@@ -360,6 +360,38 @@ export const ENGINES = {
   },
 };
 
+/**
+ * An explicit path to a build of one engine, from the environment.
+ *
+ * `MOSHCODE_ENGINE_BIN_<KEY>` (for example `MOSHCODE_ENGINE_BIN_CODEX`) names
+ * the executable to launch for that engine instead of searching for its usual
+ * name. It exists for the case where you are running your own build of an
+ * engine — a fork carrying a patch its upstream has not taken, a debug build
+ * you are bisecting — and want moshcode to open *that* rather than whichever
+ * copy happens to come first on PATH.
+ *
+ * It is unset by default, and deliberately so: with no override the name below
+ * is resolved against PATH exactly as before, which is the behaviour every
+ * engine here documents and every test relies on. An override is a statement
+ * that you know better than PATH for this one engine, so it is only ever made
+ * out loud.
+ *
+ * The value is used as-is, including `~`-free absolute paths; an empty or
+ * whitespace-only value is treated as unset rather than as an empty command.
+ */
+export function engineBinOverride(key, env = process.env) {
+  const value = env[`MOSHCODE_ENGINE_BIN_${key.toUpperCase().replace(/[^A-Z0-9]/g, "_")}`];
+  return typeof value === "string" && value.trim() ? value.trim() : null;
+}
+
+// Applied once, here, rather than at each launch site: `bin` is read directly
+// by the CLI, the TUI's error messages, the swarm and the MCP bridge, and an
+// override that only some of them honoured would be worse than none.
+for (const [key, engine] of Object.entries(ENGINES)) {
+  const override = engineBinOverride(key);
+  if (override) engine.bin = override;
+}
+
 /**
  * The command that upgrades an already-installed engine in place: its native
  * updater if it has one, else re-run the installer (they're idempotent and
 
@@ -14,7 +14,7 @@ import { fileURLToPath } from "node:url";
 import { spawn } from "node:child_process";
 import test from "node:test";
 
-import { ENGINES, agentLaunchArgs, aiExecArgs, exitReason, isInstalled, openPassthrough, pickAiEngine, ranOk, resolveEngine, runCmd } from "../src/engines.mjs";
+import { ENGINES, agentLaunchArgs, aiExecArgs, engineBinOverride, exitReason, isInstalled, openPassthrough, pickAiEngine, ranOk, resolveEngine, runCmd } from "../src/engines.mjs";
 
 const BIN = fileURLToPath(new URL("../bin/moshcode.mjs", import.meta.url));
 // The autonomous-session bypass flags each engine declares (engine.agentArgs).
@@ -62,7 +62,7 @@ process.stdout.write(JSON.stringify(process.argv.slice(2)));
   chmodSync(file, 0o755);
 }
 
-function run(args, binDir) {
+function run(args, binDir, extraEnv = {}) {
   return new Promise((resolve, reject) => {
     const ioDir = tempDir("moshcode-engine-stdio-");
     const stdinFile = path.join(ioDir, "stdin");
@@ -77,6 +77,7 @@ function run(args, binDir) {
       env: {
         ...process.env,
         PATH: `${binDir}${path.delimiter}${process.env.PATH || ""}`,
+        ...extraEnv,
       },
     });
     let failed = false;
@@ -274,3 +275,37 @@ test("PATH still wins over a tool's install dir", async () => {
     process.env.PATH = previous;
   }
 });
+
+test("an engine bin override is read from the environment, and only when it says something", () => {
+  assert.deepEqual(
+    [
+      engineBinOverride("codex", {}),
+      engineBinOverride("codex", { MOSHCODE_ENGINE_BIN_CODEX: "" }),
+      engineBinOverride("codex", { MOSHCODE_ENGINE_BIN_CODEX: "   " }),
+      engineBinOverride("codex", { MOSHCODE_ENGINE_BIN_CODEX: " /opt/codex " }),
+      engineBinOverride("privacycode", { MOSHCODE_ENGINE_BIN_PRIVACYCODE: "/opt/pc" }),
+    ],
+    [null, null, null, "/opt/codex", "/opt/pc"],
+  );
+});
+
+test("an engine bin override launches that build instead of the copy on PATH", async () => {
+  // The name every other test would get: first on PATH, and not what we asked for.
+  const pathDir = tempDir();
+  mkdirSync(pathDir, { recursive: true });
+  writeEngine(pathDir, "codex");
+
+  // Our own build, deliberately somewhere PATH would never find it.
+  const buildDir = tempDir("moshcode-engine-build-");
+  mkdirSync(buildDir, { recursive: true });
+  const build = path.join(buildDir, "codex-fork");
+  writeFileSync(build, `#!/usr/bin/env node
+process.stdout.write(JSON.stringify(["override", ...process.argv.slice(2)]));
+`);
+  chmodSync(build, 0o755);
+
+  const result = await run(["agents", "codex"], pathDir, { MOSHCODE_ENGINE_BIN_CODEX: build });
+
+  assert.equal(result.status, 0);
+  assert.deepEqual(JSON.parse(result.stdout), ["override", ...EXPECTED_LAUNCH_ARGS.codex]);
+});