feat: add /transaction/metadata-blob endpoint

docs-v2/openapi-v1.yaml

@@ -2667,6 +2667,30 @@ paths:
             application/json:
               schema:
                 $ref: '#/components/schemas/Error'
+  /transaction/metadata-blob:
+    post:
+      tags:
+      - transaction
+      summary: Get the metadata blob for offline signers.
+      description: Returns the minimal metadata ("metadata blob" or "proof") needed by offline
+        signers to decode a transaction's signing payload, along with the metadata
+        hash for the CheckMetadataHash signed extension per RFC-0078.
+      operationId: getTransactionMetadataBlob
+      requestBody:
+        $ref: '#/components/requestBodies/TransactionMetadataBlob'
+      responses:
+        "200":
+          description: successful operation
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/TransactionMetadataBlob'
+        "400":
+          description: invalid request body or metadata V15 not available
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
   /pallets/assets/{assetId}/asset-info:
     get:
       tags:
@@ -8406,6 +8430,74 @@ components:
         - `RuntimeVersion`: https://crates.parity.io/sp_version/struct.RuntimeVersion.html
         - `SignedExtension`: https://crates.parity.io/sp_runtime/traits/trait.SignedExtension.html
         -  FRAME Support: https://crates.parity.io/frame_support/metadata/index.html
+    MetadataBlobBody:
+      type: object
+      properties:
+        tx:
+          type: string
+          format: hex
+          description: Full encoded extrinsic as a hex string.
+        txAdditionalSigned:
+          type: string
+          format: hex
+          description: Optional additional signed data for the extrinsic. Used together with `tx`.
+        callData:
+          type: string
+          format: hex
+          description: Call data as hex string. Use this alongside includedInExtrinsic
+            and includedInSignedData instead of `tx`.
+        includedInExtrinsic:
+          type: string
+          format: hex
+          description: Signed extension data included in the extrinsic. Required when using callData.
+        includedInSignedData:
+          type: string
+          format: hex
+          description: Signed extension data included in the signature. Required when using callData.
+        at:
+          type: string
+          description: Block hash or number to query at. Defaults to finalized head.
+          format: unsignedInteger or $hex
+      description: >-
+        Request body for generating the metadata blob. You must provide either:
+        (1) `tx` (full extrinsic) with optional `txAdditionalSigned`, or
+        (2) `callData` with `includedInExtrinsic` and `includedInSignedData`.
+    TransactionMetadataBlob:
+      type: object
+      properties:
+        at:
+          $ref: '#/components/schemas/BlockIdentifiers'
+        metadataHash:
+          type: string
+          format: hex
+          description: The 32-byte metadata hash (hex-encoded) that should be used with
+            the CheckMetadataHash signed extension.
+        metadataBlob:
+          type: string
+          format: hex
+          description: The metadata blob (hex-encoded) containing the minimal metadata
+            required for offline signers to decode the transaction. This is a SCALE-encoded
+            Proof structure per RFC-0078.
+        specVersion:
+          type: number
+          description: The chain's spec version.
+        specName:
+          type: string
+          description: The chain's spec name.
+        base58Prefix:
+          type: number
+          description: The chain's SS58 address prefix.
+        decimals:
+          type: number
+          description: Number of decimals for the chain's native token.
+        tokenSymbol:
+          type: string
+          description: Symbol for the chain's native token.
+      description: >-
+        Response containing the metadata blob for offline signers. The metadataBlob
+        contains type definitions needed to decode the specific transaction, Merkle
+        proofs verifying these types are part of the full metadata, and extra chain
+        info.
     TransactionPool:
       type: object
       properties:
@@ -8533,6 +8625,12 @@ components:
           schema:
             $ref: '#/components/schemas/DryRunBody'
       required: true
+    TransactionMetadataBlob:
+      content:
+        application/json:
+          schema:
+            $ref: '#/components/schemas/MetadataBlobBody'
+      required: true
     ContractMetadata:
       content:
         application/json:

docs/src/openapi-v1.yaml

@@ -2667,6 +2667,44 @@ paths:
             application/json:
               schema:
                 $ref: '#/components/schemas/Error'
+  /transaction/metadata-blob:
+    post:
+      tags:
+      - transaction
+      summary: Get the metadata blob for offline signers.
+      description: >-
+        Returns the minimal metadata ("metadata blob" or "proof") needed by offline
+        signers to decode a transaction's signing payload, along with the metadata
+        hash for the CheckMetadataHash signed extension per RFC-0078.
+
+
+        The metadata blob contains type definitions needed to decode the specific
+        transaction, Merkle proofs verifying these types are part of the full metadata,
+        and extra chain info (specVersion, specName, base58Prefix, decimals, tokenSymbol).
+
+
+        Offline signers can use this to decode the transaction to display what the user
+        is signing and verify the metadata subset matches the on-chain metadata via
+        Merkle proofs.
+
+
+        Reference: RFC-0078 https://polkadot-fellows.github.io/RFCs/approved/0078-merkleized-metadata.html
+      operationId: getTransactionMetadataBlob
+      requestBody:
+        $ref: '#/components/requestBodies/TransactionMetadataBlob'
+      responses:
+        "200":
+          description: successful operation
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/TransactionMetadataBlob'
+        "400":
+          description: invalid request body or metadata V15 not available
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
   /pallets/assets/{assetId}/asset-info:
     get:
       tags:
@@ -8406,6 +8444,74 @@ components:
         - `RuntimeVersion`: https://crates.parity.io/sp_version/struct.RuntimeVersion.html
         - `SignedExtension`: https://crates.parity.io/sp_runtime/traits/trait.SignedExtension.html
         -  FRAME Support: https://crates.parity.io/frame_support/metadata/index.html
+    MetadataBlobBody:
+      type: object
+      properties:
+        tx:
+          type: string
+          format: hex
+          description: Full encoded extrinsic as a hex string.
+        txAdditionalSigned:
+          type: string
+          format: hex
+          description: Optional additional signed data for the extrinsic. Used together with `tx`.
+        callData:
+          type: string
+          format: hex
+          description: Call data as hex string. Use this alongside includedInExtrinsic
+            and includedInSignedData instead of `tx`.
+        includedInExtrinsic:
+          type: string
+          format: hex
+          description: Signed extension data included in the extrinsic. Required when using callData.
+        includedInSignedData:
+          type: string
+          format: hex
+          description: Signed extension data included in the signature. Required when using callData.
+        at:
+          type: string
+          description: Block hash or number to query at. Defaults to finalized head.
+          format: unsignedInteger or $hex
+      description: >-
+        Request body for generating the metadata blob. You must provide either:
+        (1) `tx` (full extrinsic) with optional `txAdditionalSigned`, or
+        (2) `callData` with `includedInExtrinsic` and `includedInSignedData`.
+    TransactionMetadataBlob:
+      type: object
+      properties:
+        at:
+          $ref: '#/components/schemas/BlockIdentifiers'
+        metadataHash:
+          type: string
+          format: hex
+          description: The 32-byte metadata hash (hex-encoded) that should be used with
+            the CheckMetadataHash signed extension.
+        metadataBlob:
+          type: string
+          format: hex
+          description: The metadata blob (hex-encoded) containing the minimal metadata
+            required for offline signers to decode the transaction. This is a SCALE-encoded
+            Proof structure per RFC-0078.
+        specVersion:
+          type: number
+          description: The chain's spec version.
+        specName:
+          type: string
+          description: The chain's spec name.
+        base58Prefix:
+          type: number
+          description: The chain's SS58 address prefix.
+        decimals:
+          type: number
+          description: Number of decimals for the chain's native token.
+        tokenSymbol:
+          type: string
+          description: Symbol for the chain's native token.
+      description: >-
+        Response containing the metadata blob for offline signers. The metadataBlob
+        contains type definitions needed to decode the specific transaction, Merkle
+        proofs verifying these types are part of the full metadata, and extra chain
+        info.
     TransactionPool:
       type: object
       properties:
@@ -8533,6 +8639,12 @@ components:
           schema:
             $ref: '#/components/schemas/DryRunBody'
       required: true
+    TransactionMetadataBlob:
+      content:
+        application/json:
+          schema:
+            $ref: '#/components/schemas/MetadataBlobBody'
+      required: true
     ContractMetadata:
       content:
         application/json:

package.json

@@ -51,6 +51,7 @@
     "test:test-release": "yarn build:scripts && node scripts/build/runYarnPack.js"
   },
   "dependencies": {
+    "@polkadot-api/merkleize-metadata": "^1.1.29",
     "@polkadot/api": "16.5.2",
     "@polkadot/api-augment": "16.5.2",
     "@polkadot/api-contract": "16.5.2",

src/chains-config/assetHubKusamaControllers.ts

@@ -1,4 +1,4 @@
-// Copyright 2017-2025 Parity Technologies (UK) Ltd.
+// Copyright 2017-2026 Parity Technologies (UK) Ltd.
 // This file is part of Substrate API Sidecar.
 //
 // Substrate API Sidecar is free software: you can redistribute it and/or modify
@@ -88,6 +88,7 @@ export const assetHubKusamaControllers: ControllerConfig = {
 		'TransactionDryRun',
 		'TransactionFeeEstimate',
 		'TransactionMaterial',
+		'TransactionMetadataBlob',
 		'TransactionSubmit',
 	],
 	options: {

src/chains-config/assetHubNextWestendControllers.ts

@@ -1,4 +1,4 @@
-// Copyright 2017-2025 Parity Technologies (UK) Ltd.
+// Copyright 2017-2026 Parity Technologies (UK) Ltd.
 // This file is part of Substrate API Sidecar.
 //
 // Substrate API Sidecar is free software: you can redistribute it and/or modify
@@ -86,6 +86,7 @@ export const assetHubNextWestendControllers: ControllerConfig = {
 		'TransactionDryRun',
 		'TransactionFeeEstimate',
 		'TransactionMaterial',
+		'TransactionMetadataBlob',
 		'TransactionSubmit',
 	],
 	options: {

src/chains-config/assetHubPolkadotControllers.ts

@@ -1,4 +1,4 @@
-// Copyright 2017-2025 Parity Technologies (UK) Ltd.
+// Copyright 2017-2026 Parity Technologies (UK) Ltd.
 // This file is part of Substrate API Sidecar.
 //
 // Substrate API Sidecar is free software: you can redistribute it and/or modify
@@ -88,6 +88,7 @@ export const assetHubPolkadotControllers: ControllerConfig = {
 		'TransactionDryRun',
 		'TransactionFeeEstimate',
 		'TransactionMaterial',
+		'TransactionMetadataBlob',
 		'TransactionSubmit',
 	],
 	options: {

src/chains-config/assetHubWestendControllers.ts

@@ -1,4 +1,4 @@
-// Copyright 2017-2025 Parity Technologies (UK) Ltd.
+// Copyright 2017-2026 Parity Technologies (UK) Ltd.
 // This file is part of Substrate API Sidecar.
 //
 // Substrate API Sidecar is free software: you can redistribute it and/or modify
@@ -87,6 +87,7 @@ export const assetHubWestendControllers: ControllerConfig = {
 		'TransactionDryRun',
 		'TransactionFeeEstimate',
 		'TransactionMaterial',
+		'TransactionMetadataBlob',
 		'TransactionSubmit',
 	],
 	options: {

src/chains-config/defaultControllers.ts

@@ -1,4 +1,4 @@
-// Copyright 2017-2025 Parity Technologies (UK) Ltd.
+// Copyright 2017-2026 Parity Technologies (UK) Ltd.
 // This file is part of Substrate API Sidecar.
 //
 // Substrate API Sidecar is free software: you can redistribute it and/or modify
@@ -54,6 +54,7 @@ export const defaultControllers: ControllerConfig = {
 		'TransactionDryRun',
 		'TransactionFeeEstimate',
 		'TransactionMaterial',
+		'TransactionMetadataBlob',
 		'TransactionSubmit',
 	],
 	options: {

src/chains-config/kusamaControllers.ts

@@ -1,4 +1,4 @@
-// Copyright 2017-2025 Parity Technologies (UK) Ltd.
+// Copyright 2017-2026 Parity Technologies (UK) Ltd.
 // This file is part of Substrate API Sidecar.
 //
 // Substrate API Sidecar is free software: you can redistribute it and/or modify
@@ -56,6 +56,7 @@ export const kusamaControllers: ControllerConfig = {
 		'TransactionDryRun',
 		'TransactionFeeEstimate',
 		'TransactionMaterial',
+		'TransactionMetadataBlob',
 		'TransactionSubmit',
 	],
 	options: {

src/chains-config/polkadotControllers.ts

@@ -1,4 +1,4 @@
-// Copyright 2017-2025 Parity Technologies (UK) Ltd.
+// Copyright 2017-2026 Parity Technologies (UK) Ltd.
 // This file is part of Substrate API Sidecar.
 //
 // Substrate API Sidecar is free software: you can redistribute it and/or modify
@@ -55,6 +55,7 @@ export const polkadotControllers: ControllerConfig = {
 		'TransactionDryRun',
 		'TransactionFeeEstimate',
 		'TransactionMaterial',
+		'TransactionMetadataBlob',
 		'TransactionSubmit',
 	],
 	options: {

src/chains-config/westendControllers.ts

@@ -1,4 +1,4 @@
-// Copyright 2017-2025 Parity Technologies (UK) Ltd.
+// Copyright 2017-2026 Parity Technologies (UK) Ltd.
 // This file is part of Substrate API Sidecar.
 //
 // Substrate API Sidecar is free software: you can redistribute it and/or modify
@@ -55,6 +55,7 @@ export const westendControllers: ControllerConfig = {
 		'TransactionDryRun',
 		'TransactionFeeEstimate',
 		'TransactionMaterial',
+		'TransactionMetadataBlob',
 		'TransactionSubmit',
 	],
 	options: {