diff --git a/packages/agent/src/adapters/codex/spawn.test.ts b/packages/agent/src/adapters/codex/spawn.test.ts
index 4f2653c7c1..48fb947283 100644
--- a/packages/agent/src/adapters/codex/spawn.test.ts
+++ b/packages/agent/src/adapters/codex/spawn.test.ts
@@ -38,3 +38,39 @@ describe("spawnCodexProcess MCP disable args", () => {
     expect(args.some((arg) => arg.includes("weird name"))).toBe(false);
   });
 });
+
+describe("spawnCodexProcess developer instructions", () => {
+  it("passes guidance via developer_instructions to preserve the base prompt", () => {
+    spawnMock.mockClear();
+    spawnMock.mockReturnValue(makeFakeChild());
+
+    spawnCodexProcess({
+      logger: new Logger({ debug: false }),
+      developerInstructions: "Follow PostHog signed-commit rules.",
+    });
+
+    const args: string[] = spawnMock.mock.calls[0][1];
+    expect(args).toContain(
+      'developer_instructions="Follow PostHog signed-commit rules."',
+    );
+    // The bare `instructions` and `model_instructions_file` keys replace Codex's
+    // model-optimized base prompt, so guidance must never go through them.
+    expect(args.some((arg) => arg.startsWith("instructions="))).toBe(false);
+    expect(args.some((arg) => arg.startsWith("model_instructions_file="))).toBe(
+      false,
+    );
+  });
+
+  it("escapes backslashes, newlines and quotes in developer_instructions", () => {
+    spawnMock.mockClear();
+    spawnMock.mockReturnValue(makeFakeChild());
+
+    spawnCodexProcess({
+      logger: new Logger({ debug: false }),
+      developerInstructions: 'a\\b\n"c',
+    });
+
+    const args: string[] = spawnMock.mock.calls[0][1];
+    expect(args).toContain('developer_instructions="a\\\\b\\n\\"c"');
+  });
+});
diff --git a/packages/agent/src/adapters/codex/spawn.ts b/packages/agent/src/adapters/codex/spawn.ts
index 72ff246057..3cbcf915af 100644
--- a/packages/agent/src/adapters/codex/spawn.ts
+++ b/packages/agent/src/adapters/codex/spawn.ts
@@ -12,7 +12,12 @@ export interface CodexProcessOptions {
   apiKey?: string;
   model?: string;
   reasoningEffort?: string;
-  instructions?: string;
+  /**
+   * Guidance appended on top of Codex's model-optimized base prompt via the
+   * `developer_instructions` config key. Unlike `instructions` /
+   * `model_instructions_file`, this does not replace the native base prompt.
+   */
+  developerInstructions?: string;
   binaryPath?: string;
   logger?: Logger;
   processCallbacks?: ProcessSpawnedCallback;
@@ -73,13 +78,13 @@ function buildConfigArgs(options: CodexProcessOptions): string[] {
     args.push("-c", `sandbox_workspace_write.writable_roots=[${escaped}]`);
   }
 
-  if (options.instructions) {
-    const escaped = options.instructions
+  if (options.developerInstructions) {
+    const escaped = options.developerInstructions
       .replace(/\\/g, "\\\\")
       .replace(/\n/g, "\\n")
       .replace(/\r/g, "\\r")
       .replace(/"/g, '\\"');
-    args.push("-c", `instructions="${escaped}"`);
+    args.push("-c", `developer_instructions="${escaped}"`);
   }
 
   return args;
diff --git a/packages/agent/src/agent.ts b/packages/agent/src/agent.ts
index e1264f7c47..7cb2d2b82b 100644
--- a/packages/agent/src/agent.ts
+++ b/packages/agent/src/agent.ts
@@ -136,7 +136,7 @@ export class Agent {
               binaryPath: options.codexBinaryPath,
               model: sanitizedModel,
               reasoningEffort: options.reasoningEffort,
-              instructions: options.instructions,
+              developerInstructions: options.developerInstructions,
               additionalDirectories: options.additionalDirectories,
             }
           : undefined,
diff --git a/packages/agent/src/server/agent-server.ts b/packages/agent/src/server/agent-server.ts
index 0492b9fd06..b3410b570d 100644
--- a/packages/agent/src/server/agent-server.ts
+++ b/packages/agent/src/server/agent-server.ts
@@ -992,7 +992,7 @@ export class AgentServer {
               apiKey: this.config.apiKey,
               model: this.config.model ?? DEFAULT_CODEX_MODEL,
               reasoningEffort: this.config.reasoningEffort,
-              instructions: codexInstructions,
+              developerInstructions: codexInstructions,
             }
           : undefined,
       onStructuredOutput: async (output) => {
diff --git a/packages/agent/src/types.ts b/packages/agent/src/types.ts
index 4e5a3c0190..23b14bc696 100644
--- a/packages/agent/src/types.ts
+++ b/packages/agent/src/types.ts
@@ -54,7 +54,11 @@ export interface TaskExecutionOptions {
   gatewayUrl?: string;
   codexBinaryPath?: string;
   reasoningEffort?: EffortLevel;
-  instructions?: string;
+  /**
+   * Codex-only. Appended on top of the model's base prompt via the Codex
+   * `developer_instructions` config key, preserving Codex's native base prompt.
+   */
+  developerInstructions?: string;
   processCallbacks?: ProcessSpawnedCallback;
   /** Callback invoked when the agent calls the create_output tool for structured output */
   onStructuredOutput?: (output: Record<string, unknown>) => Promise<void>;
diff --git a/packages/workspace-server/src/services/agent/agent.ts b/packages/workspace-server/src/services/agent/agent.ts
index 9a79db7ff3..f1c9419588 100644
--- a/packages/workspace-server/src/services/agent/agent.ts
+++ b/packages/workspace-server/src/services/agent/agent.ts
@@ -690,7 +690,8 @@ When creating pull requests, add the following footer at the end of the PR descr
           adapter === "codex" ? this.getCodexBinaryPath() : undefined,
         model,
         reasoningEffort: adapter === "codex" ? effort : undefined,
-        instructions: adapter === "codex" ? systemPrompt.append : undefined,
+        developerInstructions:
+          adapter === "codex" ? systemPrompt.append : undefined,
         additionalDirectories:
           adapter === "codex" ? additionalDirectories : undefined,
         onStructuredOutput: jsonSchema