protocol-droid 0.4.1

//! Routing a method name to the handler that serves it.
//!
//! # Why this is not on `Server`
//!
//! `Server::call` takes a tool name, because a tool call is the only thing a
//! handler can answer. A peer, though, sends a _method_ name, and several
//! methods never reach a tool handler at all — `tools/list` and `ping` have no
//! handler to dispatch to. The method table lives here rather than on
//! `Server` so the two stay apart: the builder is how a server is described,
//! this is how a request against it is routed.
//!
//! # What the result is, and is not
//!
//! The result is an **IR-level** [`JsonObject`], not a wire frame. There is no
//! request-side codec yet — `Codec` lifts and lowers a `ToolResult` and nothing
//! else — so a result built here is the canonical form that T13/T14's transport
//! will then lower for whichever revision negotiated. The in-process test that
//! exercises this says the same thing in its own doc comment rather than
//! pretending to cover the wire.
//!
//! # Which fields a listing carries
//!
//! Only the ones **every** revision expresses. `title` and `icons` arrived in
//! `2026-07-28`, so emitting them unconditionally would hand a `2024-11-05`
//! peer a field its revision has no name for. Gating a field on a revision is
//! the codec's job and belongs with it; this module stays the common
//! denominator, and the omission is recorded rather than papered over.

use std::sync::Arc;

use crate::application::mcp::initialize::{read_offer, server_capabilities_json};
use crate::application::mcp::input_required::{InputRound, capability_for, is_input_required};
use crate::application::registry::listing::Listing;
use crate::application::server::builder::{DispatchError, Server};
use crate::application::server::context::{PeerSession, RequestContext};
use crate::domain::feature::{Feature, FeatureProfile};
use crate::domain::idempotency::{Claim, IdempotencyKey};
use crate::domain::ir::JsonObject;
use crate::domain::ir::LogLevel;
use crate::domain::registry::{Primitive, PrimitiveKind, PromptArgument, Registered};
use crate::domain::{
    CapabilitySet, Negotiation, ProtocolVersion, SupportedVersions, TransportKind,
    client_method_defined_in, discover_defined_in, handshake_defined_in, negotiate_server, resolve,
    transport_allowed,
};

/// Routes methods to a server's handlers.
#[derive(Clone)]
pub struct Dispatcher {
    server: Arc<Server>,
}

impl core::fmt::Debug for Dispatcher {
    /// Prints the server's name and tool count.
    ///
    /// For the same reason `PeerSession` does: the registry behind it is a
    /// large snapshot, and a post-mortem wants to know which server it was.
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.debug_struct("Dispatcher")
            .field("name", &self.server.name())
            .field("tool_count", &self.server.tool_count())
            .finish_non_exhaustive()
    }
}

/// Which of the two subscription methods a request was.
///
/// They share every step — the same gate, the same existence check, the same
/// empty answer — so one handler takes this instead of a `bool`, which would
/// make a reader guess at the argument's polarity, and two near-identical
/// handlers, which would be two places for the check to drift.
#[derive(Clone, Copy)]
enum SubscribeOp {
    /// `resources/subscribe`.
    Subscribe,
    /// `resources/unsubscribe`.
    Unsubscribe,
}

impl Dispatcher {
    /// Routes to `server`.
    #[must_use]
    pub const fn new(server: Arc<Server>) -> Self {
        Self { server }
    }

    /// The server requests are routed to.
    #[must_use]
    pub const fn server(&self) -> &Arc<Server> {
        &self.server
    }

    /// The server's state map, for a transport handler that needs to
    /// hand it to a [`RequestContext`].
    ///
    /// Cloning the `Arc` is one bump; the contents of the map are not
    /// touched on the read path.
    #[must_use]
    pub fn state(&self) -> &Arc<crate::application::server::context::StateMap> {
        self.server.state()
    }

    /// Runs the handshake and returns the session narrowed to what was
    /// negotiated.
    ///
    /// [`dispatch`](Self::dispatch) answers `initialize` and discards the
    /// negotiated profile, which leaves the caller gating the rest of the
    /// connection on the profile the session was *constructed* with — a
    /// revision chosen before the peer existed. That is the right answer only
    /// when the peer happens to want the newest revision this build serves.
    ///
    /// The returned session is what the transport must carry forward: it holds
    /// the profile resolved for the revision the client actually asked for, so
    /// every later gate reads that. Transport-specific policy still belongs to
    /// the transport; the profile does not belong to the caller.
    ///
    /// # Errors
    ///
    /// As [`dispatch`](Self::dispatch) for `initialize`, and
    /// [`DispatchError::MalformedInitialize`] when the body does not carry a
    /// readable version and capability block.
    pub fn handshake(
        &self,
        ctx: &RequestContext,
        params: &JsonObject,
    ) -> Result<(JsonObject, PeerSession), DispatchError> {
        let (version, profile) = negotiate(ctx, params)?;
        let reply = self.initialize_result(version, profile)?;
        Ok((reply, ctx.peer().clone().with_profile(profile)))
    }

    /// The `InitializeResult` body for a settled negotiation.
    fn initialize_result(
        &self,
        version: ProtocolVersion,
        profile: FeatureProfile,
    ) -> Result<JsonObject, DispatchError> {
        // SEP-2575 removed the handshake in `2026-07-28`. There is no result
        // shape to build, so this is the spec's own answer — the method is not
        // defined on this revision — and not a `FeatureUnavailable` dressed up
        // as one.
        if !handshake_defined_in(version) {
            return Err(DispatchError::UnknownMethod);
        }
        let result = serde_json::json!({
            "protocolVersion": version.as_wire(),
            "capabilities": server_capabilities_json(profile),
            "serverInfo": {
                "name": self.server.name(),
                "version": self.server.version(),
            },
        });
        JsonObject::from_value(result).map_err(|_| DispatchError::MalformedInitialize)
    }

    /// Routes one request.
    ///
    /// The gate runs before anything is read out of `params`, so a method the
    /// session cannot use does not even parse its arguments — the same order
    /// `RequestContext`'s own gated calls use, and for the same reason.
    ///
    /// # Errors
    ///
    /// [`DispatchError::UnknownMethod`] for a name no method answers, and
    /// [`DispatchError::NotCallable`] for a method whose feature the session
    /// does not have. Both are refusals that wrote nothing.
    pub async fn dispatch(
        &self,
        ctx: &RequestContext,
        method: &str,
        params: &JsonObject,
    ) -> Result<JsonObject, DispatchError> {
        let version = ctx.peer().negotiated();
        let answered = self.route(ctx, method, params).await?;
        // `resultType` belongs on every result at 2026-07-28 and on none
        // before it, so it is stamped here rather than in each method: there
        // are ten of them and a per-method call is a call someone forgets.
        let stamped = stamp_result(answered, Cacheable::No, version);
        Ok(self.identify(stamped, version))
    }

    /// Adds `_meta["io.modelcontextprotocol/serverInfo"]` at 2026-07-28.
    ///
    /// SEP-2575: with no handshake there is no `InitializeResult` to carry
    /// `serverInfo`, so a server "SHOULD identify itself in each result's
    /// `_meta`". Older revisions carry it once, in the handshake, and their
    /// result schemas have no such key.
    fn identify(&self, result: JsonObject, version: ProtocolVersion) -> JsonObject {
        if !result_envelope_defined_in(version) {
            return result;
        }
        let Ok(serde_json::Value::Object(mut members)) =
            serde_json::from_str::<serde_json::Value>(result.as_str())
        else {
            return result;
        };
        let meta = members
            .entry("_meta")
            .or_insert_with(|| serde_json::Value::Object(serde_json::Map::new()));
        if let Some(meta) = meta.as_object_mut() {
            meta.insert(
                "io.modelcontextprotocol/serverInfo".to_owned(),
                serde_json::json!({
                    "name": self.server.name(),
                    "version": self.server.version(),
                }),
            );
        }
        JsonObject::from_value(serde_json::Value::Object(members)).unwrap_or(result)
    }

    /// The SEP-2322 round a retry carries, verified.
    ///
    /// Both members are optional and both are read only here. A `requestState`
    /// that does not open under this server's key is refused before any
    /// handler sees it — the handler is only ever handed a state this server
    /// issued.
    fn input_round(&self, params: &JsonObject) -> Result<InputRound, DispatchError> {
        let responses = match parsed_param(params, "inputResponses")? {
            None | Some(serde_json::Value::Null) => None,
            Some(serde_json::Value::Object(map)) => Some(map),
            Some(_) => return Err(DispatchError::MalformedParams),
        };
        let state = match parsed_param(params, "requestState")? {
            None | Some(serde_json::Value::Null) => None,
            Some(serde_json::Value::String(sealed)) => {
                let key = self
                    .server
                    .request_state_key()
                    .ok_or(DispatchError::TamperedRequestState)?;
                Some(
                    key.open(&sealed)
                        .ok_or(DispatchError::TamperedRequestState)?
                        .to_owned(),
                )
            }
            Some(_) => return Err(DispatchError::MalformedParams),
        };
        Ok(InputRound {
            responses,
            state,
            key: self.server.request_state_key().cloned(),
        })
    }

    /// Checks an interim result against what the request may receive.
    ///
    /// Two rules from SEP-2322 and one from SEP-2575. An `InputRequiredResult`
    /// exists only at `2026-07-28` and later, and only on `tools/call`,
    /// `prompts/get` and `resources/read` — "servers MUST NOT send
    /// `InputRequiredResult` responses on any client requests other than the
    /// supported ones". And every input request it carries needs the client
    /// capability for that request, which the client must have declared on
    /// *this* request: otherwise the answer is `-32021`.
    fn check_interim(
        ctx: &RequestContext,
        method: &str,
        result: JsonObject,
    ) -> Result<JsonObject, DispatchError> {
        let Ok(serde_json::Value::Object(members)) =
            serde_json::from_str::<serde_json::Value>(result.as_str())
        else {
            return Ok(result);
        };
        if !is_input_required(&members) {
            return Ok(result);
        }
        if !accepts_input_round(method) || !result_envelope_defined_in(ctx.peer().negotiated()) {
            return Err(DispatchError::HandlerFailed(
                "an InputRequiredResult is not defined for this request".to_owned(),
            ));
        }
        let mut required: Vec<&'static str> = Vec::new();
        if let Some(requests) = members
            .get("inputRequests")
            .and_then(serde_json::Value::as_object)
        {
            for request in requests.values() {
                let Some(capability) = request
                    .get("method")
                    .and_then(serde_json::Value::as_str)
                    .and_then(capability_for)
                else {
                    continue;
                };
                let feature = match capability {
                    "sampling" => Feature::Sampling,
                    "elicitation" => Feature::Elicitation,
                    _ => Feature::Roots,
                };
                if !ctx.peer().allows(feature) && !required.contains(&capability) {
                    required.push(capability);
                }
            }
        }
        if !required.is_empty() {
            return Err(DispatchError::MissingClientCapability { required });
        }
        Ok(result)
    }

    /// Routes one method to its handler.
    ///
    /// Split from [`dispatch`](Self::dispatch) so the result envelope is added
    /// once, on the way out, instead of at each of the ten places a result is
    /// built.
    async fn route(
        &self,
        ctx: &RequestContext,
        method: &str,
        params: &JsonObject,
    ) -> Result<JsonObject, DispatchError> {
        let version = ctx.peer().negotiated();
        // A method the negotiated revision does not define is `-32601`
        // whatever this server could otherwise do with it: answering `ping` on
        // `2026-07-28` would tell a client the method exists there.
        if !client_method_defined_in(version, method) {
            return Err(DispatchError::UnknownMethod);
        }
        let round;
        let ctx = if accepts_input_round(method) && result_envelope_defined_in(version) {
            round = ctx.clone().with_input_round(self.input_round(params)?);
            &round
        } else {
            ctx
        };
        let answered = self.route_defined(ctx, method, params).await?;
        Self::check_interim(ctx, method, answered)
    }

    /// Routes a method the negotiated revision defines.
    async fn route_defined(
        &self,
        ctx: &RequestContext,
        method: &str,
        params: &JsonObject,
    ) -> Result<JsonObject, DispatchError> {
        match method {
            // `initialize` is the one method with no gate, and for the
            // opposite reason to `ping`. `ping` may be answered on a session
            // that already exists; `initialize` is the request that *creates*
            // the session, so consulting the feature profile to decide whether
            // to serve it would ask the session whether it exists and take the
            // answer from something that has not been built yet. The
            // negotiation that does decide it has already happened — the
            // revision in the result is the one the session settled on — so
            // the gate has nothing left to contribute here.
            //
            // The one thing it *does* still have to contribute is which
            // revision that is: on `2026-07-28` there is no handshake to
            // perform, and the handler refuses rather than inventing one.
            "initialize" => self.initialize(ctx, params),
            // SEP-2575's replacement for the handshake, and defined only where
            // the handshake is not. The two are complementary, so a revision
            // never has both.
            "server/discover" => Self::discover(ctx),
            // `ping` is defined in every revision that has a session and
            // answers on an otherwise idle one, so gating it would refuse the
            // request it exists for.
            "ping" => Ok(JsonObject::default()),
            "tools/list" => self.list(ctx, PrimitiveKind::Tool, "tools"),
            "resources/list" => self.list(ctx, PrimitiveKind::Resource, "resources"),
            "resources/templates/list" => {
                self.list(ctx, PrimitiveKind::ResourceTemplate, "resourceTemplates")
            }
            "prompts/list" => self.list(ctx, PrimitiveKind::Prompt, "prompts"),
            "tools/call" => self.call_tool(ctx, params).await,
            "resources/read" => self.read_resource(ctx, params).await,
            "prompts/get" => self.get_prompt(ctx, params).await,
            "logging/setLevel" => Self::set_log_level(ctx, params),
            "completion/complete" => Self::complete(ctx, params),
            "resources/subscribe" => self.subscribe(ctx, params, SubscribeOp::Subscribe),
            "resources/unsubscribe" => self.subscribe(ctx, params, SubscribeOp::Unsubscribe),
            _ => Err(DispatchError::UnknownMethod),
        }
    }

    /// Serves `server/discover`: what this server speaks, and for how long a
    /// client may keep the answer.
    ///
    /// SEP-2575 removed `initialize` from `2026-07-28` and made every request
    /// carry its own version and capabilities in `_meta`. That is enough to
    /// *use* the server and not enough to *choose* one, which is the gap this
    /// fills: a client with no session and no `initialize` has no other way to
    /// learn the revisions on offer.
    ///
    /// The capabilities are the profile's own advertisement rather than a
    /// separate list, because a second list is a second opinion: a client that
    /// discovered `tools` and then found `tools/call` refused would have been
    /// told something this crate does not do.
    ///
    /// `ttlMs` is `0`. This server has no cache and re-derives the answer on
    /// every call, so advertising a lifetime it cannot honour would tell a
    /// client to reuse a result whose backing registry may since have changed.
    /// Zero is the honest value and the specification permits it: "should be
    /// considered immediately stale".
    ///
    /// `cacheScope` is `private`. The advertisement describes what this
    /// process can do, and there is nothing user-specific in it, but a
    /// capability set is also a map of what an operator has turned on, and
    /// `"public"` invites an intermediary to share it across authorization
    /// contexts. `"private"` is the safe reading of the same answer.
    fn discover(ctx: &RequestContext) -> Result<JsonObject, DispatchError> {
        if !discover_defined_in(ctx.peer().negotiated()) {
            return Err(DispatchError::UnknownMethod);
        }
        let result = serde_json::json!({
            "resultType": "complete",
            "supportedVersions": SupportedVersions::all()
                .map_err(|_| DispatchError::UnnegotiableVersion)?
                .into_iter()
                .rev()
                .map(|version| version.as_wire().into_owned())
                .collect::<Vec<_>>(),
            "ttlMs": 0,
            "cacheScope": "private",
            "capabilities": server_capabilities_json(ctx.peer().profile()),
        });
        JsonObject::from_value(result).map_err(|_| DispatchError::MalformedParams)
    }

    /// Serves `resources/subscribe` and `resources/unsubscribe`.
    ///
    /// Both answer the empty object. A subscription is recorded against the
    /// session's own sink, so [`ServerHandle::notify_resource_updated`] reaches
    /// exactly the sessions that asked, and it ends with the session.
    ///
    /// [`ServerHandle::notify_resource_updated`]: crate::application::server::ServerHandle::notify_resource_updated
    fn subscribe(
        &self,
        ctx: &RequestContext,
        params: &JsonObject,
        op: SubscribeOp,
    ) -> Result<JsonObject, DispatchError> {
        Self::require(ctx, Feature::ResourcesSubscribe)?;
        let uri = string_param(params, "uri")?;
        // The URI must be one this server lists. Accepting a subscription for
        // a resource that does not exist is how a client ends up waiting for
        // an update that can never come, and the peer cannot tell that from a
        // server that simply has nothing to report.
        if self.server.reader(&uri).is_none() {
            return Err(DispatchError::UnknownResource);
        }
        let handle = self.server.handle();
        match op {
            SubscribeOp::Subscribe => handle.watch_resource(ctx.peer().sink(), &uri, None),
            SubscribeOp::Unsubscribe => handle.unwatch_resource(ctx.peer().sink(), &uri),
        }
        Ok(JsonObject::default())
    }

    /// Serves `resources/read`.
    ///
    /// Not idempotency-keyed, unlike `tools/call`. A read has no side effect
    /// the peer can observe, so a retried read costs another read and nothing
    /// else — while a key would make two legitimate concurrent reads of the
    /// same URI collide into `DuplicateInFlight`, which is exactly wrong for a
    /// request whose whole purpose is to be repeatable.
    async fn read_resource(
        &self,
        ctx: &RequestContext,
        params: &JsonObject,
    ) -> Result<JsonObject, DispatchError> {
        Self::require(ctx, Feature::Resources)?;
        let uri = string_param(params, "uri")?;
        let reader = self
            .server
            .reader(&uri)
            .ok_or(DispatchError::UnknownResource)?;
        // One of the five cacheable results at 2026-07-28, and the only one
        // that is not a listing — which is exactly why it is marked here
        // rather than inferred from the method name.
        Ok(stamp_result(
            reader(ctx.clone(), uri).await?,
            Cacheable::Yes,
            ctx.peer().negotiated(),
        ))
    }

    /// Serves `prompts/get`.
    ///
    /// Also not idempotency-keyed, for the same reason as `read_resource`: a
    /// prompt getter produces a message list, and running it twice produces the
    /// same list twice at no cost to anybody.
    async fn get_prompt(
        &self,
        ctx: &RequestContext,
        params: &JsonObject,
    ) -> Result<JsonObject, DispatchError> {
        Self::require(ctx, Feature::Prompts)?;
        let name = string_param(params, "name")?;
        let arguments = match parsed_param(params, "arguments")? {
            None | Some(serde_json::Value::Null) => JsonObject::default(),
            Some(arguments) => {
                JsonObject::from_value(arguments).map_err(|_| DispatchError::MalformedParams)?
            }
        };
        let getter = self
            .server
            .getter(&name)
            .ok_or(DispatchError::UnknownPrompt)?;
        getter(ctx.clone(), arguments).await
    }

    /// Serves `logging/setLevel`.
    ///
    /// Answers the empty object the method's own result type is defined as,
    /// after checking the requested `level` is one the spec names. The check
    /// is here rather than ignored because a server that accepts any string
    /// reports a level it cannot honour: the client believes a filter is in
    /// place and stops sending the records it would otherwise have expected,
    /// and the peer has no way to tell "set to debug" from "set to a typo".
    ///
    /// The level is stored on the session's [`LogGate`], which every later
    /// request's context shares, so [`RequestContext::log`] filters against
    /// it from the next record on.
    ///
    /// [`LogGate`]: crate::application::server::LogGate
    fn set_log_level(
        ctx: &RequestContext,
        params: &JsonObject,
    ) -> Result<JsonObject, DispatchError> {
        Self::require(ctx, Feature::Logging)?;
        let level = string_param(params, "level")?;
        // A `level` outside the spec's own set is a client error, and answers
        // `InvalidParams` — the same answer as a missing member, because a
        // peer cannot act on either and the difference is not one it can use.
        let level = level
            .parse::<LogLevel>()
            .map_err(|_| DispatchError::MalformedParams)?;
        ctx.peer().log_gate().set(level);
        Ok(JsonObject::default())
    }

    /// Serves `completion/complete`.
    ///
    /// Returns an empty `values` array: the crate registers no completable
    /// `PromptArgument` values, so the reference is well-defined — there is
    /// nothing to offer, and the suite documents an empty array as a
    /// conforming answer.
    ///
    /// `completion` is an object holding `values`, not a bare array. An empty
    /// *array* at the top level answers `{"completion":[]}`, which is the
    /// shape a first reading of the method name suggests and the one that
    /// scores `0 passed, 2 failed` — the scenario reads
    /// `r.completion.values`, finds `[]` has no member `values`, and reports
    /// `Missing values array in completion`. The empty case is the one where
    /// the distinction is invisible: both a missing `values` and an empty one
    /// mean "no completions", so the error names a missing array that was
    /// never absent, only wrapped wrongly.
    fn complete(ctx: &RequestContext, _params: &JsonObject) -> Result<JsonObject, DispatchError> {
        Self::require(ctx, Feature::Completions)?;
        JsonObject::new(r#"{"completion":{"values":[]}}"#)
            .map_err(|_| DispatchError::MalformedParams)
    }

    /// Answers the handshake: this server, the revision it speaks, and what it
    /// can do.
    ///
    /// The result is the IR form of `InitializeResult` — `protocolVersion`,
    /// `capabilities`, `serverInfo` — which are the three fields every
    /// revision that has a handshake marks required. `instructions` is omitted
    /// rather than emitted empty: it is optional, it is a human-facing hint
    /// with no source on a [`Server`], and an empty string is worse than
    /// absent, because a client that reads it has been told the server has no
    /// instructions when what it means is that this server supplies none.
    ///
    /// # What decides the revision
    ///
    /// The session's, not the request's. `params.protocolVersion` is what the
    /// client *offered*; by the time a request reaches a dispatcher the layer
    /// that owns negotiation has already settled the revision, and the session
    /// is running on it. Answering with the offer instead would let a client
    /// move a session onto a revision the transport never negotiated for it,
    /// and the profile gating every other method would be one revision ahead of
    /// the wire.
    ///
    /// The offer is still read and still checked, for the two reasons a
    /// malformed handshake matters. The version the session speaks is settled
    /// by [`negotiate_server`] against **the client's own offer**, which is
    /// why [`check_initialize_params`] returns it rather than discarding it.
    ///
    /// # What decides `capabilities`
    ///
    /// [`FeatureProfile::advertised_by_server`] — the profile, read back. The
    /// advertisement is a fold over the same table every feature gate is a fold
    /// over, so it cannot claim a capability the session will not act on, and
    /// it cannot decline one it will.
    ///
    /// # Errors
    ///
    /// - [`DispatchError::UnnegotiableVersion`] when the session is on a
    ///   revision this build has no wire types for.
    /// - [`DispatchError::UnknownMethod`] when the session's revision has no
    ///   handshake at all — `2026-07-28` and `draft` removed `initialize` and
    ///   its result, so a request for it on one of those revisions is asking
    ///   for a method the revision does not define.
    /// - [`DispatchError::MalformedInitialize`] when `params` is not an object
    ///   with a string `protocolVersion` that parses, a `capabilities` object,
    ///   and a `clientInfo` carrying a `name`.
    fn initialize(
        &self,
        ctx: &RequestContext,
        params: &JsonObject,
    ) -> Result<JsonObject, DispatchError> {
        let (version, profile) = negotiate(ctx, params)?;
        self.initialize_result(version, profile)
    }

    /// One of the four listing methods.
    ///
    /// All four are the same operation against a different kind, so they are
    /// one function with the feature and the key as parameters rather than
    /// four near-identical bodies.
    fn list(
        &self,
        ctx: &RequestContext,
        kind: PrimitiveKind,
        key: &str,
    ) -> Result<JsonObject, DispatchError> {
        Self::require(ctx, kind.feature())?;
        let listing =
            self.server
                .registry()
                .list_visible(ctx.peer().auth(), &ctx.peer().profile(), kind);
        Ok(stamp_result(
            wrap_listing(key, &listing),
            Cacheable::Yes,
            ctx.peer().negotiated(),
        ))
    }

    /// `tools/call`, through the server's own dispatch.
    /// Runs one `tools/call`, at most once per request id.
    ///
    /// The order here is not a style choice; it is the claim:
    ///
    /// 1. **Check.** Claim the key before anything runs. Claiming after would
    ///    leave a window in which two callers both see an absent key.
    /// 2. **Execute.** Only the caller that was handed the token runs the
    ///    tool. A caller that finds a completed key answers from the recording
    ///    and never reaches the handler, and one that finds an in-flight key
    ///    is refused rather than queued — see [`DispatchError::DuplicateInFlight`].
    /// 3. **Persist.** Record the result, or release the claim, before
    ///    returning. A result that is returned but not recorded is a result a
    ///    retry will not find, and a key that is neither recorded nor released
    ///    is a key nobody can ever claim again.
    ///
    /// # Why a failed call releases rather than records
    ///
    /// An `Err` here means the tool produced no result — the gate refused it,
    /// or the handler failed. There is nothing to replay, so recording it would
    /// hand every future retry the same refusal for a condition that may have
    /// been fixed, and would make a transient failure permanent. Releasing
    /// makes the next attempt a fresh one, which is what a caller sending the
    /// same id again is asking for.
    ///
    /// The consequence is that a tool which *ran* and then failed is retried.
    /// That is the honest answer: this layer cannot tell a tool that had no
    /// effect from one that charged a card and then failed to answer, and
    /// guessing "no effect" would silently drop work. A tool that must not
    /// run twice needs a key of its own — which is the tool's own port, not
    /// this one.
    ///
    /// # What a dropped connection costs
    ///
    /// A client whose POST never got an answer retries with the same request
    /// id. It gets the recorded result, and the tool ran once. This is the
    /// only reason the store is on `Server` rather than left to the transport:
    /// the transport sees a dropped socket and knows nothing about what the
    /// tool did, and only the layer that ran it can answer.
    async fn call_tool(
        &self,
        ctx: &RequestContext,
        params: &JsonObject,
    ) -> Result<JsonObject, DispatchError> {
        Self::require(ctx, Feature::Tools)?;
        let (name, arguments) = split_call_params(params)?;

        // A request id is only unique within a session — every client numbers
        // its first request `1` — so the key carries both. Keying on the id
        // alone would answer one session's call with another's result.
        //
        // It also has to carry *what* was asked. A request id is a label for
        // one request, not a promise that the same id always names the same
        // operation: JSON-RPC 2.0 lets a peer reuse ids across a sequence, and
        // the conformance suite's client restarts its counter per scenario
        // against a connection it never closes. Keyed on the id alone, the
        // second `tools/call` a session receives is answered with the first
        // one's recording — a valid `CallToolResult` carrying another tool's
        // output, which the caller has no way to detect. Repeating the *same*
        // tool still replays, which is the case a retry needs.
        let key = IdempotencyKey::for_method_call(
            &ctx.peer().session(),
            ctx.request_id(),
            "tools/call",
            &name,
        );
        let token = match self.server.idempotency().claim(&key)? {
            // Recorded already. The answer is the recording, and the tool does
            // not run.
            Claim::Completed(recorded) => return Ok(recorded),
            // Someone else holds it. Refusing is what keeps one side effect
            // from running twice; the holder is a real request.
            Claim::InFlight => {
                return Err(DispatchError::DuplicateInFlight(
                    ctx.request_id().to_string(),
                ));
            }
            Claim::Claimed { token } => token,
        };

        let outcome = self.server.call(ctx, &name, arguments).await;
        match outcome {
            Ok(result) => {
                // The result is both the answer and the recording, and
                // `complete` takes it by value, so it is cloned out. `JsonObject`
                // holds its bytes in an `Arc`, so this is a pointer bump and
                // not a copy of the document.
                self.server.idempotency().complete(&token, result.clone())?;
                Ok(result)
            }
            Err(error) => {
                // Release before returning, so the key is claimable again. A
                // failed record or release is not swallowed: it is reported,
                // because a key stuck in `Claimed` is invisible to every later
                // caller and looks exactly like a server that stopped working.
                self.server.idempotency().abandon(&token)?;
                Err(error)
            }
        }
    }

    /// The gate every method shares, in one place.
    ///
    /// Separate from `RequestContext`'s own private `require`, which returns a
    /// `GatedError` naming the caller. A method is not a tool, so the refusal
    /// here is a `DispatchError` and the two types do not merge — a caller
    /// matching on the distinction can tell "you may not call this method"
    /// from "this tool needs something you lack".
    ///
    /// An associated function rather than a method because it reads nothing
    /// from the dispatcher: it asks the session, and a gate that could also
    /// consult the server is a gate with a second opinion.
    fn require(ctx: &RequestContext, feature: Feature) -> Result<(), DispatchError> {
        if ctx.peer().allows(feature) {
            return Ok(());
        }
        Err(DispatchError::NotCallable)
    }
}

/// Checks that `initialize` params are the shape the handshake has to be.
///
/// Every revision that has a handshake marks `capabilities`, `clientInfo` and
/// `protocolVersion` required on `InitializeRequestParams`, and `Implementation`
/// marks `name` and `version` required inside `clientInfo`. Each is read and
/// type-checked here, which is what "not a silently-ignored field" means: a
/// missing or misshapen required field is a refusal, not a default.
///
/// # Why this returns nothing
///
/// A check, not a parse, and the distinction is worth stating because the
/// crate's rule is to parse rather than validate. It returns nothing because
/// nothing in the answer comes from the request: `protocolVersion` is the
/// session's revision, `capabilities` is the session's profile, and
/// `serverInfo` is this server. The offer is a statement about the *client*,
/// and the layer that owns that is the one that negotiated the session — it
/// already holds the client's capability set and, on a `NegotiatedSession`,
/// the client's [`PeerInfo`]. Reading it again here to build a value nobody
/// consumes would be the second source of truth, not the first.
///
/// The one field whose parse *is* a decision: `protocolVersion` is run through
/// [`ProtocolVersion`]'s `FromStr`, so a client offering a revision this build
/// has never heard of — which the specification explicitly allows, and which
/// [`negotiate_server`] answers with the server's own version rather than an
/// error — still gets a session, while a string that is not a `YYYY-MM-DD`
/// date is a typo and is refused.
///
/// # Errors
///
/// Reads `name` and `arguments` out of a `tools/call` params object.
///
/// A missing `name` is a malformed request rather than an unknown tool: the
/// call named nothing at all, and reporting `UnknownTool` for it would send a
/// caller hunting for a tool that exists and works.
///
/// # Errors
///
/// [`DispatchError::MalformedParams`] when the body is not an object, when
/// `name` is absent or is not a string, and when `arguments` is present but is
/// not an object. `arguments` is optional — a tool taking no parameters is
/// called with none.
fn split_call_params(params: &JsonObject) -> Result<(String, JsonObject), DispatchError> {
    let parsed: serde_json::Value =
        serde_json::from_str(params.as_str()).map_err(|_| DispatchError::MalformedParams)?;
    let object = parsed.as_object().ok_or(DispatchError::MalformedParams)?;

    let name = object
        .get("name")
        .and_then(serde_json::Value::as_str)
        .ok_or(DispatchError::MalformedParams)?
        .to_string();

    let arguments = match object.get("arguments") {
        None | Some(serde_json::Value::Null) => JsonObject::default(),
        Some(arguments) => {
            JsonObject::from_value(arguments.clone()).map_err(|_| DispatchError::MalformedParams)?
        }
    };

    Ok((name, arguments))
}

/// The string member `key` of a params object.
fn string_param(params: &JsonObject, key: &str) -> Result<String, DispatchError> {
    parsed_param(params, key)?
        .and_then(|value| value.as_str().map(str::to_string))
        .ok_or(DispatchError::MalformedParams)
}

/// One member of a params object, already parsed.
///
/// `None` for an absent member as well as `null`, because both mean the same
/// thing to every caller here and the specification's params are optional
/// throughout. A non-object params document is malformed either way.
fn parsed_param(
    params: &JsonObject,
    key: &str,
) -> Result<Option<serde_json::Value>, DispatchError> {
    let parsed: serde_json::Value =
        serde_json::from_str(params.as_str()).map_err(|_| DispatchError::MalformedParams)?;
    let object = parsed.as_object().ok_or(DispatchError::MalformedParams)?;
    Ok(object.get(key).cloned())
}

/// Wraps a listing's visible entries under `key`.
///
/// The document is assembled from values this module built, so the only way it
/// could be invalid is a bug here. It falls back to `{}` rather than
/// propagating because a listing that cannot be encoded should read as empty
/// to the caller rather than take the whole response down.
/// Whether a result carries SEP-2549's caching hints.
///
/// A second parameter to [`stamp_result`] rather than a flag on every method:
/// the schema draws the line itself, and this names which side a result is on.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Cacheable {
    /// `ttlMs` and `cacheScope` belong on this result.
    Yes,
    /// They do not — a `CallToolResult` is not something to cache.
    No,
}

/// Adds the members **2026-07-28** requires on a result.
///
/// Two SEPs, one place:
///
/// - `resultType` is required on *every* result at this revision. Every
///   definition in the vendored schema that ends in `Result` lists it, so a
///   result without one is schema-invalid rather than merely dated.
/// - `ttlMs` and `cacheScope` are required only on the cacheable ones —
///   the four listings and `resources/read`. A `CallToolResult` carrying a
///   `ttlMs` would be telling the client to reuse a side effect.
///
/// Older revisions are untouched: both members are absent from every
/// pre-2026 result definition, and adding them would make this crate's own
/// answer invalid on the four revisions it already passes.
///
/// `ttlMs` is `0` and `cacheScope` is `private` for the same reasons as in
/// [`Dispatcher::discover`]: this server holds no cache, so the only lifetime
/// it can honour is "not at all", and the answer describes a registry whose
/// contents can change under a caller, which is not shareable across
/// authorization contexts.
fn stamp_result(result: JsonObject, cacheable: Cacheable, version: ProtocolVersion) -> JsonObject {
    if !result_envelope_defined_in(version) {
        return result;
    }
    let mut merged: serde_json::Map<String, serde_json::Value> =
        match serde_json::from_str(result.as_str()) {
            Ok(serde_json::Value::Object(members)) => members,
            _ => return result,
        };
    // A result that already names its type keeps it: an interim
    // `InputRequiredResult` says `"input_required"`, and overwriting that with
    // `"complete"` would tell the client the request finished.
    merged
        .entry("resultType")
        .or_insert_with(|| serde_json::Value::from("complete"));
    if cacheable == Cacheable::Yes {
        merged.insert("ttlMs".to_owned(), serde_json::Value::from(0));
        merged.insert("cacheScope".to_owned(), serde_json::Value::from("private"));
    }
    JsonObject::from_value(serde_json::Value::Object(merged)).unwrap_or(result)
}

/// Whether `method` may answer with a SEP-2322 `InputRequiredResult`.
///
/// `tasks/result` is the fourth the SEP names; it belongs to the tasks
/// extension, which this crate does not serve.
fn accepts_input_round(method: &str) -> bool {
    matches!(method, "tools/call" | "prompts/get" | "resources/read")
}

/// Drops members whose value is `null`.
///
/// Every optional member of a listing entry is typed as a string, a boolean or
/// an object in every revision's schema, and none admits `null`. Writing an
/// absent description as `"description": null` is therefore a schema-invalid
/// entry, and a client that validates its input — the `2026-07-28` reference
/// client does — refuses the whole listing over it. Only the top level is
/// touched: a tool's `inputSchema` is the tool author's document, and `null`
/// is a legitimate value inside one.
fn without_nulls(mut value: serde_json::Value) -> serde_json::Value {
    if let Some(members) = value.as_object_mut() {
        members.retain(|_, member| !member.is_null());
    }
    value
}

/// Whether SEP-2549's result envelope exists in this revision.
#[must_use]
pub const fn result_envelope_defined_in(version: ProtocolVersion) -> bool {
    matches!(
        version,
        ProtocolVersion::V2026_07_28 | ProtocolVersion::Unknown(_)
    )
}

fn wrap_listing(key: &str, listing: &Listing) -> JsonObject {
    let entries: Vec<serde_json::Value> = listing
        .visible()
        .iter()
        .map(|entry| without_nulls(listing_entry(entry)))
        .collect();
    JsonObject::from_value(serde_json::json!({ key: entries })).unwrap_or_default()
}

/// The common-denominator JSON for one registered primitive.
///
/// The match is exhaustive, which is the whole point: a new `Primitive` variant
/// fails to compile in this function rather than silently vanishing from every
/// listing. That is what the exhaustiveness buys, and it needs no `Option` to
/// buy it — an `Option` here would only invite a `filter_map` to swallow a
/// future `None` quietly, undoing the check.
fn listing_entry(registered: &Registered) -> serde_json::Value {
    match registered.primitive() {
        Primitive::Tool(tool) => {
            let mut entry = serde_json::json!({
                "name": tool.name.as_str(),
                "description": tool.description,
            });
            if let (serde_json::Value::Object(map), Ok(schema)) = (
                &mut entry,
                serde_json::from_str::<serde_json::Value>(tool.input_schema.as_str()),
            ) {
                // Embedded as a value, not as a string. `json!` will not do
                // this for us: `"inputSchema": text.as_str()` produces a JSON
                // *string* whose characters happen to be JSON, so every peer
                // would read a schema where the specification says an object,
                // and the failure would surface in the client as a tool with
                // no arguments rather than here.
                map.insert("inputSchema".to_string(), schema);
            }
            entry
        }
        Primitive::Resource(resource) => serde_json::json!({
            "uri": resource.uri.as_str(),
            "name": resource.name,
            "description": resource.description,
            "mimeType": resource.mime_type,
        }),
        Primitive::ResourceTemplate(template) => serde_json::json!({
            "uriTemplate": template.uri_template.as_str(),
            "name": template.name,
            "description": template.description,
            "mimeType": template.mime_type,
        }),
        Primitive::Prompt(prompt) => serde_json::json!({
            "name": prompt.name.as_str(),
            "description": prompt.description,
            "arguments": prompt.arguments.iter().map(prompt_argument).collect::<Vec<_>>(),
        }),
    }
}

/// One prompt argument, as JSON.
///
/// Written out rather than derived, because `PromptArgument` is a domain type
/// with no `Serialize`: it exists to be lowered by a codec, and deriving
/// `Serialize` on it here would make the wire shape the domain's own opinion
/// instead of the codec's. `title` is skipped for the same reason the
/// other listings skip it — `2026-07-28` only.
fn prompt_argument(argument: &PromptArgument) -> serde_json::Value {
    without_nulls(serde_json::json!({
        "name": argument.name,
        "description": argument.description,
        "required": argument.required,
    }))
}

/// The revision and profile `initialize` settles on with this peer.
///
/// Resolved for the revision that was *negotiated*, from the capabilities
/// the client actually sent, rather than read off the session. The session
/// was built before any frame arrived and knows nothing about the peer;
/// resolving from it would tell a `2024-11-05` client it had capabilities
/// that revision cannot express, which is the one thing a handshake must
/// never do.
///
/// Free rather than a method: what a revision can express does not depend
/// on which [`Server`] is serving it, and a `&self` here would imply a
/// per-server rule that does not exist.
fn negotiate(
    ctx: &RequestContext,
    params: &JsonObject,
) -> Result<(ProtocolVersion, FeatureProfile), DispatchError> {
    let transport = ctx.peer().transport();
    let supported = supported_on(transport)?;
    let offer = read_offer(params).map_err(|_| DispatchError::MalformedInitialize)?;
    let version = negotiate_server(offer.version, &supported).version();
    let profile = resolve(Negotiation {
        version,
        client_caps: offer.capabilities,
        server_caps: CapabilitySet::all_for(version),
        transport,
    });
    Ok((version, profile))
}

/// The revisions this server will agree to over `transport`.
///
/// HTTP+SSE belongs to `2024-11-05` alone, so a legacy session is never
/// answered with a revision that does not define it: a client that offers
/// `2025-11-25` over it is counter-offered `2024-11-05` and decides for itself
/// whether it can continue. Other transports answer with whatever the client
/// offered when this build speaks it.
fn supported_on(transport: TransportKind) -> Result<SupportedVersions, DispatchError> {
    let all = SupportedVersions::all().map_err(|_| DispatchError::UnnegotiableVersion)?;
    if transport != TransportKind::LegacySse {
        return Ok(all);
    }
    let defining: Vec<ProtocolVersion> = all
        .as_slice()
        .iter()
        .copied()
        .filter(|version| transport_allowed(*version, transport))
        .collect();
    SupportedVersions::from_slice(&defining).map_err(|_| DispatchError::UnnegotiableVersion)
}

#[cfg(test)]
mod tests {
    use std::num::NonZeroUsize;
    use std::sync::Arc;
    use std::sync::atomic::{AtomicUsize, Ordering};

    use super::{DispatchError, Dispatcher};
    use crate::application::server::context::{
        CallbackExchange, ExchangeError, PeerSession, RequestContext,
    };
    use crate::application::server::{Server, ToolHandler};
    use crate::application::session::{FrameSink, SinkError};
    use crate::domain::feature::{Negotiation, resolve};
    use crate::domain::ir::{JsonObject, JsonObjectError};
    use crate::domain::registry::{AuthContext, Primitive, Registered, Scope, ScopeSet, Tool};
    use crate::domain::session::{CancellationToken, SessionId};
    use crate::domain::{Capability, CapabilitySet, ProtocolVersion, RequestId, TransportKind};

    /// What a test here can fail with.
    #[derive(Debug)]
    enum Failure {
        /// The test's own complaint.
        Message(String),
        /// A fixture was not a JSON object.
        Fixture(JsonObjectError),
        /// A result the dispatcher built did not read back as JSON.
        Result(serde_json::Error),
    }

    impl core::fmt::Display for Failure {
        fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
            match self {
                Self::Message(text) => f.write_str(text),
                Self::Fixture(error) => write!(f, "bad fixture: {error}"),
                Self::Result(error) => write!(f, "unreadable result: {error}"),
            }
        }
    }

    impl std::error::Error for Failure {}

    impl From<String> for Failure {
        fn from(text: String) -> Self {
            Self::Message(text)
        }
    }

    impl From<&str> for Failure {
        fn from(text: &str) -> Self {
            Self::Message(text.to_string())
        }
    }

    impl From<JsonObjectError> for Failure {
        fn from(error: JsonObjectError) -> Self {
            Self::Fixture(error)
        }
    }

    impl From<DispatchError> for Failure {
        fn from(error: DispatchError) -> Self {
            Self::Message(error.to_string())
        }
    }

    impl From<serde_json::Error> for Failure {
        fn from(error: serde_json::Error) -> Self {
            Self::Result(error)
        }
    }

    type TestError = Failure;

    /// The test's own complaint.
    ///
    /// A helper rather than `Err("...".into())` at each site, because that
    /// spelling settles the module's error type on `&str` — which converts to
    /// both itself and `Box<dyn Error>`, so the compiler picks it from the
    /// first early return and every `?` in the body then has no valid
    /// conversion. A vague error type is a compile error that points at the
    /// `?` and not at the `Err` that caused it.
    fn fail(message: impl Into<String>) -> TestError {
        Failure::Message(message.into())
    }

    /// An exchange that refuses, so no test here can accidentally pass by
    /// having a callback answered.
    struct NoExchange;

    impl CallbackExchange for NoExchange {
        fn exchange(
            &self,
            method: &str,
            _params: &JsonObject,
        ) -> impl std::future::Future<Output = Result<JsonObject, ExchangeError>> + Send {
            let _ = method;
            std::future::ready(Err(ExchangeError::new("no callbacks here")))
        }
    }

    /// A sink that discards.
    struct NoSink;

    impl FrameSink for NoSink {
        fn send(
            &self,
            _frame: &str,
        ) -> impl std::future::Future<Output = Result<(), SinkError>> + Send {
            std::future::ready(Ok(()))
        }
    }

    /// A handler that answers with `answer`.
    fn answering(answer: &'static str) -> ToolHandler {
        let answer: JsonObject = JsonObject::new(answer).unwrap_or_default();
        Arc::new(move |_ctx, _args| {
            let answer = answer.clone();
            Box::pin(async move { answer })
        })
    }

    /// A dispatcher over one plain tool and one scope-restricted tool.
    fn dispatcher() -> Result<Dispatcher, TestError> {
        let restricted = Registered::from(Primitive::from(Tool::new(
            "secret",
            r#"{"type":"object"}"#,
        )?))
        .requiring_scope(Scope::Read);

        Ok(Dispatcher::new(Arc::new(
            Server::builder("test")
                .tool(Tool::new("echo", r#"{"type":"object"}"#)?, answering("{}"))
                .restricted_tool(restricted, answering("{}"))
                .build(),
        )))
    }

    /// A session at `version` with `scopes`, negotiating `server_caps`.
    fn peer(version: ProtocolVersion, scopes: ScopeSet, server_caps: CapabilitySet) -> PeerSession {
        PeerSession::new(
            version,
            resolve(Negotiation {
                version,
                client_caps: CapabilitySet::empty(),
                server_caps,
                transport: TransportKind::StreamableHttp,
            }),
            SessionId::new(1),
            TransportKind::Stdio,
            Arc::new(NoExchange),
            Arc::new(NoSink),
        )
        .with_auth(AuthContext::new("tester", scopes))
    }

    /// A request on `peer`.
    fn ctx(peer: &PeerSession) -> RequestContext {
        RequestContext::for_request(peer, RequestId::text("r1"), CancellationToken::new(), None)
    }

    /// The server capabilities a fully-featured session negotiates at
    /// `version`.
    fn caps(version: ProtocolVersion) -> CapabilitySet {
        CapabilitySet::all_for(version)
    }

    /// A session on session `id`, so a test can hold two at once.
    ///
    /// The existing [`peer`] pins session 1, which is fine until a test needs
    /// to prove that two sessions are not the same session.
    fn peer_on(
        version: ProtocolVersion,
        scopes: ScopeSet,
        server_caps: CapabilitySet,
        id: u64,
    ) -> PeerSession {
        PeerSession::new(
            version,
            resolve(Negotiation {
                version,
                client_caps: CapabilitySet::empty(),
                server_caps,
                transport: TransportKind::StreamableHttp,
            }),
            SessionId::new(id),
            TransportKind::Stdio,
            Arc::new(NoExchange),
            Arc::new(NoSink),
        )
        .with_auth(AuthContext::new("tester", scopes))
    }

    /// The one tool the idempotency tests call.
    fn charge_tool() -> Result<Tool, TestError> {
        Ok(Tool::new("charge", r#"{"type":"object"}"#)?)
    }

    /// `initialize` params for a client that offers `version`.
    fn init_params(version: &str) -> Result<JsonObject, TestError> {
        Ok(JsonObject::new(
            r#"{"protocolVersion":""#.to_string()
                + version
                + r#"","capabilities":{},"clientInfo":{"name":"probe","version":"0"}}"#,
        )?)
    }

    /// An `initialize` body whose client advertises `capabilities`.
    fn init_params_with(version: &str, capabilities: &str) -> Result<JsonObject, TestError> {
        Ok(JsonObject::new(
            r#"{"protocolVersion":""#.to_string()
                + version
                + r#"","capabilities":"#
                + capabilities
                + r#","clientInfo":{"name":"probe","version":"0"}}"#,
        )?)
    }

    /// An IR object read back as a `serde_json` value.
    fn parsed(object: &JsonObject) -> Result<serde_json::Value, TestError> {
        Ok(serde_json::from_str(object.as_str())?)
    }

    /// The advertised capabilities are resolved for the revision that was
    /// negotiated, so an older client is never told it has something that
    /// revision cannot express.
    ///
    /// The session here is on `2025-06-18`, which is the newest revision in
    /// this build's profile that gates `elicitation`. The client offers
    /// `2024-11-05`, which has no such capability. Reading the profile off the
    /// session — as this did — would have answered `elicitation: {}` to a
    /// client on a revision whose schema has nowhere to put it.
    #[tokio::test]
    async fn advertised_capabilities_follow_the_negotiated_revision() -> Result<(), TestError> {
        let session_version = ProtocolVersion::V2025_06_18;
        let peer = peer(
            session_version,
            ScopeSet::all(),
            CapabilitySet::all_for(session_version),
        );
        let result = dispatcher()?
            .dispatch(&ctx(&peer), "initialize", &init_params("2024-11-05")?)
            .await
            .map_err(|error| Failure::Message(format!("the handshake refused: {error}")))?;

        let capabilities = parsed(&result)?
            .get("capabilities")
            .cloned()
            .ok_or("the handshake advertised no capabilities")?;
        assert!(
            capabilities.get("elicitation").is_none(),
            "2024-11-05 has no elicitation capability: {capabilities}"
        );
        Ok(())
    }

    /// A capability the client advertises narrows what the server answers it
    /// can do, because the profile is resolved from what the client actually
    /// sent rather than from an assumption that it sent everything.
    #[tokio::test]
    async fn the_client_capabilities_are_read_not_assumed() -> Result<(), TestError> {
        let session_version = ProtocolVersion::V2025_11_25;
        let peer = peer(
            session_version,
            ScopeSet::all(),
            CapabilitySet::all_for(session_version),
        );
        dispatcher()?
            .dispatch(
                &ctx(&peer),
                "initialize",
                &init_params_with("2025-11-25", r#"{"roots":{"listChanged":true}}"#)?,
            )
            .await
            .map_err(|error| Failure::Message(format!("the handshake refused: {error}")))?;
        Ok(())
    }

    /// The handshake answers with the revision the client negotiated.
    ///
    /// The distinction is the test's whole point: a client offering a version
    /// this build cannot speak has already been negotiated down by the time a
    /// request reaches here, so echoing the *request* would tell the peer it
    /// got the revision it asked for when it did not.
    #[tokio::test]
    async fn initialize_reports_the_negotiated_revision() -> Result<(), TestError> {
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::all(), CapabilitySet::all_for(version));
        // The session is on 2025-06-18 and the client offers 2024-11-05. The
        // answer must be the client's, not the session's: the session was
        // constructed before any frame arrived and knows nothing about who is
        // calling. Negotiating against it would have every client answered with
        // whatever revision the process happened to be built for.
        let result = dispatcher()?
            .dispatch(&ctx(&peer), "initialize", &init_params("2024-11-05")?)
            .await
            .map_err(|error| Failure::Message(format!("the handshake refused: {error}")))?;

        let reported = parsed(&result)?
            .get("protocolVersion")
            .and_then(serde_json::Value::as_str)
            .ok_or("the handshake did not report a revision")?
            .to_string();
        assert_eq!(reported, "2024-11-05");
        Ok(())
    }

    /// A client newer than this build gets a session on the newest revision
    /// whose handshake exists, rather than a refusal.
    ///
    /// This is the whole point of reading the offer. A build that answered from
    /// its own pinned version could not tell this client from any other; one
    /// that counter-offered `2026-07-28` would name a revision with no
    /// `initialize` on it, so the peer would be told to use a protocol on which
    /// the handshake it just attempted does not exist.
    #[tokio::test]
    async fn a_client_newer_than_this_build_gets_the_newest_handshake() -> Result<(), TestError> {
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::all(), CapabilitySet::all_for(version));
        let result = dispatcher()?
            .dispatch(&ctx(&peer), "initialize", &init_params("2099-01-01")?)
            .await
            .map_err(|error| Failure::Message(format!("a newer client was refused: {error}")))?;

        let reported = parsed(&result)?
            .get("protocolVersion")
            .and_then(serde_json::Value::as_str)
            .ok_or("the handshake did not report a revision")?
            .to_string();
        assert_eq!(
            reported,
            ProtocolVersion::V2025_11_25.to_string(),
            "the newest revision this build can actually complete a handshake on"
        );
        Ok(())
    }

    /// The reported capabilities are the features the profile allows, not a
    /// second list kept beside it.
    ///
    /// The server here declared one tool and negotiated everything, so `tools`
    /// is present. What must *not* appear is anything client-side: a
    /// `ServerCapabilities` object has no field for `roots` or `sampling`, and
    /// a handshake that grew one would be promising a field the peer's schema
    /// has no place to read.
    #[tokio::test]
    async fn the_handshake_advertises_only_server_capabilities() -> Result<(), TestError> {
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::all(), CapabilitySet::all_for(version));
        let result = dispatcher()?
            .dispatch(&ctx(&peer), "initialize", &init_params("2025-06-18")?)
            .await
            .map_err(|error| Failure::Message(format!("the handshake refused: {error}")))?;

        let capabilities = parsed(&result)?
            .get("capabilities")
            .cloned()
            .ok_or("the handshake advertised no capabilities")?;
        for forbidden in ["roots", "sampling", "elicitation"] {
            assert!(
                capabilities.get(forbidden).is_none(),
                "a client-side capability reached the server advertisement: {forbidden}"
            );
        }
        assert!(
            capabilities.get("tools").is_some(),
            "a session that negotiated tools did not advertise them"
        );
        Ok(())
    }

    /// The handshake is what establishes the session, so it answers on a peer
    /// that negotiated no tools at all.
    ///
    /// Gating it the way every other method is gated would mean a client
    /// could never start a conversation, because a session begins with
    /// capabilities rather than with any.
    #[tokio::test]
    async fn initialize_answers_on_a_session_with_no_features() -> Result<(), TestError> {
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::all(), CapabilitySet::empty());
        let outcome = dispatcher()?
            .dispatch(&ctx(&peer), "initialize", &init_params("2025-06-18")?)
            .await;

        assert!(outcome.is_ok(), "the handshake was gated off: {outcome:?}");
        Ok(())
    }

    /// Every refusal below is a body a client could not have meant, and they
    /// are one error rather than three: a caller that distinguishes "no
    /// `clientInfo`" from "no `name` inside it" is distinguishing two spellings
    /// of the same mistake.
    #[tokio::test]
    async fn a_malformed_handshake_is_refused_whatever_is_missing() -> Result<(), TestError> {
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::all(), CapabilitySet::all_for(version));

        let bodies = [
            "{}",
            r#"{"capabilities":{},"clientInfo":{"name":"p","version":"0"}}"#,
            r#"{"protocolVersion":"2025-06-18","clientInfo":{"name":"p","version":"0"}}"#,
            r#"{"protocolVersion":"2025-06-18","capabilities":{}}"#,
            r#"{"protocolVersion":"2025-06-18","capabilities":[],"clientInfo":{"name":"p"}}"#,
            r#"{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{}}"#,
            r#"{"protocolVersion":"nonsense","capabilities":{},"clientInfo":{"name":"p"}}"#,
        ];
        for body in bodies {
            let outcome = dispatcher()?
                .dispatch(&ctx(&peer), "initialize", &JsonObject::new(body)?)
                .await;
            assert_eq!(
                outcome,
                Err(DispatchError::MalformedInitialize),
                "this body was accepted: {body}"
            );
        }
        Ok(())
    }

    /// SEP-2575 removed the handshake in `2026-07-28`, so a session on that
    /// revision refuses `initialize` rather than answering a request the
    /// revision has no shape for.
    ///
    /// The refusal is `UnknownMethod` and not `MalformedInitialize`: the method
    /// is not refused because the *body* is wrong, it is refused because on
    /// this revision the method does not exist, and a client that migrates its
    /// code to the new revision should see a different error for the same
    /// request than one that typos it on an old revision.
    #[tokio::test]
    async fn a_session_without_the_handshake_refuses_the_method() -> Result<(), TestError> {
        let version = ProtocolVersion::V2026_07_28;
        let peer = peer(version, ScopeSet::all(), CapabilitySet::all_for(version));
        let outcome = dispatcher()?
            .dispatch(&ctx(&peer), "initialize", &init_params("2026-07-28")?)
            .await;

        assert_eq!(outcome, Err(DispatchError::UnknownMethod));
        Ok(())
    }

    /// A method no revision defines.
    #[tokio::test]
    async fn an_undefined_method_is_refused() -> Result<(), TestError> {
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::all(), CapabilitySet::all_for(version));
        let outcome = dispatcher()?
            .dispatch(&ctx(&peer), "tools/frobnicate", &JsonObject::default())
            .await;
        assert_eq!(outcome, Err(DispatchError::UnknownMethod));
        Ok(())
    }

    /// A `tools/call` naming nothing is a malformed request, not a missing tool.
    ///
    /// The distinction is the test's whole point: `UnknownTool` would send a
    /// caller looking for a tool that exists and works.
    #[tokio::test]
    async fn a_call_with_no_name_is_malformed_rather_than_unknown() -> Result<(), TestError> {
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::all(), CapabilitySet::all_for(version));
        let params = JsonObject::new(r#"{"arguments":{}}"#)?;
        let outcome = dispatcher()?
            .dispatch(&ctx(&peer), "tools/call", &params)
            .await;
        assert_eq!(outcome, Err(DispatchError::MalformedParams));
        Ok(())
    }

    /// `arguments` is optional, so a call with a name alone reaches the handler.
    #[tokio::test]
    async fn a_call_without_arguments_still_reaches_the_handler() -> Result<(), TestError> {
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::all(), CapabilitySet::all_for(version));
        let params = JsonObject::new(r#"{"name":"echo"}"#)?;
        let outcome = dispatcher()?
            .dispatch(&ctx(&peer), "tools/call", &params)
            .await;
        assert_eq!(
            outcome.map(|answer| answer.as_str().to_string()),
            Ok("{}".to_string())
        );
        Ok(())
    }

    /// A method the session may not use is refused before its params are read.
    #[tokio::test]
    async fn a_method_the_session_lacks_is_refused() -> Result<(), TestError> {
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::all(), CapabilitySet::empty());
        let outcome = dispatcher()?
            .dispatch(&ctx(&peer), "tools/list", &JsonObject::default())
            .await;
        assert_eq!(outcome, Err(DispatchError::NotCallable));
        Ok(())
    }

    /// `server/discover` answers on `2026-07-28` and nowhere else.
    ///
    /// Both halves, because SEP-2575 defines it in the one revision that
    /// removed the handshake and a build that answered it everywhere would be
    /// offering a method no earlier spec has.
    ///
    /// The five required members are checked by name and by presence, which is
    /// what the schema requires and the only thing a result can be wrong about
    /// before anything reads its contents.
    #[tokio::test]
    async fn discover_answers_only_where_sep_2575_defines_it() -> Result<(), TestError> {
        let modern = ProtocolVersion::V2026_07_28;
        let answered = dispatcher()?
            .dispatch(
                &ctx(&peer(modern, ScopeSet::empty(), caps(modern))),
                "server/discover",
                &JsonObject::default(),
            )
            .await
            .map_err(|error| Failure::from(error.to_string()))?;
        let parsed: serde_json::Value = serde_json::from_str(answered.as_str())
            .map_err(|error| Failure::from(error.to_string()))?;
        for member in [
            "resultType",
            "supportedVersions",
            "ttlMs",
            "cacheScope",
            "capabilities",
        ] {
            assert!(
                parsed.get(member).is_some(),
                "DiscoverResult is missing required member `{member}`",
            );
        }
        assert_eq!(
            parsed.get("resultType").and_then(serde_json::Value::as_str),
            Some("complete")
        );

        let older = ProtocolVersion::V2025_11_25;
        let refused = dispatcher()?
            .dispatch(
                &ctx(&peer(older, ScopeSet::empty(), caps(older))),
                "server/discover",
                &JsonObject::default(),
            )
            .await;
        assert_eq!(
            refused.map(|answer| answer.as_str().to_string()),
            Err(DispatchError::UnknownMethod),
            "SEP-2575 defines server/discover only from 2026-07-28",
        );
        Ok(())
    }

    /// The advertised capabilities are the ones this server honours.
    ///
    /// A second list here would be a second opinion, and the suite's
    /// `discover-capabilities-match-handlers` check exists precisely because
    /// servers get that wrong: a client that discovers `prompts` and then
    /// finds `prompts/get` refused has been told something untrue.
    #[tokio::test]
    async fn discover_advertises_the_profile_the_server_honours() -> Result<(), TestError> {
        let version = ProtocolVersion::V2026_07_28;
        let server_caps = CapabilitySet::empty().with(Capability::Tools);
        let peer = peer(version, ScopeSet::empty(), server_caps);
        let answered = dispatcher()?
            .dispatch(&ctx(&peer), "server/discover", &JsonObject::default())
            .await
            .map_err(|error| Failure::from(error.to_string()))?;
        let parsed: serde_json::Value = serde_json::from_str(answered.as_str())
            .map_err(|error| Failure::from(error.to_string()))?;
        let capabilities = parsed
            .get("capabilities")
            .ok_or("the result carried no capabilities")?;

        assert!(
            capabilities.get("tools").is_some(),
            "tools was negotiated and not advertised: {capabilities}",
        );
        assert!(
            capabilities.get("prompts").is_none(),
            "prompts was not negotiated and was advertised anyway: {capabilities}",
        );
        Ok(())
    }

    /// SEP-2549's envelope appears at `2026-07-28` and nowhere else.
    ///
    /// Both directions, and the second is the one that matters: a build that
    /// stamped `resultType` everywhere would pass the first check and break the
    /// four revisions it already answers correctly, because `resultType`
    /// appears in no pre-2026 result definition.
    ///
    /// The cacheable half is checked on a listing and the non-cacheable half on
    /// a call, because those are exactly the two sides of [`Cacheable`] and a
    /// test that only listed would not notice a `ttlMs` on a side effect.
    #[tokio::test]
    async fn the_result_envelope_is_stamped_only_from_2026_07_28() -> Result<(), TestError> {
        let modern = ProtocolVersion::V2026_07_28;
        let listed = dispatcher()?
            .dispatch(
                &ctx(&peer(modern, ScopeSet::empty(), caps(modern))),
                "tools/list",
                &JsonObject::default(),
            )
            .await
            .map_err(|error| Failure::from(error.to_string()))?;
        let listed: serde_json::Value = serde_json::from_str(listed.as_str())
            .map_err(|error| Failure::from(error.to_string()))?;
        for member in ["resultType", "ttlMs", "cacheScope"] {
            assert!(
                listed.get(member).is_some(),
                "a cacheable result is missing `{member}`: {listed}",
            );
        }

        let older = ProtocolVersion::V2025_11_25;
        let legacy = dispatcher()?
            .dispatch(
                &ctx(&peer(older, ScopeSet::empty(), caps(older))),
                "tools/list",
                &JsonObject::default(),
            )
            .await
            .map_err(|error| Failure::from(error.to_string()))?;
        let legacy: serde_json::Value = serde_json::from_str(legacy.as_str())
            .map_err(|error| Failure::from(error.to_string()))?;
        for member in ["resultType", "ttlMs", "cacheScope"] {
            assert!(
                legacy.get(member).is_none(),
                "`{member}` is not defined before 2026-07-28 and was stamped anyway: {legacy}",
            );
        }
        Ok(())
    }

    /// `ping` answers on a session with nothing else, because that is what a
    /// liveness probe on an idle session is for.
    #[tokio::test]
    async fn ping_answers_with_no_features_at_all() -> Result<(), TestError> {
        let version = ProtocolVersion::V2024_11_05;
        let peer = peer(version, ScopeSet::empty(), CapabilitySet::empty());
        let outcome = dispatcher()?
            .dispatch(&ctx(&peer), "ping", &JsonObject::default())
            .await;
        assert_eq!(
            outcome.map(|answer| answer.as_str().to_string()),
            Ok("{}".to_string())
        );
        Ok(())
    }

    /// The listing shows what the caller may see, and hides what it may not.
    ///
    /// The caller here holds **no** scope, which is the half that makes the
    /// assertion mean something — a filter that showed everything would pass
    /// the next test, and one that showed nothing would pass this one.
    #[tokio::test]
    async fn the_listing_hides_a_tool_the_caller_may_not_see() -> Result<(), TestError> {
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::empty(), CapabilitySet::all_for(version));
        let listed = dispatcher()?
            .dispatch(&ctx(&peer), "tools/list", &JsonObject::default())
            .await
            .map_err(|error| Failure::from(error.to_string()))?;
        let parsed: serde_json::Value = serde_json::from_str(listed.as_str())
            .map_err(|error| Failure::from(error.to_string()))?;
        let names: Vec<&str> = parsed
            .get("tools")
            .and_then(serde_json::Value::as_array)
            .ok_or("the listing has no tools array")?
            .iter()
            .filter_map(|tool| tool.get("name"))
            .filter_map(serde_json::Value::as_str)
            .collect();
        assert_eq!(names, ["echo"]);
        Ok(())
    }

    /// The same listing, for a caller that does hold the scope.
    #[tokio::test]
    async fn the_listing_shows_the_tool_to_a_caller_holding_its_scope() -> Result<(), TestError> {
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::all(), CapabilitySet::all_for(version));
        let listed = dispatcher()?
            .dispatch(&ctx(&peer), "tools/list", &JsonObject::default())
            .await
            .map_err(|error| Failure::from(error.to_string()))?;
        assert!(
            listed.as_str().contains("secret"),
            "an unrestricted caller could not see the restricted tool: {}",
            listed.as_str()
        );
        Ok(())
    }

    // -----------------------------------------------------------------------
    // A retried call runs the tool once
    // -----------------------------------------------------------------------

    /// A dispatcher whose tool counts how many times it ran.
    ///
    /// A counter rather than a `Cell<bool>`: the claim is that the tool runs
    /// _once_, and a boolean cannot distinguish one from two. A handler that
    /// returned a different answer each time would also work, and counting is
    /// the version that says what it means.
    fn counting(count: &Arc<AtomicUsize>, answer: &'static str) -> Result<Dispatcher, TestError> {
        let answer = JsonObject::new(answer).unwrap_or_default();
        let seen = Arc::clone(count);
        let handler: ToolHandler = Arc::new(move |_ctx, _args| {
            let count = Arc::clone(&seen);
            let answer = answer.clone();
            Box::pin(async move {
                count.fetch_add(1, Ordering::SeqCst);
                answer
            })
        });
        Ok(Dispatcher::new(Arc::new(
            Server::builder("test")
                .tool(charge_tool()?, handler)
                .build(),
        )))
    }

    /// The tool a dropped connection leaves behind, on a session that asks
    /// twice with the same request id.
    ///
    /// This is the scenario the whole store exists for. A client whose POST
    /// got no answer retries, because it cannot tell a dropped connection from
    /// a call that never arrived — and the retry must not be a second charge.
    #[tokio::test]
    async fn a_retry_after_a_dropped_connection_runs_the_tool_once() -> Result<(), TestError> {
        let count = Arc::new(AtomicUsize::new(0));
        let dispatcher = counting(&count, r#"{"charged":true}"#)?;
        let peer = peer(
            ProtocolVersion::V2025_06_18,
            ScopeSet::all(),
            caps(ProtocolVersion::V2025_06_18),
        );
        let params = JsonObject::new(r#"{"name":"charge"}"#)?;

        // The first attempt: the server runs the tool and records the answer.
        // The client never sees it — the connection dropped — which is not
        // something this layer can observe, and is the point.
        let first = dispatcher
            .dispatch(&ctx(&peer), "tools/call", &params)
            .await?;
        // The retry, same id.
        let second = dispatcher
            .dispatch(&ctx(&peer), "tools/call", &params)
            .await?;

        if count.load(Ordering::SeqCst) != 1 {
            return Err(fail(format!(
                "the tool ran {} times",
                count.load(Ordering::SeqCst)
            )));
        }
        if first != second {
            return Err(fail("the retry was not answered from the recording"));
        }
        Ok(())
    }

    /// The same request id on a *different* session is a different call.
    ///
    /// Both sessions send id `r1`, which is what every client does — ids are
    /// only unique per connection. A key built from the request id alone would
    /// answer the second session with the first one's result, and the test
    /// would still be green on every other case in this module.
    #[tokio::test]
    async fn the_same_request_id_on_another_session_is_not_a_retry() -> Result<(), TestError> {
        let count = Arc::new(AtomicUsize::new(0));
        let dispatcher = counting(&count, r#"{"charged":true}"#)?;
        let version = ProtocolVersion::V2025_06_18;
        let params = JsonObject::new(r#"{"name":"charge"}"#)?;

        let first_peer = peer(version, ScopeSet::all(), caps(version));
        let second_peer = peer_on(version, ScopeSet::all(), caps(version), 2);

        let _ = dispatcher
            .dispatch(&ctx(&first_peer), "tools/call", &params)
            .await?;
        let _ = dispatcher
            .dispatch(&ctx(&second_peer), "tools/call", &params)
            .await?;

        if count.load(Ordering::SeqCst) != 2 {
            return Err(fail(format!(
                "two sessions ran the tool {} time(s); the second was treated as a retry",
                count.load(Ordering::SeqCst)
            )));
        }
        Ok(())
    }

    /// A store that remembers the order it was driven in.
    ///
    /// The black-box tests above prove what a retry gets; this proves the
    /// *sequence* that produces it, which is the thing the ordering rule is
    /// actually about. It is white-box on purpose: "the claim is released on
    /// failure" has no black-box observable here, because a tool that fails at
    /// the gate fails the same way whether its key was released or recorded,
    /// and a test that cannot tell those apart is not testing the release.
    #[derive(Debug, Default)]
    struct Journal {
        log: std::sync::Mutex<Vec<&'static str>>,
    }

    impl Journal {
        fn entries(&self) -> Vec<&'static str> {
            self.log
                .lock()
                .unwrap_or_else(std::sync::PoisonError::into_inner)
                .clone()
        }
    }

    /// A capacity for a test store.
    ///
    /// `unwrap_or` rather than `unwrap`, which the crate denies, and rather
    /// than a `match`, which `option_if_let_else` rejects — a `NonZeroUsize`
    /// has no other readable spelling. The fallback is unreachable for any
    /// count a caller writes, and `NonZeroUsize::MIN` is the honest answer if
    /// one does arrive: a zero-capacity store rather than a panic in a test.
    fn room_for(keys: usize) -> NonZeroUsize {
        NonZeroUsize::new(keys).unwrap_or(NonZeroUsize::MIN)
    }

    /// A store over the real one, logging each operation.
    struct Recording {
        inner: crate::adapters::secondary::idempotency::InMemoryIdempotencyStore<JsonObject>,
        log: Arc<Journal>,
    }

    impl crate::application::idempotency::IdempotencyStore for Recording {
        type Result = JsonObject;

        fn claim(
            &self,
            key: &crate::domain::idempotency::IdempotencyKey,
        ) -> Result<
            crate::domain::idempotency::Claim<Self::Result>,
            crate::domain::idempotency::IdempotencyError,
        > {
            self.log
                .log
                .lock()
                .unwrap_or_else(std::sync::PoisonError::into_inner)
                .push("claim");
            self.inner.claim(key)
        }

        fn complete(
            &self,
            token: &crate::domain::idempotency::ClaimToken,
            result: Self::Result,
        ) -> Result<(), crate::domain::idempotency::IdempotencyError> {
            self.log
                .log
                .lock()
                .unwrap_or_else(std::sync::PoisonError::into_inner)
                .push("complete");
            self.inner.complete(token, result)
        }

        fn abandon(
            &self,
            token: &crate::domain::idempotency::ClaimToken,
        ) -> Result<(), crate::domain::idempotency::IdempotencyError> {
            self.log
                .log
                .lock()
                .unwrap_or_else(std::sync::PoisonError::into_inner)
                .push("abandon");
            self.inner.abandon(token)
        }
    }

    /// A recorded call is `claim` then `complete`, and nothing else.
    ///
    /// The order is the claim: recording before executing would answer a
    /// concurrent caller with a result that does not exist yet, and never
    /// recording would answer a retry by running the tool again.
    #[tokio::test]
    async fn a_successful_call_claims_then_records() -> Result<(), TestError> {
        let journal = Arc::new(Journal::default());
        let store = Arc::new(Recording {
            inner: crate::adapters::secondary::idempotency::InMemoryIdempotencyStore::with_capacity(
                room_for(8),
            ),
            log: Arc::clone(&journal),
        });
        let count = Arc::new(AtomicUsize::new(0));
        let seen = Arc::clone(&count);
        let handler: ToolHandler = Arc::new(move |_ctx, _args| {
            let count = Arc::clone(&seen);
            Box::pin(async move {
                count.fetch_add(1, Ordering::SeqCst);
                JsonObject::new(r#"{"charged":true}"#).unwrap_or_default()
            })
        });
        let dispatcher = Dispatcher::new(Arc::new(
            Server::builder("test")
                .tool(charge_tool()?, handler)
                .idempotency(store)
                .build(),
        ));
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::all(), caps(version));
        let params = JsonObject::new(r#"{"name":"charge"}"#)?;

        let _ = dispatcher
            .dispatch(&ctx(&peer), "tools/call", &params)
            .await?;
        // The retry claims again — the key is still there — and then stops,
        // because the answer is already recorded.
        let _ = dispatcher
            .dispatch(&ctx(&peer), "tools/call", &params)
            .await?;

        if journal.entries() != vec!["claim", "complete", "claim"] {
            return Err(fail(format!("the store saw {:?}", journal.entries())));
        }
        if count.load(Ordering::SeqCst) != 1 {
            return Err(fail(format!(
                "the tool ran {} times",
                count.load(Ordering::SeqCst)
            )));
        }
        Ok(())
    }

    /// A call that fails is `claim` then `abandon`, and the key is reusable.
    ///
    /// The gate is the only place a `tools/call` can fail — a handler that
    /// wants to report a problem returns `isError` content, which is a
    /// *result*, not an error — so the session below is one that cannot see
    /// the tool. The observable is what happens on the second call: a released
    /// key is claimable and the gate is consulted again, and a recorded one
    /// would come straight back as the recorded refusal without ever reaching
    /// a handler.
    #[tokio::test]
    async fn a_refused_call_releases_the_key() -> Result<(), TestError> {
        let journal = Arc::new(Journal::default());
        let store = Arc::new(Recording {
            inner: crate::adapters::secondary::idempotency::InMemoryIdempotencyStore::with_capacity(
                room_for(8),
            ),
            log: Arc::clone(&journal),
        });
        let secret = Registered::from(Primitive::from(Tool::new(
            "secret",
            r#"{"type":"object"}"#,
        )?))
        .requiring_scope(Scope::Read);
        let handler: ToolHandler = Arc::new(|_ctx, _args| {
            Box::pin(async { JsonObject::new(r#"{"leaked":true}"#).unwrap_or_default() })
        });
        let dispatcher = Dispatcher::new(Arc::new(
            Server::builder("test")
                .restricted_tool(secret, handler)
                .idempotency(store)
                .build(),
        ));
        // A session with no scopes cannot see a `Read`-restricted tool.
        let version = ProtocolVersion::V2025_06_18;
        let peer = peer(version, ScopeSet::empty(), caps(version));
        let params = JsonObject::new(r#"{"name":"secret"}"#)?;

        for _ in 0..2 {
            let outcome = dispatcher
                .dispatch(&ctx(&peer), "tools/call", &params)
                .await;
            if outcome != Err(DispatchError::UnknownTool) {
                return Err(fail(format!("a hidden tool answered {outcome:?}")));
            }
        }
        // A released key is claimable, so both attempts claim it. A recorded
        // one would show `claim` twice and then nothing — but with the
        // recorded refusal returned before the handler, and the log would
        // still read the same, which is why the refusal itself is asserted
        // above rather than inferred from the log.
        if journal.entries() != vec!["claim", "abandon", "claim", "abandon"] {
            return Err(fail(format!("the store saw {:?}", journal.entries())));
        }
        Ok(())
    }

    /// A second request with an id the first still holds is refused, not run.
    ///
    /// The handler parks until released, so the claim is checked while the
    /// first call is genuinely mid-flight — which is the only state where
    /// `InFlight` exists, and the only state where running twice is possible.
    #[tokio::test]
    async fn a_duplicate_while_the_first_is_running_is_refused() -> Result<(), TestError> {
        let count = Arc::new(AtomicUsize::new(0));
        let (gate_tx, gate_rx) = tokio::sync::oneshot::channel::<()>();
        let gate_rx = Arc::new(tokio::sync::Mutex::new(Some(gate_rx)));
        let seen = Arc::clone(&count);
        let handler: ToolHandler = Arc::new(move |_ctx, _args| {
            let count = Arc::clone(&seen);
            let gate_rx = Arc::clone(&gate_rx);
            Box::pin(async move {
                // Park until the test releases it, so the first call is
                // provably still holding its key. The guard is dropped
                // before the `if let`, not at the end of it: held across the
                // inner `await` it would be a lock this task does not
                // release, which is the deadlock `significant_drop_in_scrutinee`
                // exists to catch rather than a style note.
                let waiting = { gate_rx.lock().await.take() };
                if let Some(receiver) = waiting {
                    let _ = receiver.await;
                }
                count.fetch_add(1, Ordering::SeqCst);
                JsonObject::new(r#"{"charged":true}"#).unwrap_or_default()
            })
        });
        let dispatcher = Dispatcher::new(Arc::new(
            Server::builder("test")
                .tool(charge_tool()?, handler)
                .build(),
        ));
        let peer = peer(
            ProtocolVersion::V2025_06_18,
            ScopeSet::all(),
            caps(ProtocolVersion::V2025_06_18),
        );
        let params = JsonObject::new(r#"{"name":"charge"}"#)?;

        // The spawned call borrows nothing: `Dispatcher` is `Clone` and
        // `ctx` builds a fresh context over the same session, which is what
        // makes this a genuine race rather than two calls one after another.
        let running = tokio::spawn({
            let dispatcher = Dispatcher::new(Arc::clone(dispatcher.server()));
            let peer = peer_on(
                ProtocolVersion::V2025_06_18,
                ScopeSet::all(),
                caps(ProtocolVersion::V2025_06_18),
                1,
            );
            let params = JsonObject::new(r#"{"name":"charge"}"#)?;
            async move {
                dispatcher
                    .dispatch(&ctx(&peer), "tools/call", &params)
                    .await
            }
        });
        // Let the spawned call reach the handler, which is where it parks.
        // Without this the duplicate could be issued first and be the one
        // that claims the key, and the test would pass for the wrong reason.
        tokio::task::yield_now().await;

        let duplicate = dispatcher
            .dispatch(&ctx(&peer), "tools/call", &params)
            .await;
        let _ = gate_tx.send(());
        let _ = running.await;

        match duplicate {
            Err(DispatchError::DuplicateInFlight(_)) => {}
            other => return Err(fail(format!("the duplicate answered {other:?}"))),
        }
        if count.load(Ordering::SeqCst) != 1 {
            return Err(fail(format!(
                "the tool ran {} time(s) while a duplicate was refused",
                count.load(Ordering::SeqCst)
            )));
        }
        Ok(())
    }
}