diff --git a/commands/container.go b/commands/container.go
index bc685b7b..9b909b75 100644
--- a/commands/container.go
+++ b/commands/container.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2025 Tulir Asokan
+// Copyright (c) 2026 Tulir Asokan
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -8,14 +8,20 @@ package commands
 
 import (
 	"fmt"
+	"slices"
 	"strings"
 	"sync"
+
+	"go.mau.fi/util/exmaps"
+
+	"maunium.net/go/mautrix/event/cmdschema"
 )
 
 type CommandContainer[MetaType any] struct {
 	commands map[string]*Handler[MetaType]
 	aliases  map[string]string
 	lock     sync.RWMutex
+	parent   *Handler[MetaType]
 }
 
 func NewCommandContainer[MetaType any]() *CommandContainer[MetaType] {
@@ -25,6 +31,29 @@ func NewCommandContainer[MetaType any]() *CommandContainer[MetaType] {
 	}
 }
 
+func (cont *CommandContainer[MetaType]) AllSpecs() []*cmdschema.EventContent {
+	data := make(exmaps.Set[*Handler[MetaType]])
+	cont.collectHandlers(data)
+	specs := make([]*cmdschema.EventContent, 0, data.Size())
+	for handler := range data.Iter() {
+		if handler.Parameters != nil {
+			specs = append(specs, handler.Spec())
+		}
+	}
+	return specs
+}
+
+func (cont *CommandContainer[MetaType]) collectHandlers(into exmaps.Set[*Handler[MetaType]]) {
+	cont.lock.RLock()
+	defer cont.lock.RUnlock()
+	for _, handler := range cont.commands {
+		into.Add(handler)
+		if handler.subcommandContainer != nil {
+			handler.subcommandContainer.collectHandlers(into)
+		}
+	}
+}
+
 // Register registers the given command handlers.
 func (cont *CommandContainer[MetaType]) Register(handlers ...*Handler[MetaType]) {
 	if cont == nil {
@@ -32,7 +61,10 @@ func (cont *CommandContainer[MetaType]) Register(handlers ...*Handler[MetaType])
 	}
 	cont.lock.Lock()
 	defer cont.lock.Unlock()
-	for _, handler := range handlers {
+	for i, handler := range handlers {
+		if handler == nil {
+			panic(fmt.Errorf("handler #%d is nil", i+1))
+		}
 		cont.registerOne(handler)
 	}
 }
@@ -45,6 +77,10 @@ func (cont *CommandContainer[MetaType]) registerOne(handler *Handler[MetaType])
 	} else if aliasTarget, alreadyExists := cont.aliases[handler.Name]; alreadyExists {
 		panic(fmt.Errorf("tried to register command %q, but it's already registered as an alias for %q", handler.Name, aliasTarget))
 	}
+	if !slices.Contains(handler.parents, cont.parent) {
+		handler.parents = append(handler.parents, cont.parent)
+		handler.nestedNameCache = nil
+	}
 	cont.commands[handler.Name] = handler
 	for _, alias := range handler.Aliases {
 		if strings.ToLower(alias) != alias {
diff --git a/commands/event.go b/commands/event.go
index 77a3c0d2..76d6c9f0 100644
--- a/commands/event.go
+++ b/commands/event.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2025 Tulir Asokan
+// Copyright (c) 2026 Tulir Asokan
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -8,6 +8,7 @@ package commands
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"strings"
 
@@ -35,6 +36,8 @@ type Event[MetaType any] struct {
 	// RawArgs is the same as args, but without the splitting by whitespace.
 	RawArgs string
 
+	StructuredArgs json.RawMessage
+
 	Ctx     context.Context
 	Log     *zerolog.Logger
 	Proc    *Processor[MetaType]
@@ -61,7 +64,7 @@ var IDHTMLParser = &format.HTMLParser{
 }
 
 // ParseEvent parses a message into a command event struct.
-func ParseEvent[MetaType any](ctx context.Context, evt *event.Event) *Event[MetaType] {
+func (proc *Processor[MetaType]) ParseEvent(ctx context.Context, evt *event.Event) *Event[MetaType] {
 	content, ok := evt.Content.Parsed.(*event.MessageEventContent)
 	if !ok || content.MsgType == event.MsgNotice || content.RelatesTo.GetReplaceID() != "" {
 		return nil
@@ -70,12 +73,34 @@ func ParseEvent[MetaType any](ctx context.Context, evt *event.Event) *Event[Meta
 	if content.Format == event.FormatHTML {
 		text = IDHTMLParser.Parse(content.FormattedBody, format.NewContext(ctx))
 	}
+	if content.MSC4391BotCommand != nil {
+		if !content.Mentions.Has(proc.Client.UserID) || len(content.Mentions.UserIDs) != 1 {
+			return nil
+		}
+		wrapped := StructuredCommandToEvent[MetaType](ctx, evt, content.MSC4391BotCommand)
+		wrapped.RawInput = text
+		return wrapped
+	}
 	if len(text) == 0 {
 		return nil
 	}
 	return RawTextToEvent[MetaType](ctx, evt, text)
 }
 
+func StructuredCommandToEvent[MetaType any](ctx context.Context, evt *event.Event, content *event.MSC4391BotCommandInput) *Event[MetaType] {
+	commandParts := strings.Split(content.Command, " ")
+	return &Event[MetaType]{
+		Event: evt,
+		// Fake a command and args to let the subcommand finder in Process work.
+		Command: commandParts[0],
+		Args:    commandParts[1:],
+		Ctx:     ctx,
+		Log:     zerolog.Ctx(ctx),
+
+		StructuredArgs: content.Arguments,
+	}
+}
+
 func RawTextToEvent[MetaType any](ctx context.Context, evt *event.Event, text string) *Event[MetaType] {
 	parts := strings.Fields(text)
 	if len(parts) == 0 {
@@ -188,3 +213,25 @@ func (evt *Event[MetaType]) UnshiftArg(arg string) {
 	evt.RawArgs = arg + " " + evt.RawArgs
 	evt.Args = append([]string{arg}, evt.Args...)
 }
+
+func (evt *Event[MetaType]) ParseArgs(into any) error {
+	return json.Unmarshal(evt.StructuredArgs, into)
+}
+
+func ParseArgs[T, MetaType any](evt *Event[MetaType]) (into T, err error) {
+	err = evt.ParseArgs(&into)
+	return
+}
+
+func WithParsedArgs[T, MetaType any](fn func(*Event[MetaType], T)) func(*Event[MetaType]) {
+	return func(evt *Event[MetaType]) {
+		parsed, err := ParseArgs[T, MetaType](evt)
+		if err != nil {
+			evt.Log.Debug().Err(err).Msg("Failed to parse structured args into struct")
+			// TODO better error, usage info? deduplicate with Process
+			evt.Reply("Failed to parse arguments: %v", err)
+			return
+		}
+		fn(evt, parsed)
+	}
+}
diff --git a/commands/handler.go b/commands/handler.go
index b01d594f..3b92a908 100644
--- a/commands/handler.go
+++ b/commands/handler.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2025 Tulir Asokan
+// Copyright (c) 2026 Tulir Asokan
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -8,6 +8,9 @@ package commands
 
 import (
 	"strings"
+
+	"maunium.net/go/mautrix/event"
+	"maunium.net/go/mautrix/event/cmdschema"
 )
 
 type Handler[MetaType any] struct {
@@ -25,12 +28,53 @@ type Handler[MetaType any] struct {
 	// Event.ShiftArg will likely be useful for implementing such parameters.
 	PreFunc func(ce *Event[MetaType])
 
+	// Description is a short description of the command.
+	Description *event.ExtensibleTextContainer
+	// Parameters is a description of structured command parameters.
+	// If set, the StructuredArgs field of Event will be populated.
+	Parameters []*cmdschema.Parameter
+
+	parents             []*Handler[MetaType]
+	nestedNameCache     []string
 	subcommandContainer *CommandContainer[MetaType]
 }
 
+func (h *Handler[MetaType]) NestedNames() []string {
+	if h.nestedNameCache != nil {
+		return h.nestedNameCache
+	}
+	nestedNames := make([]string, 0, (1+len(h.Aliases))*len(h.parents))
+	for _, parent := range h.parents {
+		if parent == nil {
+			nestedNames = append(nestedNames, h.Name)
+			nestedNames = append(nestedNames, h.Aliases...)
+		} else {
+			for _, parentName := range parent.NestedNames() {
+				nestedNames = append(nestedNames, parentName+" "+h.Name)
+				for _, alias := range h.Aliases {
+					nestedNames = append(nestedNames, parentName+" "+alias)
+				}
+			}
+		}
+	}
+	h.nestedNameCache = nestedNames
+	return nestedNames
+}
+
+func (h *Handler[MetaType]) Spec() *cmdschema.EventContent {
+	names := h.NestedNames()
+	return &cmdschema.EventContent{
+		Command:     names[0],
+		Aliases:     names[1:],
+		Parameters:  h.Parameters,
+		Description: h.Description,
+	}
+}
+
 func (h *Handler[MetaType]) initSubcommandContainer() {
 	if len(h.Subcommands) > 0 {
 		h.subcommandContainer = NewCommandContainer[MetaType]()
+		h.subcommandContainer.parent = h
 		h.subcommandContainer.Register(h.Subcommands...)
 	} else {
 		h.subcommandContainer = nil
diff --git a/commands/processor.go b/commands/processor.go
index 9341329b..0089226f 100644
--- a/commands/processor.go
+++ b/commands/processor.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2025 Tulir Asokan
+// Copyright (c) 2026 Tulir Asokan
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -72,9 +72,9 @@ func (proc *Processor[MetaType]) Process(ctx context.Context, evt *event.Event)
 	case event.EventReaction:
 		parsed = proc.ParseReaction(ctx, evt)
 	case event.EventMessage:
-		parsed = ParseEvent[MetaType](ctx, evt)
+		parsed = proc.ParseEvent(ctx, evt)
 	}
-	if parsed == nil || !proc.PreValidator.Validate(parsed) {
+	if parsed == nil || (!proc.PreValidator.Validate(parsed) && parsed.StructuredArgs == nil) {
 		return
 	}
 	parsed.Proc = proc
@@ -107,6 +107,11 @@ func (proc *Processor[MetaType]) Process(ctx context.Context, evt *event.Event)
 			break
 		}
 	}
+	if parsed.StructuredArgs != nil && len(parsed.Args) > 0 {
+		// The client sent MSC4391 data, but the target command wasn't found
+		log.Debug().Msg("Didn't find handler for MSC4391 command")
+		return
+	}
 
 	logWith := log.With().
 		Str("command", parsed.Command).
@@ -116,11 +121,31 @@ func (proc *Processor[MetaType]) Process(ctx context.Context, evt *event.Event)
 	}
 	if proc.LogArgs {
 		logWith = logWith.Strs("args", parsed.Args)
+		if parsed.StructuredArgs != nil {
+			logWith = logWith.RawJSON("structured_args", parsed.StructuredArgs)
+		}
 	}
 	log = logWith.Logger()
 	parsed.Ctx = log.WithContext(ctx)
 	parsed.Log = &log
 
+	if handler.Parameters != nil && parsed.StructuredArgs == nil {
+		// The handler wants structured parameters, but the client didn't send MSC4391 data
+		var err error
+		parsed.StructuredArgs, err = handler.Spec().ParseArguments(parsed.RawArgs)
+		if err != nil {
+			log.Debug().Err(err).Msg("Failed to parse structured arguments")
+			// TODO better error, usage info? deduplicate with WithParsedArgs
+			parsed.Reply("Failed to parse arguments: %v", err)
+			return
+		}
+		if proc.LogArgs {
+			log.UpdateContext(func(c zerolog.Context) zerolog.Context {
+				return c.RawJSON("structured_args", parsed.StructuredArgs)
+			})
+		}
+	}
+
 	log.Debug().Msg("Processing command")
 	handler.Func(parsed)
 }
diff --git a/commands/reactions.go b/commands/reactions.go
index 0df372e5..0d316219 100644
--- a/commands/reactions.go
+++ b/commands/reactions.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2025 Tulir Asokan
+// Copyright (c) 2026 Tulir Asokan
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -8,6 +8,7 @@ package commands
 
 import (
 	"context"
+	"encoding/json"
 	"strings"
 
 	"github.com/rs/zerolog"
@@ -19,6 +20,11 @@ import (
 const ReactionCommandsKey = "fi.mau.reaction_commands"
 const ReactionMultiUseKey = "fi.mau.reaction_multi_use"
 
+type ReactionCommandData struct {
+	Command string `json:"command"`
+	Args    any    `json:"args,omitempty"`
+}
+
 func (proc *Processor[MetaType]) ParseReaction(ctx context.Context, evt *event.Event) *Event[MetaType] {
 	content, ok := evt.Content.Parsed.(*event.ReactionEventContent)
 	if !ok {
@@ -67,21 +73,33 @@ func (proc *Processor[MetaType]) ParseReaction(ctx context.Context, evt *event.E
 			Msg("Reaction command not found in target event")
 		return nil
 	}
-	cmdString, ok := rawCmd.(string)
-	if !ok {
+	var wrappedEvt *Event[MetaType]
+	switch typedCmd := rawCmd.(type) {
+	case string:
+		wrappedEvt = RawTextToEvent[MetaType](ctx, evt, typedCmd)
+	case map[string]any:
+		var input event.MSC4391BotCommandInput
+		if marshaled, err := json.Marshal(typedCmd); err != nil {
+
+		} else if err = json.Unmarshal(marshaled, &input); err != nil {
+
+		} else {
+			wrappedEvt = StructuredCommandToEvent[MetaType](ctx, evt, &input)
+		}
+	}
+	if wrappedEvt == nil {
 		zerolog.Ctx(ctx).Debug().
 			Stringer("target_event_id", evtID).
 			Str("reaction_key", content.RelatesTo.Key).
 			Msg("Reaction command data is invalid")
 		return nil
 	}
-	wrappedEvt := RawTextToEvent[MetaType](ctx, evt, cmdString)
 	wrappedEvt.Proc = proc
 	wrappedEvt.Redact()
 	if !isMultiUse {
 		DeleteAllReactions(ctx, proc.Client, evt)
 	}
-	if cmdString == "" {
+	if wrappedEvt.Command == "" {
 		return nil
 	}
 	return wrappedEvt