fix(mcp): surface mid-session re-authorization and stop serving dead auth links by mattzcarey · Pull Request #2660 · cloudflare/agents

mattzcarey · 2026-10-22T12:39:48Z

When a request on a ready MCP server needs re-authorization, MCPClientManager now moves the server to authenticating, saves the new auth_url and throws MCPAuthorizationRequiredError. The OAuth callback for that link then exchanges the code instead of dropping it as a stale popup. On wake, a saved auth_url restores only while its state can still be redeemed, and a dead link is cleared so the server reconnects.

Fixes #2658. The SDK already built the step-up URL for a 403 insufficient_scope or a failed token refresh, but nobody saw it:

the server stayed ready and auth_url was never saved
the callback for that URL was consumed without a token exchange
the conformance worker patched connectionState by hand to get around it, and that patch is removed here

Other changes callers will see:

MCPAuthorizationRequiredError extends UnauthorizedError and keeps its name, so existing checks still match
callTool(), readResource() and getPrompt() reject with MCPConnectionNotFoundError for an unknown serverId, where they used to throw a TypeError
addMcpServer() returns { state: "authenticating", authUrl } when discovery gets a 401, and waits for a server that is still connecting instead of reporting it ready
requests that get a 401 at the same time now share one OAuth flow. Before, each registered its own client, so against a server that allows the handshake without a token the returned link could never complete, and two calls whose token expired together refreshed in parallel and lost the race on a rotating refresh token

import { MCPAuthorizationRequiredError } from "agents/mcp/client";

try {
  return await this.mcp.callTool({ serverId, name: "deploy", arguments: {} });
} catch (error) {
  if (error instanceof MCPAuthorizationRequiredError) {
    return { needsAuthorization: true, authUrl: error.authUrl };
  }
  throw error;
}

First of four stacked PRs, followed by #2661, #2662 and #2663. The first commit is @jonnyparris's fix from his fork, kept as he wrote it.

pnpm run check and the agents workers suite pass. The client conformance lanes pass without the old conformance patch.

changeset-bot · 2026-10-22T12:39:55Z

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Newer findings are available below. Devin Review posted a newer report on this PR, in addition to the findings presented here.

✅ Devin Review: No Issues Found

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agent-think · 2026-10-22T12:42:40Z

🟢 agents import sizes: 1 entry point changed, no growth

	Entry point	Exports	Largest gzip change	Size now
🆕	`agents/mcp/client`	2 new	—	74.7 KiB

Changed exports (2)

	Import	Gzip change	Size now
🆕	`agents/mcp/client#MCPAuthorizationRequiredError`	—	74.7 KiB
🆕	`agents/mcp/client#MCPConnectionNotFoundError`	—	74.7 KiB

How this works

Each runtime export is bundled on its own, minified, and gzipped. Changes smaller than 100 B, or smaller than 1% and 1 KiB, are ignored. Growth over 10% or 5 KiB is marked 🔴. This report is informational and does not fail CI. The workflow artifact contains every measurement.

_{Compared be174922 → 978d6f95 · workflow run · reported by agent-think[bot]}

pkg-pr-new · 2026-10-22T12:53:50Z

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agents

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@cloudflare/ai-chat

npm i https://pkg.pr.new/@cloudflare/ai-chat@2660

@cloudflare/codemode

npm i https://pkg.pr.new/@cloudflare/codemode@2660

hono-agents

npm i https://pkg.pr.new/hono-agents@2660

@cloudflare/shell

npm i https://pkg.pr.new/@cloudflare/shell@2660

@cloudflare/think

npm i https://pkg.pr.new/@cloudflare/think@2660

@cloudflare/voice

npm i https://pkg.pr.new/@cloudflare/voice@2660

@cloudflare/worker-bundler

npm i https://pkg.pr.new/@cloudflare/worker-bundler@2660

commit: 978d6f9

devin-ai-integration

Devin Review found 1 new potential issue.

…nager A request on a ready connection can fail with 403 insufficient_scope (scope step-up) or a 401 whose refresh fails. The SDK transport builds a new authorization URL, stores it on the provider and throws UnauthorizedError. MCPClientManager only surfaced auth URLs at connect time, so the URL was never shown, and a callback for it was dropped as stale because the connection was still ready. callTool(), readResource() and getPrompt() now detect this (unauthorized error plus a new provider authUrl), move the connection to authenticating, persist auth_url, and throw MCPAuthorizationRequiredError with authUrl. The normal OAuth callback then exchanges the code and reconnects. Other errors are rethrown unchanged.

- connect, discovery and mid-session requests enter authenticating through enterAuthenticating, so auth_url, state and broadcast always agree - addMcpServer returns authenticating when discovery needs OAuth, waits for in-flight connection work instead of reporting it ready, and rediscovers a connected-but-undiscovered server - restore only trusts a persisted auth_url whose state can still be redeemed; otherwise it reconnects (fixes dead links and the callback crash window) - callTool/readResource/getPrompt reject with MCPConnectionNotFoundError for an unknown serverId - conformance worker drops its requestReauth state patch

When several requests got a 401 together, as discovery's parallel catalog lists do against a server that allows the handshake without a token, each started its own OAuth flow on the shared provider: five client registrations, and a saved link whose code verifier belonged to another flow, so the callback failed with "No code verifier found for OAuth state". A 401 from the MCP endpoint now waits until the authorization an earlier one started has built a link or saved tokens, so later flows reuse that client. Other responses are not delayed.

… turns too The SSE transport posts to a per-session message endpoint, not the connection URL, so matching on the URL let concurrent 401s there refresh in parallel. A request is MCP traffic when it carries a JSON-RPC message or opens an event stream; OAuth requests are neither.

Wrap the transport's onUnauthorized instead of inferring the end of a flow from fetch responses and provider calls. A flow that failed outright used to hold later 401s until a 10 second timeout; now the next flow starts as soon as the previous one returns or throws. A flow that finds a newer access token than the one it was called with skips authorizing, so two calls hitting an expired token refresh once.

mattzcarey · 2026-10-30T15:57:27Z

Replaced by #2782 (MCP client changes) and #2783 (examples).

devin-ai-integration Bot reviewed Oct 22, 2026

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This was referenced Oct 22, 2026

refactor(mcp): split MCPClientManager and serialize connection work per server #2661

Closed

feat(mcp): durable reconnect and authorization-expiry jobs #2662

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mattzcarey force-pushed the fix/2658-mcp-midsession-reauth branch 3 times, most recently from ddeb8e2 to 4601cff Compare October 27, 2026 13:49

mattzcarey added this pull request to stack #2750 October 27, 2026 18:20

mattzcarey mentioned this pull request Oct 27, 2026

test(mcp): integration tests for MCP re-authorization against an msw server #2751

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devin-ai-integration Bot reviewed Oct 29, 2026

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Comment thread .changeset/mcp-sdk-2-4-cimd.md Resolved

jonnyparris and others added 6 commits October 29, 2026 19:24

chore: drop per-PR changesets; the stack has one, on its last PR

978d6f9

mattzcarey force-pushed the fix/2658-mcp-midsession-reauth branch from acab922 to 978d6f9 Compare October 29, 2026 19:28

mattzcarey mentioned this pull request Oct 30, 2026

fix(mcp): re-authorization and connection failures for the MCP client #2782

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mattzcarey closed this Oct 30, 2026

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fix(mcp): surface mid-session re-authorization and stop serving dead auth links - #2660

fix(mcp): surface mid-session re-authorization and stop serving dead auth links#2660
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