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Multi-round-trip requests

Sometimes a tool can't finish in one round trip. It needs something only the user has: a choice, a confirmation, a credential.

Before 2026-07-28 the server got it by calling back: opening its own request to the client (an elicitation, a sampling call) in the middle of handling the original one. The 2026-07-28 spec retires that back-channel.

Instead, the server returns.

Return, don't call back

The server answers tools/call with an InputRequiredResult instead of a CallToolResult. Two of its fields do the work:

  • input_requests: what the server still needs, as a dict keyed by names the server chose. Each value is an ElicitRequest, a CreateMessageRequest, or a ListRootsRequest.
  • request_state: an opaque token. The client echoes it back verbatim on the retry. Your server is the only thing that reads it.

The client fulfils each request, then calls the same tool again, carrying its answers in input_responses and the token in request_state. The server now has what it was missing and returns a normal CallToolResult.

That's the whole protocol. Every leg is an ordinary request from the client to the server. Nothing ever flows the other way.

The server side

On @mcp.tool() you rarely build this by hand: declare a dependency that asks the user and the SDK returns the InputRequiredResult for you - that form is the Dependencies tutorial. The two forms don't mix: a call has one input_responses/request_state channel, so a tool that uses Resolve(...) parameters cannot also return InputRequiredResult from its body. A declared InputRequiredResult return is rejected at registration (InvalidSignature), and an undeclared one fails the call at runtime. The manual form is the low-level Server, whose on_call_tool handler is allowed to return either result type:

--8<-- "docs_src/mrtr/tutorial001.py"
  • on_call_tool is typed -> CallToolResult | InputRequiredResult. Returning the second one is the entire server-side API.
  • On the first call params.input_responses is None, so the guard fires and the handler asks instead of answering.
  • On the retry, the ElicitResult the client sent is sitting under the same key ("region") that the server used in input_requests.

Everything else in that file (the explicit input_schema, the hand-built CallToolResult) is the ordinary low-level Server, covered in The low-level Server. This page only adds the second return type.

Beyond tools

tools/call is not special: at 2026-07-28 a server may answer prompts/get and resources/read the same way. On MCPServer, an @mcp.prompt() function — or an @mcp.resource() template function — returns the InputRequiredResult itself and reads the retry's answers off the context:

--8<-- "docs_src/mrtr/tutorial004.py"
  • The first round returns the InputRequiredResult. On the retry, ctx.input_responses holds the answers under the same keys and the function returns its ordinary result — prompt messages here, resource content for a template resource.
  • An @mcp.tool() function can return the result directly the same way, when the dependency form doesn't fit.
  • Static @mcp.resource() functions don't participate: they take no Context, so they could never read the retry. Only template resources can ask.
  • The era rules below apply unchanged: returning an InputRequiredResult on a pre-2026 session is the same -32603 the warning describes.

The client side

Client runs the loop for you.

Register the callbacks the server might ask for (elicitation_callback, sampling_callback, list_roots_callback) and call the tool. When an InputRequiredResult arrives, Client dispatches each entry in input_requests to the matching callback, retries with the answers and the echoed request_state, and keeps going until a CallToolResult comes back:

--8<-- "docs_src/mrtr/tutorial003.py"
  • That elicitation_callback is the same one a pre-2026 server's back-channel elicitation/create would have hit. The same is true of sampling_callback for sampling/createMessage and list_roots_callback for roots/list: at 2026-07-28 the standalone server->client RPCs are gone, but the identical ElicitRequest / CreateMessageRequest / ListRootsRequest payloads ride inside input_requests and dispatch to the same three callbacks. One set of callbacks serves both eras.
  • call_tool returns a plain CallToolResult. The intermediate rounds are invisible to the caller.
  • get_prompt and read_resource drive the same loop.

!!! check Leave the callback off and the loop fails on the first round: the SDK's stand-in callback answers every elicitation with an error, and call_tool raises MCPError with the message "Elicitation not supported".

The loop is bounded. Client(..., input_required_max_rounds=10) is the default cap; a server that keeps returning InputRequiredResult past it makes call_tool raise. If a round carries only request_state and no input_requests, Client sleeps briefly (50ms doubling to a 250ms ceiling) before retrying, so a server that is just saying "not done yet" isn't busy-polled.

Driving the loop yourself

The auto-loop is enough for a single-process client. Own the loop instead when:

  • Your client is distributed: the process that renders the question to the user is not the process that called call_tool, so a different worker issues the retry. request_state is the persistable token you carry across that boundary, through your own storage, and input_responses is what the other side sends back with it.
  • You want to inspect each round: log or audit every input_requests entry, refuse certain request kinds, or apply your own backoff between legs.
  • You want a wall-clock bound rather than a round-count bound: wrap your own loop in anyio.fail_after(...) instead of relying on input_required_max_rounds.

Drop to the underlying session, where allow_input_required=True hands you the union directly:

--8<-- "docs_src/mrtr/tutorial002.py"
  • client.session.call_tool(..., allow_input_required=True) widens the return type to CallToolResult | InputRequiredResult. The isinstance is what narrows it back.
  • request_state is now in your hands. Write it down between legs and the conversation can resume from a fresh process.
  • For every entry in input_requests you put an InputResponse under the same key in input_responses. fulfil is where your UI goes; this one hard-codes the answer.
  • Same tool name, same arguments, every leg. The retry is the original call carried out again, not a new method.

A 2026-07-28 result

InputRequiredResult only exists at protocol version 2026-07-28. The in-memory Client(server) negotiates it for you; over the wire, mode="auto" discovers it. After connecting, client.protocol_version tells you what you got.

!!! warning A pre-2026 session has nowhere to put an InputRequiredResult. Return one from your handler on a mode="legacy" connection and the runner cannot serialize it into the negotiated version; the client gets back a -32603 "Handler returned an invalid result" error. A server that serves both eras must check ctx.protocol_version before reaching for it.

!!! info URL-mode elicitation rides this exact mechanism on a 2026 connection. The entry in input_requests is an ElicitRequest whose params are ElicitRequestURLParams; the user finishes the out-of-band flow and your client retries the call. Same loop, no new API. The high-level server half is in Elicitation.

Recap

  • At 2026-07-28 a server that needs input mid-call returns an InputRequiredResult. It never opens a request to the client.
  • input_requests is what it needs. request_state is an opaque resume token only the server reads.
  • Client runs the retry loop for you: register elicitation_callback / sampling_callback / list_roots_callback and call_tool returns a plain CallToolResult. input_required_max_rounds (default 10) bounds it.
  • To inspect or persist rounds, use client.session.call_tool(..., allow_input_required=True) and own the while isinstance(result, InputRequiredResult) loop yourself.
  • On @mcp.tool(), a dependency that asks the user produces this result for you (Dependencies); the low-level Server is the manual form.
  • Prompts and resources participate too: an @mcp.prompt() or template @mcp.resource() function returns the InputRequiredResult itself and reads ctx.input_responses on the retry.

This is the mechanism that replaces server-initiated sampling and the rest of the push-style back-channel; see Deprecated features.