diff --git a/.changeset/fix-compaction-max-output-size.md b/.changeset/fix-compaction-max-output-size.md
new file mode 100644
index 000000000..cac5f917c
--- /dev/null
+++ b/.changeset/fix-compaction-max-output-size.md
@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/agent-core": patch
+---
+
+Pass `maxOutputSize` to `resolveCompletionBudget` in the compaction worker to prevent falling back to `max_context_size` as the completion token cap.
\ No newline at end of file
diff --git a/packages/agent-core/src/agent/compaction/full.ts b/packages/agent-core/src/agent/compaction/full.ts
index 3c52043a9..4ed98874e 100644
--- a/packages/agent-core/src/agent/compaction/full.ts
+++ b/packages/agent-core/src/agent/compaction/full.ts
@@ -244,6 +244,7 @@ export class FullCompaction {
       const provider = applyCompletionBudget({
         provider: this.agent.config.provider,
         budget: resolveCompletionBudget({
+          maxOutputSize: this.agent.config.maxOutputSize,
           reservedContextSize: this.agent.kimiConfig?.loopControl?.reservedContextSize,
         }),
         capability: this.agent.config.modelCapabilities,