refactor(agent-loop): project runtime context before steps

packages/core/session/src/index.ts

@@ -13,7 +13,7 @@ import { scopeOf, scopeTarget } from '@deepseek-ai/dsh-scope'
 import type { Scoped } from '@deepseek-ai/dsh-scope'
 import type { Message } from '@deepseek-ai/dsh-llm'
 import { SESSION_FORMAT_VERSION, SessionId } from './types.ts'
-import type { CreateSessionOptions, EpochHeader, SessionEvent, SessionEventMap, SessionEventType, SessionHeader, SurfaceIntent, SurfaceEventType } from './types.ts'
+import type { CreateSessionOptions, EpochHeader, RequestContext, SessionEvent, SessionEventMap, SessionEventType, SessionHeader, SurfaceIntent, SurfaceEventType } from './types.ts'
 import { snapshotJsonValue } from './json.ts'
 import { SurfaceManager } from './surface.ts'
 import type { SessionSurface } from './surface.ts'
@@ -392,7 +392,7 @@ export class Session {
   readonly firstLiveSeq: number
 
   constructor(id: SessionId, seed?: readonly SessionEvent[], header?: SessionHeader) {
-    if (seed) {
+    if (seed !== undefined) {
       // Validate the seed to the SAME invariants `append` enforces, so a
       // replay/fork (`ctx.sessions.create(id, { seed })`) cannot construct a
       // live log that no persistence backend could store: each event's `data`
@@ -429,7 +429,7 @@ export class Session {
     // captures the creation seed: no load-time write. Re-marking is skipped
     // because a cold session is resumed on first touch, so repeatedly opening
     // one must not grow its log per open.
-    if (this.firstLiveSeq > 0 && this.log.at(-1)?.type !== 'session/end-seed') {
+    if (seed !== undefined && this.log.at(-1)?.type !== 'session/end-seed') {
       this.append('session/end-seed', {})
     }
   }
@@ -566,6 +566,25 @@ export class Session {
     return this.headerFold
   }
 
+  /** Cached fold of `request/context` events. */
+  private contextFold: RequestContext | undefined
+  private contextFoldSeq = 0
+
+  /**
+   * Return the latest resolved route metadata, or `undefined` before the first
+   * `request/context` event. Each event is folded once.
+   * @returns the latest immutable route metadata.
+   */
+  requestContext(): RequestContext | undefined {
+    if (this.contextFoldSeq < this.log.length) {
+      for (const event of this.log.slice(this.contextFoldSeq)) {
+        if (event.type === 'request/context') this.contextFold = deepFreeze({ ...event.data })
+      }
+      this.contextFoldSeq = this.log.length
+    }
+    return this.contextFold
+  }
+
   /** The derived-message cache: frozen projections, extended per unseen node. */
   private derived: Message[] = []
   /** Surface position (nodes projected) the cache has reached. */

packages/core/session/src/invariant.ts

@@ -149,7 +149,8 @@ function validateEvent(
       break
     case 'steering/message':
     case 'todo/write':
-    case 'request/header': {
+    case 'request/header':
+    case 'request/context': {
       if (trace.openTurn === null) {
         fail(`${event.type} appended outside any open turn (core execution events must be turn-enclosed)`)
       }

packages/core/session/src/types.ts

@@ -151,6 +151,16 @@ export interface EpochHeader {
   tools?: ToolSchema[]
 }
 
+/** Registration-bound metadata for one resolved model route. */
+export interface RequestContext {
+  /** Registered provider route the metadata belongs to. */
+  provider: string
+  /** Provider-owned model id the metadata belongs to. */
+  model: string
+  /** Maximum combined request and response context in tokens, when advertised. */
+  contextWindow?: number
+}
+
 /**
  * Why a `request/header` snapshot was appended: `'initial'` — the log's first
  * header (a new conversation); `'resume'` — a loop instance's first request
@@ -233,6 +243,11 @@ export interface SessionEventMap {
    * It is log-only; the latest snapshot reconstructs the request header.
    */
   'request/header': { header: EpochHeader; reason: RequestHeaderReason }
+  /**
+   * Route metadata for the next request, logged only when the route or capacity
+   * changes. It does not participate in request reconstruction or header equality.
+   */
+  'request/context': RequestContext
   /**
    * Marks the end of a constructor seed. Events before it have smaller seq
    * values and came from the seed (resume, fork, or replay); this lifecycle