diff --git a/.sandcastle/context-compressor.test.ts b/.sandcastle/context-compressor.test.ts
new file mode 100644
index 0000000..f01df8e
--- /dev/null
+++ b/.sandcastle/context-compressor.test.ts
@@ -0,0 +1,121 @@
+/**
+ * Unit tests for context-compressor — the headroom-ai wrapper that shrinks
+ * compiled prompts before they reach the agent.
+ *
+ * All tests set / restore process.env.HEADROOM_MODE around each assertion so
+ * they are order-independent (getHeadroomMode() reads the env var at call time,
+ * not at module load time, which is what makes this possible).
+ *
+ * Run: npm test (picks up all *.test.ts files listed in the test script)
+ */
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import {
+  getHeadroomMode,
+  getCompressionCallback,
+  isCompressionActive,
+} from "./context-compressor.ts";
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+function withMode(mode: string | undefined, fn: () => void): void {
+  const orig = process.env.HEADROOM_MODE;
+  if (mode === undefined) {
+    delete process.env.HEADROOM_MODE;
+  } else {
+    process.env.HEADROOM_MODE = mode;
+  }
+  try {
+    fn();
+  } finally {
+    if (orig === undefined) {
+      delete process.env.HEADROOM_MODE;
+    } else {
+      process.env.HEADROOM_MODE = orig;
+    }
+  }
+}
+
+// ---------------------------------------------------------------------------
+// getHeadroomMode
+// ---------------------------------------------------------------------------
+
+test("getHeadroomMode returns 'off' when HEADROOM_MODE is unset", () => {
+  withMode(undefined, () => {
+    assert.equal(getHeadroomMode(), "off");
+  });
+});
+
+test("getHeadroomMode returns 'conservative' when HEADROOM_MODE=conservative", () => {
+  withMode("conservative", () => {
+    assert.equal(getHeadroomMode(), "conservative");
+  });
+});
+
+test("getHeadroomMode returns 'aggressive' when HEADROOM_MODE=aggressive", () => {
+  withMode("aggressive", () => {
+    assert.equal(getHeadroomMode(), "aggressive");
+  });
+});
+
+// ---------------------------------------------------------------------------
+// isCompressionActive
+// ---------------------------------------------------------------------------
+
+test("isCompressionActive returns false when HEADROOM_MODE is unset (default off)", () => {
+  withMode(undefined, () => {
+    assert.equal(isCompressionActive(), false);
+  });
+});
+
+test("isCompressionActive returns true when HEADROOM_MODE=conservative", () => {
+  withMode("conservative", () => {
+    assert.equal(isCompressionActive(), true);
+  });
+});
+
+test("isCompressionActive returns true when HEADROOM_MODE=aggressive", () => {
+  withMode("aggressive", () => {
+    assert.equal(isCompressionActive(), true);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// getCompressionCallback — structural shape tests (no live headroom-ai call)
+// ---------------------------------------------------------------------------
+
+test("getCompressionCallback returns undefined when HEADROOM_MODE is off (default off)", () => {
+  withMode(undefined, () => {
+    assert.equal(getCompressionCallback(), undefined);
+  });
+});
+
+test("getCompressionCallback returns a function when HEADROOM_MODE=conservative", () => {
+  withMode("conservative", () => {
+    const cb = getCompressionCallback();
+    assert.equal(typeof cb, "function");
+  });
+});
+
+test("getCompressionCallback returns a function when HEADROOM_MODE=aggressive", () => {
+  withMode("aggressive", () => {
+    const cb = getCompressionCallback();
+    assert.equal(typeof cb, "function");
+  });
+});
+
+test("getCompressionCallback returns an async function (returns a Promise)", () => {
+  withMode("conservative", () => {
+    const cb = getCompressionCallback();
+    assert.ok(cb !== undefined);
+    // An async function always returns a Promise synchronously, before any
+    // `await` inside it (here, the dynamic `import("headroom-ai")`) resolves
+    // or rejects — so this holds regardless of whether headroom-ai succeeds.
+    const result = cb("test prompt");
+    assert.ok(result instanceof Promise, "callback must return a Promise");
+    // Consume any rejection so node:test doesn't treat it as an unhandled error.
+    result.catch(() => {});
+  });
+});
diff --git a/.sandcastle/context-compressor.ts b/.sandcastle/context-compressor.ts
index 00448ba..c6e9f68 100644
--- a/.sandcastle/context-compressor.ts
+++ b/.sandcastle/context-compressor.ts
@@ -15,24 +15,21 @@
 /** Compression mode — off by default for safe rollout. */
 export type HeadroomMode = "off" | "conservative" | "aggressive";
 
-const HEADROOM_MODE: HeadroomMode =
-  (process.env.HEADROOM_MODE as HeadroomMode) || "off";
-
-const HEADROOM_MODEL = process.env.HEADROOM_MODEL || "claude-sonnet-4-6";
-
-/** Current headroom mode (for tests and logging). */
+/** Current headroom mode — reads env var at call time so tests can set it after module load. */
 export function getHeadroomMode(): HeadroomMode {
-  return HEADROOM_MODE;
+  return (process.env.HEADROOM_MODE as HeadroomMode) || "off";
 }
 
 /** Resolve the compression callback, or undefined if disabled. */
 export function getCompressionCallback() {
-  if (HEADROOM_MODE === "off") return undefined;
+  const mode = getHeadroomMode();
+  if (mode === "off") return undefined;
 
+  const model = process.env.HEADROOM_MODEL || "claude-sonnet-4-6";
   return async (prompt: string): Promise<string> => {
     const { compress } = await import("headroom-ai");
     const messages = [{ role: "user" as const, content: prompt }];
-    const result = await compress(messages, { model: HEADROOM_MODEL });
+    const result = await compress(messages, { model });
     // CompressResult.messages is typed `any[]` (headroom preserves whatever
     // format was passed in) — validate the shape at runtime rather than
     // trusting it, so a proxy-side format drift degrades to uncompressed
@@ -44,11 +41,17 @@ export function getCompressionCallback() {
       );
       return prompt;
     }
+    // Token savings measurement: log before/after char counts (~4 chars/token).
+    const saved = prompt.length - content.length;
+    const pct = ((saved / prompt.length) * 100).toFixed(1);
+    console.log(
+      `[context-compressor] mode=${mode} ${prompt.length}→${content.length} chars (−${saved}, −${pct}%)`,
+    );
     return content;
   };
 }
 
-/** Whether compression is active (for logging). */
+/** Whether compression is active (for proxy injection and logging). */
 export function isCompressionActive(): boolean {
-  return HEADROOM_MODE !== "off";
+  return getHeadroomMode() !== "off";
 }
diff --git a/.sandcastle/package.json b/.sandcastle/package.json
index 316f264..03007f8 100644
--- a/.sandcastle/package.json
+++ b/.sandcastle/package.json
@@ -8,7 +8,7 @@
     "postinstall": "patch-package",
     "start": "tsx main.ts",
     "typecheck": "tsc --noEmit",
-    "test": "node --import tsx --test reduce.test.ts sandbox-runner.test.ts reviewer-adapter.test.ts memory.test.ts memory-store.test.ts run-sh.test.ts init-sh.test.ts up-sh.test.ts afk-cmd.test.ts",
+    "test": "node --import tsx --test reduce.test.ts sandbox-runner.test.ts reviewer-adapter.test.ts memory.test.ts memory-store.test.ts run-sh.test.ts init-sh.test.ts up-sh.test.ts afk-cmd.test.ts context-compressor.test.ts",
     "test:integration": "SANDCASTLE_INTEGRATION=1 node --import tsx --test integration.test.ts"
   },
   "devDependencies": {
diff --git a/.sandcastle/sandbox-runner.test.ts b/.sandcastle/sandbox-runner.test.ts
index 79ce82d..aecdaa5 100644
--- a/.sandcastle/sandbox-runner.test.ts
+++ b/.sandcastle/sandbox-runner.test.ts
@@ -123,3 +123,41 @@ test("claude tier: no sandboxEnv override when HEADROOM_MODE is off (default)",
   const input = buildAgentInput({ tier: "claude" }, STUB_ISSUE);
   assert.equal(input.sandboxEnv, undefined);
 });
+
+test("claude tier agent gets ANTHROPIC_BASE_URL proxy override when HEADROOM_MODE=conservative", () => {
+  const orig = process.env.HEADROOM_MODE;
+  process.env.HEADROOM_MODE = "conservative";
+  try {
+    const input = buildAgentInput({ tier: "claude" }, STUB_ISSUE);
+    assert.deepEqual(input.sandboxEnv, { ANTHROPIC_BASE_URL: "http://host.docker.internal:8787" });
+  } finally {
+    if (orig === undefined) delete process.env.HEADROOM_MODE;
+    else process.env.HEADROOM_MODE = orig;
+  }
+});
+
+test("claude tier agent gets ANTHROPIC_BASE_URL proxy override when HEADROOM_MODE=aggressive", () => {
+  const orig = process.env.HEADROOM_MODE;
+  process.env.HEADROOM_MODE = "aggressive";
+  try {
+    const input = buildAgentInput({ tier: "claude" }, STUB_ISSUE);
+    assert.deepEqual(input.sandboxEnv, { ANTHROPIC_BASE_URL: "http://host.docker.internal:8787" });
+  } finally {
+    if (orig === undefined) delete process.env.HEADROOM_MODE;
+    else process.env.HEADROOM_MODE = orig;
+  }
+});
+
+test("local tier is never proxy-routed regardless of HEADROOM_MODE", () => {
+  const orig = process.env.HEADROOM_MODE;
+  process.env.HEADROOM_MODE = "conservative";
+  try {
+    // local tier uses opencode; ANTHROPIC_BASE_URL proxy injection only
+    // applies to the claudeCode agent path (sandbox-runner.ts L225 tier gate).
+    const input = buildAgentInput({ tier: "local" }, STUB_ISSUE);
+    assert.equal(input.agent.name, "opencode");
+  } finally {
+    if (orig === undefined) delete process.env.HEADROOM_MODE;
+    else process.env.HEADROOM_MODE = orig;
+  }
+});