diff --git a/packages/engine/src/env.ts b/packages/engine/src/env.ts
index 9573a03..32e9f59 100644
--- a/packages/engine/src/env.ts
+++ b/packages/engine/src/env.ts
@@ -59,19 +59,40 @@ export function detectProxy(base: Record<string, string | undefined>): string |
   for (const name of PROXY_VARS) {
     for (const key of [name, name.toLowerCase()]) {
       const value = base[key];
-      if (typeof value === "string" && value.trim() !== "") return proxyHost(value.trim());
+      if (typeof value === "string" && value.trim() !== "") return safeHost(value.trim());
     }
   }
   return null;
 }
 
-/** Reduce a proxy URL to `host[:port]`, dropping any embedded credentials/path. */
-function proxyHost(raw: string): string {
+/**
+ * Detect an intentional custom Anthropic endpoint (`ANTHROPIC_BASE_URL`) in the
+ * base environment (#174, sibling of the #145 proxy notice). Like a proxy, it is
+ * NOT stripped — [`sanitizeChildEnv`] deliberately preserves it and its
+ * credentials as a user's explicit choice — but it silently reroutes ALL Claude
+ * CLI transcript traffic to an arbitrary host, so the transcript must never
+ * transit it *silently*: the host surfaces a content-free notice when this
+ * returns non-null.
+ *
+ * Returns a SAFE host label (`host` or `host:port`), or null when unset/blank.
+ * The full URL is never returned or logged — it can embed credentials and a
+ * path/query — and is reduced to host[:port] via the shared [`safeHost`].
+ */
+export function detectCustomEndpoint(base: Record<string, string | undefined>): string | null {
+  const value = base.ANTHROPIC_BASE_URL;
+  if (typeof value === "string" && value.trim() !== "") return safeHost(value.trim());
+  return null;
+}
+
+/** Reduce a proxy / endpoint URL to `host[:port]`, dropping any embedded
+ *  credentials and path/query. Shared by [`detectProxy`] and
+ *  [`detectCustomEndpoint`] so both notices redact identically. */
+function safeHost(raw: string): string {
   try {
     const url = new URL(raw.includes("://") ? raw : `http://${raw}`);
     return url.port ? `${url.hostname}:${url.port}` : url.hostname;
   } catch {
-    // Unparseable value: acknowledge a proxy is set without echoing the raw string.
+    // Unparseable value: acknowledge the reroute is set without echoing the raw string.
     return "(set)";
   }
 }
diff --git a/packages/engine/src/index.ts b/packages/engine/src/index.ts
index 45dde6f..e32944f 100644
--- a/packages/engine/src/index.ts
+++ b/packages/engine/src/index.ts
@@ -18,7 +18,7 @@ export type {
 } from "./types";
 
 export { StreamJsonParser } from "./stream-parser";
-export { sanitizeChildEnv, detectProxy } from "./env";
+export { sanitizeChildEnv, detectProxy, detectCustomEndpoint } from "./env";
 export { buildClaudeArgs, ISOLATION_ARGS, DEFAULT_MODEL } from "./args";
 export type { ClaudeArgsOptions } from "./args";
 export {
diff --git a/packages/engine/test/env.test.ts b/packages/engine/test/env.test.ts
index bbaff7b..dbbb25c 100644
--- a/packages/engine/test/env.test.ts
+++ b/packages/engine/test/env.test.ts
@@ -1,6 +1,6 @@
 import { describe, it, expect } from "vitest";
 
-import { sanitizeChildEnv, detectProxy } from "../src/env";
+import { sanitizeChildEnv, detectProxy, detectCustomEndpoint } from "../src/env";
 
 describe("sanitizeChildEnv", () => {
   it("strips ANTHROPIC_API_KEY and ANTHROPIC_AUTH_TOKEN by default", () => {
@@ -157,3 +157,42 @@ describe("detectProxy (#145)", () => {
     expect(detectProxy({ HTTPS_PROXY: "://:::bogus" })).toBe("(set)");
   });
 });
+
+describe("detectCustomEndpoint (#174)", () => {
+  it("returns null when ANTHROPIC_BASE_URL is unset", () => {
+    expect(detectCustomEndpoint({ PATH: "/usr/bin" })).toBeNull();
+  });
+
+  it("returns null for an empty/whitespace value", () => {
+    expect(detectCustomEndpoint({ ANTHROPIC_BASE_URL: "" })).toBeNull();
+    expect(detectCustomEndpoint({ ANTHROPIC_BASE_URL: "   " })).toBeNull();
+  });
+
+  it("surfaces the host[:port] of a set custom endpoint", () => {
+    expect(detectCustomEndpoint({ ANTHROPIC_BASE_URL: "https://gateway.example:8443" })).toBe(
+      "gateway.example:8443",
+    );
+    expect(detectCustomEndpoint({ ANTHROPIC_BASE_URL: "https://relay.corp" })).toBe("relay.corp");
+  });
+
+  it("redacts the path/query and any embedded credentials — host[:port] only", () => {
+    const host = detectCustomEndpoint({
+      ANTHROPIC_BASE_URL: "https://user:s3cret@gateway.example:8443/v1/messages?key=abc",
+    });
+    expect(host).toBe("gateway.example:8443");
+    expect(host).not.toContain("s3cret");
+    expect(host).not.toContain("user");
+    expect(host).not.toContain("v1");
+    expect(host).not.toContain("abc");
+  });
+
+  it("accepts a scheme-less host:port value", () => {
+    expect(detectCustomEndpoint({ ANTHROPIC_BASE_URL: "gateway.example:8443" })).toBe(
+      "gateway.example:8443",
+    );
+  });
+
+  it("does not leak the raw value for an unparseable base URL", () => {
+    expect(detectCustomEndpoint({ ANTHROPIC_BASE_URL: "://:::bogus" })).toBe("(set)");
+  });
+});
diff --git a/src/host/session.ts b/src/host/session.ts
index dcdc25a..faf6756 100644
--- a/src/host/session.ts
+++ b/src/host/session.ts
@@ -12,6 +12,7 @@ import {
   computeMeetingMetrics,
   CreditAccountant,
   detectProxy,
+  detectCustomEndpoint,
   ExtrasBudget,
   ExtrasBudgetExceededError,
   ExtrasPipeline,
@@ -151,6 +152,19 @@ export function meterEngines(
   }
 }
 
+/** The content-free custom-endpoint notice (#174) for a base environment, or
+ *  null when no `ANTHROPIC_BASE_URL` is set. A set base URL silently reroutes all
+ *  CLI transcript traffic to an arbitrary host (its credentials preserved by
+ *  #145), so the CLI path surfaces this ONCE per session — carrying ONLY the safe
+ *  host (`detectCustomEndpoint` drops the URL path/query and any credentials),
+ *  mirroring the #145 proxy notice. Pure over the env so the exact status string
+ *  and its redaction are unit-testable — `HostSession.start` spawns real CLI
+ *  children and has no headless harness. */
+export function customEndpointNotice(env: Record<string, string | undefined>): string | null {
+  const host = detectCustomEndpoint(env);
+  return host === null ? null : `translation traffic is routing to a custom Anthropic endpoint: ${host}`;
+}
+
 export class HostSession {
   /** Routers for the two CLI lanes (null on the local-only path). A credit- or
    *  health-driven fallback switches BOTH to the shared local engine (#142). */
@@ -323,6 +337,12 @@ export class HostSession {
       if (proxyHost) {
         this.emit({ type: "status", detail: `translation traffic is routing through a proxy: ${proxyHost}` });
       }
+      // Custom-endpoint discipline (#174, sibling of #145): surface a set
+      // ANTHROPIC_BASE_URL ONCE per session so its silent reroute is a conscious
+      // choice. The notice string + redaction live in the pure, tested
+      // `customEndpointNotice`; here we only emit it.
+      const endpointNotice = customEndpointNotice(process.env);
+      if (endpointNotice) this.emit({ type: "status", detail: endpointNotice });
     } else {
       if (resolved.enginePref === "cli") {
         this.emit({ type: "status", detail: "no Claude CLI found — using the local model" });
diff --git a/test/custom-endpoint-notice.test.ts b/test/custom-endpoint-notice.test.ts
new file mode 100644
index 0000000..265a1f8
--- /dev/null
+++ b/test/custom-endpoint-notice.test.ts
@@ -0,0 +1,45 @@
+// #174: prove the VISIBLE HostSession notice, not just the detector. `start()`
+// spawns real CLI/llama-server children (no headless harness — see
+// session-accounting.test.ts), so the status string + its redaction are factored
+// into the pure, exported `customEndpointNotice`, which is exactly what the CLI
+// path emits: `if (endpointNotice) emit({ type: "status", detail: endpointNotice })`.
+// These assert the emitted status when ANTHROPIC_BASE_URL is set (redacted to the
+// safe host) and that NO endpoint status is produced when it is unset.
+
+import { describe, expect, it } from "vitest";
+
+import { customEndpointNotice } from "../src/host/session";
+
+describe("customEndpointNotice (#174 visible endpoint status)", () => {
+  it("produces NO endpoint status when ANTHROPIC_BASE_URL is unset", () => {
+    expect(customEndpointNotice({ PATH: "/usr/bin" })).toBeNull();
+  });
+
+  it("produces NO endpoint status for an empty/whitespace value", () => {
+    expect(customEndpointNotice({ ANTHROPIC_BASE_URL: "" })).toBeNull();
+    expect(customEndpointNotice({ ANTHROPIC_BASE_URL: "   " })).toBeNull();
+  });
+
+  it("emits exactly the redacted endpoint status when ANTHROPIC_BASE_URL is set", () => {
+    expect(customEndpointNotice({ ANTHROPIC_BASE_URL: "https://gateway.example:8443" })).toBe(
+      "translation traffic is routing to a custom Anthropic endpoint: gateway.example:8443",
+    );
+    // Bare host when no port is present.
+    expect(customEndpointNotice({ ANTHROPIC_BASE_URL: "https://relay.corp" })).toBe(
+      "translation traffic is routing to a custom Anthropic endpoint: relay.corp",
+    );
+  });
+
+  it("carries ONLY the safe host — credentials, path, and query never reach the status", () => {
+    const notice = customEndpointNotice({
+      ANTHROPIC_BASE_URL: "https://user:s3cret@gateway.example:8443/v1/messages?key=abc",
+    });
+    expect(notice).toBe(
+      "translation traffic is routing to a custom Anthropic endpoint: gateway.example:8443",
+    );
+    expect(notice).not.toContain("s3cret");
+    expect(notice).not.toContain("user");
+    expect(notice).not.toContain("v1");
+    expect(notice).not.toContain("abc");
+  });
+});