Critical: Silent Transaction Broadcast Failures in BlockstreamAPI - HTTP Payload Formatting Bug

[liosna]

Problem Statement

Users of the bit library experience silent transaction broadcast failures where .send() appears successful but the transaction never reaches the network mempool. The transaction hash is returned to the user, but blockchain explorers show "Transaction Not Found."

Symptoms

• Transaction signing completes successfully
• .send() returns a valid transaction hash (TXID)
• Checking the TXID on blockchain explorers (blockstream.info, blockchain.com) shows no transaction
• No error is raised - silent failure
• Issue reported since May 2020 (Issue Transaction broadcast failed, or Unspents were already used #110, Issue ConnectionError: Transaction broadcast failed, or Unspents were already used. #170)

Root Cause Analysis

Two concurrent issues in bit/network/services.py:

Issue 1: Malformed HTTP Request Payload

• The BlockstreamAPI.broadcast_tx() and broadcast_tx_testnet() methods wrap transaction hex in a dictionary
• Blockstream API specification requires raw hex string in request body
• Server rejects malformed request with HTTP 400
• Dispatcher interprets error as connection failure

Issue 2: Deprecated Service Provider Dependencies

• Dispatcher falls back to non-functional services:
  • Smartbit: Service permanently shut down
  • Bitcore: API endpoints unreliable/unreachable
  • Blockchair: Requires API key not implemented in library
  • Blockchain.com: Produces unreliable response patterns
• Result: Failed transaction attempts exhaust fallback list, return generic ConnectionError

Technical Details

# CURRENT (BROKEN) - Wraps hex in dictionary
r = requests.post(cls.MAIN_TX_PUSH_API, data={cls.TX_PUSH_PARAM: tx_hex}, timeout=DEFAULT_TIMEOUT)

# BLOCKSTREAM API SPEC REQUIREMENT - Raw hex string
# Reference: https://github.com/Blockstream/esplora/blob/master/API.md
# POST /tx endpoint expects: raw transaction hex as request body

Impact

• Affects all users attempting to broadcast Bitcoin transactions via bit library
• Silent failures (no exception raised) make debugging difficult
• Users unaware whether issue is code or network related
• Workaround: Use external tools to broadcast same transaction hex

Files Affected

• bit/network/services.py (2 methods, 2 configuration lists)

Related Issues

• Issue Transaction broadcast failed, or Unspents were already used #110: "Transaction broadcast failed, or Unspents were already used" (May 2020)
• Issue ConnectionError: Transaction broadcast failed, or Unspents were already used. #170: "ConnectionError: Transaction broadcast failed" (November 2022)
• Stack Overflow: Multiple reports of silent broadcast failures (2021-2023)

Severity

Critical - Core functionality (transaction broadcasting) is broken for all users

[liosna]

Proposed Solution

Implement two targeted fixes:

Fix 1: Correct HTTP Payload Format

• Modify BlockstreamAPI.broadcast_tx() to send raw hex, not dictionary
• Modify BlockstreamAPI.broadcast_tx_testnet() to send raw hex, not dictionary
• Aligns with official Blockstream API specification

Fix 2: Remove Defunct Services

• Remove non-functional providers from BROADCAST_TX_MAIN dispatcher list
• Remove non-functional providers from BROADCAST_TX_TEST dispatcher list
• Keep only Blockstream (actively maintained, reliable)

Impact of Fix:

• ✅ Transactions broadcast successfully
• ✅ Zero breaking changes
• ✅ 100% backward compatible
• ✅ No new dependencies
• ✅ ~20 lines changed in 1 file

Pull Request

github.com/ofek/bit/pull/201

Complete Technical Analysis & Verification

For comprehensive analysis, methodology, and detailed verification results, see:

📦 Repository: github.com/liosna/bit-broadcast-fix