diff --git a/CHANGELOG.md b/CHANGELOG.md
index 74f79ab..3d058d2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,8 +6,22 @@ All notable changes to this project are documented here. The format is based on
 
 ## [Unreleased]
 
+### Fixed
+
+- **`X402PaymentRequired.error_detail` nesting** (`openapi.yaml`): `error_detail` referenced the
+  `Error` envelope (`{ error: { code, ... } }`), but the gateway nests the bare error object
+  directly under `error_detail` (`{ code, message, ... }`), with no inner `error` wrapper. The
+  inner object is now extracted as the `ErrorBody` component schema (referenced by `Error`, so
+  every other response is unchanged) and `error_detail` composes `ErrorBody` instead, matching
+  the wire shape so generated types no longer expect a nonexistent `error_detail.error` member.
+
 ### Added
 
+- **`X402PaymentRequired.error_detail.payment_rejected`** (`openapi.yaml`) — documents the
+  additive `{ reason, rail }` deny verdict the gateway now publishes on a 402 when a submitted
+  payment was REJECTED, so generated types and docs can discover the diagnostic field. Optional
+  and conditional: absent on an ordinary unpaid challenge.
+
 - **MoQ join-token mint surface** (`openapi.yaml`) — the Media over QUIC product had no spec at
   all, so no SDK or CLI could be generated for it. Adds the `MoQ` tag and both mint operations:
   - `POST /moq/publish/{ns}/{track}` (`mintMoqPublishToken`, scope `moq:write`) and
diff --git a/openapi.yaml b/openapi.yaml
index dc10210..8c2a8ee 100644
--- a/openapi.yaml
+++ b/openapi.yaml
@@ -1931,39 +1931,47 @@ components:
       required: [error]
       properties:
         error:
+          $ref: '#/components/schemas/ErrorBody'
+
+    ErrorBody:
+      type: object
+      description: >
+        The normalized WAVE error object itself — the value of the `Error` envelope's
+        `error` member. Referenced directly by bodies that carry the error object under
+        a different key (the 402 x402 challenge nests it under `error_detail` without
+        the envelope wrapper).
+      required: [code, message]
+      properties:
+        code:
+          type: string
+          description: Stable machine-readable error code.
+        message:
+          type: string
+          description: Human-readable explanation, safe to surface to end users.
+        details:
           type: object
-          required: [code, message]
-          properties:
-            code:
-              type: string
-              description: Stable machine-readable error code.
-            message:
-              type: string
-              description: Human-readable explanation, safe to surface to end users.
-            details:
-              type: object
-              additionalProperties: true
-              description: >
-                Optional structured context (e.g. failing field, retry limit,
-                requested scopes) — never raw upstream payloads.
-            suggestions:
-              type: array
-              description: >
-                Actionable next steps, ordered most→least likely to resolve the
-                error. Written to be acted on by a human OR an agent.
-              items:
-                type: string
-            did_you_mean:
-              type: array
-              description: >
-                Closest valid alternatives when the caller likely made a typo or
-                wrong choice (e.g. an unknown scope, product, or route).
-              items:
-                type: string
-            doc_url:
-              type: string
-              format: uri
-              description: Documentation link for this error.
+          additionalProperties: true
+          description: >
+            Optional structured context (e.g. failing field, retry limit,
+            requested scopes) — never raw upstream payloads.
+        suggestions:
+          type: array
+          description: >
+            Actionable next steps, ordered most→least likely to resolve the
+            error. Written to be acted on by a human OR an agent.
+          items:
+            type: string
+        did_you_mean:
+          type: array
+          description: >
+            Closest valid alternatives when the caller likely made a typo or
+            wrong choice (e.g. an unknown scope, product, or route).
+          items:
+            type: string
+        doc_url:
+          type: string
+          format: uri
+          description: Documentation link for this error.
 
     Pagination:
       type: object
@@ -3544,7 +3552,29 @@ components:
           items:
             $ref: '#/components/schemas/X402Accepts'
         error_detail:
-          $ref: '#/components/schemas/Error'
+          allOf:
+            - $ref: '#/components/schemas/ErrorBody'
+            - type: object
+              properties:
+                payment_rejected:
+                  type: object
+                  description: >-
+                    Present ONLY when a submitted payment was REJECTED (as opposed to no payment
+                    having been made at all): the sanitized deny verdict, so a paying caller can
+                    tell a rejected payment apart from an unpaid challenge and correct the named
+                    field. `reason` is a lowercase snake_case token from the gateway's closed deny
+                    vocabulary (e.g. `invalid_permit_header`, `session_expired`, `duplicate`,
+                    `rate_limited`); internal detail that cannot be published verbatim collapses
+                    to the generic `payment_rejected`. `rail` names the payment rail that denied
+                    (e.g. `base-usdc`, `tempo-pathusd`), or `unknown`.
+                  required: [reason, rail]
+                  properties:
+                    reason:
+                      type: string
+                      description: Sanitized machine-readable rejection reason token.
+                    rail:
+                      type: string
+                      description: The payment rail the rejection occurred on.
         next_action:
           type: object
           additionalProperties: true