diff --git a/src/cliBrowserContext.ts b/src/cliBrowserContext.ts
index b45282dcd..f7cf9b1a0 100644
--- a/src/cliBrowserContext.ts
+++ b/src/cliBrowserContext.ts
@@ -7,9 +7,20 @@ import {
 } from "./domain/errors.js";
 import type { JsonValue } from "./domain/jsonValue.js";
 
-/** Load the configured passive browser authority and provider for one CLI operation. */
-export const browserContext = async (operation: string) => {
-  const config = parseConfig(process.env);
+/**
+ * Load the configured passive browser authority and provider for one CLI
+ * operation.
+ *
+ * `environment` defaults to the process environment so CLI callers stay
+ * unchanged, but every caller may supply the environment explicitly. That is
+ * what makes configuration-driven behaviour testable without mutating
+ * `process.env` for the duration of a test.
+ */
+export const browserContext = async (
+  operation: string,
+  environment: Readonly<Record<string, string | undefined>> = process.env,
+) => {
+  const config = parseConfig(environment);
   if (!config.ok)
     return { ok: false as const, error: browserCliError(config.error) };
   const policy = config.value.browserObservationPolicy;
diff --git a/src/cliHostInputs.test.ts b/src/cliHostInputs.test.ts
new file mode 100644
index 000000000..7aa90c9fc
--- /dev/null
+++ b/src/cliHostInputs.test.ts
@@ -0,0 +1,65 @@
+import { describe, expect, it } from "vitest";
+
+import { browserContext } from "./cliBrowserContext.js";
+import { HopperProvider } from "./hopper/HopperProvider.js";
+import { silentLogger } from "./logger.js";
+import { parseConfig } from "./config.js";
+
+const config = (env: Record<string, string | undefined> = {}) => {
+  const parsed = parseConfig(env);
+  if (!parsed.ok) throw new Error("expected valid configuration");
+  return parsed.value;
+};
+
+describe("host platform is injected, not read from the ambient process", () => {
+  it("reports Hopper as unsupported for a platform it cannot run on", () => {
+    const provider = new HopperProvider(config(), silentLogger, "win32");
+    const availability = provider.inspectAvailability();
+    expect(availability).toMatchObject({
+      status: "unavailable",
+      code: "unsupported_host",
+    });
+    expect(JSON.stringify(availability)).toContain("win32");
+  });
+
+  it.each(["darwin", "linux"] as const)(
+    "does not gate Hopper as unsupported on %s",
+    (platform) => {
+      const provider = new HopperProvider(
+        config({ hopper_launcher_path: "/nonexistent/hopper-launcher" }),
+        silentLogger,
+        platform,
+      );
+      expect(provider.inspectAvailability().code).not.toBe("unsupported_host");
+    },
+  );
+});
+
+describe("configuration is read from an injected environment", () => {
+  it("lets the supplied environment decide whether the operation is available", async () => {
+    // The error projection deliberately redacts specifics, so the meaningful
+    // signal is the outcome: the same helper fails or succeeds purely as a
+    // function of the environment it was handed.
+    const disabled = await browserContext("inspect_web_page", {});
+    expect(disabled.ok).toBe(false);
+
+    const enabled = await browserContext("inspect_web_page", {
+      REA_BROWSER_OBSERVE_ENABLED: "true",
+      REA_BROWSER_CDP_ENDPOINTS_JSON: '["http://127.0.0.1:9222"]',
+      REA_BROWSER_ALLOWED_ORIGINS_JSON: '["https://app.example.test"]',
+    });
+    expect(enabled.ok).toBe(true);
+  });
+
+  it("reports a configuration error from the supplied environment", async () => {
+    const invalid = await browserContext("inspect_web_page", {
+      REA_BROWSER_ALLOWED_ORIGINS: "https://example.test",
+      REA_BROWSER_MAX_BYTES: "not-a-number",
+    });
+    expect(invalid.ok).toBe(false);
+    if (invalid.ok) return;
+    // Projected through the shared analysis-error contract, like every other
+    // CLI failure path.
+    expect(JSON.stringify(invalid.error)).toContain("code");
+  });
+});
diff --git a/src/hopper/HopperProvider.ts b/src/hopper/HopperProvider.ts
index 28882dc5d..63f71ab53 100644
--- a/src/hopper/HopperProvider.ts
+++ b/src/hopper/HopperProvider.ts
@@ -84,9 +84,16 @@ const CAPABILITIES: readonly CapabilityDescriptor[] = Object.freeze(
 
 /** Concrete analysis provider backed by REA's private Hopper bridge. */
 export class HopperProvider implements AnalysisProviderCandidate {
+  /**
+   * Host platform, injected so availability and launch-mode decisions can be
+   * exercised for any host rather than only the machine running the suite.
+   * Mirrors `GhidraProvider`, which already threads its installation host's
+   * platform the same way.
+   */
   constructor(
     private readonly config: AppConfig,
     private readonly logger: Logger,
+    private readonly platform: NodeJS.Platform = process.platform,
   ) {}
 
   identity(): ProviderIdentity {
@@ -100,13 +107,13 @@ export class HopperProvider implements AnalysisProviderCandidate {
   inspectAvailability(): ProviderAvailability {
     const diagnostics = {
       launcher_path: this.config.hopperLauncherPath,
-      platform: process.platform,
+      platform: this.platform,
     };
-    if (process.platform !== "darwin" && process.platform !== "linux")
+    if (this.platform !== "darwin" && this.platform !== "linux")
       return {
         status: "unavailable",
         code: "unsupported_host",
-        reason: `Hopper integration is not supported on ${process.platform}.`,
+        reason: `Hopper integration is not supported on ${this.platform}.`,
         diagnostics,
       };
     try {
@@ -205,7 +212,7 @@ export class HopperProvider implements AnalysisProviderCandidate {
         bridgeScriptPath: fileURLToPath(
           new URL("../../bridge/hopper_bridge.py", import.meta.url),
         ),
-        ...(process.platform === "linux"
+        ...(this.platform === "linux"
           ? {
               launchMode: "verified_linux_demo" as const,
               demoHelperPath: fileURLToPath(