diff --git a/src/gateway/server-methods/agent.ts b/src/gateway/server-methods/agent.ts
index d159d1f78..de7e3098f 100644
--- a/src/gateway/server-methods/agent.ts
+++ b/src/gateway/server-methods/agent.ts
@@ -10,6 +10,7 @@ import {
   updateSessionStore,
 } from "../../config/sessions.js";
 import { registerAgentRunContext } from "../../infra/agent-events.js";
+import { describeNetworkError } from "../../infra/errors.js";
 import {
   resolveAgentDeliveryPlan,
   resolveAgentOutboundTarget,
@@ -398,11 +399,12 @@ export const agentHandlers: GatewayRequestHandlers = {
         respond(true, payload, undefined, { runId });
       })
       .catch((err) => {
-        const error = errorShape(ErrorCodes.UNAVAILABLE, String(err));
+        const summary = describeNetworkError(err);
+        const error = errorShape(ErrorCodes.UNAVAILABLE, summary);
         const payload = {
           runId,
           status: "error" as const,
-          summary: String(err),
+          summary,
         };
         context.dedupe.set(`agent:${idem}`, {
           ts: Date.now(),
diff --git a/src/infra/errors.test.ts b/src/infra/errors.test.ts
new file mode 100644
index 000000000..f1b07cbc8
--- /dev/null
+++ b/src/infra/errors.test.ts
@@ -0,0 +1,91 @@
+import { describe, expect, it } from "vitest";
+import {
+  describeNetworkError,
+  extractErrorCode,
+  formatErrorMessage,
+  formatUncaughtError,
+} from "./errors.js";
+
+describe("extractErrorCode", () => {
+  it("returns string code", () => {
+    expect(extractErrorCode({ code: "ECONNREFUSED" })).toBe("ECONNREFUSED");
+  });
+  it("returns number code as string", () => {
+    expect(extractErrorCode({ code: 42 })).toBe("42");
+  });
+  it("returns undefined for missing code", () => {
+    expect(extractErrorCode({})).toBeUndefined();
+  });
+  it("returns undefined for non-object", () => {
+    expect(extractErrorCode(null)).toBeUndefined();
+    expect(extractErrorCode("str")).toBeUndefined();
+  });
+});
+
+describe("formatErrorMessage", () => {
+  it("extracts Error.message", () => {
+    expect(formatErrorMessage(new Error("boom"))).toBe("boom");
+  });
+  it("returns string as-is", () => {
+    expect(formatErrorMessage("oops")).toBe("oops");
+  });
+  it("stringifies primitives", () => {
+    expect(formatErrorMessage(123)).toBe("123");
+  });
+});
+
+describe("formatUncaughtError", () => {
+  it("returns message for INVALID_CONFIG", () => {
+    const err = Object.assign(new Error("bad config"), { code: "INVALID_CONFIG" });
+    expect(formatUncaughtError(err)).toBe("bad config");
+  });
+  it("returns stack for generic errors", () => {
+    const err = new Error("fail");
+    expect(formatUncaughtError(err)).toContain("fail");
+  });
+});
+
+describe("describeNetworkError", () => {
+  it("returns plain message when no cause", () => {
+    const err = new Error("Connection error.");
+    expect(describeNetworkError(err)).toBe("Connection error.");
+  });
+
+  it("appends cause error code", () => {
+    const cause = Object.assign(new Error("fetch failed"), {
+      code: "UNABLE_TO_VERIFY_LEAF_SIGNATURE",
+    });
+    const err = Object.assign(new Error("Connection error."), { cause });
+    expect(describeNetworkError(err)).toBe("Connection error. (UNABLE_TO_VERIFY_LEAF_SIGNATURE)");
+  });
+
+  it("appends cause message when no code", () => {
+    const cause = new Error("fetch failed");
+    const err = Object.assign(new Error("Connection error."), { cause });
+    expect(describeNetworkError(err)).toBe("Connection error. (fetch failed)");
+  });
+
+  it("prefers code over message", () => {
+    const cause = Object.assign(new Error("connect timeout"), {
+      code: "UND_ERR_CONNECT_TIMEOUT",
+    });
+    const err = Object.assign(new Error("Connection error."), { cause });
+    expect(describeNetworkError(err)).toBe("Connection error. (UND_ERR_CONNECT_TIMEOUT)");
+  });
+
+  it("does not duplicate detail already in message", () => {
+    const cause = new Error("Connection error.");
+    const err = Object.assign(new Error("Connection error."), { cause });
+    expect(describeNetworkError(err)).toBe("Connection error.");
+  });
+
+  it("handles non-Error cause with code", () => {
+    const cause = { code: "ECONNREFUSED" };
+    const err = Object.assign(new Error("Connection error."), { cause });
+    expect(describeNetworkError(err)).toBe("Connection error. (ECONNREFUSED)");
+  });
+
+  it("handles non-error input", () => {
+    expect(describeNetworkError("plain string")).toBe("plain string");
+  });
+});
diff --git a/src/infra/errors.ts b/src/infra/errors.ts
index 9175b3622..fa1aa50b8 100644
--- a/src/infra/errors.ts
+++ b/src/infra/errors.ts
@@ -21,6 +21,18 @@ export function formatErrorMessage(err: unknown): string {
   }
 }
 
+/** Extract diagnostic detail from a network error's cause chain (e.g. APIConnectionError). */
+export function describeNetworkError(err: unknown): string {
+  const msg = formatErrorMessage(err);
+  const cause = (err as { cause?: unknown })?.cause;
+  if (!cause) return msg;
+  const code = extractErrorCode(cause);
+  const causeMsg = cause instanceof Error ? cause.message : undefined;
+  const detail = code ?? causeMsg;
+  if (!detail || msg.includes(detail)) return msg;
+  return `${msg} (${detail})`;
+}
+
 export function formatUncaughtError(err: unknown): string {
   if (extractErrorCode(err) === "INVALID_CONFIG") {
     return formatErrorMessage(err);