diff --git a/src/process/ProviderProcess.ts b/src/process/ProviderProcess.ts
index 390be712e5..7bb1f1ba2d 100644
--- a/src/process/ProviderProcess.ts
+++ b/src/process/ProviderProcess.ts
@@ -20,6 +20,7 @@ import {
   systemProcessOwnershipHost,
 } from "./ProcessOwnershipObservation.js";
 import { ProcessOwnershipEvidenceOwner } from "./ProcessOwnershipEvidence.js";
+import { mergeEnvironmentOverrides } from "./snapshotEnvironment.js";
 
 const DEFAULT_TERMINATION_GRACE_MS = 250;
 const DEFAULT_KILL_GRACE_MS = 1_000;
@@ -267,11 +268,18 @@ export const spawnOwnedProviderProcess = async (
     options.signal?.throwIfAborted();
   }
   const hostEnvironment = options.hostEnvironment ?? process.env;
-  const environment = {
-    ...hostEnvironment,
-    ...options.env,
-    REA_PROCESS_RUN_ID: options.runId,
-  };
+  const selectedEnvironment = mergeEnvironmentOverrides(
+    hostEnvironment,
+    options.env ?? {},
+    platform,
+  );
+  // Ownership is the final layer, including when a caller used another casing.
+  // Resolve undefined overrides before the launchers omit unset variables.
+  const environment = mergeEnvironmentOverrides(
+    selectedEnvironment,
+    { REA_PROCESS_RUN_ID: options.runId },
+    platform,
+  );
   if (platform === "win32" && process.platform === "win32") {
     const child = new WindowsOwnedProcess(
       options.command,
diff --git a/tests/boundary/process/providerProcessEnvironment.test.ts b/tests/boundary/process/providerProcessEnvironment.test.ts
new file mode 100644
index 0000000000..ddd2e3e599
--- /dev/null
+++ b/tests/boundary/process/providerProcessEnvironment.test.ts
@@ -0,0 +1,89 @@
+import { expect, it } from "vitest";
+
+import {
+  ProviderProcessSupervisor,
+  spawnOwnedProviderProcess,
+} from "../../../src/process/ProviderProcess.js";
+
+const hostEnvironment = {
+  PATH: "host-search-path",
+  REA_CASE_TEST: "host-case",
+  REA_EMPTY_TEST: "host-empty",
+  REA_UNSET_TEST: "host-unset",
+  REA_EXACT_UNSET_TEST: "host-exact-unset",
+  REA_PROCESS_RUN_ID: "host-run-id",
+};
+
+const observeEnvironment = async (
+  platform: NodeJS.Platform,
+  path: string | undefined,
+): Promise<Record<string, string>> => {
+  const launched = await spawnOwnedProviderProcess({
+    command: process.execPath,
+    arguments: [
+      "-e",
+      `process.stdout.write(JSON.stringify(Object.fromEntries(Object.entries(process.env).filter(([key]) => key.toUpperCase() === "PATH" || key.toUpperCase().startsWith("REA_")))));`,
+    ],
+    hostEnvironment,
+    env: {
+      Path: path,
+      rea_case_test: "caller-case",
+      rea_empty_test: "",
+      rea_unset_test: undefined,
+      REA_EXACT_UNSET_TEST: undefined,
+      REA_PROCESS_RUN_ID: "caller-uppercase-run-id",
+      rea_process_run_id: "caller-lowercase-run-id",
+    },
+    platform,
+    runId: "owned-run-id",
+  });
+  const supervisor = new ProviderProcessSupervisor({
+    ...launched,
+    ownsProcessLifetime: true,
+  });
+  try {
+    expect(await supervisor.waitForOutputClose(2_000)).toBe(true);
+    const snapshot = supervisor.snapshot();
+    expect(snapshot.exitCode, snapshot.stderr.text).toBe(0);
+    return JSON.parse(snapshot.stdout.text) as Record<string, string>;
+  } finally {
+    const stopped = await supervisor.stop();
+    expect(stopped.status).not.toBe("incomplete");
+  }
+};
+
+it.each([undefined, ""])(
+  "preserves Windows caller PATH %s and ownership token precedence at real process creation",
+  async (path) => {
+    // On Windows this exercises the native Job Object launcher. On POSIX the
+    // configured-platform seam exposes the exact producer environment to a real child.
+    const observed = await observeEnvironment("win32", path);
+    const entries = Object.entries(observed);
+    const canonical = Object.fromEntries(
+      entries.map(([name, value]) => [name.toUpperCase(), value]),
+    );
+    expect(canonical).toEqual({
+      ...(path === undefined ? {} : { PATH: "" }),
+      REA_CASE_TEST: "caller-case",
+      REA_EMPTY_TEST: "",
+      REA_PROCESS_RUN_ID: "owned-run-id",
+    });
+    expect(entries).toHaveLength(Object.keys(canonical).length);
+  },
+);
+
+it.skipIf(process.platform === "win32")(
+  "keeps case-distinct POSIX variables and exact-key removal through real process creation",
+  async () => {
+    expect(await observeEnvironment(process.platform, undefined)).toEqual({
+      PATH: "host-search-path",
+      REA_CASE_TEST: "host-case",
+      REA_EMPTY_TEST: "host-empty",
+      REA_UNSET_TEST: "host-unset",
+      REA_PROCESS_RUN_ID: "owned-run-id",
+      rea_case_test: "caller-case",
+      rea_empty_test: "",
+      rea_process_run_id: "caller-lowercase-run-id",
+    });
+  },
+);