State Model

All state in AHP is identified by URIs. Clients subscribe to a URI to receive its current state snapshot and subsequent action updates. This is the single universal mechanism for state synchronization.

Root State

Subscribable at agenthost:/root. Contains global, lightweight data that all clients need. Does not contain the session list — that is fetched imperatively via RPC (see Commands).

RootState {
  agents: AgentInfo[]
}

Each AgentInfo includes the models available for that agent:

AgentInfo {
  provider: string         // e.g. 'copilot'
  displayName: string
  description: string
  models: ModelInfo[]
  customizations?: CustomizationRef[]  // Open Plugins
}

ModelInfo {
  id: string
  provider: string
  name: string
  maxContextWindow?: number
  supportsVision?: boolean
  policyState?: 'enabled' | 'disabled' | 'unconfigured'
  configSchema?: ConfigSchema   // model-specific options (e.g. thinking level)
  _meta?: Record<string, unknown>  // intrinsic facts (e.g. pricing); see below
}

ConfigSchema {
  type: 'object'
  properties: Record<string, ConfigPropertySchema>
  required?: string[]
}

ConfigPropertySchema {
  type: 'string'
  title: string
  description?: string
  default?: string
  enum: string[]                 // allowed values
  enumLabels?: string[]          // display labels (parallel array)
  enumDescriptions?: string[]    // descriptions (parallel array)
  readOnly?: boolean
}

When a model has a configSchema, clients present it as a form and pass the resolved values in a ModelSelection (see Session Summary).

_meta carries additional provider-specific metadata. Clients MAY look for well-known keys here to provide enhanced UI — for example, a pricing key may carry model pricing metadata.

Root state is mutated only by server-originated actions (e.g. root/agentsChanged).

Session State

Subscribable at the session's URI (e.g. copilot:/<uuid>). Contains the full state for a single session.

SessionState {
  summary: SessionSummary
  lifecycle: 'creating' | 'ready' | 'creationFailed'
  creationError?: ErrorInfo
  turns: Turn[]
  activeTurn: ActiveTurn | undefined
  inputRequests?: SessionInputRequest[]
  customizations?: SessionCustomization[]  // active session plugins
}

Lifecycle

The lifecycle field tracks the asynchronous creation process. When a client creates a session, it picks a URI, sends the command, and subscribes immediately. The initial snapshot has lifecycle: 'creating'. The server asynchronously initializes the backend and dispatches session/ready or session/creationFailed.

Session Summary

Lightweight metadata used in the session list and embedded within session state:

SessionSummary {
  resource: URI
  provider: string
  title: string
  status: number  // SessionStatus bitset
  activity?: string
  createdAt: number
  modifiedAt: number
  project?: ProjectInfo
  model?: ModelSelection
  workingDirectory?: URI
}

ModelSelection {
  id: string                             // model ID
  config?: Record<string, string>        // model-specific config values
}

ProjectInfo {
  uri: URI
  displayName: string
}

The status bitset encodes both the session's activity state and metadata flags like read/archived state. See the Session Status Bitset table below for details.

Session Status Bitset

summary.status is a numeric bitset. Clients SHOULD use bitwise checks instead of string or equality checks for activity states:

Name	Value	Bits	Meaning
SessionStatus.Idle	1	1 << 0	No active turn and no pending input request.
SessionStatus.Error	2	1 << 1	The most recent turn ended with an error.
SessionStatus.InProgress	8	1 << 3	A turn is active.
SessionStatus.InputNeeded	24	(1 << 3) | (1 << 4)	A turn is active and either at least one user input request is open, or at least one tool call is awaiting user confirmation (pre- or post-execution). Includes the InProgress bit.
SessionStatus.IsRead	32	1 << 5	The client has viewed this session since its last modification. Cleared automatically when a new turn starts or an input request arrives. Toggled via session/isReadChanged.
SessionStatus.IsArchived	64	1 << 6	The session has been archived by the client. Toggled via session/isArchivedChanged.

Bits 0–4 encode mutually-exclusive activity status (exactly one is set at a time). Bits 5+ encode orthogonal metadata flags that may be combined with any activity status via bitwise OR.

For example, (status & SessionStatus.InProgress) !== 0 is true for both InProgress and InputNeeded. A session that is idle, read, and archived has status 1 | 32 | 64 = 97.

Turns

A turn represents one request/response cycle between user and agent.

Completed Turn

Turn {
  id: string
  userMessage: UserMessage
  responseParts: ResponsePart[]     // all content in stream order
  usage: UsageInfo | undefined
  state: 'complete' | 'cancelled' | 'error'
  error?: ErrorInfo
}

Active Turn

An in-progress turn where the assistant is actively streaming:

ActiveTurn {
  id: string
  userMessage: UserMessage
  responseParts: ResponsePart[]     // all content in stream order
  usage: UsageInfo | undefined
}

User Messages

UserMessage {
  text: string
  attachments?: MessageAttachment[]
}

// Discriminated union — see types/state.ts for full definitions.
type MessageAttachment =
  | SimpleMessageAttachment            // type: 'simple'
  | MessageEmbeddedResourceAttachment  // type: 'embeddedResource'
  | MessageResourceAttachment          // type: 'resource'

// Common fields shared by all variants:
MessageAttachmentBase {
  label: string                  // human-readable label, e.g. filename
  range?: TextRange              // range in `text` that references this attachment
  displayKind?: 'image' | 'document' | 'symbol' | 'directory' | 'selection' | string
  _meta?: Record<string, unknown>
}

TextRange {
  start: { line: number, character: number }  // zero-based text position
  end: { line: number, character: number }
}

TextSelection {
  range: TextRange
}

MessageResourceAttachment {
  type: 'resource'
  uri: URI
  displayKind?: 'selection'
  selection?: TextSelection
}

Attachments MAY be referenced inline by text via the optional range field, which points at a span in the message text. This is a text range, not a byte range. Attachments without a range are still associated with the message but are not anchored to a specific span.

Resource and embedded-resource attachments MAY also include selection to identify a selected range within the attached textual resource. This is distinct from range, which only describes where the attachment is referenced in the user message text. Selected text is not embedded inline; consumers can resolve the resource and read the selected range when needed. selection is only meaningful for textual resources; binary resources may still use resource or embedded-resource attachments, but they should not use this text selection field.

Use SimpleMessageAttachment for opaque attachments whose model representation is supplied by the producer, MessageEmbeddedResourceAttachment for small inline base64 payloads (e.g. a pasted image), and MessageResourceAttachment to reference a resource by URI (the content is fetched via resourceRead when needed).

Attachments produced by the completions command MAY include a _meta blob; clients MUST preserve every property of _meta when echoing the attachment back in the user message.

User-Message Completions

To support @-mention pickers and similar inline-completion experiences, the client can call the completions command while the user is composing a message:

CompletionsParams {
  kind: 'userMessage'      // CompletionItemKind.UserMessage
  session: URI
  text: string             // full text typed so far
  offset: number           // cursor offset (UTF-16 code units)
}

CompletionsResult {
  items: CompletionItem[]
}

CompletionItem {
  insertText: string
  rangeStart?: number      // range in `text` to replace; insertion at cursor if omitted
  rangeEnd?: number
  attachment: MessageAttachment
}

Servers advertise the characters that should auto-trigger this request via InitializeResult.completionTriggerCharacters (e.g. ['@', '#']). Clients MAY also issue completions calls in response to explicit user actions (such as a keyboard shortcut). When the user accepts an item, the client replaces [rangeStart, rangeEnd) in the input with insertText and associates the item's attachment with the resulting UserMessage.

Response Parts

All response content — text, tool calls, reasoning, and content references — lives in a single responseParts array in stream order. This mirrors how LLM APIs (e.g. OpenAI) represent responses as a unified list of typed items.

// Inline markdown content
MarkdownResponsePart {
  kind: 'markdown'
  id: string               // targeted by session/delta for text appends
  content: string
}

// Reasoning/thinking content from the model
ReasoningResponsePart {
  kind: 'reasoning'
  id: string               // targeted by session/reasoning for text appends
  content: string
}

// Tool call (see Tool Call Lifecycle below)
ToolCallResponsePart {
  kind: 'toolCall'
  toolCall: ToolCallState   // full lifecycle state
}

// Reference to large content stored outside the state tree
ContentRef {
  kind: 'contentRef'
  uri: string              // scheme://sessionId/contentId
  sizeHint?: number
  mimeType?: string
}

Text content uses a create-then-append pattern: the server first emits a session/responsePart action to create a new markdown (or reasoning) part with an id, then streams text into it via session/delta (or session/reasoning) actions targeting that partId. This pattern is extensible to future streaming content types.

Clients fetch ContentRef content separately via the resourceRead(uri) command. This keeps the state tree small and serializable.

Consumers can derive display text by concatenating all markdown parts, find tool calls by filtering for toolCall parts, and access reasoning by filtering for reasoning parts.

Tool Call Lifecycle

Tool calls are represented as a discriminated union on status, where each state only exposes the fields valid for that phase.

States

Status	Key Fields	Description
streaming	partialInput?	LM is streaming tool call parameters. partialInput accumulates via toolCallDelta.
pending-confirmation	invocationMessage, toolInput?, edits?, editable?, options?	Parameters complete or mid-execution confirmation needed. edits previews file changes. editable indicates the client may edit parameters before confirming. options provides server-defined choices beyond simple approve/deny (see below). Uses _meta for additional context.
running	confirmed, selectedOption?	Tool is executing. confirmed records how it was approved. selectedOption holds the chosen confirmation option, if any.
pending-result-confirmation	success, pastTenseMessage, content?, selectedOption?	Execution finished, waiting for client to approve the result.
completed	success, pastTenseMessage, content?, selectedOption?	Terminal state. Tool finished.
cancelled	reason, reasonMessage?, userSuggestion?, selectedOption?	Terminal state. reason is 'denied', 'skipped', or 'result-denied'.

Mid-execution Re-confirmation

When a running tool needs additional user approval (e.g. a shell permission), the server dispatches session/toolCallReady again without confirmed. This transitions the tool call from running back to pending-confirmation, updating invocationMessage and _meta with context about what needs approval. The client uses the standard session/toolCallConfirmed flow to approve or deny.

Editable Parameters

When editable is true on a pending-confirmation tool call, the client may allow the user to modify the tool's input parameters before confirming. If the user edits the parameters, the client includes editedToolInput on the session/toolCallConfirmed action. The reducer uses editedToolInput (if present) in place of the original toolInput when transitioning to running.

When a turn completes, non-terminal tool calls in responseParts are force-cancelled with reason 'skipped'.

Confirmation Options

By default, clients render a binary approve/deny UI for pending-confirmation tool calls. The server can provide richer choices via options — an array of ConfirmationOption objects, each with:

Field	Type	Description
id	string	Unique identifier, returned in the session/toolCallConfirmed action as selectedOptionId.
label	string	Human-readable text for the button or menu item. The server SHOULD localise this using the client's locale (sent in initialize).
kind	'approve' | 'deny'	Classifies the option so the server and client know whether it represents approval or denial.
group	number?	Logical group number. Clients SHOULD display options in the order they are defined and MAY use differing group numbers to insert dividers between logical clusters.

For example, a server might offer "Approve", "Approve in this Session", "Deny", and "Deny with reason". When the user picks an option, the client dispatches session/toolCallConfirmed with selectedOptionId set to the chosen option's id. The reducer resolves the full ConfirmationOption object and stores it as selectedOption on the resulting running or cancelled state, and it carries through to completed.

Session Input Requests

Sessions can request structured input from the user by storing live requests in top-level session state:

SessionState {
  // ...existing fields...
  inputRequests?: SessionInputRequest[]
}

SessionInputRequest {
  id: string
  message: string
  url?: URI
  questions?: SessionInputQuestion[]
  answers?: Record<string, SessionInputAnswer>
}

See Elicitation for the request lifecycle, question and answer shapes, URL requests, multi-client draft synchronization, and validation rules.

Usage Info

Token usage reported per turn:

UsageInfo {
  inputTokens?: number
  outputTokens?: number
  model?: string
  cacheReadTokens?: number
  _meta?: Record<string, unknown>
}

_meta carries provider-specific metadata for the usage report. Clients may inspect well-known optional keys to provide enhanced UI.

Session List

The session list can be arbitrarily large and is not part of the state tree. Instead:

• Clients fetch the list imperatively via listSessions() RPC.
• The server sends lightweight notifications to keep connected clients' caches in sync without re-fetching:
  • notify/sessionAdded and notify/sessionRemoved signal lifecycle (creation and disposal).
  • notify/sessionSummaryChanged streams partial updates to an existing session's summary (title, status, modifiedAt, project, model, working directory, diffs) so clients that are displaying a session list can stay in sync without subscribing to every session URI individually. Only fields present in changes carry new values; omitted fields are unchanged. The server SHOULD emit this notification whenever any mutable summary field changes, and MAY coalesce or debounce noisy updates (for example, rapid modifiedAt bumps while a turn is streaming) at its discretion.

Notifications are ephemeral — not processed by reducers, not stored in state, not replayed on reconnect. On reconnect, clients re-fetch the list.

Pending Messages

Sessions maintain two optional arrays of pending messages — instructions queued for future delivery to the agent:

SessionState {
  // ...existing fields...
  steeringMessage?: PendingMessage      // inject into current turn
  queuedMessages?: PendingMessage[]     // start as new turns
}

PendingMessage {
  id: string
  userMessage: UserMessage
}

Steering Message

The steering message is injected into the current turn at a convenient point. Clients set a steering message to guide the agent mid-flight — for example, telling it to focus on a specific file or change approach. Only one steering message exists at a time; adding a new one replaces any existing one.

• When the session has an active turn, the server consumes the steering message at its discretion, dispatching session/pendingMessageRemoved when it does.
• When set while idle, the steering message is silently stored until a turn starts.

Queued Messages

Queued messages are automatically started as new turns after the current turn finishes. The server processes them FIFO (by arrival order).

• When a turn completes and queued messages exist, the server removes the first queued message and starts a new turn from it.
• When a queued message is added while the session is idle, the server SHOULD immediately consume it and start a turn.
• The resulting session/turnStarted action includes a queuedMessageId field linking back to the source queued message.

Management

Clients can set or remove both steering and queued messages at any time using the session/pendingMessageSet (upsert) and session/pendingMessageRemoved actions with a kind discriminant ('steering' or 'queued').

Session Truncation

The session/truncated action removes turn history from a session. It is client-dispatchable — either side can truncate. If the session has an active turn it is silently dropped and the session status returns to idle.

• With turnId — keeps all turns up to and including the specified turn; removes everything after it.
• Without turnId — removes all turns (empties the session).

A common pattern is to truncate and then immediately start a new turn with an edited message:

If the turnId is not found in the completed turns array, the action is a no-op.

Session Forking

A new session can be created as a fork of an existing session by providing the optional fork field in createSession. The server populates the new session with content from the source session up to and including the response of the specified turn.

createSession({
  session: 'copilot:/<new-uuid>',
  provider: 'copilot',
  fork: {
    session: 'copilot:/<source-uuid>',
    turnId: 't-3',     // copy turns through t-3
  },
})

The forked session is an independent copy — subsequent changes to either session do not affect the other. The server broadcasts notify/sessionAdded for the new session as usual.

Next Steps

• Actions — How state is mutated.
• Elicitation — How sessions request user input.
• Customizations — Extending sessions with Open Plugins.
• Write-Ahead Reconciliation — How clients stay in sync.
• State Types Reference — Complete type definitions.