diff --git a/src/adapters/qoder/adapter.ts b/src/adapters/qoder/adapter.ts
index 20c0b5581b..d42d9054fd 100644
--- a/src/adapters/qoder/adapter.ts
+++ b/src/adapters/qoder/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";
@@ -12,7 +15,7 @@ export function buildQoderChildEnv(profile: QoderProfile, apiKey: string): Recor
 }
 
 /** Single-shot, tools-disabled Qoder CLI invocation; Codex remains the tool owner. */
-export function buildQoderArgs(parsed: OcxParsedRequest, provider: OcxProviderConfig): string[] {
+export function buildQoderArgs(parsed: OcxParsedRequest, provider: OcxProviderConfig, systemPromptFile?: string): string[] {
   const args = [
     "-p",
     "--output-format", "stream-json",
@@ -26,8 +29,7 @@ export function buildQoderArgs(parsed: OcxParsedRequest, provider: OcxProviderCo
   ];
   const effort = mapReasoningEffort(provider, parsed.modelId, parsed.options.reasoning);
   if (effort) args.push("--reasoning-effort", effort);
-  const system = buildSystemPrompt(parsed);
-  if (system) args.push("--append-system-prompt", system);
+  if (systemPromptFile) args.push("--append-system-prompt-file", systemPromptFile);
   return args;
 }
 
@@ -55,16 +57,39 @@ export function createQoderAdapter(provider: OcxProviderConfig, deps: QoderAdapt
         });
         return;
       }
-      await runCodingAgentTurn({
-        profiles: QODER_PROFILES,
-        provider,
-        parsed,
-        incoming,
-        emit,
-        buildArgs: (_profile, req, prov) => buildQoderArgs(req, prov),
-        buildEnv: (profile, apiKey) => buildQoderChildEnv(profile as QoderProfile, apiKey),
-        deps,
-      });
+      const system = buildSystemPrompt(parsed);
+      let promptDir: string | undefined;
+      let promptFile: string | undefined;
+      try {
+        promptDir = system ? await mkdtemp(join(tmpdir(), "ocx-qoder-prompt-")) : undefined;
+        promptFile = promptDir ? join(promptDir, "system-prompt.txt") : undefined;
+        if (promptFile) await writeFile(promptFile, system!, { encoding: "utf8", mode: 0o600 });
+      } catch {
+        if (promptDir) await rm(promptDir, { recursive: true, force: true }).catch(() => {});
+        emit({
+          type: "error",
+          message: "Qoder system prompt could not be prepared securely.",
+          status: 500,
+          errorType: "upstream_error",
+          code: "prompt_file_failed",
+          retryable: false,
+        });
+        return;
+      }
+      try {
+        await runCodingAgentTurn({
+          profiles: QODER_PROFILES,
+          provider,
+          parsed,
+          incoming,
+          emit,
+          buildArgs: (_profile, req, prov) => buildQoderArgs(req, prov, promptFile),
+          buildEnv: (profile, apiKey) => buildQoderChildEnv(profile as QoderProfile, apiKey),
+          deps,
+        });
+      } finally {
+        if (promptDir) await rm(promptDir, { recursive: true, force: true }).catch(() => {});
+      }
     },
   };
 }
diff --git a/tests/providers/qoder-adapter.test.ts b/tests/providers/qoder-adapter.test.ts
index 5411504354..4066fc5e99 100644
--- a/tests/providers/qoder-adapter.test.ts
+++ b/tests/providers/qoder-adapter.test.ts
@@ -1,6 +1,7 @@
 import { beforeEach, describe, expect, test } from "bun:test";
 import { EventEmitter } from "node:events";
 import { Readable, Writable } from "node:stream";
+import { readFile } from "node:fs/promises";
 import type { ChildProcess } from "node:child_process";
 import { buildQoderArgs, buildQoderChildEnv, createQoderAdapter } from "../../src/adapters/qoder/adapter";
 import { clearQoderBinaryCache, QODER_CN_PROFILE, QODER_GLOBAL_PROFILE, resolveQoderProfile } from "../../src/adapters/qoder/profiles";
@@ -44,6 +45,37 @@ describe("qoder adapter", () => {
     expect(args).not.toContain("--dangerously-skip-permissions");
   });
 
+  test("keeps system and developer prompts out of child-process arguments", async () => {
+    const secretSystem = "private system instructions";
+    const secretDeveloper = "private developer context";
+    let args: readonly string[] = [];
+    let promptFromFile: Promise<string> | undefined;
+    const adapter = createQoderAdapter(provider(), {
+      which: () => "/bin/qoder",
+      spawn: (_command, childArgs) => {
+        args = childArgs;
+        const flag = childArgs.indexOf("--append-system-prompt-file");
+        promptFromFile = readFile(childArgs[flag + 1]!, "utf8");
+        return fakeChild(['{"type":"result","subtype":"success","is_error":false}\n']);
+      },
+    });
+    await adapter.runTurn!(parsed({
+      context: {
+        systemPrompt: [secretSystem],
+        messages: [
+          { role: "developer", content: secretDeveloper, timestamp: 0 },
+          { role: "user", content: "hello", timestamp: 0 },
+        ],
+      },
+    }), { headers: new Headers(), translatorBudget: createTestTranslatorBudget() }, () => {});
+
+    const promptPath = args[args.indexOf("--append-system-prompt-file") + 1]!;
+    expect(args.join(" ")).not.toContain(secretSystem);
+    expect(args.join(" ")).not.toContain(secretDeveloper);
+    expect(await readFile(promptPath, "utf8").catch(() => "removed")).toBe("removed");
+    expect(await promptFromFile).toBe(`${secretSystem}\n\n${secretDeveloper}`);
+  });
+
   test("keeps Global and CN profiles, executables, destinations, and PAT variables isolated", async () => {
     expect(resolveQoderProfile("https://qoder.com/")).toBe(QODER_GLOBAL_PROFILE);
     expect(resolveQoderProfile("https://qoder.cn/")).toBe(QODER_CN_PROFILE);