fix(agent-loop): admit prompts under the prepared route

Resolve agent/request and prepareCall inside the accepted open step before committing system and user input. Preserve prompt-assembly model selection, bind capability and dispatch to the same prepared adapter, then log the envelope and derive the request synchronously. Preparation cancellation leaves a balanced empty step with no admitted input.

Normalize incapable-route retained prompt versions with logged per-node empty replacements and a current head, including unchanged rendering and resumed history. Dormant empty tails do not override the effective prompt. No new event or delete operation.

Tests: 389 agent-loop cases pass with exact agent.ts/runtime-context.ts coverage at 100% for all metrics; tsc -b packages/core/agent-loop/tsconfig.json passes. Seven actual-loop MockAdapter cases exercise pi-ai conversion, source-event reconstruction, both route transitions, resume, cancellation barriers, and bound selection. Stale-capability negative control fails all three transition regressions. Update EN/ZH request visibility, lifecycle generator, and owning rationale. Broad catalogs, recorded SDK/GUI and aggregate gates remain parent-owned.

packages/core/agent-loop/src/runtime-context.ts

@@ -33,7 +33,7 @@ export interface SystemPromptCommit {
 
 /** The request-series facts one prompt decision is made under. */
 export interface SystemPromptDecisionInput {
-  /** Whether the route that served the last request reads a later `system` message as the effective prompt. */
+  /** Whether the prepared route for this attempt reads a later `system` message as the effective prompt. */
   inHistory: boolean
   /**
    * Whether this step's request starts a new model-message series: a pre-step
@@ -54,7 +54,8 @@ function eventsNewestFirst(session: Session): readonly SessionEvent[] {
  * replaces the latest surviving system node in place, except on an
  * `in-history` route while the request series continues, where it appends a
  * new `system/message` after the cached history; a series start folds the
- * prompt back into node 0 when no later system node survives.
+ * prompt back into node 0 when no later system node survives. Incapable routes
+ * normalize nonempty prompts to the first node and empty all later active nodes.
  */
 export class SystemPromptProjection {
   constructor(private readonly session: Session) {}
@@ -74,25 +75,35 @@ export class SystemPromptProjection {
    * Create an uncommitted system node when absent, even for an empty prompt, or changed.
    * @param rendered - the fully rendered system prompt; `''` when none is active.
    * @param input - the route capability and series facts for this step.
-   * @returns the message and its surface intent, or `undefined` when no update is needed.
+   * @returns ordered per-node updates; an empty list means no update is needed.
    */
-  project(rendered: string, input: SystemPromptDecisionInput): SystemPromptCommit | undefined {
+  project(rendered: string, input: SystemPromptDecisionInput): SystemPromptCommit[] {
     const nodes = this.systemNodes()
-    const latest = nodes.at(-1)
-    if (latest === undefined) {
-      return { message: createSystemMessage(rendered, SOURCE), intent: { surfaceOp: 'append' } }
+    const head = nodes[0]
+    if (head === undefined) {
+      return [{ message: createSystemMessage(rendered, SOURCE), intent: { surfaceOp: 'append' } }]
     }
-    if (latest.text === rendered) return
+    const latest = nodes.findLast(node => node.text.length > 0) ?? head
+    if (!input.inHistory && rendered.length > 0) {
+      const updates = nodes.slice(1).filter(node => node.text.length > 0)
+        .map(node => this.replace(node.seq, ''))
+      if (head.text !== rendered) updates.push(this.replace(head.seq, rendered))
+      return updates
+    }
+    if (latest.text === rendered) return []
     const message = createSystemMessage(rendered, SOURCE)
     // An empty node projects to no message, so clearing the prompt must rewrite
     // the surviving node; a series start with one system node re-baselines it.
     const append = input.inHistory
       && rendered.length > 0
       && (!input.startsSeries || nodes.length > 1)
-    if (append) return { message, intent: { surfaceOp: 'append' } }
-    const { seq } = latest
+    if (append) return [{ message, intent: { surfaceOp: 'append' } }]
+    return [this.replace(latest.seq, rendered)]
+  }
+
+  private replace(seq: SessionSeq, text: string): SystemPromptCommit {
     return {
-      message,
+      message: createSystemMessage(text, SOURCE),
       intent: { surfaceOp: { op: 'replace', start: seq, end: seq }, sourceEventSeqs: [seq] },
     }
   }