fix(tools,client): chain-mode resolution and a live preset label

modeFor read only the exact scope's layer, so a `code`-preset session
advertised the native catalog: the mode is declared on the preset's STANDING
scope, and the agent only parents to it. Nearest scope wins along the chain —
the mode decides what the model SEES, which is the class of fact the chain
inherits. Caught live (the model politely computed with bash and said
run_code was not in its list); the chain test pins it.

The client half of the label fix: the create echo and the session-added
frame's agentPreset now reach the session list (newest wins in the upsert —
every producer of the field reports the CURRENT composition), and a confirmed
blank-session switch publishes through the new ISessions.noteAgentPreset, so
the header label moves with the composition instead of waiting for a reload.

packages/client/runtime/src/client/contract/sessions.ts

@@ -62,6 +62,15 @@ export interface ISessions {
    * @returns completion of the current or newly started refresh.
    */
   refreshSubagents(parentSessionId: SessionId): Promise<void>
+
+  /**
+   * Record the composition one session now runs. The agent-preset seat calls
+   * this after a successful blank-session switch, so the header label moves
+   * with the composition instead of waiting for the next full list refresh.
+   * @param sessionId - the switched session.
+   * @param agentPreset - the preset id the host confirmed.
+   */
+  noteAgentPreset(sessionId: SessionId, agentPreset: string): void
   /** Clear the current selection into the no-session view state. */
   clear(): void
   /**

packages/client/runtime/src/client/sessions/manager.ts

@@ -523,6 +523,7 @@ export class SessionManager {
         this.recordMutation({ kind: 'upsert', summary: {
           sessionId: result.value.sessionId, updatedAt: Date.now(), running: false, blank: true,
           ...(opts.cwd !== undefined ? { cwd: opts.cwd } : {}),
+          ...(result.value.agentPreset !== undefined ? { agentPreset: result.value.agentPreset } : {}),
         } })
       } else {
         const publishedSessionId = workspaceAttachSessionId(result.error)
@@ -588,6 +589,17 @@ export class SessionManager {
     this.recordMutation({ kind: 'upsert', summary })
   }
 
+  /**
+   * Record a host-confirmed composition switch (see ISessions.noteAgentPreset).
+   * @param sessionId - the switched session.
+   * @param agentPreset - the preset id the host confirmed.
+   */
+  noteAgentPreset(sessionId: SessionId, agentPreset: string): void {
+    this.recordMutation({ kind: 'upsert', summary: {
+      sessionId, updatedAt: Date.now(), running: false, blank: true, agentPreset,
+    } })
+  }
+
   /** Apply immediately and retain for replay when a list response is in flight. */
   private recordMutation(mutation: SessionListMutation): void {
     this.listMutations?.push(mutation)
@@ -743,6 +755,7 @@ export class SessionManager {
           ...(frame.parentSessionId !== undefined ? { parentSessionId: frame.parentSessionId } : {}),
           ...(frame.origin !== undefined ? { origin: frame.origin } : {}),
           ...(frame.cwd !== undefined ? { cwd: frame.cwd } : {}),
+          ...(frame.agentPreset !== undefined ? { agentPreset: frame.agentPreset } : {}),
         })
         this.sessions.get(frame.sessionId)?.handleBlank(frame.blank)
         if (frame.origin === 'subagent' && frame.parentSessionId !== undefined) {
@@ -1027,9 +1040,15 @@ function applyMutation(summaries: readonly SessionSummary[], mutation: SessionLi
           ? { parentSessionId: mutation.summary.parentSessionId } : {}),
         ...(existing.origin === undefined && mutation.summary.origin !== undefined
           ? { origin: mutation.summary.origin } : {}),
+        // Newest wins, not fill-only: a blank-session preset switch replaces
+        // the creation-time value, and every producer of this field (the
+        // create echo, the select echo, a list row) reports the CURRENT one.
+        ...(mutation.summary.agentPreset !== undefined
+          ? { agentPreset: mutation.summary.agentPreset } : {}),
       }
       if (filled.cwd === existing.cwd && filled.parentSessionId === existing.parentSessionId
-        && filled.origin === existing.origin && filled.blank === existing.blank) return [...summaries]
+        && filled.origin === existing.origin && filled.blank === existing.blank
+        && filled.agentPreset === existing.agentPreset) return [...summaries]
       return summaries.map(summary => summary.sessionId === mutation.summary.sessionId ? filled : summary)
     }
     case 'remove':

packages/client/runtime/src/client/sessions/service.ts

@@ -365,6 +365,10 @@ export class SessionsService implements ISessions {
     return this.manager.refreshSubagents(parentSessionId)
   }
 
+  noteAgentPreset(sessionId: SessionId, agentPreset: string): void {
+    this.manager.noteAgentPreset(sessionId, agentPreset)
+  }
+
   /**
    * Clear the current selection so the layout shows the no-session empty
    * state (new-session affordance and the workspace preselection flow).