GPT-5 Codex vs GPT-3.5 Turbo

1. Purpose-Built for Agentic Coding

• Optimized specifically for scenarios where the model must act as an autonomous or semi-autonomous coding agent.
• Tailored for Codex workflows such as planning, editing, debugging, and multi-step tool-driven code tasks.

2. Advanced Coding Reasoning

• Extends GPT-5's higher reasoning mode to better handle complex software logic and multi-file dependencies.
• Produces more accurate, structured, and maintainable code across modern programming languages.

3. Strong Tool Use in Developer-Like Environments

• Designed for Codex's agent environment, enabling the model to:
  • Read and modify files
  • Follow function signatures and API contracts
  • Navigate codebases with awareness of context and structure

4. Large Context Window for Full-Project Understanding

• 400,000-token context allows ingestion of:
  • Entire repositories
  • Multiple files at once
  • Architectural descriptions
• Enables long-range reasoning across codebases rather than isolated snippets.

5. Multimodal Capability for Development Tasks

• Accepts text and image as input (great for screenshots of error logs, UI mocks, whiteboards).
• Outputs text only, focusing its output precision on code, reasoning, and documentation.

6. Continuous Snapshot Updates

• The underlying model version is regularly upgraded behind the scenes.
• Ensures developers always use the best coding-enhanced GPT-5 variant without changing model names.

7. Reliable Instruction Following

• Very strong adherence to constraints like:
  • File/folder structure requirements
  • Framework conventions
  • Naming patterns
  • Linting rules
• Makes it suitable for production coding agents.

8. Broad API Integration

• Available only in the Responses API, giving you:
  • Streaming
  • Structured outputs
  • Function calling
• Allows creation of interactive coding tools and agent workflows with tight model control.

1. Extremely low-cost text model

• One of the cheapest legacy models available.
• Suitable for very high-volume workloads with simple requirements.

2. Good for lightweight NLP tasks

• Classification, summarization, rewriting, paraphrasing, intent detection.
• Works for simple logic tasks and short reasoning sequences.

3. Works well for basic chatbots

• Optimized for Chat Completions API, originally powering early ChatGPT use cases.
• Good for rule-based or templated conversation flows.

4. Stable and predictable outputs

• Legacy behavior makes it suitable for systems built years ago that rely on its quirks.
• Good for backward compatibility or long-term enterprise pipelines.

5. Supports fine-tuning

• Useful for teams maintaining older fine-tuned GPT-3.5 models.
• Allows domain-specific compression of older datasets.

6. Limited capabilities compared to newer models

• No vision, no audio, no streaming, and no function calling.
• Much weaker reasoning and correctness vs GPT-4o mini or GPT-5.1.

7. Small context window (16K)

• Limited for multi-document tasks or long conversations.
• Best used for short, simple prompts or structured tasks.

8. Recommended migration path

• OpenAI explicitly recommends using GPT-4o mini instead.
• 4o mini is cheaper, smarter, faster, multimodal, and far more capable.