Use RTK automatically when rtk is on PATH

Flip resolveRtkPrefix from opt-in to auto-detect: an installed rtk on PATH is
used by default. POSTHOG_RTK now only overrides — "0"/"false" opts out even
when rtk is installed, and any other value is still an explicit binary path.
When rtk is not on PATH the resolver returns undefined and the hook is never
registered, so there is no behavior change on machines without rtk.

Generated-By: PostHog Code
Task-Id: 2cfc5453-0307-4c67-a6eb-6456bc96cf1a

Author: gantoine

packages/agent/src/adapters/claude/session/rtk.test.ts

@@ -86,24 +86,30 @@ describe("resolveRtkPrefix", () => {
   });
 
   test.each([
-    ["undefined", undefined],
+    ["unset", undefined],
     ["empty", ""],
-    ["zero", "0"],
-    ["false", "false"],
-    ["FALSE", "FALSE"],
-  ])("is disabled when POSTHOG_RTK is %s", (_label, value) => {
-    expect(resolveRtkPrefix({ POSTHOG_RTK: value })).toBeUndefined();
+    ["1", "1"],
+    ["true", "true"],
+  ])("auto-detects rtk on PATH when POSTHOG_RTK is %s", (_label, value) => {
+    expect(resolveRtkPrefix({ POSTHOG_RTK: value, PATH: dir })).toBe(binary);
   });
 
-  test("auto-detects rtk on PATH when enabled", () => {
-    expect(resolveRtkPrefix({ POSTHOG_RTK: "1", PATH: dir })).toBe(binary);
+  test("returns undefined when rtk is not on PATH", () => {
+    expect(resolveRtkPrefix({ PATH: "/nonexistent" })).toBeUndefined();
   });
 
-  test("returns undefined when enabled but rtk is not on PATH", () => {
-    expect(
-      resolveRtkPrefix({ POSTHOG_RTK: "true", PATH: "/nonexistent" }),
-    ).toBeUndefined();
-  });
+  test.each([
+    ["zero", "0"],
+    ["false", "false"],
+    ["FALSE", "FALSE"],
+  ])(
+    "opts out when POSTHOG_RTK is %s, even with rtk on PATH",
+    (_label, value) => {
+      expect(
+        resolveRtkPrefix({ POSTHOG_RTK: value, PATH: dir }),
+      ).toBeUndefined();
+    },
+  );
 
   test("uses an explicit path that exists", () => {
     expect(resolveRtkPrefix({ POSTHOG_RTK: binary })).toBe(binary);

packages/agent/src/adapters/claude/session/rtk.ts

@@ -10,7 +10,7 @@ import { gitSubcommand } from "../git-command";
  * available we rewrite eligible `Bash` calls to run through it, so the savings
  * happen at the source — the verbose output is never generated into context.
  *
- * Opt-in and dormant by default: nothing changes unless `POSTHOG_RTK` is set.
+ * Used automatically when `rtk` is on PATH; set `POSTHOG_RTK=0` to opt out.
  */
 
 // Commands RTK compresses faithfully and that have no side effects, so wrapping
@@ -100,27 +100,33 @@ function findOnPath(bin: string, env: NodeJS.ProcessEnv): string | undefined {
 }
 
 /**
- * Resolves the RTK binary to route shell output through, from `POSTHOG_RTK`:
- *   unset / "" / "0" / "false" → disabled (undefined)
- *   "1" / "true"               → auto-detect `rtk` on PATH
- *   any other value            → an explicit path to the binary
- *
- * Opt-in by design: a user who merely has `rtk` installed sees no behavior
- * change until they ask for it, since rewriting alters how their commands run.
+ * Resolves the RTK binary to route shell output through. Auto-detects `rtk` on
+ * PATH by default, so an installed `rtk` is used automatically. `POSTHOG_RTK`
+ * overrides:
+ *   unset / "" / "1" / "true" → auto-detect `rtk` on PATH
+ *   "0" / "false"             → disabled (opt out)
+ *   any other value           → an explicit path to the binary
  */
 export function resolveRtkPrefix(env: NodeJS.ProcessEnv): string | undefined {
   const raw = env.POSTHOG_RTK?.trim();
-  if (!raw || raw === "0" || raw.toLowerCase() === "false") return undefined;
-  if (raw === "1" || raw.toLowerCase() === "true") {
-    return findOnPath("rtk", env);
-  }
-  try {
-    if (fs.statSync(raw).isFile()) return raw;
-  } catch {
-    // Explicit path doesn't exist — treat as disabled rather than emit a
-    // command that would fail with "rtk: not found".
+  const lowered = raw?.toLowerCase();
+
+  // Explicit opt-out, even when rtk is installed.
+  if (lowered === "0" || lowered === "false") return undefined;
+
+  // An explicit binary-path override (anything other than a bare enable flag).
+  if (raw && lowered !== "1" && lowered !== "true") {
+    try {
+      if (fs.statSync(raw).isFile()) return raw;
+    } catch {
+      // Explicit path doesn't exist — treat as disabled rather than emit a
+      // command that would fail with "rtk: not found".
+    }
+    return undefined;
   }
-  return undefined;
+
+  // Default (unset) or explicit enable: use rtk if it is on PATH.
+  return findOnPath("rtk", env);
 }
 
 export const createRtkRewriteHook =