How GPT-4.1 Differs
Literal Instruction-Following
Treats every line as law—ambiguous wording can derail answers.
Higher Steerability
A single, firm sentence can bring the model back on course.
Explicit Reasoning
Not a "reasoning model" out of the box—you must ask for it.
Non-negotiable Agent Reminders
Add these to every agentic system prompt:
Persistence
Prevents the model from handing control back before completion. GPT-4.1 may try to conclude interactions prematurely, especially with complex tasks. This reminder ensures it continues working until the full request is addressed.
Tool-calling
Stops hallucinated answers and forces real data. GPT-4.1 can be overconfident in its knowledge. This reminder encourages the model to use available tools to verify information rather than relying solely on its parameters, significantly reducing hallucinations.
Planning
Gives transparent reasoning and easier debugging. Unlike GPT-4, GPT-4.1 doesn't automatically show its reasoning process. This reminder forces the model to explicitly plan its approach, making its thought process visible and allowing you to catch errors before they happen.
Best Practices
API Tools Field
- • Name tools verb-first with sentence-case descriptions
- • Put examples under an # Examples heading outside JSON schema
- • Keep descriptions concise
Chain-of-Thought
Add explicit instructions like:
Expect +4% accuracy on coding tasks (but budget for extra tokens).
Long-context Hygiene
- • Instruction placement: best = both top and bottom
- • Small contexts: Use Markdown headings & code blocks
- • Huge document batches: Use simple XML
- • Avoid JSON for multi-file long contexts
Bulletproof Instructions
- • Start with a "Response Rules" section (bullets)
- • Add sub-sections for tone, workflow steps, or sample phrases
- • Use ordered lists for multi-step behavior
- • Reserve ALL-CAPS or emojis for genuine edge-cases
Common Failure Modes & Fixes
| Symptom | Cause | Fix |
|---|---|---|
| Hallucinated or null tool calls | "Always call a tool" rule with no escape hatch | Add "If you lack sufficient info, ask the user..." |
| Repetitive, copy-pasted phrases | Model latches onto provided examples | Explicitly instruct "vary phrasing" or supply multiple variants |
| Unwanted explanatory prose | No length/format guardrails | Declare exact format (JSON, Markdown) and forbid extras |
Diff & Patch Generation
- • Use line-number-free formats showing full old and new text blocks
- • Options: OpenAI's V4A diff, Aider's SEARCH/REPLACE, or pseudo-XML
- • When applying patches, wrap them in exact sentinel lines:
*** End Patch
Bringing It All Together
Think of these pieces as modular toggles: you'll rarely need everything, but omitting the persistence & tool-calling reminders, or letting instructions conflict, is where most "why is my agent acting weird?" bugs live.
Happy prompting—push hard, measure often, and iterate!