diff --git a/src/client.ts b/src/client.ts
index 3d30cb4..a6616fa 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -18,8 +18,41 @@ export interface InterfazeOptions extends ClientOptions {
   adminKey?: string;
 }
 
+function envVar(name: string): string | undefined {
+  return typeof process !== "undefined" && process.env ? process.env[name] : undefined;
+}
+
+/**
+ * The openai client merges `OPENAI_CUSTOM_HEADERS` into every request. Those headers
+ * are for api.openai.com (or a gateway in front of it), so null them out here; a
+ * null default header tells the openai client to drop it.
+ */
+function openaiEnvHeaders(): Record<string, null> {
+  const out: Record<string, null> = {};
+  for (const line of envVar("OPENAI_CUSTOM_HEADERS")?.split("\n") ?? []) {
+    const colon = line.indexOf(":");
+    if (colon > 0) out[line.slice(0, colon).trim()] = null;
+  }
+  return out;
+}
+
+/**
+ * `defaultHeaders` is openai's `HeadersLike`, so it may be a `Headers` instance or an
+ * array of tuples. Spreading either gives `{}` or `{"0": [name, value]}`, which silently
+ * drops every header the caller set.
+ */
+function headerRecord(given: ClientOptions["defaultHeaders"]): Record<string, unknown> {
+  if (!given) return {};
+  if ((typeof Headers !== "undefined" && given instanceof Headers) || Array.isArray(given)) {
+    const out: Record<string, string> = {};
+    new Headers(given as HeadersInit).forEach((value, name) => (out[name] = value));
+    return out;
+  }
+  return { ...given };
+}
+
 function envKey(): string | undefined {
-  return typeof process !== "undefined" && process.env ? process.env["INTERFAZE_API_KEY"] : undefined;
+  return envVar("INTERFAZE_API_KEY");
 }
 
 /** Interfaze API client — an OpenAI-compatible client wired to the Interfaze endpoint. */
@@ -37,7 +70,7 @@ export class Interfaze {
       throw new InterfazeError("Missing API key. Pass `new Interfaze({ apiKey })` or set the INTERFAZE_API_KEY environment variable.");
     }
 
-    const headers: Record<string, string> = { ...(defaultHeaders as Record<string, string> | undefined) };
+    const headers = { ...openaiEnvHeaders(), ...headerRecord(defaultHeaders) } as Record<string, string | null>;
     if (showAdditionalInfo) headers[HEADERS.showAdditionalInfo] = "true";
     if (bypassMoA) headers[HEADERS.bypassMoA] = "true";
     if (bypassCache) headers[HEADERS.bypassCache] = "true";
@@ -47,6 +80,10 @@ export class Interfaze {
       ...rest,
       apiKey: resolvedKey,
       baseURL: baseURL ?? INTERFAZE_BASE_URL,
+      // Left undefined, these fall back to OPENAI_ORG_ID / OPENAI_PROJECT_ID and get
+      // sent to Interfaze as OpenAI-Organization / OpenAI-Project headers.
+      organization: rest.organization ?? null,
+      project: rest.project ?? null,
       defaultHeaders: headers,
       timeout: rest.timeout ?? DEFAULT_TIMEOUT_MS,
     });
diff --git a/test/client.test.ts b/test/client.test.ts
index 7889538..0ffa0c7 100644
--- a/test/client.test.ts
+++ b/test/client.test.ts
@@ -1,5 +1,6 @@
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
-import { Interfaze, InterfazeError } from "../src/index.js";
+import { Interfaze, InterfazeError, type InterfazeOptions } from "../src/index.js";
+import { completion, jsonResponse, mockInterfaze } from "./helpers.js";
 
 describe("client construction", () => {
   const saved = process.env["INTERFAZE_API_KEY"];
@@ -54,3 +55,60 @@ describe("client construction", () => {
     expect((i.openai as unknown as { timeout: number }).timeout).toBe(30_000);
   });
 });
+
+describe("OPENAI_* environment isolation", () => {
+  const vars = ["OPENAI_ORG_ID", "OPENAI_PROJECT_ID", "OPENAI_CUSTOM_HEADERS"] as const;
+  const saved = Object.fromEntries(vars.map((v) => [v, process.env[v]]));
+  beforeEach(() => {
+    process.env["OPENAI_ORG_ID"] = "org-from-env";
+    process.env["OPENAI_PROJECT_ID"] = "proj-from-env";
+    process.env["OPENAI_CUSTOM_HEADERS"] = "Helicone-Auth: Bearer sk-gateway\nx-team: platform";
+  });
+  afterEach(() => {
+    for (const v of vars) {
+      if (saved[v] === undefined) delete process.env[v];
+      else process.env[v] = saved[v];
+    }
+  });
+
+  it("does not send OpenAI org, project or custom env headers to Interfaze", async () => {
+    const { interfaze, calls } = mockInterfaze(() => jsonResponse(completion()));
+    await interfaze.chat.completions.create({ messages: [{ role: "user", content: "hi" }] });
+
+    const headers = calls[0]!.headers;
+    expect(headers.get("openai-organization")).toBeNull();
+    expect(headers.get("openai-project")).toBeNull();
+    expect(headers.get("helicone-auth")).toBeNull();
+    expect(headers.get("x-team")).toBeNull();
+    expect(headers.get("authorization")).toBe("Bearer test-key");
+  });
+
+  it("still sends values the caller passes explicitly", async () => {
+    const { interfaze, calls } = mockInterfaze(() => jsonResponse(completion()), {
+      organization: "org-explicit",
+      defaultHeaders: { "x-team": "explicit" },
+    });
+    await interfaze.chat.completions.create({ messages: [{ role: "user", content: "hi" }] });
+
+    const headers = calls[0]!.headers;
+    expect(headers.get("openai-organization")).toBe("org-explicit");
+    expect(headers.get("x-team")).toBe("explicit");
+  });
+});
+
+describe("defaultHeaders", () => {
+  it.each([
+    ["a plain object", { "x-trace": "abc" }],
+    ["a Headers instance", new Headers({ "x-trace": "abc" })],
+    ["an array of tuples", [["x-trace", "abc"]]],
+  ])("forwards headers given as %s", async (_label, defaultHeaders) => {
+    const { interfaze, calls } = mockInterfaze(() => jsonResponse(completion()), {
+      defaultHeaders: defaultHeaders as InterfazeOptions["defaultHeaders"],
+      bypassCache: true,
+    });
+    await interfaze.chat.completions.create({ messages: [{ role: "user", content: "hi" }] });
+
+    expect(calls[0]!.headers.get("x-trace")).toBe("abc");
+    expect(calls[0]!.headers.get("x-interfaze-bypass-cache")).toBe("true");
+  });
+});