diff --git a/src/agents/pi-embedded-runner/extensions.ts b/src/agents/pi-embedded-runner/extensions.ts
index 0364d880c..c6e7a637f 100644
--- a/src/agents/pi-embedded-runner/extensions.ts
+++ b/src/agents/pi-embedded-runner/extensions.ts
@@ -81,8 +81,16 @@ export function buildEmbeddedExtensionPaths(params: {
   const paths: string[] = [];
   if (resolveCompactionMode(params.cfg) === "safeguard") {
     const compactionCfg = params.cfg?.agents?.defaults?.compaction;
+    const contextWindowInfo = resolveContextWindowInfo({
+      cfg: params.cfg,
+      provider: params.provider,
+      modelId: params.modelId,
+      modelContextWindow: params.model?.contextWindow,
+      defaultTokens: DEFAULT_CONTEXT_TOKENS,
+    });
     setCompactionSafeguardRuntime(params.sessionManager, {
       maxHistoryShare: compactionCfg?.maxHistoryShare,
+      contextWindowTokens: contextWindowInfo.tokens,
     });
     paths.push(resolvePiExtensionPath("compaction-safeguard"));
   }
diff --git a/src/agents/pi-extensions/compaction-safeguard-runtime.ts b/src/agents/pi-extensions/compaction-safeguard-runtime.ts
index 450ab6f8f..bda1b1de6 100644
--- a/src/agents/pi-extensions/compaction-safeguard-runtime.ts
+++ b/src/agents/pi-extensions/compaction-safeguard-runtime.ts
@@ -1,5 +1,6 @@
 export type CompactionSafeguardRuntimeValue = {
   maxHistoryShare?: number;
+  contextWindowTokens?: number;
 };
 
 // Session-scoped runtime registry keyed by object identity.
diff --git a/src/agents/pi-extensions/compaction-safeguard.ts b/src/agents/pi-extensions/compaction-safeguard.ts
index c824c4735..2c2e39194 100644
--- a/src/agents/pi-extensions/compaction-safeguard.ts
+++ b/src/agents/pi-extensions/compaction-safeguard.ts
@@ -195,11 +195,15 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
     }
 
     try {
-      const contextWindowTokens = resolveContextWindowTokens(model);
+      const runtime = getCompactionSafeguardRuntime(ctx.sessionManager);
+      const modelContextWindow = resolveContextWindowTokens(model);
+      const contextWindowTokens = Math.min(
+        runtime?.contextWindowTokens ?? modelContextWindow,
+        modelContextWindow,
+      );
       const turnPrefixMessages = preparation.turnPrefixMessages ?? [];
       let messagesToSummarize = preparation.messagesToSummarize;
 
-      const runtime = getCompactionSafeguardRuntime(ctx.sessionManager);
       const maxHistoryShare = runtime?.maxHistoryShare ?? 0.5;
 
       const tokensBefore =