Compatibility and current limits

Separate protocol support, runtime commitments, and verified client behavior.

Toolgrain targets two explicit MCP revisions. Protocol support, runtime support, and interoperability evidence are different claims: a supported protocol does not establish every client’s UI or cancellation behavior.

Protocol scope

Revision	Model	Transports
2026-07-28	Per-request metadata, discovery, listing, and calls	Stdio and HTTP
2025-06-18	Legacy initialization-based stdio negotiation; independent legacy HTTP requests	Stdio and HTTP

Other revisions are unsupported. Legacy initialization can negotiate down to the supported older revision; this does not mean Toolgrain implements the client’s originally offered revision. Modern stdio requests can interleave with legacy traffic after era selection.

Modern listings include cache/server metadata and icons. Legacy results omit modern envelope metadata; listings omit icons and non-object-root output schemas. Legacy structured content is object-only; other JSON results remain text. Tool lists are a single page and nonempty cursors are rejected.

Client evidence

Inspector 2.11.0 passes the existing checks in both revisions and automatic discovery. Tested Codex and Claude Code tool flows work with documented timeout/presentation limitations. Cursor discovery/auth checks passed; execution and cancellation remain unverified. Modern Claude Code coverage, live OAuth login, GUI flows, and remote deployment remain open in the report.

See the 2026-10-22 client report for exact versions, assertion counts, controls, and workarounds, and protocol verification for setup and repeatable Inspector commands. Do not silently promote an old partial result into a current complete compatibility claim.

Application requirements

Node ^24.20.0 and Bun ≥1.4.4 <2 are the supported application runtimes. Strict TypeScript 5.5.4 is the declaration floor; Zod 4 is the peer requirement. Node 26 remains preview. Hosted combinations need passing CI evidence before verification claims; see the support policy.

Features outside the current scope

Resources and prompts methods, elicitation/multi-round-trip input, progress, subscriptions/change notifications, HTTP sessions, SSE streaming, and resumability are separate future work. There is no built-in per-tool timeout API or automatic provider cleanup. Endpoint auth is implemented; token issuance and per-tool scope-policy configuration are not.