Explained: Copilot's Agent Harness in Microsoft Foundry

A top model alone gives a confident answer with no sources. What turns it into finished work is the harness: instructions, context, tools, a sandbox and the agent loop. On 28 October 2026 Microsoft put the Microsoft Copilot harness, the same one behind the GitHub Copilot app, the Copilot CLI and VS Code, into Foundry Agent Service as a public preview. We follow one research-brief agent (Microsoft's own example) from a weak answer to a sourced brief, see who runs what, the four building blocks, the azure.yaml line that picks the harness, the three Foundry options, a model swap that leaves the loop alone, and how hosted agents restart after an interruption (long-running resilience is generally available) without duplicate side effects. On screen, not spoken • azure.yaml: harness: {type: microsoft_copilot_preview} (preview azd surface); azd deploy --all, then azd ai agent invoke research-brief "…" • 20+ frontier models plus bring your own key • Preview limits: choose the harness when creating the agent; existing agents are unchanged; switching after creation is not supported yet • Pricing: harness runtime free before 1 December 2026; from 1 December sandbox compute and memory are USD $0.0994 per vCPU-hour and $0.0118 per GiB-hour; model inference, separately billed tools and connected services remain subject to their own charges; prices may vary by region and agreement The research-brief run, the 'Model A / Model B' swap and the invoice scenario are illustrations. 'My take' is opinion. Not covered in the video • Microsoft says the harness runtime was recently ported to Rust; it published no performance figures. Chapters: 0:00 Model vs harness 1:21 Why harnesses are hard 1:46 The 28 Oct announcement 2:27 Four blocks, one YAML file 3:43 Three options 4:30 Restarts and idempotency 5:44 Your turn 6:46 Catches and my take In-video check: a hosted agent saves a checkpoint after 'Build invoice', then 'Email customer' sends the invoice, then the work is interrupted during 'Update ledger'. After Foundry restarts it, how many emails does the customer get? The answer is in the video. Sources: Microsoft blog, 'Introducing the Microsoft Copilot harness', 2026-10-28: https://blogs.microsoft.com/blog/2026... Microsoft Foundry blog, 'Microsoft Copilot harness in Foundry', 2026-10-28: https://devblogs.microsoft.com/foundr... Visual Studio Magazine, 2026-10-28: https://visualstudiomagazine.com/arti... Instagram: @whiteboardbytes #aiagents #githubcopilot #microsoftfoundry