Stateless MCP Migration: How to Regression Test Your Server

Migrating an MCP server to the stateless 2026-07-28 revision requires more than confirming that MCP Inspector connects, because Inspector defaults to the legacy protocol era and may only test the retained initialize-based path. Under the new revision, requests carry protocol and capability information in `_meta`, sessions and server-initiated requests such as elicitation are removed, `server/discover` supports version discovery, and workflows needing user input must use Multi Round-Trip Requests with securely validated request state. The recommended approach is to capture a legacy baseline of tool lists, successful and failing calls, error envelopes, resources, prompts, and stateful flows; upgrade the SDK separately from enabling the new protocol; then test both legacy and modern clients explicitly against a dual-era server. Regression suites should cover removed or changed features including session-header replay, logging behavior, subscriptions, tasks, HTTP headers, error-code changes, authorization requirements, and deprecated capabilities, while normalizing unavoidable era-specific output differences before diffing results. Conformance tests should be pinned to each protocol revision and integrated into CI, and agent-level testing should verify that clients using the server still make correct tool choices and produce observable outcomes, since wire-level compatibility alone does not ensure unchanged agent behavior.

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