Share

Every agent deployment estimate runs the same arithmetic. Per-step success rate, raised to the number of steps, watch the floor open. Directionally right, and nobody should be argued out of it. But it borrows two things the evidence hasn't supplied: one per-step rate to raise, and steps that fail independently of each other. Five runs of the same task, same agent, same instruction, different bill. Horizon is doing most of the damage in the reliability results, and it is the variable those studies measure least directly.

A basket, a second item, a card that only the person at the keyboard can produce, and a refund that may take a day to land. Four calls, one story.

The 2026-07-28 revision removed the connection that used to hold that story together. Nothing was actually subtracted. The handshake, the session, the durable job each have a successor, and every one of those successors now sits on your side of the line. What follows is what a shopping cart looks like once the protocol stops keeping it for you.

A basket, a second item, a card that only the person at the keyboard can produce, and a refund that may take a day to land. Four calls, one story.

A wrong parameter on a single tool call is annoying. A lost constraint propagating through a five-step chain is dangerous. A compressed summary that quietly drops the one fact preventing a catastrophic action twenty steps later is something else entirely. The failure modes change character as tasks get longer, not just frequency.

Recent research quantifies what practitioners already feel in their debugging sessions: capability gains have outpaced reliability gains by roughly seven to one, and failures compound nonlinearly with step count. Scaffolding that fixes one band's problems can introduce new failure surfaces at the next. These six cards map each step-count band to its dominant failure mode and the specific instrumentation check worth running this week.

The MCP 2026-07-28 revision removes protocol-level sessions from Streamable HTTP. Everything that used to ride implicitly on a persistent connection now needs to be declared, carried, or echoed back. Otherwise it simply vanishes between requests.

Where things landed varies. Some moved into per-request headers. Others became opaque handles the client passes as tool arguments. A few live in entirely new extension mechanisms. But the pattern holds: ambient context is gone, and explicit declaration is the only thing left.

Each card below maps one implicit dependency to its new location and the concrete breakage a team hits by leaving it unaddressed.

Foundations

Foundations

The Exponent Borrows Two Things Nobody Has Measured

The Exponent Borrows Two Things Nobody Has Measured

Somebody Has to Hold the Basket

Somebody Has to Hold the Basket