diff --git a/README.md b/README.md
index 3c668cb..a69acb9 100644
--- a/README.md
+++ b/README.md
@@ -163,6 +163,17 @@ The log captures the request (model id, body byte length), each HTTP attempt (st
 wait, elapsed ms), every stream event payload, and the terminal finish reason + usage. It is
 useful for diagnosing model selection, retry, tool-call, and finish-reason issues.
 
+## ZDR header toggle
+
+The `x-cmd-zdr: 1` request header is **omitted by default** because it is rejected by some
+models. Set `COMMANDCODE_ZDR=1` (or `true`/`yes`) when launching opencode to opt back in. The
+value is read once at provider load, so restart opencode after changing it. Per-provider
+overrides still win: `headers: { "x-cmd-zdr": "..." }` in `opencode.json` takes precedence over
+the environment variable.
+
+With `COMMANDCODE_DEBUG` enabled, the trace logs a `zdr on|off` line per request so the toggle
+state is observable.
+
 ## Features
 
 | Capability | Status |
diff --git a/src/model.ts b/src/model.ts
index 35578c0..9519337 100644
--- a/src/model.ts
+++ b/src/model.ts
@@ -23,6 +23,10 @@ import { redact } from "./redact.js";
 import { transform } from "./transform.js";
 import { finishReasonFrom, usageFromFinish, type FinishEvent } from "./usage.js";
 
+// x-cmd-zdr breaks some models; off by default (server.py never sends it).
+// COMMANDCODE_ZDR=1|true|yes opts back in. Read at load, like COMMANDCODE_DEBUG.
+const ZDR_ENABLED = /^(1|true|yes)$/i.test(process.env["COMMANDCODE_ZDR"] ?? "");
+
 export type CommandCodeOptions = {
   name?: string;
   apiKey?: string;
@@ -139,9 +143,10 @@ class CommandCodeLanguageModel implements LanguageModelV3 {
       "x-project-slug": "project",
       "x-taste-learning": "true",
       "x-co-flag": "false",
-      "x-cmd-zdr": "1",
+      ...(ZDR_ENABLED ? { "x-cmd-zdr": "1" } : {}),
       ...this.opts.headers,
     };
+    debug("fetch", "zdr", ZDR_ENABLED ? "on" : "off");
     const auth = bearer(this.opts.apiKey);
     if (auth) headers["Authorization"] = auth;
     for (const [k, v] of Object.entries(extra ?? {})) if (v !== undefined) headers[k] = v;