diff --git a/src/shared/deep-merge.test.ts b/src/shared/deep-merge.test.ts
new file mode 100644
index 000000000..f78e621c8
--- /dev/null
+++ b/src/shared/deep-merge.test.ts
@@ -0,0 +1,336 @@
+import { describe, expect, test } from "bun:test"
+import { deepMerge, isPlainObject } from "./deep-merge"
+
+type AnyObject = Record<string, unknown>
+
+describe("isPlainObject", () => {
+  test("returns false for null", () => {
+    //#given
+    const value = null
+
+    //#when
+    const result = isPlainObject(value)
+
+    //#then
+    expect(result).toBe(false)
+  })
+
+  test("returns false for undefined", () => {
+    //#given
+    const value = undefined
+
+    //#when
+    const result = isPlainObject(value)
+
+    //#then
+    expect(result).toBe(false)
+  })
+
+  test("returns false for string", () => {
+    //#given
+    const value = "hello"
+
+    //#when
+    const result = isPlainObject(value)
+
+    //#then
+    expect(result).toBe(false)
+  })
+
+  test("returns false for number", () => {
+    //#given
+    const value = 42
+
+    //#when
+    const result = isPlainObject(value)
+
+    //#then
+    expect(result).toBe(false)
+  })
+
+  test("returns false for boolean", () => {
+    //#given
+    const value = true
+
+    //#when
+    const result = isPlainObject(value)
+
+    //#then
+    expect(result).toBe(false)
+  })
+
+  test("returns false for array", () => {
+    //#given
+    const value = [1, 2, 3]
+
+    //#when
+    const result = isPlainObject(value)
+
+    //#then
+    expect(result).toBe(false)
+  })
+
+  test("returns false for Date", () => {
+    //#given
+    const value = new Date()
+
+    //#when
+    const result = isPlainObject(value)
+
+    //#then
+    expect(result).toBe(false)
+  })
+
+  test("returns false for RegExp", () => {
+    //#given
+    const value = /test/
+
+    //#when
+    const result = isPlainObject(value)
+
+    //#then
+    expect(result).toBe(false)
+  })
+
+  test("returns true for plain object", () => {
+    //#given
+    const value = { a: 1 }
+
+    //#when
+    const result = isPlainObject(value)
+
+    //#then
+    expect(result).toBe(true)
+  })
+
+  test("returns true for empty object", () => {
+    //#given
+    const value = {}
+
+    //#when
+    const result = isPlainObject(value)
+
+    //#then
+    expect(result).toBe(true)
+  })
+
+  test("returns true for nested object", () => {
+    //#given
+    const value = { a: { b: 1 } }
+
+    //#when
+    const result = isPlainObject(value)
+
+    //#then
+    expect(result).toBe(true)
+  })
+})
+
+describe("deepMerge", () => {
+  describe("basic merging", () => {
+    test("merges two simple objects", () => {
+      //#given
+      const base: AnyObject = { a: 1 }
+      const override: AnyObject = { b: 2 }
+
+      //#when
+      const result = deepMerge(base, override)
+
+      //#then
+      expect(result).toEqual({ a: 1, b: 2 })
+    })
+
+    test("override value takes precedence", () => {
+      //#given
+      const base = { a: 1 }
+      const override = { a: 2 }
+
+      //#when
+      const result = deepMerge(base, override)
+
+      //#then
+      expect(result).toEqual({ a: 2 })
+    })
+
+    test("deeply merges nested objects", () => {
+      //#given
+      const base: AnyObject = { a: { b: 1, c: 2 } }
+      const override: AnyObject = { a: { b: 10 } }
+
+      //#when
+      const result = deepMerge(base, override)
+
+      //#then
+      expect(result).toEqual({ a: { b: 10, c: 2 } })
+    })
+
+    test("handles multiple levels of nesting", () => {
+      //#given
+      const base: AnyObject = { a: { b: { c: { d: 1 } } } }
+      const override: AnyObject = { a: { b: { c: { e: 2 } } } }
+
+      //#when
+      const result = deepMerge(base, override)
+
+      //#then
+      expect(result).toEqual({ a: { b: { c: { d: 1, e: 2 } } } })
+    })
+  })
+
+  describe("edge cases", () => {
+    test("returns undefined when both are undefined", () => {
+      //#given
+      const base = undefined
+      const override = undefined
+
+      //#when
+      const result = deepMerge<AnyObject>(base, override)
+
+      //#then
+      expect(result).toBeUndefined()
+    })
+
+    test("returns override when base is undefined", () => {
+      //#given
+      const base = undefined
+      const override = { a: 1 }
+
+      //#when
+      const result = deepMerge<AnyObject>(base, override)
+
+      //#then
+      expect(result).toEqual({ a: 1 })
+    })
+
+    test("returns base when override is undefined", () => {
+      //#given
+      const base = { a: 1 }
+      const override = undefined
+
+      //#when
+      const result = deepMerge<AnyObject>(base, override)
+
+      //#then
+      expect(result).toEqual({ a: 1 })
+    })
+
+    test("preserves base value when override value is undefined", () => {
+      //#given
+      const base = { a: 1, b: 2 }
+      const override = { a: undefined, b: 3 }
+
+      //#when
+      const result = deepMerge(base, override)
+
+      //#then
+      expect(result).toEqual({ a: 1, b: 3 })
+    })
+
+    test("does not mutate base object", () => {
+      //#given
+      const base = { a: 1, b: { c: 2 } }
+      const override = { b: { c: 10 } }
+      const originalBase = JSON.parse(JSON.stringify(base))
+
+      //#when
+      deepMerge(base, override)
+
+      //#then
+      expect(base).toEqual(originalBase)
+    })
+  })
+
+  describe("array handling", () => {
+    test("replaces arrays instead of merging them", () => {
+      //#given
+      const base = { arr: [1, 2] }
+      const override = { arr: [3, 4, 5] }
+
+      //#when
+      const result = deepMerge(base, override)
+
+      //#then
+      expect(result).toEqual({ arr: [3, 4, 5] })
+    })
+
+    test("replaces nested arrays", () => {
+      //#given
+      const base = { a: { arr: [1, 2, 3] } }
+      const override = { a: { arr: [4] } }
+
+      //#when
+      const result = deepMerge(base, override)
+
+      //#then
+      expect(result).toEqual({ a: { arr: [4] } })
+    })
+  })
+
+  describe("prototype pollution protection", () => {
+    test("ignores __proto__ key", () => {
+      //#given
+      const base: AnyObject = { a: 1 }
+      const override: AnyObject = JSON.parse('{"__proto__": {"polluted": true}, "b": 2}')
+
+      //#when
+      const result = deepMerge(base, override)
+
+      //#then
+      expect(result).toEqual({ a: 1, b: 2 })
+      expect(({} as AnyObject).polluted).toBeUndefined()
+    })
+
+    test("ignores constructor key", () => {
+      //#given
+      const base: AnyObject = { a: 1 }
+      const override: AnyObject = { constructor: { polluted: true }, b: 2 }
+
+      //#when
+      const result = deepMerge(base, override)
+
+      //#then
+      expect(result!.b).toBe(2)
+      expect(result!["constructor"]).not.toEqual({ polluted: true })
+    })
+
+    test("ignores prototype key", () => {
+      //#given
+      const base: AnyObject = { a: 1 }
+      const override: AnyObject = { prototype: { polluted: true }, b: 2 }
+
+      //#when
+      const result = deepMerge(base, override)
+
+      //#then
+      expect(result!.b).toBe(2)
+      expect(result!.prototype).toBeUndefined()
+    })
+  })
+
+  describe("depth limit", () => {
+    test("returns override when depth exceeds MAX_DEPTH", () => {
+      //#given
+      const createDeepObject = (depth: number, leaf: AnyObject): AnyObject => {
+        if (depth === 0) return leaf
+        return { nested: createDeepObject(depth - 1, leaf) }
+      }
+      // Use different keys to distinguish base vs override
+      const base = createDeepObject(55, { baseKey: "base" })
+      const override = createDeepObject(55, { overrideKey: "override" })
+
+      //#when
+      const result = deepMerge(base, override)
+
+      //#then
+      // Navigate to depth 55 (leaf level, beyond MAX_DEPTH of 50)
+      let current: AnyObject = result as AnyObject
+      for (let i = 0; i < 55; i++) {
+        current = current.nested as AnyObject
+      }
+      // At depth 55, only override's key should exist because
+      // override replaced base entirely at depth 51+ (beyond MAX_DEPTH)
+      expect(current.overrideKey).toBe("override")
+      expect(current.baseKey).toBeUndefined()
+    })
+  })
+})