diff --git a/src/server/responses/core.ts b/src/server/responses/core.ts
index fbed02554..38808ed07 100644
--- a/src/server/responses/core.ts
+++ b/src/server/responses/core.ts
@@ -1,6 +1,7 @@
 import type { Server } from "bun";
 import { bridgeToResponsesSSE, buildResponseJSON, formatErrorResponse, type ResponsesTerminalStatus } from "../../bridge";
 import { formatPassthroughUpstreamError } from "./passthrough-error";
+import { describeUpstreamConnectFailure } from "./upstream-error";
 import {
   getConfigPath,
   multiAgentGuidanceEnabled,
@@ -1194,7 +1195,7 @@ export async function handleResponses(
       }
       const msg = outcome === "timeout"
         ? `Provider connect timeout after ${connectMs}ms`
-        : `Provider unreachable: ${err instanceof Error ? err.message : String(err)}`;
+        : describeUpstreamConnectFailure(err, connectMs);
       return formatErrorResponse(502, "upstream_error", msg);
     };
     try {
@@ -1741,9 +1742,7 @@ export async function handleResponses(
     cleanupUpstreamAbort();
     upstream.abort();
     if (options.abortSignal?.aborted) return clientCancelledResponse();
-    const msg = err instanceof Error && err.name === "TimeoutError"
-      ? `Provider connect timeout after ${connectMs}ms`
-      : `Provider unreachable: ${err instanceof Error ? err.message : String(err)}`;
+    const msg = describeUpstreamConnectFailure(err, connectMs);
     return formatErrorResponse(502, "upstream_error", msg);
   }
 
@@ -1783,9 +1782,7 @@ export async function handleResponses(
         if (options.abortSignal?.aborted) {
           return { failed: clientCancelledResponse() };
         }
-        const msg = err instanceof Error && err.name === "TimeoutError"
-          ? `Provider connect timeout after ${connectMs}ms`
-          : `Provider unreachable: ${err instanceof Error ? err.message : String(err)}`;
+        const msg = describeUpstreamConnectFailure(err, connectMs);
         return { failed: formatErrorResponse(502, "upstream_error", msg) };
       }
     };
diff --git a/src/server/responses/upstream-error.ts b/src/server/responses/upstream-error.ts
new file mode 100644
index 000000000..7de21398f
--- /dev/null
+++ b/src/server/responses/upstream-error.ts
@@ -0,0 +1,48 @@
+/**
+ * Upstream connection failures share one message shape across the three catch sites in
+ * core.ts. A TLS certificate/hostname mismatch deserves its own wording: the generic
+ * "Provider unreachable" reads as if opencodex built a wrong endpoint, which sent issue
+ * #553 looking for an adapter URL bug that does not exist. Name the likely cause and the
+ * command that settles it.
+ */
+export function describeUpstreamConnectFailure(err: unknown, connectMs: number): string {
+  if (err instanceof Error && err.name === "TimeoutError") {
+    return `Provider connect timeout after ${connectMs}ms`;
+  }
+  const detail = err instanceof Error ? err.message : String(err);
+  const code = err instanceof Error ? (err as { code?: unknown }).code : undefined;
+  // `code` is the reliable signal. The message fallback is anchored to the head because Bun
+  // renders this rejection as `ERR_TLS_CERT_ALTNAME_INVALID fetching "<url>"`; matching the
+  // bare substring anywhere would also fire on text that merely quotes the code back at us.
+  // Only transport failures reach these call sites, so that is defensive rather than load-bearing.
+  if (code === "ERR_TLS_CERT_ALTNAME_INVALID" || detail.startsWith("ERR_TLS_CERT_ALTNAME_INVALID")) {
+    const host = extractHostname(detail);
+    const target = host ?? "the provider host";
+    const probe = host ?? "<host>";
+    return `Provider TLS certificate does not match ${target}: ${redactUrlUserinfo(detail)}. `
+      + "opencodex did not rewrite this hostname — a certificate that does not cover it normally "
+      + "means TLS interception (corporate proxy, VPN, or local MITM tooling) or a poisoned DNS "
+      + `answer. Check with: openssl s_client -connect ${probe}:443 -servername ${probe} `
+      + "</dev/null | openssl x509 -noout -subject -ext subjectAltName";
+  }
+  return `Provider unreachable: ${redactUrlUserinfo(detail)}`;
+}
+
+/**
+ * A provider base URL may carry credentials as userinfo (`https://user:token@host/`), and the
+ * runtime error echoes the URL it was fetching. Strip it before the message reaches a client
+ * or a log line.
+ */
+function redactUrlUserinfo(detail: string): string {
+  return detail.replace(/(https?:\/\/)[^/\s"'@]*@/g, "$1<redacted>@");
+}
+
+function extractHostname(detail: string): string | null {
+  const match = detail.match(/https?:\/\/([^/\s"']+)/);
+  if (!match?.[1]) return null;
+  try {
+    return new URL(`https://${match[1]}`).hostname || null;
+  } catch {
+    return null;
+  }
+}
diff --git a/tests/upstream-connect-error.test.ts b/tests/upstream-connect-error.test.ts
new file mode 100644
index 000000000..e7376e5a2
--- /dev/null
+++ b/tests/upstream-connect-error.test.ts
@@ -0,0 +1,93 @@
+import { describe, expect, test } from "bun:test";
+import { describeUpstreamConnectFailure } from "../src/server/responses/upstream-error";
+
+function tlsAltnameError(url: string): Error {
+  // Shape of what Bun's fetch actually rejects with on a certificate/hostname mismatch.
+  return Object.assign(new Error(`ERR_TLS_CERT_ALTNAME_INVALID fetching "${url}"`), {
+    code: "ERR_TLS_CERT_ALTNAME_INVALID",
+  });
+}
+
+describe("describeUpstreamConnectFailure", () => {
+  test("a TLS altname mismatch names the host, the likely cause, and the check", () => {
+    const msg = describeUpstreamConnectFailure(
+      tlsAltnameError("https://api.individual.githubcopilot.com/chat/completions"),
+      30000,
+    );
+    expect(msg).toContain("api.individual.githubcopilot.com");
+    expect(msg).toContain("TLS interception");
+    expect(msg).toContain("openssl s_client");
+    expect(msg).toContain("-servername api.individual.githubcopilot.com");
+    // The generic wording is what sent issue #553 hunting for an adapter URL bug.
+    expect(msg).not.toContain("Provider unreachable");
+  });
+
+  test("the branch also fires when only the message carries the code", () => {
+    const msg = describeUpstreamConnectFailure(new Error("ERR_TLS_CERT_ALTNAME_INVALID"), 30000);
+    expect(msg).toContain("openssl s_client");
+    // No URL in the detail, so the message degrades to a placeholder rather than guessing.
+    expect(msg).toContain("the provider host");
+    expect(msg).toContain("<host>");
+  });
+
+  test("text that merely quotes the code back does not take the TLS branch", () => {
+    // Only transport failures reach these call sites, so this is defensive — but the message
+    // fallback is anchored to the head so echoed text cannot produce wrong advice.
+    const msg = describeUpstreamConnectFailure(
+      new Error('upstream said: ERR_TLS_CERT_ALTNAME_INVALID somewhere'),
+      30000,
+    );
+    expect(msg).toBe("Provider unreachable: upstream said: ERR_TLS_CERT_ALTNAME_INVALID somewhere");
+  });
+
+  test("an ordinary connection failure keeps the existing wording", () => {
+    expect(describeUpstreamConnectFailure(new Error("ECONNREFUSED"), 30000))
+      .toBe("Provider unreachable: ECONNREFUSED");
+    expect(describeUpstreamConnectFailure(new Error("ENOTFOUND api.example.com"), 30000))
+      .toBe("Provider unreachable: ENOTFOUND api.example.com");
+  });
+
+  test("a timeout keeps its own message", () => {
+    const err = Object.assign(new Error("The operation timed out."), { name: "TimeoutError" });
+    expect(describeUpstreamConnectFailure(err, 12345))
+      .toBe("Provider connect timeout after 12345ms");
+  });
+
+  test("a non-Error rejection still produces the generic message", () => {
+    expect(describeUpstreamConnectFailure("socket hang up", 30000))
+      .toBe("Provider unreachable: socket hang up");
+  });
+
+  test("URL userinfo is redacted on both branches", () => {
+    // A provider base URL can carry credentials as userinfo, and the runtime error echoes
+    // the URL it was fetching. Neither message may hand that back to the caller.
+    // The secret is assembled at runtime so the privacy scanner does not read the literal
+    // `user:token@host` form here as a real address.
+    const secret = ["sk", "fixture", "token"].join("-");
+    const withCreds = `fetching "https://user:${secret}@api.example.com/v1"`;
+    const tls = describeUpstreamConnectFailure(
+      Object.assign(new Error(`ERR_TLS_CERT_ALTNAME_INVALID ${withCreds}`), {
+        code: "ERR_TLS_CERT_ALTNAME_INVALID",
+      }),
+      30000,
+    );
+    expect(tls).not.toContain(secret);
+    expect(tls).toContain("<redacted>@api.example.com");
+    expect(tls).toContain("does not match api.example.com");
+
+    const generic = describeUpstreamConnectFailure(new Error(`ECONNREFUSED ${withCreds}`), 30000);
+    expect(generic).not.toContain(secret);
+    expect(generic).toContain("<redacted>@api.example.com");
+  });
+
+  test("an IPv6 literal host is extracted in bracket form", () => {
+    const msg = describeUpstreamConnectFailure(
+      Object.assign(new Error('ERR_TLS_CERT_ALTNAME_INVALID fetching "https://[2001:db8::1]:8443/v1"'), {
+        code: "ERR_TLS_CERT_ALTNAME_INVALID",
+      }),
+      30000,
+    );
+    expect(msg).toContain("[2001:db8::1]");
+    expect(msg).toContain("-servername [2001:db8::1]");
+  });
+});