diff --git a/src/adapters/codebuddy/adapter.ts b/src/adapters/codebuddy/adapter.ts
index 234e06907e..5c42b5e5db 100644
--- a/src/adapters/codebuddy/adapter.ts
+++ b/src/adapters/codebuddy/adapter.ts
@@ -1,4 +1,7 @@
 import type { AdapterEvent, OcxParsedRequest, OcxProviderConfig } from "../../types";
+import { mkdtemp, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
 import type { AdapterRequest, ProviderAdapter } from "../base";
 import { mapReasoningEffort } from "../../reasoning-effort";
 import { buildSystemPrompt } from "../coding-agent/protocol";
@@ -34,7 +37,12 @@ export function buildChildEnv(profile: CodeBuddyProfile, apiKey: string): Record
  * would require authorization is blocked. The turn is a single text/reasoning pass over stream-json;
  * Codex's tool catalog is not advertised in v1 (the control-protocol tool bridge is a fast-follow).
  */
-export function buildArgs(profile: CodeBuddyProfile, parsed: OcxParsedRequest, provider: OcxProviderConfig): string[] {
+export function buildArgs(
+  profile: CodeBuddyProfile,
+  parsed: OcxParsedRequest,
+  provider: OcxProviderConfig,
+  systemPromptFile?: string,
+): string[] {
   const args: string[] = [
     "-p",
     "--output-format", "stream-json",
@@ -49,8 +57,7 @@ export function buildArgs(profile: CodeBuddyProfile, parsed: OcxParsedRequest, p
   ];
   const effort = mapReasoningEffort(provider, parsed.modelId, parsed.options.reasoning);
   if (effort) args.push("--effort", effort);
-  const system = buildSystemPrompt(parsed);
-  if (system) args.push("--append-system-prompt", system);
+  if (systemPromptFile) args.push("--system-prompt-file", systemPromptFile);
   // profile is retained for symmetry with the region-isolated design and future per-region flags.
   void profile;
   return args;
@@ -70,16 +77,42 @@ export function createCodeBuddyAdapter(provider: OcxProviderConfig, deps: CodeBu
     },
 
     async runTurn(parsed, incoming, emit): Promise<void> {
-      await runCodingAgentTurn({
-        profiles: CODEBUDDY_PROFILES,
-        provider,
-        parsed,
-        incoming,
-        emit,
-        buildArgs: (resolved, req, prov) => buildArgs(resolved as CodeBuddyProfile, req, prov),
-        buildEnv: (resolved, apiKey) => buildChildEnv(resolved as CodeBuddyProfile, apiKey),
-        deps,
-      });
+      const system = buildSystemPrompt(parsed);
+      let promptDir: string | undefined;
+      let promptFile: string | undefined;
+      if (system) {
+        try {
+          promptDir = await mkdtemp(join(tmpdir(), "ocx-codebuddy-prompt-"));
+          promptFile = join(promptDir, "system-prompt.txt");
+          await writeFile(promptFile, system, { encoding: "utf8", mode: 0o600, flag: "wx" });
+        } catch {
+          if (promptDir) await rm(promptDir, { recursive: true, force: true }).catch(() => {});
+          emit({
+            type: "error",
+            message: "CodeBuddy system prompt could not be staged securely.",
+            status: 500,
+            errorType: "upstream_error",
+            code: "system_prompt_staging_failed",
+            retryable: false,
+          });
+          return;
+        }
+      }
+
+      try {
+        await runCodingAgentTurn({
+          profiles: CODEBUDDY_PROFILES,
+          provider,
+          parsed,
+          incoming,
+          emit,
+          buildArgs: (resolved, req, prov) => buildArgs(resolved as CodeBuddyProfile, req, prov, promptFile),
+          buildEnv: (resolved, apiKey) => buildChildEnv(resolved as CodeBuddyProfile, apiKey),
+          deps,
+        });
+      } finally {
+        if (promptDir) await rm(promptDir, { recursive: true, force: true }).catch(() => {});
+      }
     },
   };
 }
diff --git a/tests/providers/codebuddy-adapter.test.ts b/tests/providers/codebuddy-adapter.test.ts
index 0da898ea82..cae5d98484 100644
--- a/tests/providers/codebuddy-adapter.test.ts
+++ b/tests/providers/codebuddy-adapter.test.ts
@@ -1,4 +1,5 @@
 import { beforeEach, describe, expect, test } from "bun:test";
+import { existsSync, readFileSync, statSync } from "node:fs";
 import { EventEmitter } from "node:events";
 import { Readable, Writable } from "node:stream";
 import type { ChildProcess } from "node:child_process";
@@ -113,14 +114,16 @@ describe("codebuddy headless arguments keep tool ownership with Codex", () => {
     expect(args[args.indexOf("--model") + 1]).toBe("glm-5.3");
   });
 
-  test("maps Codex reasoning effort onto --effort and folds the system prompt", () => {
+  test("maps Codex reasoning effort and references a private system-prompt file", () => {
     const args = buildArgs(
       CODEBUDDY_GLOBAL_PROFILE,
       parsed({ options: { reasoning: "high" }, context: { systemPrompt: ["Be terse."], messages: [] } }),
       provider(),
+      "/private/system-prompt.txt",
     );
     expect(args[args.indexOf("--effort") + 1]).toBe("high");
-    expect(args[args.indexOf("--append-system-prompt") + 1]).toBe("Be terse.");
+    expect(args[args.indexOf("--system-prompt-file") + 1]).toBe("/private/system-prompt.txt");
+    expect(args).not.toContain("Be terse.");
   });
 });
 
@@ -184,6 +187,7 @@ describe("codebuddy runTurn fails closed before any spawn", () => {
     let command = "";
     let args: readonly string[] = [];
     let options: import("node:child_process").SpawnOptions | undefined;
+    let promptFile = "";
     const adapter = createCodeBuddyAdapter(provider(), {
       platform: "win32",
       which: () => "C:\\npm\\codebuddy.cmd",
@@ -191,6 +195,10 @@ describe("codebuddy runTurn fails closed before any spawn", () => {
         command = seenCommand;
         args = seenArgs;
         options = seenOptions;
+        const commandLine = seenArgs[3] ?? "";
+        const match = commandLine.match(/--system-prompt-file\s+"([^"]+)"/);
+        promptFile = match?.[1] ?? "";
+        expect(readFileSync(promptFile, "utf8")).toBe('Say "hello" & stop');
         return fakeChild([enc.encode('{"type":"result","subtype":"success"}\n')]) as unknown as ChildProcess;
       },
       killGraceMs: 20,
@@ -200,8 +208,30 @@ describe("codebuddy runTurn fails closed before any spawn", () => {
     expect(command.toLowerCase()).toContain("cmd.exe");
     expect(args.slice(0, 3)).toEqual(["/d", "/s", "/c"]);
     expect(args[3]).toContain("codebuddy.cmd");
-    expect(args[3]).toContain("Say");
+    expect(args[3]).not.toContain("Say");
     expect(options?.windowsVerbatimArguments).toBe(true);
+    expect(existsSync(promptFile)).toBe(false);
+  });
+
+  test("keeps request-derived prompts out of argv and removes the private staging file", async () => {
+    let promptFile = "";
+    const secret = "private-system-instruction";
+    const adapter = createCodeBuddyAdapter(provider(), {
+      which: () => "/usr/bin/codebuddy",
+      spawn: (_command, args) => {
+        expect(args).not.toContain(secret);
+        const index = args.indexOf("--system-prompt-file");
+        expect(index).toBeGreaterThanOrEqual(0);
+        promptFile = args[index + 1] ?? "";
+        expect(readFileSync(promptFile, "utf8")).toBe(secret);
+        if (process.platform !== "win32") expect(statSync(promptFile).mode & 0o777).toBe(0o600);
+        return fakeChild([enc.encode('{"type":"result","subtype":"success"}\n')]) as unknown as ChildProcess;
+      },
+      killGraceMs: 20,
+    });
+
+    await run(adapter, parsed({ context: { systemPrompt: [secret], messages: [] } }));
+    expect(existsSync(promptFile)).toBe(false);
   });
 });