diff --git a/canvas/src/components/__tests__/TestConnectionButton.test.tsx b/canvas/src/components/__tests__/TestConnectionButton.test.tsx
index 15f1dd9cf..b4dd0cda0 100644
--- a/canvas/src/components/__tests__/TestConnectionButton.test.tsx
+++ b/canvas/src/components/__tests__/TestConnectionButton.test.tsx
@@ -11,13 +11,21 @@ import { render, screen, fireEvent, cleanup, act } from "@testing-library/react"
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { TestConnectionButton } from "../ui/TestConnectionButton";
 import type { SecretGroup } from "@/types/secrets";
-import { validateSecret } from "@/lib/api/secrets";
+import { validateSecret, ApiError } from "@/lib/api/secrets";
 
 // ─── Mock validateSecret ──────────────────────────────────────────────────────
 // vi.mock is hoisted, so validateSecret (imported above) refers to the mocked
 // namespace value once vi.mock runs. Use vi.mocked() to access it in tests.
 vi.mock("@/lib/api/secrets", () => ({
   validateSecret: vi.fn(),
+  ApiError: class ApiError extends Error {
+    status: number;
+    constructor(status: number, message: string) {
+      super(message);
+      this.name = "ApiError";
+      this.status = status;
+    }
+  },
 }));
 
 // SecretGroup is a string literal type: 'github' | 'anthropic' | 'openrouter' | 'custom'
@@ -102,7 +110,7 @@ describe("TestConnectionButton — state machine", () => {
     expect(screen.getByText("Permission denied")).toBeTruthy();
   });
 
-  it("shows generic error message on unexpected exception", async () => {
+  it("shows a connectivity message on a genuine network exception", async () => {
     vi.mocked(validateSecret).mockRejectedValue(new Error("timeout"));
     render(<TestConnectionButton provider={toGroup("anthropic")} secretValue="sk-..." />);
 
@@ -110,8 +118,23 @@ describe("TestConnectionButton — state machine", () => {
     await act(async () => { /* flush */ });
 
     expect(screen.getByRole("alert")).toBeTruthy();
-    // The error detail is hardcoded to "Connection timed out. Service may be down."
-    expect(document.body.querySelector('[role="alert"]')?.textContent).toMatch(/timed out/i);
+    // A real thrown network error → honest connectivity message (not a
+    // fabricated "service down"); see internal#492.
+    expect(document.body.querySelector('[role="alert"]')?.textContent).toMatch(
+      /could not reach the validation service/i,
+    );
+  });
+
+  it("does not claim a timeout when the validate endpoint 404s (internal#492)", async () => {
+    vi.mocked(validateSecret).mockRejectedValue(new ApiError(404, "Not Found"));
+    render(<TestConnectionButton provider={toGroup("anthropic")} secretValue="sk-..." />);
+
+    fireEvent.click(screen.getByRole("button"));
+    await act(async () => { /* flush */ });
+
+    const alert = document.body.querySelector('[role="alert"]')?.textContent ?? "";
+    expect(alert).not.toMatch(/timed out/i);
+    expect(alert).toMatch(/not available/i);
   });
 });
 
diff --git a/canvas/src/components/ui/TestConnectionButton.tsx b/canvas/src/components/ui/TestConnectionButton.tsx
index 940c06e49..a85714ef8 100644
--- a/canvas/src/components/ui/TestConnectionButton.tsx
+++ b/canvas/src/components/ui/TestConnectionButton.tsx
@@ -2,7 +2,7 @@
 
 import { useState, useCallback, useRef, useEffect } from 'react';
 import type { TestConnectionState, SecretGroup } from '@/types/secrets';
-import { validateSecret } from '@/lib/api/secrets';
+import { validateSecret, ApiError } from '@/lib/api/secrets';
 
 interface TestConnectionButtonProps {
   provider: SecretGroup;
@@ -55,9 +55,23 @@ export function TestConnectionButton({
       }
       onResult?.(result.valid);
       resetTimerRef.current = setTimeout(() => setState('idle'), RESET_DELAYS[nextState]!);
-    } catch {
+    } catch (err) {
+      // Distinguish a real failure shape rather than always claiming a
+      // timeout. A reachable server that answered with an HTTP status
+      // (ApiError) did NOT time out — most commonly the validation route
+      // is not available (404/501), which must not masquerade as
+      // "service down". Only an actual thrown network/abort error is a
+      // connectivity failure.
       setState('failure');
-      setErrorDetail('Connection timed out. Service may be down.');
+      if (err instanceof ApiError) {
+        setErrorDetail(
+          err.status === 404 || err.status === 501
+            ? 'Key validation is not available for this service yet. The key was not tested.'
+            : `Could not verify key (server returned ${err.status}). Saving is unaffected.`,
+        );
+      } else {
+        setErrorDetail('Could not reach the validation service. Check your connection and try again.');
+      }
       onResult?.(false);
       resetTimerRef.current = setTimeout(() => setState('idle'), RESET_DELAYS.failure);
     }
diff --git a/canvas/src/components/ui/__tests__/TestConnectionButton.test.tsx b/canvas/src/components/ui/__tests__/TestConnectionButton.test.tsx
index 62d61ff11..21114ef1d 100644
--- a/canvas/src/components/ui/__tests__/TestConnectionButton.test.tsx
+++ b/canvas/src/components/ui/__tests__/TestConnectionButton.test.tsx
@@ -28,8 +28,20 @@ const mockValidateSecret = vi.fn();
 
 vi.mock("@/lib/api/secrets", () => ({
   validateSecret: (...args: unknown[]) => mockValidateSecret(...args),
+  ApiError: class ApiError extends Error {
+    status: number;
+    constructor(status: number, message: string) {
+      super(message);
+      this.name = "ApiError";
+      this.status = status;
+    }
+  },
 }));
 
+// Re-import the mocked ApiError so test cases construct the same class the
+// component's `instanceof` check sees.
+import { ApiError } from "@/lib/api/secrets";
+
 beforeEach(() => {
   vi.useFakeTimers();
   vi.clearAllMocks();
@@ -201,8 +213,27 @@ describe("TestConnectionButton — failure path", () => {
 });
 
 describe("TestConnectionButton — catch path", () => {
-  it("shows 'Connection timed out' on network error", async () => {
-    mockValidateSecret.mockRejectedValue(new Error("timeout"));
+  it("does NOT claim a timeout when the validate endpoint 404s (regression: internal#492)", async () => {
+    // The validate route is unimplemented on the server and returns a fast
+    // 404. Before the fix this rendered the misleading hardcoded string
+    // "Connection timed out. Service may be down." It must instead state
+    // honestly that validation isn't available and the key was not tested.
+    mockValidateSecret.mockRejectedValue(new ApiError(404, "Not Found"));
+    render(
+      <TestConnectionButton provider="anthropic" secretValue="sk-ant-xxx" />,
+    );
+    fireEvent.click(document.querySelector('button[type="button"]')!);
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(0);
+    });
+    expect(document.body.textContent).not.toContain("Connection timed out");
+    expect(document.body.textContent).not.toContain("Service may be down");
+    expect(document.body.textContent).toContain("not available");
+    expect(document.body.textContent).toContain("not tested");
+  });
+
+  it("reports a non-404 server error with its status, not a timeout", async () => {
+    mockValidateSecret.mockRejectedValue(new ApiError(500, "Internal Server Error"));
     render(
       <TestConnectionButton provider="github" secretValue="ghp_xxx" />,
     );
@@ -210,7 +241,20 @@ describe("TestConnectionButton — catch path", () => {
     await act(async () => {
       await vi.advanceTimersByTimeAsync(0);
     });
-    expect(document.body.textContent).toContain("Connection timed out");
+    expect(document.body.textContent).toContain("500");
+    expect(document.body.textContent).not.toContain("Connection timed out");
+  });
+
+  it("shows a connectivity message on a genuine network error", async () => {
+    mockValidateSecret.mockRejectedValue(new Error("network down"));
+    render(
+      <TestConnectionButton provider="github" secretValue="ghp_xxx" />,
+    );
+    fireEvent.click(document.querySelector('button[type="button"]')!);
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(0);
+    });
+    expect(document.body.textContent).toContain("Could not reach the validation service");
   });
 
   it("calls onResult(false) on network error", async () => {