diff --git a/python/numbersprotocol_capture/client.py b/python/numbersprotocol_capture/client.py
index 529391a..f211784 100644
--- a/python/numbersprotocol_capture/client.py
+++ b/python/numbersprotocol_capture/client.py
@@ -5,10 +5,11 @@
 from __future__ import annotations
 
 import json
+import logging
 import mimetypes
 from pathlib import Path
 from typing import Any
-from urllib.parse import urlencode
+from urllib.parse import urlencode, urlparse
 
 import httpx
 
@@ -30,6 +31,8 @@
     UpdateOptions,
 )
 
+logger = logging.getLogger(__name__)
+
 DEFAULT_BASE_URL = "https://api.numbersprotocol.io/api/v3"
 HISTORY_API_URL = "https://e23hi68y55.execute-api.us-east-1.amazonaws.com/default/get-commits-storage-backend-jade-near"
 MERGE_TREE_API_URL = "https://us-central1-numbers-protocol-api.cloudfunctions.net/get-full-asset-tree"
@@ -164,6 +167,15 @@ def __enter__(self) -> Capture:
     def __exit__(self, *args: Any) -> None:
         self.close()
 
+    def __repr__(self) -> str:
+        if not self._token:
+            masked = "***"
+        elif len(self._token) < 4:
+            masked = "*" * len(self._token)
+        else:
+            masked = f"{'*' * (len(self._token) - 4)}{self._token[-4:]}"
+        return f"Capture(token='{masked}', base_url='{self._base_url}')"
+
     def close(self) -> None:
         """Close the HTTP client."""
         self._client.close()
@@ -213,8 +225,8 @@ def _request(
             try:
                 error_data = response.json()
                 message = error_data.get("detail") or error_data.get("message") or message
-            except Exception:
-                pass
+            except Exception as exc:
+                logger.debug("Could not parse error response body: %s", exc)
             raise create_api_error(response.status_code, message, nid)
 
         result: dict[str, Any] = response.json()
@@ -296,11 +308,11 @@ def register(
         if options.headline:
             form_data["headline"] = options.headline
 
-        # Handle signing if private key provided
-        if options.sign and options.sign.private_key:
+        # Handle signing if sign options provided (private_key or custom signer)
+        if options.sign and (options.sign.private_key or options.sign.signer):
             proof_hash = sha256(data)
             proof = create_integrity_proof(proof_hash, mime_type)
-            signature = sign_integrity_proof(proof, options.sign.private_key)
+            signature = sign_integrity_proof(proof, options.sign)
 
             proof_dict = {
                 "proof_hash": proof.proof_hash,
@@ -336,7 +348,7 @@ def update(
         caption: str | None = None,
         headline: str | None = None,
         commit_message: str | None = None,
-        custom_metadata: dict[str, Any] | None = None,
+        custom_metadata: dict[str, str | int | float | bool] | None = None,
         options: UpdateOptions | None = None,
     ) -> Asset:
         """
@@ -384,7 +396,12 @@ def update(
         if options.commit_message:
             form_data["commit_message"] = options.commit_message
         if options.custom_metadata:
-            form_data["nit_commit_custom"] = json.dumps(options.custom_metadata)
+            serialized = json.dumps(options.custom_metadata)
+            if len(serialized.encode("utf-8")) > 10 * 1024:
+                raise ValidationError(
+                    "custom_metadata must not exceed 10 KB when serialized"
+                )
+            form_data["nit_commit_custom"] = serialized
 
         response = self._request(
             "PATCH",
@@ -661,6 +678,9 @@ def search_asset(
         # Add input source
         files_data: dict[str, Any] | None = None
         if options.file_url:
+            parsed = urlparse(options.file_url)
+            if parsed.scheme not in ("http", "https"):
+                raise ValidationError("file_url must use http or https scheme")
             form_data["url"] = options.file_url
         elif options.nid:
             form_data["nid"] = options.nid
@@ -703,8 +723,8 @@ def search_asset(
                     or error_data.get("error")
                     or message
                 )
-            except Exception:
-                pass
+            except Exception as exc:
+                logger.debug("Could not parse asset search error response body: %s", exc)
             raise create_api_error(response.status_code, message)
 
         data = response.json()
@@ -763,8 +783,8 @@ def search_nft(self, nid: str) -> NftSearchResult:
                     or error_data.get("error")
                     or message
                 )
-            except Exception:
-                pass
+            except Exception as exc:
+                logger.debug("Could not parse NFT search error response body: %s", exc)
             raise create_api_error(response.status_code, message, nid)
 
         data = response.json()
diff --git a/python/numbersprotocol_capture/crypto.py b/python/numbersprotocol_capture/crypto.py
index 597523c..f20c816 100644
--- a/python/numbersprotocol_capture/crypto.py
+++ b/python/numbersprotocol_capture/crypto.py
@@ -4,13 +4,20 @@
 
 import hashlib
 import json
+import logging
 import time
+from typing import TYPE_CHECKING
 
 from eth_account import Account
 from eth_account.messages import encode_defunct
 
 from .types import AssetSignature, IntegrityProof
 
+if TYPE_CHECKING:
+    from .types import SignOptions
+
+logger = logging.getLogger(__name__)
+
 
 def sha256(data: bytes | bytearray) -> str:
     """
@@ -45,22 +52,27 @@ def create_integrity_proof(proof_hash: str, mime_type: str) -> IntegrityProof:
     )
 
 
-def sign_integrity_proof(proof: IntegrityProof, private_key: str) -> AssetSignature:
+def sign_integrity_proof(
+    proof: IntegrityProof,
+    private_key_or_options: "str | SignOptions",
+) -> AssetSignature:
     """
     Signs an integrity proof using EIP-191 standard.
 
+    Accepts either a raw private key string (legacy) or a :class:`SignOptions`
+    object.  When a ``SignOptions.signer`` callback is provided the private key
+    never enters this process, reducing the window of key exposure in memory.
+
     Args:
         proof: IntegrityProof object to sign.
-        private_key: Ethereum private key (hex string with or without 0x prefix).
+        private_key_or_options: Ethereum private key string **or** a
+            :class:`~numbersprotocol_capture.types.SignOptions` instance with
+            either ``private_key`` or ``signer`` + ``address``.
 
     Returns:
         AssetSignature containing the signature data.
     """
-    # Ensure private key has 0x prefix
-    if not private_key.startswith("0x"):
-        private_key = f"0x{private_key}"
-
-    account = Account.from_key(private_key)
+    from .types import SignOptions as _SignOptions
 
     # Compute integrity hash of the signed metadata JSON
     proof_dict = {
@@ -71,15 +83,45 @@ def sign_integrity_proof(proof: IntegrityProof, private_key: str) -> AssetSignat
     proof_json = json.dumps(proof_dict, separators=(",", ":"))
     integrity_sha = sha256(proof_json.encode("utf-8"))
 
-    # Sign the integrity hash using EIP-191
-    message = encode_defunct(text=integrity_sha)
-    signed = account.sign_message(message)
+    if isinstance(private_key_or_options, str):
+        # Legacy path: raw private key string
+        pk = private_key_or_options
+        if not pk.startswith("0x"):
+            pk = f"0x{pk}"
+        account = Account.from_key(pk)
+        message = encode_defunct(text=integrity_sha)
+        signed = account.sign_message(message)
+        sig_hex: str = signed.signature.hex()
+        public_key: str = account.address
+    elif isinstance(private_key_or_options, _SignOptions):
+        opts = private_key_or_options
+        if opts.signer is not None and opts.address is not None:
+            # Custom signer path – private key stays out of this process
+            sig_hex = opts.signer(integrity_sha)
+            public_key = opts.address
+        elif opts.private_key is not None:
+            pk = opts.private_key
+            if not pk.startswith("0x"):
+                pk = f"0x{pk}"
+            account = Account.from_key(pk)
+            message = encode_defunct(text=integrity_sha)
+            signed = account.sign_message(message)
+            sig_hex = signed.signature.hex()
+            public_key = account.address
+        else:
+            raise ValueError(
+                "sign_integrity_proof: provide either private_key or both signer and address"
+            )
+    else:
+        raise TypeError(
+            f"sign_integrity_proof: unexpected argument type {type(private_key_or_options)}"
+        )
 
     return AssetSignature(
         proof_hash=proof.proof_hash,
         provider="capture-sdk",
-        signature=signed.signature.hex(),
-        public_key=account.address,
+        signature=sig_hex,
+        public_key=public_key,
         integrity_sha=integrity_sha,
     )
 
@@ -104,5 +146,6 @@ def verify_signature(message: str, signature: str, expected_address: str) -> boo
         msg = encode_defunct(text=message)
         recovered: str = Account.recover_message(msg, signature=signature)
         return recovered.lower() == expected_address.lower()
-    except Exception:
+    except Exception as exc:
+        logger.debug("verify_signature failed: %s", exc, exc_info=True)
         return False
diff --git a/python/numbersprotocol_capture/types.py b/python/numbersprotocol_capture/types.py
index 0eff64d..692b291 100644
--- a/python/numbersprotocol_capture/types.py
+++ b/python/numbersprotocol_capture/types.py
@@ -4,6 +4,7 @@
 
 from __future__ import annotations
 
+from collections.abc import Callable
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Any
@@ -35,10 +36,24 @@ class CaptureOptions:
 
 @dataclass
 class SignOptions:
-    """Options for signing asset registration."""
+    """Options for signing asset registration.
 
-    private_key: str
-    """Ethereum private key for EIP-191 signing."""
+    Provide either a ``private_key`` (held in memory only for the duration of
+    the signing operation) **or** a ``signer`` callback together with an
+    ``address`` so that the private key never enters this process at all.
+    """
+
+    private_key: str | None = None
+    """Ethereum private key for EIP-191 signing (with or without 0x prefix)."""
+
+    signer: Callable[[str], str] | None = None
+    """Custom signer callback – receives the hex-encoded integrity hash and
+    must return an EIP-191 signature hex string.  Use this to keep the private
+    key entirely out of the SDK process."""
+
+    address: str | None = None
+    """Ethereum address that corresponds to the ``signer`` callback.
+    Required when ``signer`` is provided."""
 
 
 @dataclass
@@ -74,8 +89,8 @@ class UpdateOptions:
     commit_message: str | None = None
     """Description of the changes."""
 
-    custom_metadata: dict[str, Any] | None = None
-    """Custom metadata fields."""
+    custom_metadata: dict[str, str | int | float | bool] | None = None
+    """Custom metadata fields (values must be str, int, float, or bool; max 10 KB serialized)."""
 
 
 @dataclass
diff --git a/ts/src/client.ts b/ts/src/client.ts
index c86440e..20bdbcf 100644
--- a/ts/src/client.ts
+++ b/ts/src/client.ts
@@ -248,11 +248,11 @@ export class Capture {
     const publicAccess = options?.publicAccess ?? true
     formData.append('public_access', String(publicAccess))
 
-    // Handle signing if private key provided
-    if (options?.sign?.privateKey) {
+    // Handle signing if sign options provided (privateKey or custom signer)
+    if (options?.sign && (options.sign.privateKey || options.sign.signer)) {
       const proofHash = await sha256(data)
       const proof = createIntegrityProof(proofHash, mimeType)
-      const signature = await signIntegrityProof(proof, options.sign.privateKey)
+      const signature = await signIntegrityProof(proof, options.sign)
 
       formData.append('signed_metadata', JSON.stringify(proof))
       formData.append('signature', JSON.stringify([signature]))
@@ -303,7 +303,13 @@ export class Capture {
       formData.append('commit_message', options.commitMessage)
     }
     if (options.customMetadata) {
-      formData.append('nit_commit_custom', JSON.stringify(options.customMetadata))
+      const serialized = JSON.stringify(options.customMetadata)
+      if (new TextEncoder().encode(serialized).length > 10 * 1024) {
+        throw new ValidationError(
+          'customMetadata must not exceed 10 KB when serialized'
+        )
+      }
+      formData.append('nit_commit_custom', serialized)
     }
 
     const response = await this.request<AssetApiResponse>(
@@ -497,6 +503,12 @@ export class Capture {
 
     // Add input source
     if (options.fileUrl) {
+      const parsed = new URL(options.fileUrl)
+      if (parsed.protocol !== 'http:' && parsed.protocol !== 'https:') {
+        throw new ValidationError(
+          'fileUrl must use http: or https: scheme'
+        )
+      }
       formData.append('url', options.fileUrl)
     } else if (options.nid) {
       formData.append('nid', options.nid)
diff --git a/ts/src/crypto.ts b/ts/src/crypto.ts
index de214ab..1c4f809 100644
--- a/ts/src/crypto.ts
+++ b/ts/src/crypto.ts
@@ -1,5 +1,5 @@
 import { Wallet, verifyMessage } from 'ethers'
-import type { IntegrityProof, AssetSignature } from './types.js'
+import type { IntegrityProof, AssetSignature, SignOptions } from './types.js'
 
 /**
  * Computes SHA-256 hash of data using Web Crypto API.
@@ -32,27 +32,50 @@ export function createIntegrityProof(
 
 /**
  * Signs an integrity proof using EIP-191 standard.
- * Returns the signature data required for asset registration.
+ * Accepts either a raw private key string or a custom signer callback (via SignOptions)
+ * to minimise the lifetime of key material in memory.
+ *
+ * @param proof - Integrity proof to sign.
+ * @param privateKeyOrOptions - Ethereum private key string **or** a SignOptions object
+ *   with a `signer` callback and `address`.
  */
 export async function signIntegrityProof(
   proof: IntegrityProof,
-  privateKey: string
+  privateKeyOrOptions: string | SignOptions
 ): Promise<AssetSignature> {
-  const wallet = new Wallet(privateKey)
-
   // Compute integrity hash of the signed metadata JSON
   const proofJson = JSON.stringify(proof)
   const proofBytes = new TextEncoder().encode(proofJson)
   const integritySha = await sha256(proofBytes)
 
-  // Sign the integrity hash using EIP-191
-  const signature = await wallet.signMessage(integritySha)
+  let signature: string
+  let address: string
+
+  if (typeof privateKeyOrOptions === 'string') {
+    // Legacy path: private key passed directly
+    const wallet = new Wallet(privateKeyOrOptions)
+    signature = await wallet.signMessage(integritySha)
+    address = wallet.address
+  } else if (privateKeyOrOptions.signer && privateKeyOrOptions.address) {
+    // Custom signer path: private key never enters this process
+    signature = await privateKeyOrOptions.signer(integritySha)
+    address = privateKeyOrOptions.address
+  } else if (privateKeyOrOptions.privateKey) {
+    // SignOptions with privateKey field
+    const wallet = new Wallet(privateKeyOrOptions.privateKey)
+    signature = await wallet.signMessage(integritySha)
+    address = wallet.address
+  } else {
+    throw new Error(
+      'signIntegrityProof: provide either privateKey or both signer and address'
+    )
+  }
 
   return {
     proofHash: proof.proof_hash,
     provider: 'capture-sdk',
     signature,
-    publicKey: wallet.address,
+    publicKey: address,
     integritySha,
   }
 }
diff --git a/ts/src/types.ts b/ts/src/types.ts
index bc57312..7676d13 100644
--- a/ts/src/types.ts
+++ b/ts/src/types.ts
@@ -22,10 +22,22 @@ export interface CaptureOptions {
 
 /**
  * Options for signing asset registration.
+ * Provide either a `privateKey` (the key is held in memory only for the duration of signing)
+ * or a `signer` callback to keep the private key out of this process entirely.
  */
 export interface SignOptions {
   /** Ethereum private key for EIP-191 signing */
-  privateKey: string
+  privateKey?: string
+  /**
+   * Custom signer callback – use this to keep the private key out of the SDK process.
+   * The callback receives the hex-encoded integrity hash and must return an EIP-191 signature.
+   */
+  signer?: (message: string) => Promise<string>
+  /**
+   * Ethereum address corresponding to the `signer` callback.
+   * Required when using the `signer` callback.
+   */
+  address?: string
 }
 
 /**
@@ -54,8 +66,8 @@ export interface UpdateOptions {
   headline?: string
   /** Description of the changes */
   commitMessage?: string
-  /** Custom metadata fields */
-  customMetadata?: Record<string, unknown>
+  /** Custom metadata fields (values must be string, number, or boolean; max 10 KB serialized) */
+  customMetadata?: Record<string, string | number | boolean>
 }
 
 /**
diff --git a/ts/tsup.config.ts b/ts/tsup.config.ts
index 01f694b..ad1ebb8 100644
--- a/ts/tsup.config.ts
+++ b/ts/tsup.config.ts
@@ -5,7 +5,7 @@ export default defineConfig({
   format: ['esm', 'cjs'],
   dts: true,
   splitting: false,
-  sourcemap: true,
+  sourcemap: false,
   clean: true,
   treeshake: true,
   minify: false,