MCP Tasks, Elicitation and Apps

Explicit scope, MRTR elicitation, durable Tasks, Apps UI resources and safe cancellation on MCP 2026-07-28.

MRTR elicitation round trip#

Command	What it does
`tasks/get`	Full snapshot. Outer `resultType` is always complete
`tasks/update`	Sends `inputResponses`. Returns an empty ack
`tasks/cancel`	Records cancel intent. The ack is not a final status
`subscriptions/listen`	POST with a response SSE stream of task snapshots
`notifications/tasks`	Full snapshot tagged with the listen request id
`notifications/cancelled`	Request cancel on stdio only. Never for a Task
Close the response stream	Request cancel on Streamable HTTP
`tasks/status`, `tasks/result`, `tasks/list`	Removed. Method not found

service = TaskService(directory)file-backed TaskStore

created = service.dispatch(make_request(1, "tools/call",

{"name": "generate_report", "arguments": {"size": "large"}}))

task_id = created["result"]["taskId"]resultType task, working

service = TaskService(directory)new instance reloads it

service.advance_worker(task_id)working to input_required

service.dispatch(make_request(3, "tasks/get", {"taskId": task_id}))

service.dispatch(make_request(4, "tasks/update", {"taskId": task_id,

"inputResponses": {"approve_outline": {"action": "accept",

"content": {"approved": True}}}}))

service.advance_worker(task_id)stage 2 to completed

body, headers = make_http_request(5, "tasks/get", {"taskId": task_id})

headers["Mcp-Name"]equals params.taskId

Extension: io.modelcontextprotocol/ui, declared by both peers on each request.
Binding: _meta.ui.resourceUri on the tool in tools/list, before any call.
Resource: ui://notes/timeline.html with MIME text/html;profile=mcp-app.
CSP: empty connectDomains, resourceDomains, frameDomains, baseUriDomains by default.
Bridge: View sends ui/initialize, then ui/notifications/initialized after the host answers.
Fallback: no ui capability gives the tool without _meta.ui and a text result.
Cache key: ui:// URI, server identity and version, content digest, principal if private.

Roots are deprecated. Put scope in a workspaceUri argument or a resource URI.
Enforce three layers: authorization, containment and an OS sandbox.
Normalize the URI first. Then compare path components, not string prefixes.
Never use form mode for passwords, API keys, tokens or payment data.
URL mode accept means the user agreed to open the URL. It is not proof of completion.
An empty "elicitation": {} means form support. URL-only does not.

ledger = MutationLedger(Path(directory) / "mutations.sqlite3")

ledger.charge("acct-7", 1200, rpc_id=41, idempotency_key="checkout-7")

ledger.charge("acct-7", 1200, rpc_id=42, idempotency_key="checkout-7")

ledger.executionsone execution, same receipt

classify_retry(side_effect=True, idempotency_key=None)unsafe

buffer = BoundedSseBuffer(3)capacity at least two

buffer.push_final(response)final response never dropped

reconnect_plan(previous_subscription_id, affected_resources,

unsafe_mutation_was_in_flight=True)sendLastEventId False

retry_delay_ms("<client>", attempt)capped at 8000 ms

Factor	accept	decline	cancel
Meaning	User submitted	User refused	User dismissed
Server action	Validate content, continue	Complete refusal result	Stop safely
Nonce	Consumed with the delete	Consumed, no delete	Not consumed
Retry later	No	No	Yes, until expiry
Result	`deleted: True`	`deleted: False`	`deleted: False`

Setting	Default	Note
Protocol version	2026-07-28	In `_meta` on every request
Task `ttl_ms`	900_000	Measured from creation
Task `poll_interval_ms`	1_000	Minimum suggested poll delay
`server/discover` `ttlMs`	300_000	`cacheScope` public
`tools/list` `ttlMs`	60_000	Sorted by tool name
`requestState` expiry	300	Seconds after the form
`ReplayStore` `max_entries`	1_000	Full store gives -32023
Backoff cap	8_000	250 ms times 2 per attempt

Symptom	Cause	Fix
`-32602`	Missing or non-string protocol version	Send the version in `_meta`
`-32022`	Unsupported version string	Read `supported` and `requested` data
`-32021`	Missing form or tasks or ui capability	Declare it in this request
`-32020`	Header and body disagree	Match `Mcp-Method` and `Mcp-Name`
Task id gives not found	Handle sent before the store could read it	Persist before return
Mutation runs twice	New JSON-RPC id used as dedupe	Add a durable idempotency key
Hung work never ends	Progress resets every deadline	Keep an absolute maximum timeout
UI says stopped, worker runs	Cancel ack read as final	Poll the Task to a terminal status

Who decides that a call becomes a Task?: The server. The client only declares the extension.
Where is the final task result?: Under result in the next tasks/get snapshot.
Input before or after task creation?: Before: MRTR retry. After: tasks/update.
Does ui/initialize start an MCP session?: No. It is the iframe bridge handshake only.
Is an isError: true tool result a failed Task?: No. The Task is completed.
Resume a dropped stream with Last-Event-ID?: No. Open a new listen request and refetch.