diff --git a/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts b/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts
index c8fbc48f..f2f48e2e 100644
--- a/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts
+++ b/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts
@@ -196,7 +196,12 @@ raise SystemExit(0 if len(session_id) == 64 and denied else 1)
     env: {
       ...process.env,
       PATH: `${binDir}:${process.env.PATH ?? ''}`,
-      MOSAIC_HOME: frameworkRoot,
+      // Point MOSAIC_HOME at the per-entry temp root, NOT the shipped framework
+      // tree: launch-runtime.py appends its launch ledger to
+      // $MOSAIC_HOME/fleet/run/sessions/events.ndjson, and writing that into
+      // framework/ pollutes the tree manifest.spec.ts walks. Nothing in the
+      // launch chain resolves tools via MOSAIC_HOME (scripts use SCRIPT_DIR).
+      MOSAIC_HOME: root,
       MOSAIC_PRDY_RUNTIME: 'claude',
       MOSAIC_LEASE_BROKER_SOCKET: socket,
       MOSAIC_RUNTIME_GENERATION: '1',