. It reads the shared AGENTS.md instructions file, speaks the Model Context Protocol (MCP), and can spin up a cloud code review on a GitHub pull request that comments inline and proposes fixes. Codex runs on OpenAI's GPT models — currently GPT-5.5 as the recommended frontier model — and is included with ChatGPT plans, from the free tier up through Pro. ## Mellum by JetBrains (/tools/mellum-by-jetbrains) Tagline: Fast LLMs for low-latency and high-performance workflows Category: AI Coding Agents Pricing model: not recorded Starts at: Not recorded Platforms: not recorded Public repo: none listed Docs: none listed Verified plans: none on file Strengths on file: Mellum is an open-source code language model released by JetBrains on Hugging Face.; It is purpose-built for low-latency code completion in IDE workflows.; It integrates with JetBrains IDEs through the JetBrains AI Assistant plugin. Limits on file: It is focused on code completion rather than broader agentic coding tasks.; Its capability breadth is narrower than larger frontier code models. Mellum by JetBrains is an open-source code language model, released by JetBrains on Hugging Face, built for low-latency code completion in IDE workflows. It is purpose-built for low-latency completion rather than general chat, and it integrates with JetBrains IDEs through the JetBrains AI Assistant plugin. It fits developers already in JetBrains IDEs who want fast inline completion. It is not aimed at broader agentic coding, since it targets completion rather than agent tasks, and its capability breadth is narrower than larger frontier code models.
Change log
- 2026-09-02 — First published, compiled from the catalogue entries for Codex and Mellum by JetBrains.