diff --git a/.changeset/interaction-allow-custom.md b/.changeset/interaction-allow-custom.md
new file mode 100644
index 0000000..c760198
--- /dev/null
+++ b/.changeset/interaction-allow-custom.md
@@ -0,0 +1,5 @@
+---
+"@tangle-network/agent-interface": minor
+---
+
+Add `allowCustom` to select interaction fields: when set, `validateInteractionAnswer` accepts non-blank write-in values beyond the declared options, and `questionAnswerSpec` propagates the flag from `LegacyQuestion.allowCustom`. Enables "Other…" style questions where the user supplies their own text as the answer value.
diff --git a/packages/agent-interface/src/interaction.test.ts b/packages/agent-interface/src/interaction.test.ts
new file mode 100644
index 0000000..2926b14
--- /dev/null
+++ b/packages/agent-interface/src/interaction.test.ts
@@ -0,0 +1,114 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  InteractionFieldSchema,
+  questionAnswerSpec,
+  validateInteractionAnswer,
+  type InteractionAnswerSpec,
+} from "./interaction.js";
+
+const selectSpec = (overrides?: {
+  allowCustom?: boolean;
+  multi?: boolean;
+}): InteractionAnswerSpec => ({
+  fields: [
+    {
+      type: "select",
+      name: "choice",
+      label: "Pick one",
+      required: true,
+      multi: overrides?.multi,
+      allowCustom: overrides?.allowCustom,
+      options: [
+        { value: "a", label: "A" },
+        { value: "b", label: "B" },
+      ],
+    },
+  ],
+});
+
+describe("select field schema", () => {
+  it("accepts allowCustom on select fields", () => {
+    const parsed = InteractionFieldSchema.parse({
+      type: "select",
+      name: "choice",
+      label: "Pick one",
+      allowCustom: true,
+      options: [{ value: "a", label: "A" }],
+    });
+    expect(parsed).toMatchObject({ type: "select", allowCustom: true });
+  });
+});
+
+describe("validateInteractionAnswer select", () => {
+  it("rejects out-of-options values when allowCustom is unset", () => {
+    const result = validateInteractionAnswer(selectSpec(), { choice: ["write-in"] });
+    expect(result).toEqual({
+      ok: false,
+      errors: ['field "choice" has invalid option "write-in"'],
+    });
+  });
+
+  it("accepts declared options regardless of allowCustom", () => {
+    expect(validateInteractionAnswer(selectSpec(), { choice: ["a"] })).toEqual({ ok: true });
+    expect(
+      validateInteractionAnswer(selectSpec({ allowCustom: true }), { choice: ["b"] }),
+    ).toEqual({ ok: true });
+  });
+
+  it("accepts a write-in value when allowCustom is true", () => {
+    const result = validateInteractionAnswer(selectSpec({ allowCustom: true }), {
+      choice: ["my own answer"],
+    });
+    expect(result).toEqual({ ok: true });
+  });
+
+  it("accepts a mix of declared and write-in values on multi selects", () => {
+    const result = validateInteractionAnswer(
+      selectSpec({ allowCustom: true, multi: true }),
+      { choice: ["a", "something else"] },
+    );
+    expect(result).toEqual({ ok: true });
+  });
+
+  it("rejects blank write-ins even when allowCustom is true", () => {
+    const result = validateInteractionAnswer(selectSpec({ allowCustom: true }), {
+      choice: ["   "],
+    });
+    expect(result).toEqual({
+      ok: false,
+      errors: ['field "choice" has blank write-in value'],
+    });
+  });
+
+  it("still enforces single-value and required rules with allowCustom", () => {
+    expect(
+      validateInteractionAnswer(selectSpec({ allowCustom: true }), {
+        choice: ["x", "y"],
+      }),
+    ).toEqual({ ok: false, errors: ['field "choice" accepts a single value'] });
+    expect(
+      validateInteractionAnswer(selectSpec({ allowCustom: true }), { choice: [] }),
+    ).toEqual({ ok: false, errors: ['field "choice" requires a selection'] });
+  });
+});
+
+describe("questionAnswerSpec", () => {
+  it("propagates allowCustom onto the select field", () => {
+    const spec = questionAnswerSpec([
+      {
+        question: "Which color?",
+        options: [{ label: "red" }, { label: "blue" }],
+        allowCustom: true,
+      },
+    ]);
+    expect(spec.fields[0]).toMatchObject({ type: "select", allowCustom: true });
+  });
+
+  it("omits allowCustom when the question does not opt in", () => {
+    const spec = questionAnswerSpec([
+      { question: "Which color?", options: [{ label: "red" }] },
+    ]);
+    expect(spec.fields[0]).not.toHaveProperty("allowCustom");
+  });
+});
diff --git a/packages/agent-interface/src/interaction.ts b/packages/agent-interface/src/interaction.ts
index a3e565f..c9b1a41 100644
--- a/packages/agent-interface/src/interaction.ts
+++ b/packages/agent-interface/src/interaction.ts
@@ -68,6 +68,12 @@ export const InteractionFieldSchema = z.discriminatedUnion("type", [
       .min(1),
     /** When true the user may pick more than one option. */
     multi: z.boolean().optional(),
+    /**
+     * When true the answer may contain write-in values outside `options`
+     * (e.g. a rendered "Other…" choice carrying the user's own text).
+     * Write-ins must still be non-empty strings.
+     */
+    allowCustom: z.boolean().optional(),
     default: z.array(z.string()).optional(),
   }),
   /** Like `text` but the value is sensitive (token/key) and must be masked. */
@@ -211,6 +217,8 @@ export type LegacyQuestion = {
   question: string;
   options?: Array<{ label: string; description?: string }>;
   multiSelect?: boolean;
+  /** When true the select field accepts write-in answers beyond `options`. */
+  allowCustom?: boolean;
 };
 
 /**
@@ -228,6 +236,7 @@ export function questionAnswerSpec(questions: LegacyQuestion[]): InteractionAnsw
         label: q.question,
         required: true,
         multi: q.multiSelect === true,
+        ...(q.allowCustom === true ? { allowCustom: true } : {}),
         options: q.options.map((o) => ({ value: o.label, label: o.label, description: o.description })),
       };
     }
@@ -284,7 +293,15 @@ export function validateInteractionAnswer(
         if (field.required && v.length === 0) errors.push(`field "${field.name}" requires a selection`);
         const allowed = new Set(field.options.map((o) => o.value));
         for (const choice of v) {
-          if (!allowed.has(choice)) errors.push(`field "${field.name}" has invalid option "${choice}"`);
+          if (allowed.has(choice)) continue;
+          if (field.allowCustom === true) {
+            // Write-ins are open but stay fail-closed on shape: string, non-blank.
+            if (typeof choice !== "string" || choice.trim() === "") {
+              errors.push(`field "${field.name}" has blank write-in value`);
+            }
+            continue;
+          }
+          errors.push(`field "${field.name}" has invalid option "${choice}"`);
         }
         break;
       }