diff --git a/.changeset/add-addnote-method.md b/.changeset/add-addnote-method.md
new file mode 100644
index 0000000..3bfbab7
--- /dev/null
+++ b/.changeset/add-addnote-method.md
@@ -0,0 +1,5 @@
+---
+"@deessejs/errors": minor
+---
+
+Add `ErrorInstance.addNote(note)` for attaching runtime context to errors (PEP 678, mirrors Python 3.11). Returns the instance for chaining. The `notes: string[]` property was already implemented; the method was missing despite being documented. Closes #29.
diff --git a/packages/errors/src/error/error.ts b/packages/errors/src/error/error.ts
index 45abf37..9f420a8 100644
--- a/packages/errors/src/error/error.ts
+++ b/packages/errors/src/error/error.ts
@@ -119,6 +119,12 @@ export const error = <const T extends Record<string, unknown> = Record<string, n
       return instance;
     };
 
+    // Add .addNote() method for runtime context (PEP 678)
+    instance.addNote = (note: string): ErrorInstance<T> => {
+      instance.notes.push(note);
+      return instance;
+    };
+
     // Mark this instance as created by this factory (for is() checks)
     // Use callable to avoid generic parameter conflicts
     (instance as unknown as Record<typeof FACTORY_SYMBOL, () => unknown>)[FACTORY_SYMBOL] =
diff --git a/packages/errors/src/error/types.ts b/packages/errors/src/error/types.ts
index d024dd0..41ab0f6 100644
--- a/packages/errors/src/error/types.ts
+++ b/packages/errors/src/error/types.ts
@@ -36,16 +36,29 @@ export type ErrorFactory<TFields extends Record<string, unknown> = Record<string
 /**
  * Error instance returned by an ErrorFactory.
  * Contains all standard Error properties plus additional domain-specific fields.
- *
- * Note: .addNote() is implemented in a separate task.
  */
 export type ErrorInstance<TFields extends Record<string, unknown> = Record<string, never>> =
   ErrorInstanceCore & {
     /** User-defined fields from Standard Schema */
     fields: TFields;
-    // TODO: Implement .addNote() method (Task XX)
     /** Additional notes added via .addNote() */
     notes: string[];
+    /**
+     * Adds a note to this error instance.
+     *
+     * Notes provide runtime context that complements the structured fields.
+     * Patterned after Python 3.11's `BaseException.add_note()` (PEP 678).
+     *
+     * @param note - The note text to attach
+     * @returns This error instance for chaining
+     *
+     * @example
+     * ```typescript
+     * const err = AppError().addNote('Attempt 1 failed').addNote('Retrying...');
+     * // err.notes === ['Attempt 1 failed', 'Retrying...']
+     * ```
+     */
+    addNote(note: string): ErrorInstance<TFields>;
     /**
      * Chains a cause error to this error.
      *
diff --git a/packages/errors/tests/error.test.ts b/packages/errors/tests/error.test.ts
index 864c921..c885ea5 100644
--- a/packages/errors/tests/error.test.ts
+++ b/packages/errors/tests/error.test.ts
@@ -86,6 +86,46 @@ describe('error() factory function', () => {
     });
   });
 
+  describe('.addNote()', () => {
+    it('should attach a single note to a fresh instance', () => {
+      const AppError = error({ name: 'AppError' });
+      const instance = AppError();
+
+      const returned = instance.addNote('first attempt failed');
+
+      expect(returned).toBe(instance);
+      expect(instance.notes).toEqual(['first attempt failed']);
+    });
+
+    it('should append multiple notes in order', () => {
+      const AppError = error({ name: 'AppError' });
+      const instance = AppError()
+        .addNote('Attempt 1 failed')
+        .addNote('Retrying...')
+        .addNote('Attempt 2 failed');
+
+      expect(instance.notes).toEqual(['Attempt 1 failed', 'Retrying...', 'Attempt 2 failed']);
+    });
+
+    it('should preserve notes through .from() chaining', () => {
+      const AppError = error({ name: 'AppError' });
+      const cause = new Error('underlying failure');
+      const instance = AppError().addNote('context A').from(cause).addNote('context B');
+
+      expect(instance.notes).toEqual(['context A', 'context B']);
+      expect(instance.cause).toBe(cause);
+    });
+
+    it('should isolate notes between sibling instances', () => {
+      const AppError = error({ name: 'AppError' });
+      const a = AppError().addNote('only on a');
+      const b = AppError();
+
+      expect(a.notes).toEqual(['only on a']);
+      expect(b.notes).toEqual([]);
+    });
+  });
+
   describe('inherits option', () => {
     it('should support single inheritance', () => {
       const AppError = error({ name: 'AppError' });