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MCP changed its protocol model on 2026-07-28: the specification now treats each request independently, rather than relying on an initialization handshake or a transport session. That change can expose harnesses that quietly depend on a connection, process, or hidden context surviving between calls—but it does not make an AI agent or its application workflow stateless.
What “stateless MCP” means in the 2026-07-28 specification
The Model Context Protocol specification dated 2026-07-28 says: “The Model Context Protocol (MCP) is a stateless protocol: all the information needed to process a request is contained in the request itself.” A server processes requests independently; it must not infer protocol context from earlier requests on the same connection or treat connection or process identity as evidence that two requests belong to the same conversation.
The protocol’s version boundary matters. In the 2026-07-28 specification, the initialize/initialized exchange and Mcp-Session-Id header are retired. Protocol version, client information, and client capabilities are carried in per-request metadata, and a discovery RPC is available when a client wants server capabilities in advance. The MCP project’s release announcement describes requests as routable to any server instance without protocol-level sticky sessions or a shared session store.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchThis is different from the 2025-11-25 Streamable HTTP transport specification, which described a server optionally assigning an MCP-Session-Id during initialization and clients returning it on subsequent requests. Do not combine that older session-header flow with the 2026-07-28 rules. When upgrading, first verify which protocol version the actual client and server implement; migration instructions depend on that version.
What changed—and what did not
| Area | Earlier session-oriented flow | 2026-07-28 stateless protocol |
|---|---|---|
| Where protocol context lives | Initialization establishes context; in the older Streamable HTTP flow, an optional session ID can be returned on later requests. | Each request carries the information needed to process it; prior requests and connection identity do not supply protocol context. |
| Routing | A client and server may need session continuity, depending on the implemented flow. | Requests can be routed independently to server instances; the project announcement says protocol-level sticky sessions or a shared session store are not required. |
| Workflow state | Applications may have relied on implicit continuity in addition to protocol session behavior. | Application state is still possible, but the application must identify and manage it explicitly rather than relying on a protocol session. |
| Lifecycle work | Implementations using sessions must manage their session behavior and lifecycle. | Applications using explicit handles must decide how to authorize, persist, expire, clean up, and reject invalid handles; MCP does not prescribe a wire-level handle type or method. |
Stateless protocol behavior does not require stateless applications. SEP-2567 describes an explicit-handle pattern: a tool creates application state, returns an identifier, and the client supplies that identifier in later tool arguments. The model can see and carry the handle. The handle is an application-defined reference, not a protocol-defined session object; the application must enforce ownership and authorization rather than treating possession of an identifier as permission by default.
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Why a hidden state assumption can break an agent
“Why your AI agents crash” is a useful hook, not an established general diagnosis. The sources cited here do not provide a crash-rate statistic or establish that MCP session assumptions are a general cause of agent crashes. The narrower, supportable failure mechanism is conditional: a harness can malfunction if it expects an earlier request, a particular connection, or a still-running process to supply context that a later request does not carry.
That dependency becomes visible when calls are interleaved with other tasks, retried, routed to another server instance, or resumed in a different runtime. The fix is not to keep every component alive indefinitely. It is to make the different lifetimes explicit:
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- Protocol request: one MCP operation with the metadata and inputs required by its protocol version.
- Transport connection: a communication channel, not proof of conversation identity.
- Process: for example, a long-lived child launched over stdio; its continued existence is not a durable workflow store.
- Agent task or session: an application/runtime concept. OpenAI’s Agents API architecture guide describes a hosted harness that runs a model/tool loop and maintains an agent session; that does not restore an MCP transport session.
- Application workflow: a multi-step activity that may outlive a request, connection, process, or runtime and therefore needs an explicit durable reference if it must resume.
Build the harness around explicit state and recoverable calls
- Make protocol version visible. Record the client and server versions in deployment configuration and logs. Construct each request with the metadata required by the version in use; for the 2026-07-28 HTTP flow, follow that specification’s per-request metadata and header requirements rather than caching protocol context in process-local memory.
- Pass workflow references explicitly. If a task spans tool calls, create an application handle and return it to the agent, then require it in every relevant later call. Define authorization, ownership checks, persistence, expiry, cleanup, and invalid-handle responses for that application.
- Design side-effecting tools for retries. A timeout can leave the caller unsure whether an operation completed. Use stable operation identifiers where appropriate and define duplicate-call behavior—such as returning the original result or safely rejecting a duplicate—so a retry does not silently repeat an action. This is an engineering recommendation arising from independent routing and retries, not an MCP requirement.
- Represent failures as outcomes. Handle cancellation, child-process termination, malformed output, unavailable tools, and nonzero exit status deliberately. Keep diagnostics on stderr when stdout carries machine-readable tool output, and do not treat an alive process as proof that workflow state is intact.
- Preserve status during cleanup. Capture the child command’s status before logging or cleanup changes it, and return that original status to the caller. Test normal exit, launch failure, nonzero tool exit, interruption, and cleanup under the exact shell invocation used in deployment.
Use Zsh semantics deliberately
The Zsh Manual, version 5.9.2 and updated July 12, 2026, documents that Zsh can emulate POSIX shells but its default mode is not POSIX-compatible. If a harness relies on Zsh syntax or behavior, invoke Zsh explicitly rather than assuming /bin/sh will behave the same way.
A small forwarding wrapper can preserve arguments and the child status without building an eval string:
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#!/usr/bin/env zsh
emulate -L zsh
if (( $# == 0 )); then
print -ru2 -- "usage: zsh-harness command [args ...]"
exit 64
fi
"$@"
status=$?
if (( status == 127 )); then
print -ru2 -- "child command reported status 127 (often command not found)"
elif (( status == 126 )); then
print -ru2 -- "child command reported status 126 (often not executable)"
fi
exit "$status"The Zsh manual documents status 127 for a command not found and 126 for an unexecutable file or certain unrecognized executable formats. Those numbers are useful signals, not a complete diagnosis: a program can itself exit with 126 or 127. Keep the child’s stderr and other launch diagnostics available so the caller can distinguish shell lookup trouble from a tool’s own failure.
For wrappers that install an exit trap, Zsh’s TRAPEXIT receives the shell’s exit status in $? at the start of trap execution. Capture it before cleanup and deliberately return it. Do not assume a Bash trap recipe has identical behavior: Zsh’s TRAPZERR does not run for every nonzero status, including commands in sublists ending in && or ||, and signal traps have their own return-status semantics. Test the actual invocation mode and interruption path rather than relying on a trap to catch every failure.
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Migration checks for teams moving between versions
- Identify the protocol version implemented by every client and server in the deployed path.
- For a 2026-07-28 implementation, remove reliance on the retired initialization exchange and
Mcp-Session-Idflow; follow the current specification for request metadata and capability discovery. - For a 2025-11-25 Streamable HTTP implementation, treat its optional initialization/session-header behavior as version-specific rather than applying it to the newer protocol.
- Trace a multi-step workflow across separate requests and confirm every required application reference is included explicitly.
- Exercise retries, duplicate side effects, routing to another instance, unavailable tools, nonzero exits, cancellation, and process cleanup.
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