diff --git a/server/__tests__/codex-cli.test.js b/server/__tests__/codex-cli.test.js
index 77e8448e..2f60de0d 100644
--- a/server/__tests__/codex-cli.test.js
+++ b/server/__tests__/codex-cli.test.js
@@ -189,6 +189,20 @@ describe("codex-cli argv construction", () => {
     }
   });
 
+  it("accepts an env value starting with '-' — it's the RHS of one KEY=VALUE argv element, never a flag", async () => {
+    const restore = stubCodex();
+    try {
+      const result = await cli.mcpAdd({
+        name: "docs",
+        command: "node",
+        env: { OFFSET: "-1" },
+      });
+      assert.deepEqual(argvOf(result), ["mcp", "add", "docs", "--env", "OFFSET=-1", "--", "node"]);
+    } finally {
+      restore();
+    }
+  });
+
   it("builds mcp list / remove argvs", async () => {
     const restore = stubCodex();
     try {
diff --git a/server/lib/codex-cli.js b/server/lib/codex-cli.js
index 7236e3bc..49e916a0 100644
--- a/server/lib/codex-cli.js
+++ b/server/lib/codex-cli.js
@@ -94,6 +94,24 @@ function assertEnvKey(value, label) {
   return value;
 }
 
+/**
+ * An environment variable VALUE. Unlike `assertArg`, a leading "-" is fine:
+ * the value is the right-hand side of one `KEY=VALUE` argv element (built
+ * by envFlags below), never its own argv position, so it can never be
+ * parsed as a flag. Rejecting it anyway (as `assertArg` would) refused
+ * ordinary values like "-1" or a base64 string that happens to start with
+ * "-" for no real reason.
+ */
+function assertEnvValue(value, label) {
+  if (typeof value !== "string") throw makeError("EBADARG", `${label} is required`);
+  if (value.length > 512) throw makeError("EBADARG", `${label} is too long`);
+  // eslint-disable-next-line no-control-regex
+  if (/[\u0000-\u001f\u007f]/.test(value)) {
+    throw makeError("EBADARG", `${label} contains control characters`);
+  }
+  return value;
+}
+
 /** An http(s) URL — `new URL()` both validates and normalizes the parse. */
 function assertUrl(value, label) {
   assertArg(value, label);
@@ -219,7 +237,7 @@ async function mcpAdd({ name, url, bearerTokenEnvVar, command, args = [], env })
   const cmdArgs = args.map((a, i) => assertArg(a, `args[${i}]`));
   const envFlags = Object.entries(env || {}).flatMap(([k, v]) => {
     const key = assertEnvKey(k, "env key");
-    assertArg(String(v), `env.${k}`);
+    assertEnvValue(String(v), `env.${k}`);
     return ["--env", `${key}=${v}`];
   });
   return serialize(() => runCodex(["mcp", "add", n, ...envFlags, "--", cmd, ...cmdArgs]));