diff --git a/packages/cli/src/tts/python.test.ts b/packages/cli/src/tts/python.test.ts
index b53d0396a5..bfb2ba7a90 100644
--- a/packages/cli/src/tts/python.test.ts
+++ b/packages/cli/src/tts/python.test.ts
@@ -15,7 +15,13 @@ vi.mock("node:child_process", async (importOriginal) => {
   return { ...actual, execFileSync: execFileSyncMock };
 });
 
-const { findPython, describeRejectedPythonOverride } = await import("./python.js");
+const {
+  findPython,
+  describeRejectedPythonOverride,
+  hasPythonPackage,
+  hasPythonModules,
+  workingPythonEnv,
+} = await import("./python.js");
 
 describe("findPython / describeRejectedPythonOverride", () => {
   beforeEach(() => {
@@ -76,3 +82,58 @@ describe("findPython / describeRejectedPythonOverride", () => {
     expect(execFileSyncMock).toHaveBeenCalledTimes(1);
   });
 });
+
+describe("PYTHONPATH when probing for Python packages", () => {
+  const PYTHON = "/opt/venv/bin/python";
+
+  // The import works only for the environment `works` accepts, like a real interpreter would.
+  function importsWhen(works: (env: NodeJS.ProcessEnv) => boolean) {
+    execFileSyncMock.mockReset();
+    execFileSyncMock.mockImplementation((_cmd: string, args: string[], options: any) => {
+      if (args[0] === "-c" && !works(options.env)) throw new Error("ModuleNotFoundError");
+      return "Python 3.11.0";
+    });
+  }
+
+  beforeEach(() => {
+    process.env.HYPERFRAMES_PYTHON = PYTHON;
+  });
+
+  afterEach(() => {
+    delete process.env.PYTHONPATH;
+    delete process.env.HYPERFRAMES_PYTHON;
+  });
+
+  it("keeps a PYTHONPATH the user set on purpose when the package needs it", () => {
+    process.env.PYTHONPATH = "/deliberate/site-packages";
+    importsWhen((env) => env.PYTHONPATH === "/deliberate/site-packages");
+
+    expect(hasPythonPackage(PYTHON, "kokoro_onnx")).toBe(true);
+    expect(hasPythonModules(["kokoro_onnx"])).toBe(true);
+    expect(workingPythonEnv(PYTHON, ["kokoro_onnx"], { A: "1" })).toMatchObject({
+      PYTHONPATH: "/deliberate/site-packages",
+      A: "1",
+    });
+  });
+
+  it("drops a foreign PYTHONPATH that stops the package from importing", () => {
+    process.env.PYTHONPATH = "/other/python/site-packages";
+    importsWhen((env) => env.PYTHONPATH === undefined);
+
+    expect(hasPythonPackage(PYTHON, "kokoro_onnx")).toBe(true);
+    expect(hasPythonModules(["kokoro_onnx"])).toBe(true);
+    const env = workingPythonEnv(PYTHON, ["kokoro_onnx"], { ORT_DISABLE_TELEMETRY: "1" });
+    expect(env).not.toHaveProperty("PYTHONPATH");
+    expect(env).toMatchObject({ PATH: process.env.PATH, ORT_DISABLE_TELEMETRY: "1" });
+  });
+
+  it("tries the inherited environment first and reports a package neither finds", () => {
+    process.env.PYTHONPATH = "/other/python/site-packages";
+    importsWhen(() => false);
+
+    expect(hasPythonPackage(PYTHON, "kokoro_onnx")).toBe(false);
+    expect(workingPythonEnv(PYTHON, ["kokoro_onnx"])).toBeNull();
+    const [first] = execFileSyncMock.mock.calls.filter(([, args]) => args[0] === "-c");
+    expect(first?.[2].env.PYTHONPATH).toBe("/other/python/site-packages");
+  });
+});
diff --git a/packages/cli/src/tts/python.ts b/packages/cli/src/tts/python.ts
index 3e7ec13095..563578af13 100644
--- a/packages/cli/src/tts/python.ts
+++ b/packages/cli/src/tts/python.ts
@@ -77,19 +77,38 @@ export function describeRejectedPythonOverride(): string | null {
   return `HYPERFRAMES_PYTHON="${override}" was rejected: ${validation.reason}`;
 }
 
-/** True if `import <pkg>` succeeds — actually executes the module. */
-export function hasPythonPackage(python: string, pkg: string): boolean {
+function withoutPythonPath(): NodeJS.ProcessEnv {
+  const { PYTHONPATH: _foreign, ...env } = process.env;
+  return env;
+}
+
+function runs(python: string, code: string, env: NodeJS.ProcessEnv): boolean {
   try {
-    execFileSync(python, ["-c", `import ${pkg}`], {
-      stdio: ["pipe", "pipe", "pipe"],
-      timeout: 10_000,
-    });
+    execFileSync(python, ["-c", code], { stdio: ["pipe", "pipe", "pipe"], timeout: 10_000, env });
     return true;
   } catch {
     return false;
   }
 }
 
+function environmentThatRuns(python: string, code: string): NodeJS.ProcessEnv | null {
+  return [process.env, withoutPythonPath()].find((env) => runs(python, code, env)) ?? null;
+}
+
+export function workingPythonEnv(
+  python: string,
+  modules: string[],
+  extra: NodeJS.ProcessEnv = {},
+): NodeJS.ProcessEnv | null {
+  const env = environmentThatRuns(python, `import ${modules.join(", ")}`);
+  return env && { ...env, ...extra };
+}
+
+/** True if `import <pkg>` succeeds — actually executes the module. */
+export function hasPythonPackage(python: string, pkg: string): boolean {
+  return environmentThatRuns(python, `import ${pkg}`) !== null;
+}
+
 /**
  * True if every module is installed, checked via `importlib.util.find_spec`
  * so heavy packages (torch) are never imported — fast enough for a preflight.
@@ -100,13 +119,5 @@ export function hasPythonModules(modules: string[]): boolean {
   if (!python) return false;
   const list = JSON.stringify(modules);
   const probe = `import importlib.util,sys; sys.exit(0 if all(importlib.util.find_spec(m) for m in ${list}) else 1)`;
-  try {
-    execFileSync(python, ["-c", probe], {
-      stdio: ["pipe", "pipe", "pipe"],
-      timeout: 10_000,
-    });
-    return true;
-  } catch {
-    return false;
-  }
+  return environmentThatRuns(python, probe) !== null;
 }
diff --git a/packages/cli/src/tts/synthesize.cache.test.ts b/packages/cli/src/tts/synthesize.cache.test.ts
index fa2199ccea..ebf44e98c4 100644
--- a/packages/cli/src/tts/synthesize.cache.test.ts
+++ b/packages/cli/src/tts/synthesize.cache.test.ts
@@ -14,7 +14,8 @@ vi.mock("node:fs", async (importOriginal) => {
   return { ...original, existsSync: vi.fn(original.existsSync) };
 });
 vi.mock("node:child_process", () => ({ execFileSync: vi.fn() }));
-vi.mock("./python.js", () => ({
+vi.mock("./python.js", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("./python.js")>()),
   findPython: () => "test-python",
   hasPythonPackage: () => true,
 }));
@@ -53,6 +54,28 @@ describe("synthesis script cache", () => {
     fs.rmSync(paths.home, { recursive: true, force: true });
   });
 
+  it("runs the synthesis process in the environment the packages imported under", async () => {
+    vi.mocked(execFileSync).mockImplementation((_cmd, args, options) => {
+      const importing = String(args?.[0]) === "-c";
+      const env = (options as { env?: NodeJS.ProcessEnv }).env;
+      if (importing && env?.PYTHONPATH !== undefined) throw new Error("ModuleNotFoundError");
+      fs.writeFileSync(outputPath, "stub audio");
+      return JSON.stringify({
+        outputPath,
+        sampleRate: 24000,
+        durationSeconds: 1,
+        langApplied: true,
+      });
+    });
+    vi.stubEnv("PYTHONPATH", "/other/python/site-packages");
+    const { synthesize } = await import("./synthesize.js");
+    await synthesize("Hello", outputPath);
+
+    const options = vi.mocked(execFileSync).mock.calls.at(-1)?.[2];
+    expect(options?.env).not.toHaveProperty("PYTHONPATH");
+    vi.unstubAllEnvs();
+  });
+
   it("turns off onnxruntime telemetry for the synthesis process", async () => {
     const { synthesize } = await import("./synthesize.js");
     await synthesize("Hello", outputPath);
diff --git a/packages/cli/src/tts/synthesize.ts b/packages/cli/src/tts/synthesize.ts
index 8394ad7ae0..4ec225a248 100644
--- a/packages/cli/src/tts/synthesize.ts
+++ b/packages/cli/src/tts/synthesize.ts
@@ -10,7 +10,7 @@ import {
   inferLangFromVoiceId,
   type SupportedLang,
 } from "./manager.js";
-import { findPython, hasPythonPackage } from "./python.js";
+import { findPython, hasPythonPackage, workingPythonEnv } from "./python.js";
 import { writeNewFileSync } from "../utils/writeNewFile.js";
 
 // ---------------------------------------------------------------------------
@@ -189,13 +189,15 @@ export async function synthesize(
     );
   }
 
-  if (!hasPythonPackage(python, "kokoro_onnx")) {
-    throw new Error(
-      "The kokoro-onnx package is not installed. Run: pip install kokoro-onnx soundfile (or point HYPERFRAMES_PYTHON at a venv python that has them)",
-    );
-  }
-
-  if (!hasPythonPackage(python, "soundfile")) {
+  const pythonEnv = workingPythonEnv(python, ["kokoro_onnx", "soundfile"], {
+    ORT_DISABLE_TELEMETRY: "1",
+  });
+  if (!pythonEnv) {
+    if (!hasPythonPackage(python, "kokoro_onnx")) {
+      throw new Error(
+        "The kokoro-onnx package is not installed. Run: pip install kokoro-onnx soundfile (or point HYPERFRAMES_PYTHON at a venv python that has them)",
+      );
+    }
     throw new Error("The soundfile package is not installed. Run: pip install soundfile");
   }
 
@@ -222,7 +224,7 @@ export async function synthesize(
         encoding: "utf-8",
         timeout: 300_000,
         stdio: ["pipe", "pipe", "pipe"],
-        env: { ...process.env, ORT_DISABLE_TELEMETRY: "1" },
+        env: pythonEnv,
       },
     );