diff --git a/src/app.ts b/src/app.ts
index 91d2a30248..b3d6af2043 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -26,7 +26,12 @@ import { listCommand as teamListCommand } from "./commands/team/list.js";
 import { traceRoute } from "./commands/trace/index.js";
 import { listCommand as traceListCommand } from "./commands/trace/list.js";
 import { CLI_VERSION } from "./lib/constants.js";
-import { AuthError, CliError, getExitCode } from "./lib/errors.js";
+import {
+  AuthError,
+  CliError,
+  getExitCode,
+  stringifyUnknown,
+} from "./lib/errors.js";
 import { error as errorColor } from "./lib/formatters/colors.js";
 
 /** Top-level route map containing all CLI commands */
@@ -88,7 +93,7 @@ const customText: ApplicationText = {
     if (exc instanceof Error) {
       return `Unexpected error: ${exc.stack ?? exc.message}`;
     }
-    return `Unexpected error: ${String(exc)}`;
+    return `Unexpected error: ${stringifyUnknown(exc)}`;
   },
 };
 
diff --git a/src/commands/auth/status.ts b/src/commands/auth/status.ts
index 396c93ff6e..6cbd945fba 100644
--- a/src/commands/auth/status.ts
+++ b/src/commands/auth/status.ts
@@ -18,7 +18,7 @@ import {
 } from "../../lib/db/defaults.js";
 import { getDbPath } from "../../lib/db/index.js";
 import { getUserInfo } from "../../lib/db/user.js";
-import { AuthError } from "../../lib/errors.js";
+import { AuthError, stringifyUnknown } from "../../lib/errors.js";
 import { error, muted, success } from "../../lib/formatters/colors.js";
 import {
   formatExpiration,
@@ -112,7 +112,7 @@ async function verifyCredentials(
       stdout.write(`  ... and ${orgs.length - maxDisplay} more\n`);
     }
   } catch (err) {
-    const message = err instanceof Error ? err.message : String(err);
+    const message = stringifyUnknown(err);
     stderr.write(`\n${error("✗")} Could not verify credentials: ${message}\n`);
   }
 }
diff --git a/src/commands/log/list.ts b/src/commands/log/list.ts
index 17b6156289..b9fb5054f2 100644
--- a/src/commands/log/list.ts
+++ b/src/commands/log/list.ts
@@ -11,7 +11,7 @@ import type { SentryContext } from "../../context.js";
 import { findProjectsBySlug, listLogs } from "../../lib/api-client.js";
 import { parseOrgProjectArg } from "../../lib/arg-parsing.js";
 import { buildCommand } from "../../lib/command.js";
-import { AuthError, ContextError } from "../../lib/errors.js";
+import { AuthError, ContextError, stringifyUnknown } from "../../lib/errors.js";
 import {
   formatLogRow,
   formatLogsHeader,
@@ -226,7 +226,7 @@ async function executeFollowMode(options: FollowModeOptions): Promise<void> {
       Sentry.captureException(error);
 
       // Always write to stderr (doesn't interfere with JSON on stdout)
-      const message = error instanceof Error ? error.message : String(error);
+      const message = stringifyUnknown(error);
       stderr.write(`Error fetching logs: ${message}\n`);
       // Continue polling on transient errors (network, etc.)
     }
diff --git a/src/lib/api-client.ts b/src/lib/api-client.ts
index 3e095fcafb..0aec7b3ebc 100644
--- a/src/lib/api-client.ts
+++ b/src/lib/api-client.ts
@@ -48,7 +48,7 @@ import {
 } from "../types/index.js";
 
 import type { AutofixResponse, AutofixState } from "../types/seer.js";
-import { ApiError, AuthError } from "./errors.js";
+import { ApiError, AuthError, stringifyUnknown } from "./errors.js";
 import { resolveOrgRegion } from "./region.js";
 import {
   getApiBaseUrl,
@@ -84,8 +84,8 @@ function throwApiError(
   const status = response?.status ?? 0;
   const detail =
     error && typeof error === "object" && "detail" in error
-      ? String((error as { detail: unknown }).detail)
-      : String(error);
+      ? stringifyUnknown((error as { detail: unknown }).detail)
+      : stringifyUnknown(error);
   throw new ApiError(
     `${context}: ${status} ${response?.statusText ?? "Unknown"}`,
     status,
diff --git a/src/lib/binary.ts b/src/lib/binary.ts
index 4a899491e7..df4edafeae 100644
--- a/src/lib/binary.ts
+++ b/src/lib/binary.ts
@@ -15,7 +15,7 @@ import {
 import { chmod, mkdir, unlink } from "node:fs/promises";
 import { delimiter, join, resolve } from "node:path";
 import { getUserAgent } from "./constants.js";
-import { UpgradeError } from "./errors.js";
+import { stringifyUnknown, UpgradeError } from "./errors.js";
 
 /** Known directories where the curl installer may place the binary */
 export const KNOWN_CURL_DIRS = [".local/bin", "bin", ".sentry/bin"];
@@ -143,7 +143,7 @@ export async function fetchWithUpgradeError(
     if (error instanceof Error && error.name === "AbortError") {
       throw error;
     }
-    const msg = error instanceof Error ? error.message : String(error);
+    const msg = stringifyUnknown(error);
     throw new UpgradeError(
       "network_error",
       `Failed to connect to ${serviceName}: ${msg}`
diff --git a/src/lib/db/schema.ts b/src/lib/db/schema.ts
index 32d5000a79..ba833c7560 100644
--- a/src/lib/db/schema.ts
+++ b/src/lib/db/schema.ts
@@ -12,6 +12,7 @@
  */
 
 import type { Database } from "bun:sqlite";
+import { stringifyUnknown } from "../errors.js";
 
 export const CURRENT_SCHEMA_VERSION = 5;
 
@@ -391,7 +392,7 @@ function repairMissingTables(db: Database, result: RepairResult): void {
       db.exec(ddl);
       result.fixed.push(`Created table ${tableName}`);
     } catch (e) {
-      const msg = e instanceof Error ? e.message : String(e);
+      const msg = stringifyUnknown(e);
       result.failed.push(`Failed to create table ${tableName}: ${msg}`);
     }
   }
@@ -410,7 +411,7 @@ function repairMissingColumns(db: Database, result: RepairResult): void {
         db.exec(`ALTER TABLE ${tableName} ADD COLUMN ${col.name} ${col.type}`);
         result.fixed.push(`Added column ${tableName}.${col.name}`);
       } catch (e) {
-        const msg = e instanceof Error ? e.message : String(e);
+        const msg = stringifyUnknown(e);
         result.failed.push(
           `Failed to add column ${tableName}.${col.name}: ${msg}`
         );
diff --git a/src/lib/errors.ts b/src/lib/errors.ts
index e9b1c8a8c9..de3d61f2ff 100644
--- a/src/lib/errors.ts
+++ b/src/lib/errors.ts
@@ -311,9 +311,39 @@ export class SeerError extends CliError {
 
 // Error Utilities
 
+/**
+ * Convert an unknown value to a human-readable string.
+ *
+ * Handles Error instances (`.message`), plain objects (`JSON.stringify`),
+ * strings (as-is), and other primitives (`String()`).
+ * Use this instead of bare `String(value)` when the value might be a
+ * plain object — `String({})` produces the unhelpful `"[object Object]"`.
+ *
+ * @param value - Any thrown or unknown value
+ * @returns Human-readable string representation
+ */
+export function stringifyUnknown(value: unknown): string {
+  if (typeof value === "string") {
+    return value;
+  }
+  if (value instanceof Error) {
+    return value.message;
+  }
+  if (value && typeof value === "object") {
+    // JSON.stringify can throw on circular references or BigInt values.
+    // Fall back to String() which is always safe.
+    try {
+      return JSON.stringify(value);
+    } catch {
+      return String(value);
+    }
+  }
+  return String(value);
+}
+
 /**
  * Format any error for user display.
- * Uses CliError.format() for CLI errors, message for standard errors.
+ * Uses CliError.format() for CLI errors, falls back to stringifyUnknown.
  *
  * @param error - Any thrown value
  * @returns Formatted error string
@@ -322,10 +352,7 @@ export function formatError(error: unknown): string {
   if (error instanceof CliError) {
     return error.format();
   }
-  if (error instanceof Error) {
-    return error.message;
-  }
-  return String(error);
+  return stringifyUnknown(error);
 }
 
 /**
diff --git a/test/lib/binary.test.ts b/test/lib/binary.test.ts
index 8d884d3ec5..0c9c056d4f 100644
--- a/test/lib/binary.test.ts
+++ b/test/lib/binary.test.ts
@@ -227,7 +227,7 @@ describe("fetchWithUpgradeError", () => {
   test("wraps non-Error thrown values as UpgradeError", async () => {
     globalThis.fetch = (async () => {
       // biome-ignore lint/style/useThrowOnlyError: intentionally testing non-Error throw
-      throw { toString: () => "custom thrown value" };
+      throw { code: "ECONNRESET", reason: "connection reset" };
     }) as typeof globalThis.fetch;
 
     try {
@@ -235,7 +235,7 @@ describe("fetchWithUpgradeError", () => {
       expect.unreachable("Should have thrown");
     } catch (error) {
       expect(error).toBeInstanceOf(UpgradeError);
-      expect((error as UpgradeError).message).toContain("custom thrown value");
+      expect((error as UpgradeError).message).toContain("ECONNRESET");
     }
   });
 });
diff --git a/test/lib/errors.test.ts b/test/lib/errors.test.ts
index 8f13e515d0..0bb9675834 100644
--- a/test/lib/errors.test.ts
+++ b/test/lib/errors.test.ts
@@ -9,6 +9,7 @@ import {
   formatError,
   getExitCode,
   SeerError,
+  stringifyUnknown,
   UpgradeError,
   ValidationError,
 } from "../../src/lib/errors.js";
@@ -253,6 +254,60 @@ describe("SeerError", () => {
   });
 });
 
+describe("stringifyUnknown", () => {
+  test("returns strings as-is", () => {
+    expect(stringifyUnknown("hello")).toBe("hello");
+    expect(stringifyUnknown("")).toBe("");
+  });
+
+  test("extracts message from Error instances", () => {
+    expect(stringifyUnknown(new Error("something broke"))).toBe(
+      "something broke"
+    );
+    expect(stringifyUnknown(new TypeError("bad type"))).toBe("bad type");
+  });
+
+  test("serializes plain objects to JSON", () => {
+    expect(stringifyUnknown({ code: "not_found" })).toBe(
+      '{"code":"not_found"}'
+    );
+    expect(stringifyUnknown({ detail: { message: "Forbidden" } })).toBe(
+      '{"detail":{"message":"Forbidden"}}'
+    );
+  });
+
+  test("serializes empty objects", () => {
+    expect(stringifyUnknown({})).toBe("{}");
+  });
+
+  test("serializes arrays to JSON", () => {
+    expect(stringifyUnknown(["error1", "error2"])).toBe('["error1","error2"]');
+  });
+
+  test("converts primitives via String()", () => {
+    expect(stringifyUnknown(42)).toBe("42");
+    expect(stringifyUnknown(null)).toBe("null");
+    expect(stringifyUnknown(undefined)).toBe("undefined");
+    expect(stringifyUnknown(true)).toBe("true");
+    expect(stringifyUnknown(0)).toBe("0");
+  });
+
+  test("falls back to String() for circular references", () => {
+    const circular: Record<string, unknown> = { name: "loop" };
+    circular.self = circular;
+    // Should not throw — falls back to String() which returns [object Object]
+    expect(() => stringifyUnknown(circular)).not.toThrow();
+    expect(stringifyUnknown(circular)).toBe("[object Object]");
+  });
+
+  test("falls back to String() for BigInt values", () => {
+    const obj = { count: BigInt(42) };
+    // JSON.stringify throws on BigInt — should fall back gracefully
+    expect(() => stringifyUnknown(obj)).not.toThrow();
+    expect(stringifyUnknown(obj)).toBe("[object Object]");
+  });
+});
+
 describe("formatError", () => {
   test("uses format() for CliError subclasses", () => {
     const err = new ApiError("API failed", 500, "Server error");
@@ -270,6 +325,11 @@ describe("formatError", () => {
     expect(formatError(null)).toBe("null");
     expect(formatError(undefined)).toBe("undefined");
   });
+
+  test("serializes plain objects instead of [object Object]", () => {
+    expect(formatError({ code: "not_found" })).toBe('{"code":"not_found"}');
+    expect(formatError({})).toBe("{}");
+  });
 });
 
 describe("getExitCode", () => {