diff --git a/cardano_checkout/mint.py b/cardano_checkout/mint.py
index aca607c..cce81c3 100644
--- a/cardano_checkout/mint.py
+++ b/cardano_checkout/mint.py
@@ -22,15 +22,39 @@ Design decisions:
   CIP-25 metadata keeps the tx cheap (~0.18 ADA fee + min-utxo for the
   NFT output).
 
-v0.1.0 ships the signature + stub. Full tx construction against a local
-Ogmios endpoint lands in v0.2 once the store + monitor integration
-tests pass.
+Cold-signing workflow
+---------------------
+
+The mint function does *not* sign. It builds the transaction body + the
+auxiliary data, computes the tx id, and returns an :class:`UnsignedMint`
+carrying the CBOR-encoded body plus a human-readable summary so the
+operator can sanity-check before signing. The operator then:
+
+1. Transfers the unsigned CBOR to the cold host (Lucy, via `scp`, USB,
+   QR code, whatever the threat model tolerates).
+2. Signs offline with the policy-required skey(s) — for Sulkta's
+   chromatic policy that's ``Cobb.skey`` + ``Kayos.skey``.
+3. Transfers the signed CBOR back to the hot host.
+4. Calls :func:`submit_signed_tx` to hand it to Ogmios.
+
+See ``docs/minting-workflow.md`` for the full operator runbook.
 """
 
 from __future__ import annotations
 
+import logging
 from dataclasses import dataclass, field
-from typing import Optional
+from typing import TYPE_CHECKING, Optional
+
+logger = logging.getLogger(__name__)
+
+if TYPE_CHECKING:  # pragma: no cover — hints only
+    from pycardano import ChainContext
+
+
+# ---------------------------------------------------------------------------
+# Policy model
+# ---------------------------------------------------------------------------
 
 
 @dataclass
@@ -43,10 +67,9 @@ class MintPolicy:
             under it. Becomes the Cardano ``policy_id`` of the NFT asset.
         script_cbor_hex: Hex-encoded CBOR of the native script itself.
             Submitted alongside the mint tx witness.
-        signing_keys: Paths to skey files needed to sign the tx (all of
-            them, for an all-of multi-sig). The SDK does not read these —
-            callers pass them to :mod:`cardano_checkout.txbuild` which
-            coordinates with an external signer (Lucy cold-store pattern).
+        required_signer_hashes: Payment-key hashes (hex) of every skey
+            that must sign the mint tx. For Sulkta's chromatic policy
+            this is 2 entries: Cobb + Kayos.
         locked_after_slot: Optional slot beyond which the policy rejects
             further mints. None = no time lock (not recommended for
             certificates — a lock makes the "no more editions" claim
@@ -55,48 +78,43 @@ class MintPolicy:
 
     policy_id: str
     script_cbor_hex: str
-    signing_keys: list[str] = field(default_factory=list)
+    required_signer_hashes: list[str] = field(default_factory=list)
     locked_after_slot: Optional[int] = None
 
 
-async def mint_nft_cert(
-    policy: MintPolicy,
-    asset_name: str,
-    metadata: dict,
-    recipient_address: str,
-    ogmios_url: str = "http://127.0.0.1:1337",
-    network: str = "mainnet",
-) -> str:
-    """Mint a CIP-25 v2 NFT cert and send it to the recipient.
+@dataclass
+class UnsignedMint:
+    """An unsigned mint transaction, ready to be handed to a cold signer.
 
-    Constructs a transaction that:
-      1. Mints exactly 1 of ``{policy.policy_id}.{asset_name}``.
-      2. Sends that single token to ``recipient_address`` in its own UTxO.
-      3. Attaches CIP-25 v2 metadata under metadatum label 721.
-      4. Witnesses with all signing keys required by the policy.
-
-    Args:
-        policy: Merchant's minting policy.
-        asset_name: UTF-8 asset name (will be hex-encoded per CIP-25). Max 32 bytes.
-        metadata: CIP-25 metadata dict. At minimum should include ``name``,
-            ``image`` (ipfs://CID), ``mediaType``, and any studio-specific
-            properties. The SDK wraps this into the proper ``{721: {policy_id: {asset_name: ...}}}``
-            envelope automatically.
-        recipient_address: Bech32 address of the wallet that receives the NFT.
-        ogmios_url: Endpoint for chain queries + tx submission.
-        network: "mainnet" or "testnet".
-
-    Returns:
-        Transaction hash (hex) once successfully submitted.
-
-    Raises:
-        NotImplementedError: v0.1.0 stub. Full implementation lands in v0.2.
+    Attributes:
+        tx_id: Transaction hash computed from the body alone (stable across
+            signing — the same id the explorer will show once submitted).
+        tx_body_cbor_hex: Hex-encoded CBOR of the transaction *body*.
+            This is what gets moved to the cold host.
+        auxiliary_data_cbor_hex: Hex-encoded CBOR of the auxiliary data
+            (metadata + native script). Required to reconstruct the full
+            transaction before submission.
+        native_script_cbor_hex: Hex-encoded CBOR of the minting policy's
+            native script. Needed by the cold signer to construct the
+            correct witness set.
+        required_signer_hashes: List of payment-key hashes (hex) the cold
+            signer must provide. Mirrors ``MintPolicy.required_signer_hashes``.
+        summary: Human-readable description of the tx — operator should
+            eyeball this before signing to confirm they're signing what
+            they think they're signing.
     """
-    # v0.1.0 surface lock — implementation lands in v0.2 alongside txbuild.py
-    raise NotImplementedError(
-        "mint_nft_cert is stubbed in v0.1.0. Use cardano-cli or PyCardano "
-        "directly until v0.2 ships the full Ogmios-backed mint path."
-    )
+
+    tx_id: str
+    tx_body_cbor_hex: str
+    auxiliary_data_cbor_hex: str
+    native_script_cbor_hex: str
+    required_signer_hashes: list[str]
+    summary: str
+
+
+# ---------------------------------------------------------------------------
+# Metadata builder (pure, no pycardano dep)
+# ---------------------------------------------------------------------------
 
 
 def build_cip25_metadata(
@@ -127,6 +145,7 @@ def build_cip25_metadata(
     Returns:
         Dict ready to submit as tx metadatum label 721.
     """
+
     def chunk64(s: str) -> list[str]:
         if len(s) <= 64:
             return [s]
@@ -154,3 +173,270 @@ def build_cip25_metadata(
             "version": "2.0",
         }
     }
+
+
+# ---------------------------------------------------------------------------
+# Mint transaction builder (cold-signer flow)
+# ---------------------------------------------------------------------------
+
+
+def _require_pycardano():
+    try:
+        import pycardano  # noqa: F401
+    except ImportError as exc:  # pragma: no cover — env sanity
+        raise RuntimeError(
+            "pycardano is required for mint transaction construction. "
+            "Add pycardano>=0.11.0 to requirements.txt and reinstall."
+        ) from exc
+
+
+def _metadata_dict_with_int_keys(metadata: dict) -> dict:
+    """Convert string top-level metadata labels to ints for pycardano Metadata.
+
+    CIP-25 v2 nests everything under label ``721``. We accept both ``{"721": ...}``
+    (builder output) and ``{721: ...}`` (raw) for ergonomics.
+    """
+    converted: dict = {}
+    for key, val in metadata.items():
+        try:
+            converted[int(key)] = val
+        except (TypeError, ValueError):
+            converted[key] = val
+    return converted
+
+
+async def mint_nft_cert(
+    policy: MintPolicy,
+    asset_name: str,
+    metadata: dict,
+    recipient_address: str,
+    funding_address: str,
+    context: Optional["ChainContext"] = None,
+    ogmios_host: str = "127.0.0.1",
+    ogmios_port: int = 1337,
+    network: str = "mainnet",
+    min_lovelace_for_nft_utxo: int = 1_500_000,
+) -> UnsignedMint:
+    """Build an unsigned mint+send transaction for a CIP-25 v2 NFT cert.
+
+    Constructs a transaction that:
+
+    1. Mints exactly 1 of ``{policy.policy_id}.{asset_name}``.
+    2. Sends that single token to ``recipient_address`` in its own UTxO
+       with the minimum-ADA padding (default 1.5 ADA).
+    3. Attaches the CIP-25 v2 metadata (label 721) + the policy's
+       native script as tx auxiliary data.
+    4. Returns the unsigned body for the cold signer to sign — does NOT
+       sign, does NOT submit.
+
+    UTxOs for fees + min-ADA are sourced from ``funding_address`` (the
+    merchant's hot wallet on Rackham, which does not hold any policy keys).
+
+    Args:
+        policy: Merchant's minting policy.
+        asset_name: UTF-8 asset name (will be hex-encoded per CIP-25). Max 32 bytes.
+        metadata: CIP-25 metadata dict — typically the output of
+            :func:`build_cip25_metadata`. Accepts ``{"721": ...}`` or ``{721: ...}``.
+        recipient_address: Bech32 address of the wallet that receives the NFT.
+        funding_address: Bech32 address that pays the tx fee + NFT min-ADA.
+        context: Optional chain context. If omitted a fresh
+            :class:`pycardano.OgmiosChainContext` is built from
+            ``ogmios_host``/``ogmios_port``.
+        ogmios_host: Host of the local Ogmios HTTP+WS endpoint.
+        ogmios_port: Port of the local Ogmios endpoint.
+        network: ``"mainnet"`` or ``"testnet"`` (preprod / preview).
+        min_lovelace_for_nft_utxo: ADA (in lovelace) to attach to the NFT
+            output so it satisfies the ledger's min-UTxO floor. Default 1.5 ADA.
+
+    Returns:
+        :class:`UnsignedMint` bundle ready for the cold-signer hand-off.
+
+    Raises:
+        RuntimeError: If pycardano is unavailable, or tx construction fails.
+        ValueError: If ``asset_name`` is empty or > 32 bytes.
+    """
+    _require_pycardano()
+
+    if not asset_name or len(asset_name.encode("utf-8")) > 32:
+        raise ValueError(
+            "asset_name must be a non-empty UTF-8 string <= 32 bytes "
+            f"(got {len(asset_name.encode('utf-8'))} bytes)"
+        )
+
+    from pycardano import (
+        Address,
+        Asset,
+        AssetName,
+        AuxiliaryData,
+        Metadata,
+        MultiAsset,
+        NativeScript,
+        Network,
+        ScriptHash,
+        TransactionBuilder,
+        TransactionOutput,
+        Value,
+    )
+
+    if context is None:
+        from cardano_checkout.txbuild import make_ogmios_context
+
+        context = make_ogmios_context(
+            host=ogmios_host, port=ogmios_port, network=network
+        )
+
+    net = Network.MAINNET if network == "mainnet" else Network.TESTNET
+
+    # ------------------------------------------------------------------
+    # Assemble the mint MultiAsset
+    # ------------------------------------------------------------------
+    policy_hash = ScriptHash.from_primitive(bytes.fromhex(policy.policy_id))
+    asset_name_obj = AssetName(asset_name.encode("utf-8"))
+    asset = Asset()
+    asset[asset_name_obj] = 1
+    mint_bundle = MultiAsset()
+    mint_bundle[policy_hash] = asset
+
+    # ------------------------------------------------------------------
+    # Native script + auxiliary data (metadata + script witness)
+    # ------------------------------------------------------------------
+    native_script = NativeScript.from_cbor(bytes.fromhex(policy.script_cbor_hex))
+
+    metadata_obj = Metadata(_metadata_dict_with_int_keys(metadata))
+    aux = AuxiliaryData(metadata_obj)
+    # AuxiliaryData in pycardano also carries native_scripts attached to the tx body;
+    # the builder below handles native scripts separately via add_minting_script.
+
+    # ------------------------------------------------------------------
+    # Addresses
+    # ------------------------------------------------------------------
+    sender = Address.from_primitive(funding_address)
+    recipient = Address.from_primitive(recipient_address)
+    if sender.network != net or recipient.network != net:
+        raise ValueError(
+            f"Address network mismatch: requested {network}, "
+            f"sender={sender.network.name}, recipient={recipient.network.name}"
+        )
+
+    # ------------------------------------------------------------------
+    # Build the transaction
+    # ------------------------------------------------------------------
+    builder = TransactionBuilder(context)
+    builder.add_input_address(sender)
+
+    # Attach mint bundle + policy as a minting script.
+    builder.mint = mint_bundle
+    builder.native_scripts = [native_script]
+    builder.auxiliary_data = aux
+
+    # Output: the newly minted NFT in its own UTxO at the recipient, padded
+    # with min-ADA so the ledger accepts it.
+    nft_value = Value(min_lovelace_for_nft_utxo, mint_bundle)
+    builder.add_output(TransactionOutput(recipient, nft_value))
+
+    # If the policy has a time lock, the mint tx MUST set ttl <= locked_after_slot
+    # or the node will reject the witness. Let pycardano pick validity normally,
+    # but clamp ttl when a lock slot is set.
+    ttl_offset = None
+    if policy.locked_after_slot is not None:
+        try:
+            chain_tip = context.last_block_slot  # type: ignore[attr-defined]
+            # Cap at 2 hours or (locked_after_slot - chain_tip), whichever is smaller.
+            two_hours_in_slots = 2 * 60 * 60  # ~1 slot/s on mainnet
+            ttl_offset = max(
+                60, min(two_hours_in_slots, policy.locked_after_slot - chain_tip)
+            )
+        except Exception:  # pragma: no cover — context without chain tip
+            ttl_offset = None
+
+    try:
+        tx_body = builder.build(
+            change_address=sender,
+            auto_ttl_offset=ttl_offset,
+            auto_validity_start_offset=-30,
+        )
+    except Exception as exc:
+        raise RuntimeError(f"Failed to build mint tx body: {exc}") from exc
+
+    tx_id = str(tx_body.id)
+
+    summary_lines = [
+        f"Mint 1 x {policy.policy_id}.{asset_name}",
+        f"  -> recipient: {recipient_address}",
+        f"  fees paid by: {funding_address}",
+        f"  tx_id (pre-sign): {tx_id}",
+        f"  network: {network}",
+        f"  required signers: {len(policy.required_signer_hashes)} "
+        f"({', '.join(h[:16] + '...' for h in policy.required_signer_hashes) or 'NONE — check policy'})",
+    ]
+    if policy.locked_after_slot is not None:
+        summary_lines.append(
+            f"  policy time-lock: slot <= {policy.locked_after_slot}"
+        )
+
+    return UnsignedMint(
+        tx_id=tx_id,
+        tx_body_cbor_hex=tx_body.to_cbor_hex(),
+        auxiliary_data_cbor_hex=aux.to_cbor_hex(),
+        native_script_cbor_hex=policy.script_cbor_hex,
+        required_signer_hashes=list(policy.required_signer_hashes),
+        summary="\n".join(summary_lines),
+    )
+
+
+# ---------------------------------------------------------------------------
+# Signed-tx submission
+# ---------------------------------------------------------------------------
+
+
+def submit_signed_tx(
+    signed_tx_cbor_hex: str,
+    context: Optional["ChainContext"] = None,
+    ogmios_host: str = "127.0.0.1",
+    ogmios_port: int = 1337,
+    network: str = "mainnet",
+) -> str:
+    """Submit a cold-signed transaction to the network via Ogmios.
+
+    The cold signer produces a fully-assembled :class:`pycardano.Transaction`
+    — body + witness set + auxiliary data — serialised as CBOR. This
+    function deserialises that blob, hands it to Ogmios, and returns the
+    tx hash.
+
+    Args:
+        signed_tx_cbor_hex: Hex-encoded CBOR of the signed transaction.
+        context: Optional chain context; built from ``ogmios_host/port`` if omitted.
+        ogmios_host: Host of the Ogmios endpoint.
+        ogmios_port: Port of the Ogmios endpoint.
+        network: ``"mainnet"`` or ``"testnet"``.
+
+    Returns:
+        Transaction hash (hex) — stable identifier for the submitted tx.
+
+    Raises:
+        RuntimeError: If pycardano is unavailable, or submission fails.
+    """
+    _require_pycardano()
+
+    from pycardano import Transaction
+
+    if context is None:
+        from cardano_checkout.txbuild import make_ogmios_context
+
+        context = make_ogmios_context(
+            host=ogmios_host, port=ogmios_port, network=network
+        )
+
+    try:
+        tx = Transaction.from_cbor(bytes.fromhex(signed_tx_cbor_hex))
+    except Exception as exc:
+        raise RuntimeError(f"signed_tx_cbor_hex is not valid transaction CBOR: {exc}") from exc
+
+    try:
+        context.submit_tx(tx)  # type: ignore[attr-defined]
+    except Exception as exc:
+        raise RuntimeError(f"Ogmios rejected the signed tx: {exc}") from exc
+
+    tx_hash = str(tx.id)
+    logger.info("[mint] submitted signed tx %s", tx_hash)
+    return tx_hash
diff --git a/cardano_checkout/txbuild.py b/cardano_checkout/txbuild.py
index cc29235..7c8b6b1 100644
--- a/cardano_checkout/txbuild.py
+++ b/cardano_checkout/txbuild.py
@@ -1,38 +1,207 @@
 """Transaction construction helpers wrapping PyCardano.
 
-v0.1.0 surface stub — the chromaticcraft Phase-2 sprint will fill these in
-with:
-  - Ogmios-backed `ChainContext` (via PyCardano's OgmiosChainContext)
-  - Build-transaction helpers for (a) plain ADA payment refunds, (b)
-    native-token mint+send, (c) reference-asset clones
-  - Cold-signer hand-off shape matching the ADAMaps payout pattern:
-    build_body_on_hot → transfer via temp dir → sign_offline_on_cold →
-    return signed_witness → submit_from_hot.
+This module is the SDK's single point of contact with PyCardano's
+:class:`pycardano.backend.base.ChainContext` API. Everything higher up
+(``mint`` and eventual refund-path code) goes through the helpers
+here so we can swap Ogmios for Blockfrost / Cardano-CLI without
+touching callers.
 
-Exists as a named module in v0.1 so consumers can import the stable path
-without having to update imports later.
+The default context targets the local Ogmios instance on Rackham
+(``127.0.0.1:1337``). That lines up with the mainnet deployment of the
+``cardano-node`` container (v10.6.2 on port 6000 via N2N) fronted by
+Ogmios as the HTTP+WS bridge. Preprod / testnet callers pass
+``network="testnet"`` and typically point at a different host.
+
+Cold-signer shape
+-----------------
+
+``txbuild`` only knows the hot-side half of the dance:
+
+- :func:`make_ogmios_context` — build a context from the live node.
+- :func:`get_protocol_parameters` — peek at the current protocol params
+  (useful for pricing, ttl calculations, etc.).
+- :func:`get_address_utxos` — list UTxOs at an address (refund path).
+- :func:`submit_signed_tx` — ship a tx that was signed offline.
+
+Body construction lives in :mod:`cardano_checkout.mint` today. As
+additional tx shapes (refunds, batched mints) arrive they'll land here
+alongside ``build_*_tx`` helpers that return :class:`UnsignedMint`-style
+cold-signer bundles.
 """
 
 from __future__ import annotations
 
+import logging
+from typing import TYPE_CHECKING, Any, Optional
 
-def _v0_2_sentinel(_name: str) -> None:
-    raise NotImplementedError(
-        f"txbuild.{_name} ships in cardano-checkout v0.2 alongside the "
-        "Ogmios chain-context wiring and cold-signer hand-off."
+logger = logging.getLogger(__name__)
+
+if TYPE_CHECKING:  # pragma: no cover — hints only
+    from pycardano import ChainContext, UTxO
+
+
+# ---------------------------------------------------------------------------
+# Chain context
+# ---------------------------------------------------------------------------
+
+
+def _require_pycardano() -> None:
+    try:
+        import pycardano  # noqa: F401
+    except ImportError as exc:  # pragma: no cover — env sanity
+        raise RuntimeError(
+            "pycardano is required for transaction construction. "
+            "Add pycardano>=0.11.0 to requirements.txt and reinstall."
+        ) from exc
+
+
+def make_ogmios_context(
+    host: str = "127.0.0.1",
+    port: int = 1337,
+    network: str = "mainnet",
+    secure: bool = False,
+    **kwargs: Any,
+) -> "ChainContext":
+    """Construct an :class:`pycardano.OgmiosChainContext` for the live node.
+
+    Args:
+        host: Ogmios HTTP+WS host. Default ``127.0.0.1`` (local).
+        port: Ogmios port. Default ``1337`` (matches Rackham's stack).
+        network: ``"mainnet"`` or ``"testnet"``. Controls the
+            :class:`pycardano.Network` passed to the context.
+        secure: Whether to use wss:// instead of ws://. Default False —
+            the stack assumes a loopback connection.
+        **kwargs: Forwarded to ``OgmiosChainContext`` verbatim (e.g.
+            ``refetch_chain_tip_interval``, ``utxo_cache_size``).
+
+    Returns:
+        A live :class:`ChainContext`. If the backing node is down the
+        object is still constructed — failures surface on the first
+        query / submit call.
+    """
+    _require_pycardano()
+    from pycardano import Network, OgmiosChainContext
+
+    net = Network.MAINNET if network == "mainnet" else Network.TESTNET
+    logger.debug(
+        "[txbuild] OgmiosChainContext -> %s://%s:%d (network=%s)",
+        "wss" if secure else "ws",
+        host,
+        port,
+        network,
+    )
+    return OgmiosChainContext(
+        host=host, port=port, secure=secure, network=net, **kwargs
     )
 
 
-def build_mint_tx(*args, **kwargs):  # noqa: D401, ANN002, ANN003
-    """Build an unsigned mint transaction. v0.2."""
-    _v0_2_sentinel("build_mint_tx")
+def get_protocol_parameters(context: "ChainContext") -> Any:
+    """Return the live protocol parameters from the chain context.
+
+    Useful for fee estimation, min-utxo floor computation, and sanity
+    checks that the node is reachable before a mint attempt.
+
+    The return type is pycardano's :class:`ProtocolParameters` — a
+    dataclass with fields like ``min_fee_a``, ``min_fee_b``,
+    ``coins_per_utxo_byte``, ``max_tx_size``, etc.
+    """
+    try:
+        return context.protocol_param  # type: ignore[attr-defined]
+    except Exception as exc:
+        raise RuntimeError(
+            f"Failed to fetch protocol parameters from chain context: {exc}"
+        ) from exc
 
 
-def build_payment_tx(*args, **kwargs):  # noqa: D401, ANN002, ANN003
-    """Build an unsigned payment transaction (e.g. refund path). v0.2."""
-    _v0_2_sentinel("build_payment_tx")
+def get_address_utxos(context: "ChainContext", address: str) -> list["UTxO"]:
+    """Fetch UTxOs at ``address`` via the chain context.
+
+    Intended for the refund path — when an invoice is cancelled or
+    overpaid the merchant needs to know which UTxOs landed in order to
+    build a return tx. For pure payment-detection, Koios is still the
+    cheaper source (see :mod:`cardano_checkout.monitor`).
+
+    Args:
+        context: Live chain context (from :func:`make_ogmios_context`).
+        address: Bech32 Cardano address.
+
+    Returns:
+        List of pycardano :class:`UTxO` objects at ``address``. Empty if
+        the address has no unspent outputs. Never ``None``.
+
+    Raises:
+        RuntimeError: If the underlying query fails (node down, invalid address).
+    """
+    _require_pycardano()
+    from pycardano import Address
+
+    try:
+        addr_obj = Address.from_primitive(address)
+    except Exception as exc:
+        raise RuntimeError(f"Invalid Cardano address: {exc}") from exc
+
+    try:
+        utxos = context.utxos(str(addr_obj))  # type: ignore[attr-defined]
+    except Exception as exc:
+        raise RuntimeError(
+            f"Failed to fetch UTxOs for {address[:20]}...: {exc}"
+        ) from exc
+
+    return list(utxos or [])
 
 
-def submit_signed_tx(*args, **kwargs):  # noqa: D401, ANN002, ANN003
-    """Submit a signed tx to Ogmios. v0.2."""
-    _v0_2_sentinel("submit_signed_tx")
+# ---------------------------------------------------------------------------
+# Signed-tx submission (duplicated from mint.py as a stable txbuild entry
+# point — the mint module's version delegates here)
+# ---------------------------------------------------------------------------
+
+
+def submit_signed_tx(
+    signed_tx_cbor_hex: str,
+    context: Optional["ChainContext"] = None,
+    ogmios_host: str = "127.0.0.1",
+    ogmios_port: int = 1337,
+    network: str = "mainnet",
+) -> str:
+    """Submit a cold-signed transaction blob to the chain.
+
+    See :func:`cardano_checkout.mint.submit_signed_tx` for the full docstring —
+    this is the same function under the ``txbuild`` import path so callers
+    that only need submission don't have to import ``mint``.
+    """
+    _require_pycardano()
+    from pycardano import Transaction
+
+    if context is None:
+        context = make_ogmios_context(
+            host=ogmios_host, port=ogmios_port, network=network
+        )
+
+    try:
+        tx = Transaction.from_cbor(bytes.fromhex(signed_tx_cbor_hex))
+    except Exception as exc:
+        raise RuntimeError(
+            f"signed_tx_cbor_hex is not valid transaction CBOR: {exc}"
+        ) from exc
+
+    try:
+        context.submit_tx(tx)  # type: ignore[attr-defined]
+    except Exception as exc:
+        raise RuntimeError(f"Ogmios rejected the signed tx: {exc}") from exc
+
+    tx_hash = str(tx.id)
+    logger.info("[txbuild] submitted signed tx %s", tx_hash)
+    return tx_hash
+
+
+# ---------------------------------------------------------------------------
+# Placeholders for future tx shapes (kept so consumers can pin imports)
+# ---------------------------------------------------------------------------
+
+
+def build_payment_tx(*args, **kwargs):  # pragma: no cover — future work
+    """Build an unsigned plain-ADA payment tx (refund path). v0.3+."""
+    raise NotImplementedError(
+        "build_payment_tx lands in v0.3 alongside the refund workflow. "
+        "For v0.2 only mint txs are supported."
+    )