@@ -0,0 +1,17 @@
+---
+'@objectstack/formula': minor
+'@objectstack/spec': minor
+---
+
+Expression refusals now carry a stable `code` and typed `params` beside their English `message`, so a localized author surface can render its own words: `validateExpression` (every entry in `errors[]` and `warnings[]`), `collectCelRootIdentifiers` (its `ok: false` arm), `predicateSlotRefusal` and `structuralConditionRefusal` (#20291)
+
+Clause-②: yes
+
+Until now the only thing a refusal said was an English sentence, and a designer running in another locale could only show it verbatim, beside its own translated headings. Each refusal now also names its code — one of a closed, kebab-case set — and the values its sentence interpolates, so a consumer keys a catalogue row to the code and fills it from the params. The `message` is the same sentence, byte for byte; nothing is removed or renamed, so no existing reader changes.
+
+- `@objectstack/formula` exports `EXPRESSION_REFUSAL_CODES` (the closed set as a frozen list) and the types `ExpressionRefusalCode`, `ExpressionRefusalParams` (code → params), `ExpressionRefusal`, `ExprValidationCode`, `CelRootsRefusalCode`, `CelFieldRole`, `ExpressionSourceKind` and `CelRootIdentifiersResult`. `ExprValidationError` gains `code` and `params`.
+- `@objectstack/spec/automation` exports `FLOW_SLOT_REFUSAL_CODES` (the closed set of both flow-slot refusal producers, as a frozen list) and the types `FlowSlotRefusalCode`, `FlowSlotRefusalParams` (code → params), `PredicateSlotRefusalCode`, `PredicateSlotRefusal`, `PredicateSlotValueKind`, `StructuralConditionRefusalCode`, `StructuralConditionRefusal` and `StructuralConditionValueKind`. `predicateSlotRefusal` returns `PredicateSlotRefusal` and `structuralConditionRefusal` returns `StructuralConditionRefusal`; each is its previous `{ message, source }` plus `code` and `params`.
+- Narrowing on `code` narrows `params`. A code never changes once published: a reworded message keeps its code, and a new refusal gets a new one.
+- A `detail` param is the CEL or template engine's own diagnostic, in English, passed through as the message carries it.
+- These are authoring diagnostics returned as values, not ADR-0112 request error codes, which is why they are kebab-case.
+- The `validate_expression` MCP tool forwards `validateExpression`'s `errors` and `warnings` as they are, so each entry in its answer now also carries `code` and `params`.
@@ -20,6 +20,7 @@ import type { Expression } from '@objectstack/spec';
 import { buildScope, registerNumericCoercions, registerStdLib } from './stdlib';
 import type { PermissionBinding } from './stdlib';
 import type { DialectEngine, EvalContext, EvalResult } from './types';
+import type { CelRootsRefusalCode, ExpressionRefusal } from './expression-refusal';
 
 /**
  * Default execution bounds. Picked conservatively — every metadata-authored
@@ -333,12 +334,13 @@ export function firstUndeclaredReference(
  *
  * Returns `{ ok: false }` with the classifier's message when the source does
  * not parse — callers surface that as a config error, not an empty root set.
+ * The failure also carries a refusal `code` and its `params`
+ * (`expression-refusal.ts`), for a consumer that renders its own words
+ * instead of the English `error`.
  */
-export function collectCelRootIdentifiers(
-  source: string,
-): { ok: true; roots: string[] } | { ok: false; error: string } {
+export function collectCelRootIdentifiers(source: string): CelRootIdentifiersResult {
   if (typeof source !== 'string' || !source.trim()) {
-    return { ok: false, error: 'expression is empty' };
+    return { ok: false, error: 'expression is empty', code: 'empty-expression', params: {} };
   }
   try {
     // Same nullable-ternary rewrite as compile/evaluate so "what parses" agrees
@@ -358,10 +360,20 @@ export function collectCelRootIdentifiers(
     return { ok: true, roots: [...roots] };
   } catch (err) {
     const classified = classifyError(err);
-    return { ok: false, error: classified.ok === false ? classified.error.message : String(err) };
+    const detail = classified.ok === false ? classified.error.message : String(err);
+    return { ok: false, error: detail, code: 'cel-parse-failed', params: { detail } };
   }
 }
 
+/**
+ * What {@link collectCelRootIdentifiers} answers: the roots, or a refusal
+ * whose `error` is the English reason and whose `code` / `params` say the
+ * same thing for a consumer that renders its own words.
+ */
+export type CelRootIdentifiersResult =
+  | { ok: true; roots: string[] }
+  | ({ ok: false; error: string } & ExpressionRefusal<CelRootsRefusalCode>);
+
 /**
  * A parsed CEL AST node, re-exported so a consumer can name the type this
  * package already hands it without importing `@marcbachmann/cel-js` itself.