What Copilot Chat Developer Mode Actually Shows You

I spent a solid hour last month watching a declarative agent ignore an API plugin I’d wired up. The instructions were clear, the manifest looked right, the OpenAPI spec was valid. Every prompt came back with a confident answer that had nothing to do with my data. Copilot was making things up rather than calling my endpoint, and I had no idea why.

Then I typed -developer on into Copilot Chat, and the mystery evaporated in about thirty seconds.

Seeing the Orchestrator Think

Developer mode is a built-in debugging tool in Microsoft 365 Copilot Chat. Type that command and every response from your agent comes back with a debug card — a detailed breakdown of what the orchestrator did behind the scenes to produce its answer.

The card shows three things that matter. First, agent metadata and capabilities: which knowledge sources are active, whether web search is enabled, and the identifiers you need for tracking a specific conversation. Second, function matching: did the orchestrator consider your API plugin’s functions relevant to the user’s prompt? Third, execution details: if a function was selected, what HTTP request did Copilot actually send, what response came back, and how long did it take?

That last section is where most of the debugging value lives. You can see the endpoint that was called, the request headers with auth tokens redacted, and the full response body. When something fails, you’re reading the receipt — not guessing.

The Problem It Actually Solves

Before developer mode, troubleshooting a misbehaving agent in Copilot meant staring at manifest files trying to work out what went sideways. Your agent returned a wrong answer and you couldn’t tell where the chain broke. Was the orchestrator not matching your function? Matching but failing on auth? Getting bad data back from the API? The orchestration layer was a black box.

Now you see exactly where it breaks. In my case, the debug card showed “No matched functions” — meaning the orchestrator never even considered calling my API. The problem wasn’t auth or endpoints or response formatting. It was my description_for_model field. The description said “Returns project data” but my prompt asked “What’s the status of the Henderson build?” The orchestrator couldn’t bridge that semantic gap.

I rewrote the description to cover the ways people actually ask about project status, and the next prompt hit the API cleanly.

Where It Fits in Your Workflow

Developer mode works directly in the browser. Open Copilot Chat, select your agent, type -developer on, and start testing prompts. If you’re using the Microsoft 365 Agents Toolkit in Visual Studio Code, press F5 to launch your agent and the same command activates in the chat window. The debug panel in the toolkit gives you a matching view with downloadable diagnostic logs.

A couple of patterns worth knowing: when the debug card shows “No functions selected for execution,” your function descriptions likely aren’t semantically close enough to the prompt. When it shows a function was selected but execution failed, the HTTP status code in the card usually tells you what went wrong — a 401 means your OAuth registration doesn’t match, a timeout means your API needs to respond faster.

What I’m Watching

Microsoft keeps expanding what the debug card reveals, and the Quick Copy feature now lets you export the full debugging JSON to share with a colleague or attach to a support ticket. For anyone building agents that connect to external APIs through Copilot, this is the single most useful diagnostic tool in the stack. It turns “something’s broken” into “here’s exactly what happened, and here’s why.”

If you haven’t typed -developer on yet, start there.

Microsoft Learn — Test and debug agents using Developer Mode https://learn.microsoft.com/en-us/microsoft-365/copilot/extensibility/debugging-agents-copilot-studio(opens in new window) The primary documentation page. Covers enabling/disabling developer mode, the debug info card fields, troubleshooting common failures, and how to report issues.
Microsoft Learn — Test and debug agents in Microsoft 365 Agents Toolkit using developer mode https://learn.microsoft.com/en-us/microsoft-365/copilot/extensibility/debugging-agents-vscode(opens in new window) Focuses on using developer mode alongside the Agents Toolkit in VS Code (F5 launch, debug panel, diagnostic logs).
Microsoft 365 Developer Blog — Introducing the agent debugging experience in Microsoft 365 Copilot (April 9, 2025) https://devblogs.microsoft.com/microsoft365dev/introducing-the-agent-debugging-experience-in-microsoft-365-copilot/(opens in new window) The GA announcement post by Carol Mbasinge Kigoonya. Covers new features including agent configuration insights, execution monitoring, latency tracking, and Quick Copy Debugging JSON.
Microsoft 365 Roadmap — Feature ID 474450 https://www.microsoft.com/microsoft-365/roadmap?featureid=474450(opens in new window) The original roadmap entry for Copilot Chat developer mode, listed as GA from January 2025.
Microsoft Learn — Set up your development environment for Microsoft 365 Copilot https://learn.microsoft.com/en-us/microsoft-365/copilot/extensibility/prerequisites(opens in new window) Broader context on the Copilot development environment, licensing, and extensibility options that developer mode supports.