diff --git a/test/engines.test.mjs b/test/engines.test.mjs
index e6cd7406..231c5ab6 100644
--- a/test/engines.test.mjs
+++ b/test/engines.test.mjs
@@ -14,7 +14,14 @@ import { fileURLToPath } from "node:url";
 import { spawn } from "node:child_process";
 import test from "node:test";
 
-import { ENGINES, agentLaunchArgs, aiExecArgs, engineBinOverride, exitReason, isInstalled, openPassthrough, pickAiEngine, ranOk, resolveEngine, runCmd } from "../src/engines.mjs";
+// engines.mjs applies MOSHCODE_ENGINE_BIN_<KEY> to ENGINES.*.bin at import, and
+// run() hands process.env to every child. An override exported in the shell
+// running the suite (a local codex build, say) would otherwise swap the stubs
+// below for that build. Scrub them first; the override tests set their own.
+for (const name of Object.keys(process.env)) {
+  if (name.startsWith("MOSHCODE_ENGINE_BIN_")) delete process.env[name];
+}
+const { ENGINES, agentLaunchArgs, aiExecArgs, engineBinOverride, exitReason, isInstalled, openPassthrough, pickAiEngine, ranOk, resolveEngine, runCmd } = await import("../src/engines.mjs");
 
 const BIN = fileURLToPath(new URL("../bin/moshcode.mjs", import.meta.url));
 // The autonomous-session bypass flags each engine declares (engine.agentArgs).
diff --git a/test/mcp.test.mjs b/test/mcp.test.mjs
index 6d55d53c..6d725e98 100644
--- a/test/mcp.test.mjs
+++ b/test/mcp.test.mjs
@@ -13,6 +13,14 @@ import { parseMcp } from "../src/integrations.mjs";
 
 const BIN = fileURLToPath(new URL("../bin/moshcode.mjs", import.meta.url));
 
+// The fan-out test below puts stub engines on PATH and hands process.env to the
+// CLI. A MOSHCODE_ENGINE_BIN_<KEY> exported in the shell running the suite
+// would send that engine's `mcp add` to the real build instead of the stub —
+// writing into the operator's own engine config.
+for (const name of Object.keys(process.env)) {
+  if (name.startsWith("MOSHCODE_ENGINE_BIN_")) delete process.env[name];
+}
+
 test("deriveName pulls a sane name from a remote host", () => {
   assert.equal(deriveName("https://mcp.sentry.dev/mcp"), "sentry");
   assert.equal(deriveName("https://api.githubcopilot.com/mcp/"), "githubcopilot");
diff --git a/test/tui-prd-errors.test.mjs b/test/tui-prd-errors.test.mjs
index f52284e2..88921612 100644
--- a/test/tui-prd-errors.test.mjs
+++ b/test/tui-prd-errors.test.mjs
@@ -29,6 +29,13 @@ const replied = (out) => strip(out).split("to leave")[1] || "";
 // the child nothing else.
 const NO_ENGINES = mkdtempSync(join(tmpdir(), "moshcode-no-engines-"));
 
+// An empty PATH is not the whole of "no engines": MOSHCODE_ENGINE_BIN_<KEY>
+// names an engine by absolute path, so one exported in the shell running the
+// suite made codex "installed" here and /prd launched that real build.
+for (const name of Object.keys(process.env)) {
+  if (name.startsWith("MOSHCODE_ENGINE_BIN_")) delete process.env[name];
+}
+
 function runTui(input, cwd) {
   return new Promise((resolve, reject) => {
     const child = spawn(process.execPath, [BIN], {