compact: the summary's provenance records its call envelope

compact/summary gains { model, maxTokens? } — the envelope the
summarize call actually used, reported by the backend that made the
call: summarize() now returns { summary, model, maxTokens? } instead of
bare blocks, so an overriding backend (template or remote summarizer)
reports its own envelope honestly and compactRegion logs it. 'Which
model wrote this summary' becomes answerable from the log alone, and
the one-shot summarize request — outside the loop's header-event fold
by design — is reconstructable from log + code (the reconstructability
RFC's scope statement).

packages/compact/compact-basic/src/index.ts

@@ -287,8 +287,15 @@ export class BasicCompactService extends CompactService {
    *
    * Forwards `signal` into `GenerateOptions.signal` so an abort/dispose tears
    * down the in-flight summarization rather than orphaning the model call.
+   *
+   * Returns the summary blocks TOGETHER with the call envelope it actually
+   * used (`model`, `maxTokens`) — the caller logs the envelope on the
+   * `compact/summary` provenance event, so an overriding subclass (template
+   * or remote summarizer) reports its own envelope honestly.
    */
-  async summarize(text: string, agent: Agent, signal?: AbortSignal): Promise<ContentBlock[]> {
+  async summarize(
+    text: string, agent: Agent, signal?: AbortSignal,
+  ): Promise<{ summary: ContentBlock[]; model: string; maxTokens?: number }> {
     const assembler = new BlockAssembler()
     const options: GenerateOptions = {
       model: this.config.summarizationModel || agent.options.model || '',
@@ -318,7 +325,11 @@ export class BasicCompactService extends CompactService {
       throw new Error('summarization produced no text summary content')
     }
 
-    return summary
+    return {
+      summary,
+      model: options.model,
+      ...options.maxTokens !== undefined ? { maxTokens: options.maxTokens } : {},
+    }
   }
 
   // ---- Core API (implements the abstract contract) ----
@@ -449,7 +460,7 @@ export class BasicCompactService extends CompactService {
     try {
       // --- Extract text and summarize ---
       const text = this._extractText(session, shadowedSeqs)
-      const summary = await this.summarize(text, agent, signal)
+      const { summary, model, maxTokens } = await this.summarize(text, agent, signal)
 
       // Estimate token count of the shadowed content for provenance.
       let shadowedTokenCount = 0
@@ -471,6 +482,8 @@ export class BasicCompactService extends CompactService {
         shadowedRange: { start, end },
         shadowedSeqs,
         shadowedTokenCount,
+        model,
+        ...maxTokens !== undefined ? { maxTokens } : {},
       })
 
       // --- Surface replacement ---