fix(desktop): cancel MCP OAuth flows server-side so a retry doesn't 409

apps/desktop/src/app/chat/composer/mcp-suggestion-pills.tsx

@@ -3,11 +3,11 @@ import { useState } from 'react'
 import { composerFloatingPill } from '@/components/chat/composer-dock'
 import { Codicon } from '@/components/ui/codicon'
 import { Tip } from '@/components/ui/tooltip'
-import { addMcpServer, authMcpServer, getMcpOAuthFlow, removeMcpServer } from '@/hermes'
+import { addMcpServer, authMcpServer, cancelMcpOAuthFlow, getMcpOAuthFlow, removeMcpServer } from '@/hermes'
 import { useI18n } from '@/i18n'
 import { triggerHaptic } from '@/lib/haptics'
 import { brandFor, brandGlyphStyle } from '@/lib/mcp-brands'
-import { completeMcpDesktopOAuth } from '@/lib/mcp-dashboard-oauth'
+import { completeMcpDesktopOAuth, McpOAuthCancelled } from '@/lib/mcp-dashboard-oauth'
 import { directoryEntry } from '@/lib/mcp-directory'
 import { prettyName } from '@/lib/text'
 import { useSessionSlice } from '@/lib/use-session-slice'
@@ -70,15 +70,9 @@ export function McpSuggestionPills({ sessionId }: { sessionId: null | string })
         await completeMcpDesktopOAuth({
           serverName: known.name,
           start: authMcpServer,
-          status: async flowId => {
-            const flow = await getMcpOAuthFlow(flowId)
-
-            if (cancels.get(server)) {
-              throw CANCELLED
-            }
-
-            return flow
-          },
+          status: getMcpOAuthFlow,
+          cancelled: () => cancels.get(server) === true,
+          cancel: cancelMcpOAuthFlow,
           openExternal: url => window.hermesDesktop.openExternal(url)
         })
       } catch (error) {
@@ -102,7 +96,7 @@ export function McpSuggestionPills({ sessionId }: { sessionId: null | string })
     } catch (error) {
       setPhase(server, 'idle')
 
-      if (error !== CANCELLED) {
+      if (!(error instanceof McpOAuthCancelled)) {
         notifyError(error, copy.connectFailed(prettyName(server)))
       }
     }
@@ -146,7 +140,3 @@ export function McpSuggestionPills({ sessionId }: { sessionId: null | string })
     )
   })
 }
-
-// Thrown by the poll wrapper when the user cancels — the rollback has its own
-// path, so the catch must swallow this rather than toast it.
-const CANCELLED = Symbol('mcp-pill-cancelled')

apps/desktop/src/components/assistant-ui/mcp-setup-tool.tsx

@@ -13,6 +13,7 @@ import { Input } from '@/components/ui/input'
 import {
   addMcpServer,
   authMcpServer,
+  cancelMcpOAuthFlow,
   getActionStatus,
   getMcpCatalog,
   getMcpOAuthFlow,
@@ -25,7 +26,7 @@ import { useI18n } from '@/i18n'
 import { triggerHaptic } from '@/lib/haptics'
 import { AlertCircle, CheckCircle2, Loader2 } from '@/lib/icons'
 import { brandFor, brandGlyphStyle } from '@/lib/mcp-brands'
-import { completeMcpDesktopOAuth } from '@/lib/mcp-dashboard-oauth'
+import { completeMcpDesktopOAuth, McpOAuthCancelled } from '@/lib/mcp-dashboard-oauth'
 import { directoryEntry } from '@/lib/mcp-directory'
 import { prettyName } from '@/lib/text'
 import { cn } from '@/lib/utils'
@@ -253,9 +254,8 @@ function McpSetupPending({ args }: ToolCallMessagePartProps) {
     cancelRef.current = false
     setWorking(true)
 
-    // Poll-boundary abort for the two long flows (OAuth browser round-trip,
-    // background install). Wrapping the status fns keeps the loops themselves
-    // untouched — they throw the sentinel instead of returning stale progress.
+    // Poll-boundary abort for the background-install loop; the OAuth flows
+    // carry their own cancel via completeMcpDesktopOAuth's `cancelled`.
     const throwIfCancelled = <T,>(value: T): T => {
       if (cancelRef.current) {
         throw CANCELLED
@@ -277,7 +277,9 @@ function McpSetupPending({ args }: ToolCallMessagePartProps) {
         const flow = await completeMcpDesktopOAuth({
           serverName: server,
           start: authMcpServer,
-          status: flowId => getMcpOAuthFlow(flowId).then(throwIfCancelled),
+          status: getMcpOAuthFlow,
+          cancelled: () => cancelRef.current,
+          cancel: cancelMcpOAuthFlow,
           openExternal: url => window.hermesDesktop.openExternal(url)
         })
 
@@ -322,7 +324,9 @@ function McpSetupPending({ args }: ToolCallMessagePartProps) {
           flow = await completeMcpDesktopOAuth({
             serverName: known.name,
             start: authMcpServer,
-            status: flowId => getMcpOAuthFlow(flowId).then(throwIfCancelled),
+            status: getMcpOAuthFlow,
+            cancelled: () => cancelRef.current,
+            cancel: cancelMcpOAuthFlow,
             openExternal: url => window.hermesDesktop.openExternal(url)
           })
         } catch (error) {
@@ -372,7 +376,7 @@ function McpSetupPending({ args }: ToolCallMessagePartProps) {
     } catch (error) {
       // User cancel: the declined respond is already on the wire — the
       // abandoned flow just stops, nothing to report.
-      if (error === CANCELLED) {
+      if (error === CANCELLED || error instanceof McpOAuthCancelled) {
         return
       }
 

apps/desktop/src/hermes.ts

@@ -1151,6 +1151,16 @@ export function getMcpOAuthFlow(flowId: string): Promise<McpOAuthFlow> {
   })
 }
 
+/** Cancel an in-flight MCP OAuth flow server-side, freeing the per-server
+ *  "already in progress" slot so a retry doesn't 409. */
+export function cancelMcpOAuthFlow(flowId: string): Promise<{ ok: boolean; status: string }> {
+  return window.hermesDesktop.api<{ ok: boolean; status: string }>({
+    ...profileScoped(),
+    path: `/api/mcp/oauth/flows/${encodeURIComponent(flowId)}`,
+    method: 'DELETE'
+  })
+}
+
 export function getToolsets(): Promise<ToolsetInfo[]> {
   return window.hermesDesktop.api<ToolsetInfo[]>({
     ...profileScoped(),

apps/desktop/src/lib/mcp-dashboard-oauth.ts

@@ -12,10 +12,26 @@ interface CompleteOptions {
   start: (name: string) => Promise<McpOAuthFlow>
   status: (flowId: string) => Promise<McpOAuthFlow>
   openExternal: (url: string) => Promise<void>
+  /** Polled between status checks. Returning true cancels: the flow is
+   *  cancelled SERVER-SIDE (freeing the per-server in-progress slot — without
+   *  it a retry 409s until the backend's callback timeout) and the promise
+   *  rejects with `McpOAuthCancelled`. */
+  cancelled?: () => boolean
+  /** Server-side flow cancel, wired to DELETE /api/mcp/oauth/flows/{id}. */
+  cancel?: (flowId: string) => Promise<unknown>
   sleep?: (milliseconds: number) => Promise<void>
   maxPollFailures?: number
 }
 
+/** Thrown when the caller's `cancelled()` tripped — callers branch on this to
+ *  skip error toasts for a deliberate user cancel. */
+export class McpOAuthCancelled extends Error {
+  constructor() {
+    super('OAuth cancelled by user')
+    this.name = 'McpOAuthCancelled'
+  }
+}
+
 const defaultSleep = (milliseconds: number) => new Promise<void>(resolve => window.setTimeout(resolve, milliseconds))
 
 export async function completeMcpDesktopOAuth({
@@ -23,6 +39,8 @@ export async function completeMcpDesktopOAuth({
   start,
   status,
   openExternal,
+  cancelled,
+  cancel,
   sleep = defaultSleep,
   maxPollFailures = 3
 }: CompleteOptions): Promise<McpOAuthFlow> {
@@ -41,6 +59,13 @@ export async function completeMcpDesktopOAuth({
   let pollFailures = 0
 
   for (;;) {
+    if (cancelled?.()) {
+      // Free the backend slot before rejecting; best-effort — the flow also
+      // dies on its own timeout if this request is lost.
+      await cancel?.(started.flow_id).catch(() => {})
+      throw new McpOAuthCancelled()
+    }
+
     let current: McpOAuthFlow
 
     try {