🤖 feat: add MUX_AGENT=1 env var to all command execution

Commands executed by the Mux AI agent can now detect they're running
in an agent context via the MUX_AGENT=1 environment variable.

Changes:
- Add MUX_AGENT=1 to NON_INTERACTIVE_ENV_VARS constant
- LocalRuntime already spreads NON_INTERACTIVE_ENV_VARS (no change needed)
- Update SSHRuntime to merge NON_INTERACTIVE_ENV_VARS with user env
- Add unit test for env constant
- Add integration test verifying MUX_AGENT is set in both local and SSH runtimes

Author: kylecarbs

src/common/constants/env.test.ts

@@ -0,0 +1,18 @@
+import { NON_INTERACTIVE_ENV_VARS } from "./env";
+
+describe("NON_INTERACTIVE_ENV_VARS", () => {
+  it("should include MUX_AGENT=1", () => {
+    expect(NON_INTERACTIVE_ENV_VARS.MUX_AGENT).toBe("1");
+  });
+
+  it("should include all expected env vars", () => {
+    expect(NON_INTERACTIVE_ENV_VARS).toMatchObject({
+      MUX_AGENT: "1",
+      GIT_EDITOR: "true",
+      GIT_SEQUENCE_EDITOR: "true",
+      EDITOR: "true",
+      VISUAL: "true",
+      GIT_TERMINAL_PROMPT: "0",
+    });
+  });
+});

src/common/constants/env.ts

@@ -3,6 +3,8 @@
  * These prevent tools from blocking on editor/credential prompts.
  */
 export const NON_INTERACTIVE_ENV_VARS = {
+  // Sentinel variable to indicate commands are running under Mux AI agent
+  MUX_AGENT: "1",
   // Prevent interactive editors from blocking execution
   // Critical for git operations like rebase/commit that try to open editors
   GIT_EDITOR: "true", // Git-specific editor (highest priority)

src/node/runtime/SSHRuntime.ts

@@ -16,6 +16,7 @@ import type {
 } from "./Runtime";
 import { RuntimeError as RuntimeErrorClass } from "./Runtime";
 import { EXIT_CODE_ABORTED, EXIT_CODE_TIMEOUT } from "@/common/constants/exitCodes";
+import { NON_INTERACTIVE_ENV_VARS } from "@/common/constants/env";
 import { log } from "@/node/services/log";
 import { checkInitHookExists, createLineBufferedLoggers, getInitHookEnv } from "./initHook";
 import { streamProcessToLogger } from "./streamProcess";
@@ -106,11 +107,10 @@ export class SSHRuntime implements Runtime {
     // Add cd command if cwd is specified
     parts.push(cdCommandForSSH(options.cwd));
 
-    // Add environment variable exports
-    if (options.env) {
-      for (const [key, value] of Object.entries(options.env)) {
-        parts.push(`export ${key}=${shescape.quote(value)}`);
-      }
+    // Add environment variable exports (NON_INTERACTIVE_ENV_VARS + user-provided env)
+    const allEnv = { ...NON_INTERACTIVE_ENV_VARS, ...(options.env ?? {}) };
+    for (const [key, value] of Object.entries(allEnv)) {
+      parts.push(`export ${key}=${shescape.quote(value)}`);
     }
 
     // Add the actual command

tests/runtime/runtime.test.ts

@@ -100,6 +100,18 @@ describeIntegration("Runtime integration tests", () => {
           expect(result.exitCode).toBe(0);
         });
 
+        test.concurrent("automatically sets MUX_AGENT=1 environment variable", async () => {
+          const runtime = createRuntime();
+          await using workspace = await TestWorkspace.create(runtime, type);
+
+          const result = await execBuffered(runtime, 'echo "$MUX_AGENT"', {
+            cwd: workspace.path,
+            timeout: 30,
+          });
+
+          expect(result.stdout.trim()).toBe("1");
+        });
+
         test.concurrent("passes environment variables", async () => {
           const runtime = createRuntime();
           await using workspace = await TestWorkspace.create(runtime, type);