diff --git a/packages/agent/src/adapters/claude/session/rtk.ts b/packages/agent/src/adapters/claude/session/rtk.ts
index c609d7ee4d..0b7d256826 100644
--- a/packages/agent/src/adapters/claude/session/rtk.ts
+++ b/packages/agent/src/adapters/claude/session/rtk.ts
@@ -15,7 +15,8 @@ import { gitSubcommand } from "../git-command";
 
 // Commands RTK compresses faithfully and that have no side effects, so wrapping
 // them changes only how much output reaches the model, never what runs.
-const RTK_PLAIN_COMMANDS = new Set(["grep", "find", "ls"]);
+// Exported so the instruction-level Codex guidance advertises the same set.
+export const RTK_PLAIN_COMMANDS = new Set(["grep", "find", "ls"]);
 
 // Git subcommands whose output is worth compressing and that RTK handles
 // faithfully. The criterion is compressible output, NOT read-only: RTK never
@@ -23,7 +24,8 @@ const RTK_PLAIN_COMMANDS = new Set(["grep", "find", "ls"]);
 // `git reflog expire`) still executes its write — its output is just shorter.
 // Excludes commit/push: negligible output to compress, and the cloud
 // signed-commit guard keys on a leading `git` token that `rtk git …` would hide.
-const GIT_COMPRESSIBLE_SUBCOMMANDS = new Set([
+// Exported so the instruction-level Codex guidance advertises the same set.
+export const GIT_COMPRESSIBLE_SUBCOMMANDS = new Set([
   "status",
   "diff",
   "log",
@@ -44,7 +46,8 @@ const GIT_COMPRESSIBLE_SUBCOMMANDS = new Set([
 // wrapping only its head would change the meaning of the rest.
 const SHELL_OPERATORS = /[|&;<>`\n]|\$\(/;
 
-function shQuote(value: string): string {
+// Exported so the instruction-level Codex guidance quotes the prefix the same way.
+export function shQuote(value: string): string {
   if (/^[\w./-]+$/.test(value)) return value;
   return `'${value.replace(/'/g, `'\\''`)}'`;
 }
diff --git a/packages/agent/src/adapters/rtk-guidance.test.ts b/packages/agent/src/adapters/rtk-guidance.test.ts
new file mode 100644
index 0000000000..1664ca1a1c
--- /dev/null
+++ b/packages/agent/src/adapters/rtk-guidance.test.ts
@@ -0,0 +1,97 @@
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import { afterAll, beforeAll, describe, expect, test } from "vitest";
+import {
+  GIT_COMPRESSIBLE_SUBCOMMANDS,
+  RTK_PLAIN_COMMANDS,
+} from "./claude/session/rtk";
+import { appendRtkGuidanceForCodex, buildRtkGuidance } from "./rtk-guidance";
+
+describe("rtk guidance for codex", () => {
+  let dir: string;
+  let binary: string;
+
+  beforeAll(() => {
+    dir = fs.mkdtempSync(path.join(os.tmpdir(), "rtk-guidance-test-"));
+    binary = path.join(dir, "rtk");
+    fs.writeFileSync(binary, "#!/bin/sh\n");
+  });
+
+  afterAll(() => {
+    fs.rmSync(dir, { recursive: true, force: true });
+  });
+
+  describe("buildRtkGuidance", () => {
+    // The guidance must advertise exactly the Claude hook's eligibility sets,
+    // so the token-usage cohorts stay comparable across adapters.
+    test("advertises every command the Claude hook rewrites", () => {
+      const guidance = buildRtkGuidance("/usr/local/bin/rtk");
+      for (const command of RTK_PLAIN_COMMANDS) {
+        expect(guidance).toContain(command);
+      }
+      for (const sub of GIT_COMPRESSIBLE_SUBCOMMANDS) {
+        expect(guidance).toContain(sub);
+      }
+    });
+
+    test("uses the resolved binary path in the examples", () => {
+      const guidance = buildRtkGuidance("/usr/local/bin/rtk");
+      expect(guidance).toContain("`/usr/local/bin/rtk git status`");
+    });
+
+    // A desktop install can resolve a path with spaces; unquoted it would
+    // split into multiple shell tokens and every guided command would fail.
+    test("shell-quotes a binary path containing spaces", () => {
+      const guidance = buildRtkGuidance("/Apps/PostHog Code/rtk");
+      expect(guidance).toContain("`'/Apps/PostHog Code/rtk' git status`");
+      expect(guidance).not.toContain("`/Apps/PostHog Code/rtk git status`");
+    });
+
+    // Parity with the Claude hook's exclusion: prefixing commit/push would
+    // hide the leading `git` token from the cloud signed-commit guard.
+    test("forbids prefixing git commit and git push", () => {
+      const guidance = buildRtkGuidance("rtk");
+      expect(guidance).toContain("Never prefix `git commit`, `git push`");
+    });
+  });
+
+  describe("appendRtkGuidanceForCodex", () => {
+    test("appends guidance when rtk is on PATH", () => {
+      const result = appendRtkGuidanceForCodex("base instructions", {
+        PATH: dir,
+      });
+      expect(result.startsWith("base instructions\n\n")).toBe(true);
+      expect(result).toContain("rtk command-output compression");
+      expect(result).toContain(binary);
+    });
+
+    // POSTHOG_RTK=0 is set per run from the cloud kill-switch flag; it must
+    // silence the guidance too, which is why the gate is resolveRtkPrefix
+    // rather than detectRtkBinary.
+    test.each([["0"], ["false"]])(
+      "returns instructions unchanged when POSTHOG_RTK is %s",
+      (value) => {
+        expect(
+          appendRtkGuidanceForCodex("base instructions", {
+            POSTHOG_RTK: value,
+            PATH: dir,
+          }),
+        ).toBe("base instructions");
+      },
+    );
+
+    test("returns instructions unchanged when rtk is not installed", () => {
+      expect(
+        appendRtkGuidanceForCodex("base instructions", {
+          PATH: "/nonexistent",
+        }),
+      ).toBe("base instructions");
+    });
+
+    test("does not leave a leading separator when instructions are empty", () => {
+      const result = appendRtkGuidanceForCodex("", { PATH: dir });
+      expect(result.startsWith("## rtk")).toBe(true);
+    });
+  });
+});
diff --git a/packages/agent/src/adapters/rtk-guidance.ts b/packages/agent/src/adapters/rtk-guidance.ts
new file mode 100644
index 0000000000..962913a32c
--- /dev/null
+++ b/packages/agent/src/adapters/rtk-guidance.ts
@@ -0,0 +1,59 @@
+import {
+  GIT_COMPRESSIBLE_SUBCOMMANDS,
+  RTK_PLAIN_COMMANDS,
+  resolveRtkPrefix,
+  shQuote,
+} from "./claude/session/rtk";
+
+/**
+ * Instruction-level RTK integration for Codex sessions.
+ *
+ * The Claude adapter routes eligible commands through RTK deterministically
+ * with a PreToolUse hook that rewrites the Bash input. Codex executes shell
+ * commands internally over JSON-RPC and its app-server protocol has no
+ * command-rewrite channel — the adapter can only approve or deny — so the
+ * only integration point is the developer instructions: tell the model to
+ * prefix eligible commands itself.
+ *
+ * The advertised command set and rules mirror the Claude hook exactly
+ * (RTK_PLAIN_COMMANDS + GIT_COMPRESSIBLE_SUBCOMMANDS, bare invocations only,
+ * never commit/push), so token-usage cohorts stay comparable across adapters.
+ */
+export function buildRtkGuidance(rtkPrefix: string): string {
+  // Same quoting as the Claude rewrite hook: a resolved path containing
+  // spaces must stay one shell token in the commands the model copies.
+  const prefix = shQuote(rtkPrefix);
+  const plainCommands = [...RTK_PLAIN_COMMANDS].join("`, `");
+  const gitSubcommands = [...GIT_COMPRESSIBLE_SUBCOMMANDS].join(", ");
+
+  return `## rtk command-output compression
+
+\`${prefix}\` is installed. It runs a command unchanged and compresses its output before you read it, so prefixed commands cost far less context. When you execute one of these as a single, bare command, prefix it with \`${prefix}\`:
+
+- \`${plainCommands}\`
+- these git subcommands: ${gitSubcommands}
+
+Examples: \`${prefix} git status\`, \`${prefix} grep -rn "foo" src\`, \`${prefix} ls -la\`.
+
+Rules:
+- Only prefix a single bare invocation. Never use it when the command is part of a pipe, uses \`&&\`, \`;\`, or redirection, or when another program parses the output — compression would corrupt what the consumer reads.
+- Never prefix \`git commit\`, \`git push\`, or any other command not listed above.
+- Skip the prefix when you need the exact, complete output (for example, copying a diff verbatim).`;
+}
+
+/**
+ * Appends the RTK guidance to Codex developer instructions when an RTK binary
+ * is usable. Gated on `resolveRtkPrefix` — not `detectRtkBinary` — so the
+ * per-run `POSTHOG_RTK=0` opt-out (the cloud kill-switch flag) disables the
+ * guidance along with everything else.
+ */
+export function appendRtkGuidanceForCodex(
+  instructions: string,
+  env: NodeJS.ProcessEnv = process.env,
+): string {
+  const rtkPrefix = resolveRtkPrefix(env);
+  if (!rtkPrefix) return instructions;
+  return [instructions, buildRtkGuidance(rtkPrefix)]
+    .filter(Boolean)
+    .join("\n\n");
+}
diff --git a/packages/agent/src/server/agent-server.ts b/packages/agent/src/server/agent-server.ts
index 7de8e1eb78..55a88fe26d 100644
--- a/packages/agent/src/server/agent-server.ts
+++ b/packages/agent/src/server/agent-server.ts
@@ -43,6 +43,7 @@ import {
   classifyAgentError,
   isPromptTooLongError,
 } from "../adapters/error-classification";
+import { appendRtkGuidanceForCodex } from "../adapters/rtk-guidance";
 import {
   SIGNED_COMMIT_QUALIFIED_TOOL_NAME,
   SIGNED_MERGE_QUALIFIED_TOOL_NAME,
@@ -2941,9 +2942,11 @@ export class AgentServer {
   private buildCodexInstructions(
     systemPrompt: string | { append: string },
   ): string {
-    return typeof systemPrompt === "string"
-      ? systemPrompt
-      : systemPrompt.append;
+    const instructions =
+      typeof systemPrompt === "string" ? systemPrompt : systemPrompt.append;
+    // Codex has no command-rewrite hook (see rtk-guidance.ts), so RTK is
+    // adopted through the developer instructions instead.
+    return appendRtkGuidanceForCodex(instructions);
   }
 
   /**