GPT-5.1 Codex vs o3

1. Purpose-Built for Agentic Coding

• Designed specifically for environments where the model acts as an autonomous or semi-autonomous coding agent.
• Optimized for multi-step reasoning in code tasks such as planning, refactoring, debugging, file generation, and tool coordination.

2. Enhanced Coding Intelligence

• Extends GPT-5.1's advanced reasoning capabilities to handle complex software architecture decisions.
• Better accuracy in code generation across languages (JavaScript, Python, TypeScript, Go, Rust, etc.).
• Produces cleaner, more idiomatic code aligned with modern frameworks and best practices.

3. Superior Tool Use & Code Navigation

• Excels at reading, understanding, and transforming multi-file codebases.
• Works well with Codex workflows that simulate real developer tooling.
• Strong at following function signatures, constraints, and code patterns within an existing project.

4. Long-Range Context Awareness

• 400,000-token context window enables the model to ingest large repositories or multiple files simultaneously.
• Supports deep analysis of project structures, dependencies, and cross-file logic.

5. Multi-Modal Development Capabilities

• Accepts text + image input and output - suitable for tasks like:
  • Reading UI mockups or screenshots to generate code
  • Understanding architectural diagrams
  • Reviewing images of whiteboard sessions

6. Agentic Workflow Optimization

• Built to manage longer chains of thought and execution typically required in:
  • Automated code repair
  • Project bootstrapping
  • Linting and migration tasks
  • Long-running coding agents using planning + execution loops

7. Continually Updated Model Snapshot

• Codex-specific version receives regular upgrades behind the scenes.
• Ensures the latest coding improvements without requiring developers to update model names.

8. Reliable Instruction Following

• Highly consistent in honoring explicit constraints:
  • Code styles
  • Folder structures
  • API contracts
  • Framework conventions

9. Broad API Support

• Works across Chat Completions, Responses API, Realtime, Assistants, and more.
• Ideal for apps that need live, reasoning-heavy coding agents or generative dev environments.

1. Advanced reasoning capability

• Designed for multi-step thinking across text, code, and visual inputs.
• Excels at math, science, logic puzzles, and complex analytical workflows.

2. Strong performance across domains

• Highly capable in technical writing, data analysis, and structured problem-solving.
• Useful for research, engineering tasks, and intricate instruction-following.

3. Visual reasoning support

• Accepts image inputs, enabling tasks such as diagram analysis, chart interpretation, and visual logic assessments.

4. High output capacity

• Up to 100,000 output tokens, supporting long-form content, technical breakdowns, and multi-part solutions.

5. Excellent instruction following

• Produces detailed, step-by-step responses for tasks requiring precision and clarity.
• Ideal for educational explanations, system design reasoning, and code walkthroughs.

6. Large 200K context window

• Handles long documents, multi-file reasoning, or extended conversations with minimal loss of context.

7. Broad API support

• Works with Chat Completions, Responses, Realtime, Assistants, Batch, Embeddings, Image Generation, and more.
• Supports streaming and function calling for advanced workflows.

8. Positioned as a legacy reasoning model

• Remains extremely capable but formally succeeded by GPT-5, which offers stronger reasoning and performance.