diff --git a/src/format.ts b/src/format.ts
new file mode 100644
index 0000000..04f48a1
--- /dev/null
+++ b/src/format.ts
@@ -0,0 +1,37 @@
+export const DEFAULT_ROW_LIMIT = 100
+export const MAX_ROW_LIMIT = 1000
+
+export const clampRowLimit = (requested?: number): number => {
+    if (requested === undefined) return DEFAULT_ROW_LIMIT
+    return Math.max(1, Math.min(Math.trunc(requested), MAX_ROW_LIMIT))
+}
+
+export const truncateRows = <T>(rows: T[], limit: number): { rows: T[]; truncated: boolean } =>
+    rows.length > limit
+        ? { rows: rows.slice(0, limit), truncated: true }
+        : { rows, truncated: false }
+
+export const normalizeCell = (value: unknown): unknown => {
+    if (typeof value === 'bigint') return value.toString()
+    if (value instanceof Date) return value.toISOString()
+    if (Buffer.isBuffer(value)) return `\\x${value.toString('hex')}`
+    return value
+}
+
+type DbErrorShape = Error & {
+    code?: string | number
+    detail?: string
+    hint?: string
+    sqlMessage?: string
+}
+
+export const formatDbError = (engine: string, error: unknown): string => {
+    if (error instanceof Error) {
+        const e = error as DbErrorShape
+        const code = e.code !== undefined && e.code !== null ? ` ${e.code}` : ''
+        const message = e.sqlMessage ?? e.message
+        const extras = [e.detail, e.hint].filter(Boolean).join(' | ')
+        return `[${engine}${code}] ${message}${extras ? ` (${extras})` : ''}`
+    }
+    return `[${engine}] ${String(error)}`
+}
diff --git a/test/unit/format.test.ts b/test/unit/format.test.ts
new file mode 100644
index 0000000..726f008
--- /dev/null
+++ b/test/unit/format.test.ts
@@ -0,0 +1,80 @@
+import { describe, expect, it } from 'vitest'
+import {
+    clampRowLimit,
+    DEFAULT_ROW_LIMIT,
+    formatDbError,
+    MAX_ROW_LIMIT,
+    normalizeCell,
+    truncateRows
+} from '../../src/format.js'
+
+describe('clampRowLimit', () => {
+    it('defaults when undefined', () => {
+        expect(clampRowLimit(undefined)).toBe(DEFAULT_ROW_LIMIT)
+    })
+
+    it('clamps to max and min and truncates fractions', () => {
+        expect(clampRowLimit(99999)).toBe(MAX_ROW_LIMIT)
+        expect(clampRowLimit(0)).toBe(1)
+        expect(clampRowLimit(-5)).toBe(1)
+        expect(clampRowLimit(10.9)).toBe(10)
+    })
+})
+
+describe('truncateRows', () => {
+    it('passes short arrays through', () => {
+        expect(truncateRows([1, 2], 5)).toEqual({ rows: [1, 2], truncated: false })
+    })
+
+    it('slices long arrays and flags truncation', () => {
+        const { rows, truncated } = truncateRows([1, 2, 3], 2)
+        expect(rows).toEqual([1, 2])
+        expect(truncated).toBe(true)
+    })
+})
+
+describe('normalizeCell', () => {
+    it('converts bigint to string', () => {
+        expect(normalizeCell(123n)).toBe('123')
+    })
+
+    it('converts Date to ISO string', () => {
+        expect(normalizeCell(new Date('2026-01-02T03:04:05Z'))).toBe('2026-01-02T03:04:05.000Z')
+    })
+
+    it('converts Buffer to hex string', () => {
+        expect(normalizeCell(Buffer.from([0xde, 0xad]))).toBe('\\xdead')
+    })
+
+    it('passes primitives and null through', () => {
+        expect(normalizeCell('x')).toBe('x')
+        expect(normalizeCell(5)).toBe(5)
+        expect(normalizeCell(null)).toBeNull()
+    })
+})
+
+describe('formatDbError', () => {
+    it('formats code, detail and hint', () => {
+        const error = Object.assign(new Error('relation "x" does not exist'), {
+            code: '42P01',
+            hint: 'check the table name'
+        })
+        expect(formatDbError('postgres', error)).toBe(
+            '[postgres 42P01] relation "x" does not exist (check the table name)'
+        )
+    })
+
+    it('prefers sqlMessage for mysql errors', () => {
+        const error = Object.assign(new Error('outer'), {
+            code: 'ER_NO_SUCH_TABLE',
+            sqlMessage: "Table 'a.b' doesn't exist"
+        })
+        expect(formatDbError('mysql', error)).toBe(
+            "[mysql ER_NO_SUCH_TABLE] Table 'a.b' doesn't exist"
+        )
+    })
+
+    it('handles non-Error values', () => {
+        expect(formatDbError('mysql', 'boom')).toBe('[mysql] boom')
+    })
+})