Fix #732 by de-duplicating duplicated JSON responses from AWS Bedrock in structured data extraction

[kbenoit]

Problem

AWS Bedrock occasionally returns duplicate identical JSON objects during structured data extraction, causing
extract_data() to fail with the error:

  Data extraction failed: 2 data results received.

This causes parallel_chat_structured() to fail completely when processing batches of prompts.

Evidence

See also the reprex based on my own data, in #732 (comment).

Captured failure case from real-world usage (Spanish political manifesto analysis), before I simplified the type_object by flattening it, as in the reprex linked above:

# The Turn object contains two identical ContentJson objects:
turn@contents:
  [[1]] <ellmer::ContentJson>
  @value: List of 9
$ Country_LLM         : chr "Spain"
$ Party_LLM           : chr "Partido Popular"
$ is_populist         : logi FALSE
$ is_populist_evidence: chr "The party does not consistently frame politics in populist terms..."
$ antielitism         :List of 2
..$ evidence: chr "The party criticizes the current government's policy of 'cesiones humillantes'..."
..$ score   : int 4
$ generalwill         :List of 2
..$ evidence: chr "The party emphasizes the importance of unity and the general will of the people..."
..$ score   : int 5
$ indivisible         :List of 2
..$ evidence: chr "The party portrays Spain as a single nation with a common identity..."
..$ score   : int 6
$ manichean           :List of 2
..$ evidence: chr "The party criticizes the current government's policies and actions..."
..$ score   : int 3
$ peoplecentrism      :List of 2
..$ evidence: chr "The party emphasizes the importance of listening to the people..."
..$ score   : int 5

[[2]] <ellmer::ContentJson>
  @value: List of 9

# IDENTICAL structure and values as [[1]]
# Verification: identical(json1, json2) == TRUE

Scale: Captured 11 failure cases vs 3 success cases during batch processing, indicating this is a significant
intermittent issue with Bedrock's tool calling mechanism.

Solution

Updated extract_data() in R/chat-structured.R to gracefully handle multiple JSON responses:

  if (n == 2) {
    # Check if the two JSON objects are identical (duplicate case)
    if (identical(val1, val2)) {
      warning("Found duplicate JSON responses, using the first one", call. = FALSE)
      out <- val1
    } else {
      # Different JSON objects - use the last one (likely the final response)  
      warning("Found multiple different JSON responses, using the last one", call. = FALSE)
      out <- val2
    }
  }

Tests

Added comprehensive tests in test-chat-structured.R:

• Handle duplicate identical JSON responses with warning
• Handle different JSON responses (use last one)
• Include prompt index in warning messages for parallel operations
• Error appropriately on >2 JSON responses

Impact

Before:

# This would fail completely
parallel_chat_structured(chat, prompts, type = my_type)
#> Error: Data extraction failed: 2 data results received.

After:

# This now succeeds with warning
result <- parallel_chat_structured(chat, prompts, type = my_type)
#> Warning: Found duplicate JSON responses, using the first one (prompt 43).
#> ... successful extraction continues

This ensures robust structured data extraction for production use, even when providers exhibit intermittent duplication behaviour.

Interactions with other (current) PRs

This works best when combined with #725. #705 does not fix this issue, but if the other two PRs are accepted, I will redo #705 and integrate them into a single, more robust parallel_chat_structured().