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Your Agent Defaulted to the Cheap Model. Here's How We Found Out.

Learn to choose between the Responses API and the Agents SDK, pick models on purpose, and wire up tools that actually run.

MCB Editorial

Tech Buzz Daily Desk · 10 min read ·

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Learning Objectives & Key Takeaways

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What you will learn
  1. The Agents SDK defaults to gpt-5.6-luna with reasoning effort none unless you set a model explicitly.
  2. tool_search only works on gpt-5.4 and later models, so check support before wiring it in.
  3. The Responses API makes you own the tool loop; the Agents SDK runner owns it for you.
  4. Built-in tools, function tools, tool search, PTC, and remote MCP servers are five separate tool paths.
  5. OpenAIResponsesModel is the Agents SDK's recommended OpenAI wrapper, not Chat Completions.

The Silent Downgrade

A support team shipped a customer-facing agent built on the Agents SDK and assumed it was running their strongest reasoning model, since nobody had typed a model name anywhere in the agent definition. Within a week, billing disputes that needed careful multi-step reasoning were getting flat, one-line answers that missed obvious context from earlier in the conversation.

The on-call engineer pulled the request logs and found the model field quietly set to gpt-5.6-luna, with reasoning.effort set to none and verbosity set to low. Nothing was broken. This is the documented default for an Agents SDK agent that never specifies a model, and it is tuned for cost-sensitive, high-volume workflows rather than nuanced customer escalations.

In the rest of this story
  1. Pick Your Control Boundary First
  2. Models Have Opinions by Default
  3. Five Doors Into Tool Calling
  4. A Complete Agents SDK Example
  5. Owning the Loop Yourself with the Responses API
2 code examples · 10 min read in full
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MCB Editorial

Tech Buzz Daily Desk. Contributing deep-dive engineering architectures, cloud paradigms, and system design insights to Master Class Buddy.