diff --git a/changelog.d/fixes/port-2132-headroom-developer-role.md b/changelog.d/fixes/port-2132-headroom-developer-role.md
new file mode 100644
index 0000000000..c9064e43c5
--- /dev/null
+++ b/changelog.d/fixes/port-2132-headroom-developer-role.md
@@ -0,0 +1 @@
+- **fix(compression):** the Headroom SmartCrusher tabular-compaction guard now also skips `role: "developer"` messages, not just `role: "system"` — Codex CLI sends its instructions/tool-schema turn as `developer` (the Responses-API equivalent of `system`), so an embedded JSON array (e.g. an `update_plan` example) could get tabular-compacted, corrupting the model's tool-calling instructions and breaking Codex CLI plan mode. (thanks @SingCJ)
diff --git a/open-sse/services/compression/engines/headroom/smartcrusher.ts b/open-sse/services/compression/engines/headroom/smartcrusher.ts
index c40ea82928..a14c851b6b 100644
--- a/open-sse/services/compression/engines/headroom/smartcrusher.ts
+++ b/open-sse/services/compression/engines/headroom/smartcrusher.ts
@@ -160,7 +160,7 @@ export function collectCompactableArrays(
     while ((m = regex.exec(text)) !== null) pushIfCompactable(m[1].trim());
   };
   for (const msg of messages) {
-    if (msg.role === "system") continue;
+    if (msg.role === "system" || msg.role === "developer") continue;
     if (typeof msg.content === "string") scanText(msg.content);
     else if (Array.isArray(msg.content)) {
       for (const part of msg.content) {
@@ -218,8 +218,12 @@ export function crushMessages(
   let changed = false;
 
   const result = messages.map((msg): MessageLike => {
-    // Guard: never touch system messages
-    if (msg.role === "system") return { ...msg };
+    // Guard: never touch system messages. "developer" is the Responses-API equivalent of
+    // "system" used by newer models (e.g. Codex CLI, see open-sse/executors/codex.ts) and
+    // carries the same kind of instructions/tool-schema content — compacting a JSON array
+    // embedded there (e.g. an update_plan example) can corrupt the model's tool-calling
+    // instructions (9router#2132: broke Codex CLI plan mode).
+    if (msg.role === "system" || msg.role === "developer") return { ...msg };
 
     if (typeof msg.content === "string") {
       const crushed = crushText(msg.content, minRows);
diff --git a/tests/unit/compression/headroom-developer-role-2132.test.ts b/tests/unit/compression/headroom-developer-role-2132.test.ts
new file mode 100644
index 0000000000..5f797d3f1a
--- /dev/null
+++ b/tests/unit/compression/headroom-developer-role-2132.test.ts
@@ -0,0 +1,91 @@
+/**
+ * Regression test for upstream 9router#2132 (ported): "Token saver Headroom ruins plan mode
+ * in Codex CLI".
+ *
+ * Root cause: SmartCrusher's system-message guard only checked `role === "system"`. Codex CLI
+ * (open-sse/executors/codex.ts) sends its instructions/tool-schema turn with role "developer"
+ * (the Responses-API equivalent of "system" used by newer models). Every other guard in this
+ * codebase that excludes "system" also excludes "developer" (see roleNormalizer.ts,
+ * contextManager.ts, claudeUpstreamMessages.ts, etc.) — SmartCrusher was the exception, so it
+ * happily tabular-compacted JSON arrays (e.g. the update_plan tool schema/examples) embedded in
+ * the developer-role turn, corrupting the instructions the model needs to call the plan tool.
+ */
+
+import { describe, it, before } from "node:test";
+import assert from "node:assert/strict";
+
+let crushMessages: typeof import("../../../open-sse/services/compression/engines/headroom/smartcrusher.ts").crushMessages;
+let collectCompactableArrays: typeof import("../../../open-sse/services/compression/engines/headroom/smartcrusher.ts").collectCompactableArrays;
+let headroomEngine: import("../../../open-sse/services/compression/engines/headroom/index.ts").headroomEngine;
+
+before(async () => {
+  const mod = await import("../../../open-sse/services/compression/engines/headroom/smartcrusher.ts");
+  crushMessages = mod.crushMessages;
+  collectCompactableArrays = mod.collectCompactableArrays;
+
+  const engineMod = await import("../../../open-sse/services/compression/engines/headroom/index.ts");
+  headroomEngine = engineMod.headroomEngine;
+});
+
+/** A homogeneous array big enough (>= default minRows=8) to trigger compaction. */
+function makePlanSchemaExample(): Record<string, unknown>[] {
+  return Array.from({ length: 10 }, (_, i) => ({
+    step: `step-${i + 1}`,
+    status: i === 0 ? "in_progress" : "pending",
+  }));
+}
+
+describe("headroom SmartCrusher — developer-role guard (9router#2132)", () => {
+  it("does NOT compact JSON arrays embedded in a developer-role message (crushMessages)", () => {
+    const json = JSON.stringify(makePlanSchemaExample());
+    const messages = [
+      {
+        role: "developer",
+        content: `Use the update_plan tool. Example plan:\n\`\`\`json\n${json}\n\`\`\``,
+      },
+      { role: "user", content: "Refactor the auth module." },
+    ];
+
+    const { messages: result, changed } = crushMessages(messages, 8);
+
+    assert.equal(changed, false, "developer-role content must not be touched");
+    assert.equal(result[0].content, messages[0].content);
+  });
+
+  it("still compacts the same payload when placed under role: system (control case)", () => {
+    // Sanity check: this proves the array itself WOULD be compactable — the guard, not the
+    // shape of the payload, is what must change.
+    const json = JSON.stringify(makePlanSchemaExample());
+    const messages = [{ role: "user", content: `\`\`\`json\n${json}\n\`\`\`` }];
+
+    const { changed } = crushMessages(messages, 8);
+    assert.equal(changed, true, "control case: user-role content of the same shape IS compacted");
+  });
+
+  it("collectCompactableArrays does not surface arrays from developer-role messages", () => {
+    const json = JSON.stringify(makePlanSchemaExample());
+    const messages = [
+      { role: "developer", content: `\`\`\`json\n${json}\n\`\`\`` },
+    ];
+    const found = collectCompactableArrays(messages, 8);
+    assert.equal(found.length, 0);
+  });
+
+  it("headroomEngine.apply leaves a Codex-CLI-shaped developer turn untouched end-to-end", () => {
+    const json = JSON.stringify(makePlanSchemaExample());
+    const body: Record<string, unknown> = {
+      model: "gpt-5-codex",
+      messages: [
+        {
+          role: "developer",
+          content: `Instructions with an embedded schema example:\n\`\`\`json\n${json}\n\`\`\``,
+        },
+        { role: "user", content: "Implement the feature." },
+      ],
+    };
+
+    const result = headroomEngine.apply(body);
+    assert.equal(result.compressed, false);
+    assert.deepEqual(result.body, body);
+  });
+});