Codex Knowledge Base
GPT-5.5 Drops: What Changes for Codex Users
Six weeks. That is the gap between GPT-5.4 and GPT-5.5. OpenAI released its newest frontier model on 23 April 2026, rolling it out simultaneously to ChatGPT.
Legacy Code Archaeology with Codex CLI: Understanding, Documenting, and Safely Modernising Unfamiliar Codebases
Every senior developer has faced it: a critical system written by people who left years ago, sparse documentation, no tests.
MCP Schema Bloat and System Prompt Tax: Performance Impact of Tool Definitions
Every MCP server you connect to Codex CLI injects its full tool manifest — JSON schemas with parameter descriptions, type annotations, enum constraints.
Contract-Driven API Development with Codex CLI: Using Specmatic MCP for Spec-First Full-Stack Builds
Most agentic coding workflows suffer from the same failure mode: the agent generates code that compiles, passes its own tests.
Codex CLI Hooks Graduate to Stable: MCP Observation, Inline Config, and Auto-Review in v0.124
With the release of Codex CLI v0.124.0 on 23 April 2026, hooks have officially graduated from experimental to stable.
Codex CLI Custom Model Providers: The Complete Configuration Guide
Codex CLI ships with three built-in providers, but the real power lies in its extensible provider framework. Point Codex at any Responses API endpoint with a custom config.toml block.
Agent Identity Key Rotation and Security Operations for Codex CLI
The v0.123 release of Codex CLI introduced AuthMode::AgentIdentity, giving each agent its own Ed25519 key pair and replacing forwarded bearer tokens with.
Prompt Injection Defence for Codex CLI: Attack Vectors, Real CVEs, and Practical Hardening
Prompt injection remains OWASP's number-one vulnerability for LLM applications in 2026, appearing in an estimated 73% of production AI deployments.
Mutation Testing with Codex CLI: Why Your AI-Generated Tests Are Lying and How to Fix Them
AI coding agents are excellent at generating tests that hit every line of your code. They are also excellent at generating tests that assert almost nothing.
Context Fragment Injection: Modular DeveloperInstructions via Plugins
Until today, Codex CLI assembled its system prompt as a single monolithic DeveloperInstructions blob — one giant string stuffed into the developer-role.