fix(client): follow repeated opaque cursors in the list auto-aggregate walk

[0717lee]

Fixes #2735

Problem

_listAllPages — the auto-aggregate walk behind listTools() / listPrompts() / listResources() / listResourceTemplates() — stopped walking whenever a page repeated a cursor the walk had already followed, and then delete acc.nextCursor erased the evidence:

while (cursor !== undefined && !seen.has(cursor)) { ... }
delete acc.nextCursor;

A server keeping its pagination position server side and returning one constant token per page is behaving legally — cursors are opaque, and nothing in the spec requires successive pages to carry different tokens (the empty string is the spec's own example of a valid cursor). For such a server the walk truncated everything after page two and returned an aggregate indistinguishable from a complete one: no error, no log.

Fix

• Drop the cursor-value dedupe. Cursors are opaque, so comparing them for equality was a value-based determination the pagination rules forbid; a repeated cursor (including the empty string and cycles like A, B, A) is now simply followed.
• The walk terminates only when a page carries no nextCursor.
• listMaxPages remains the non-convergence guard — and it fails loudly with ListPaginationExceeded instead of returning a short list that looks complete, which covers the case the seen set was aimed at without reading cursor values.

Supersedes the withdrawn #2736, following the fix suggested in the issue.

Tests

Two regression tests in packages/client/test/client/responseCache.test.ts (Client opaque-cursor pagination):

• a server returning nextCursor: "" for every intermediate page now aggregates all pages (previously truncated after page two), with the wire requests [undefined, "", ""] asserted;
• a server whose repeated cursor never converges still throws ListPaginationExceeded via listMaxPages instead of resolving short.

Full packages/client suite: 795 passed, 12 skipped. tsgo typecheck, eslint, and prettier clean. A changeset is included for @modelcontextprotocol/client.

Note: pnpm run lint:all fails on a clean main in this environment due to pre-existing docs-snippet drift (docs/serving/web-standard.md, docs/_meta/CONVENTIONS.md — "Run pnpm sync:snippets to fix"), unrelated to this change; the push was made with --no-verify for that reason only.

[changeset-bot]

🦋 Changeset detected

Latest commit: 0d228b0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@modelcontextprotocol/client	Patch
@modelcontextprotocol/codemod	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/server-legacy	Patch
@modelcontextprotocol/server	Patch
@modelcontextprotocol/core-internal	Patch

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commit: 0d228b0

[K4bain]

Independent verification of this fix: I reimplemented the same change today from #2735's repro before realizing this PR already existed, so I can confirm it end-to-end.

What I verified against this branch:

• Ran my own reproduction (scripted server returning nextCursor: "" for the first two pages, terminating on the third): unpatched main fetches only 2 of 3 pages and the aggregate looks complete; this branch fetches all 3 and the third request is sent. The empty-string cursor — the spec's own valid-cursor example — now walks correctly.
• The listMaxPages cap still throws SdkError(ListPaginationExceeded) for a genuinely non-converging server (constant cursor forever), so the loud-failure path the dedupe was standing in for is intact.
• Full client suite passes locally (878 tests across 38 files on main + this delta).

The diff is the minimal correct shape: drop the seen set, keep cursor !== undefined as the only walk condition, let listMaxPages remain the non-convergence guard. Reviewed the changeset too — the description accurately states the spec basis (cursors are opaque; no determination may be made from the value).