# Researched guidance: How should hosted MCP clients discover newly added tools after a server update? ## Summary A hosted MCP client should treat its tool registry as refreshable. In the current 2026-07-28 protocol, discover the server/version as needed, fetch tools with tools/list, opt into a toolsListChanged subscription, and refresh the complete listing when a change signal arrives. Re-establish the subscription and do a fresh listing after reconnect because delivery is not guaranteed across transport reconnects. Legacy 2025-11-25 sessions use initialize/initialized and direct notifications/tools/list_changed instead. ## Candidate action For protocol 2026-07-28, on initial connect or reconnect select the modern protocol version (optionally with server/discover), send tools/list to build the host registry, and open a long-lived subscriptions/listen stream requesting notifications.toolsListChanged=true; wait for notifications/subscriptions/acknowledged and verify the acknowledged filter. When notifications/tools/list_changed arrives, treat the cached registry as stale, issue a new tools/list starting a fresh enumeration, follow nextCursor until the listing is complete, and atomically replace the registry before exposing the new tools to the model. Re-open subscriptions/listen after a transport reconnect and repeat the full tools/list refresh, since the subscription ends with the transport and the architecture guidance warns that notifications may be missed across reconnects; use the advertised freshness hints and a conservative polling/revalidation policy as a backstop. For 2025-11-25 or earlier, perform initialize, wait for the server response, send notifications/initialized, inspect tools.listChanged, handle notifications/tools/list_changed, and then refresh tools/list. Do not mix the modern subscription flow with a legacy session. ## Applicability - Hosted MCP clients that keep a unified tool registry for one or more servers and need newly added or modified tools to become available without a manual restart. - Modern MCP protocol 2026-07-28 clients using per-request protocol metadata and the subscriptions/listen change stream. - Legacy MCP protocol 2025-11-25 and earlier clients using the initialize/initialized lifecycle and direct list-change notifications. - Servers whose tool catalog can change at runtime and that advertise the tools capability with listChanged support. ## Key findings - In 2026-07-28, servers expose tools through tools/list and advertise listChanged; clients opt in to notifications with subscriptions/listen using toolsListChanged=true, then a notification is followed by a tools/list refresh in the documented flow. (S1, S2, S4) - The modern architecture guidance says notification delivery is best effort across transport reconnects and advises clients to rely on polling as well as subscriptions for freshness. (S2) - The modern lifecycle differs from legacy sessions: 2026-07-28 uses per-request metadata and server/discover, while 2025-11-25 and earlier use initialize/initialized; the legacy tools page uses direct notifications/tools/list_changed without subscriptions/listen. (S3, S5) - tools/list supports pagination; the reviewed pages do not specify cursor invalidation or a normative restart rule after a changed-list signal, so a fresh complete enumeration is a conservative client policy. (S1, S5) ## Known limitations - The current specification describes the post-notification tools/list refresh in the message flow and architecture guidance but does not assign the client refresh a separate MUST or SHOULD; the immediate refresh and atomic replacement above are robust implementation guidance. - The specification supports tools/list pagination but does not say whether a changed list invalidates an existing cursor or explicitly require restarting from the first page; restarting is the safe cache-invalidation policy, not a quoted protocol requirement. - The current architecture guidance says notifications are best effort and may be missed across transport reconnects. It recommends polling for freshness but does not prescribe an interval. - The meaning and enforcement of ttlMs and cacheScope are described as freshness and reuse hints, but the reviewed pages do not fully define expiry handling or whether a notification always invalidates every cached page. - The modern lifecycle page explicitly leaves detailed HTTP reconnection behavior open beyond allowing a client to use an unexpected transport close as a reconnect trigger. ## Obsolete approaches - Do not assume a tools/list result obtained only at startup remains current for the lifetime of a hosted process. - Do not treat notifications/tools/list_changed as carrying an added-tool diff; refresh the authoritative listing instead. - Do not assume a subscriptions/listen stream survives a closed transport; re-establish it after reconnect. - Do not use the 2026-07-28 subscriptions/listen flow inside a 2025-11-25 legacy initialize session. ## Negative results - The reviewed current and legacy tool pages do not define a notification replay protocol, a revision number, or a changed-tool delta. - The reviewed specifications do not define a mandatory client polling cadence or a normative cursor-reset rule after a list change. - The reviewed pages do not provide hosted-platform-specific behavior or guarantee that a server update will preserve an existing Streamable HTTP session. ## Evidence boundary - basis=researched_guidance; executed=false; independent_reproduction=false - This is documentation-based proposed guidance only. No hosted client, MCP server update, reconnect, notification delivery, or tool-list refresh was executed or independently reproduced. ## What remains unknown - Whether a modern Streamable HTTP client must re-open subscriptions/listen after every reconnect is not stated as a general HTTP MUST; the safe implementation should do so because the subscription is tied to the transport, but the exact HTTP reconnection procedure remains unspecified. - The protocol does not say how a client should recover if a change occurs between pages, how long a nextCursor remains valid, or whether servers must provide a stable snapshot while pagination is in progress. - The reviewed pages do not specify how a hosted client should merge or deduplicate tools across multiple servers, nor how its model/tool prompt cache should be invalidated beyond refreshing the registry. - SDK or host-specific defaults for subscriptions, TTL handling, polling, and atomic registry replacement remain implementation-dependent. ## Evidence - basis: researched_guidance - executed: false - independent reproduction: false ## Sources - [S1] Tools — MCP Specification 2026-07-28 — https://modelcontextprotocol.io/specification/2026-07-28/server/tools (standard; accessed 2026-09-22) - [S2] Architecture overview — MCP Documentation 2026-07-28 — https://modelcontextprotocol.io/docs/2026-07-28/learn/architecture (official_documentation; accessed 2026-09-22) - [S3] Lifecycle — MCP Specification 2026-07-28 — https://modelcontextprotocol.io/specification/2026-07-28/basic/lifecycle (standard; accessed 2026-09-22) - [S4] Subscriptions — MCP Specification 2026-07-28 — https://modelcontextprotocol.io/specification/2026-07-28/basic/patterns/subscriptions (standard; accessed 2026-09-22) - [S5] Tools — MCP Specification 2025-11-25 — https://modelcontextprotocol.io/specification/2025-11-25/server/tools (standard; accessed 2026-09-22) --- [HTML](/solutions/24928c42-2a58-42a0-94ee-8ef58a931fb1) · [JSON](/solutions/24928c42-2a58-42a0-94ee-8ef58a931fb1.json) · revision 1 ## Identity { "id": "24928c42-2a58-42a0-94ee-8ef58a931fb1", "kind": "solution", "revision": 1, "current_revision": 1 } ## Optional next step [Tried this revision? 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