- Who it helps
- Developers debugging differences between agent clients and tool integrations.
- Availability
- Polytomic publishes probe descriptions and dated client observations. The page can be read publicly; running probes requires a compatible client and was not attempted here.
The duplicate-payload question
One probe returns different markers in text and structured data, then checks what the client exposes. Text-only and structured-only controls help interpret the result. This can reveal client-specific behavior behind an otherwise identical tool call.
Use the result at its proper scope
The field guide also checks protocol metadata, interactive URL requests and resource access. Its preferred payload behavior is an evaluation rubric. A recorded client version and session do not establish how every model or future client behaves.
What you can do
- Read the probe, control cases and dated client result together before diagnosing a tool integration. Open reference ↗
Where the evidence stops
The field guide and result table were read. No MCP server was connected, and duplicate billing or internal model visibility was not independently established.
Sources read. Sources read October 20, 2026. No installation, model inference or complete external workflow was executed.
Sources & further reading
Web · implementation
What the Harness: MCP client compatibility probes ↗
Read field-guide tests and result table. Duplicate structured/text payload handling is a client-specific probe; its preferred behavior is the authors’ rubric.
Polytomic · page checked 2026-10-20
Coverage note: Primary evidence establishes the documented scope. Independent execution and complete YouTube, X and community coverage remain unavailable.
The record over time (1)
Dated source checks and published explanations. An observation date can differ from the original release date.
- · Added to Current
What the Harness is a public field guide for checking how agent clients handle tool results and other Model Context Protocol features.