---
title: "Parallel API batching for AI motion summarization with adaptive throughput"
date: "2026-05-05"
category: workflow-issues
module: summarizer
problem_type: workflow_issue
component: service_object
severity: medium
applies_when:
  - "Backfilling large numbers of AI-generated summaries via an API"
  - "Rate limits or slow throughput bottlenecking batch processing"
  - "Need to process 10,000+ items with an LLM API"
tags:
  - parallelization
  - batching
  - openrouter
  - mistral
  - throughput
  - cost-optimization
---

# Parallel API Batching for AI Motion Summarization

## Context

Generating layman-friendly explanations for 29,000+ parliamentary motions via an LLM API. Initial approach processed one motion per API call, yielding ~700 motions/hour with significant per-request overhead. At this rate, the full backfill would take ~40 hours and cost ~$15-20. The budget tracking was also inaccurate — estimated $5.00 cap but actual API spend was only ~$1.78 when the cap was hit.

## Guidance

### 1. Application-level batching (not native API batching)

OpenAI's `/chat/completions` endpoint does not support multiple independent conversations in one request. Instead, pack 10-20 motions into a single prompt and request structured JSON output:

```python
# Build one prompt with N motions
prompt = f"""Je krijgt {len(motions)} moties.
Schrijf voor ELKE motie 2-3 zinnen uitleg.
Geef antwoord als JSON: {{"motion_id": "uitleg", ...}}

{motions_block}
"""

# Request JSON mode
payload = {
    "model": model,
    "messages": messages,
    "response_format": {"type": "json_object"}
}
```

This eliminates 90%+ of per-request HTTP overhead and context-window overhead.

### 2. Parallel API requests with ThreadPoolExecutor

When the API supports concurrent requests, use `ThreadPoolExecutor` to saturate the connection:

```python
from concurrent.futures import ThreadPoolExecutor

def chat_completion_json_parallel(
    message_batches, model=None, max_workers=3
):
    def _fetch_one(messages):
        return chat_completion_json(messages, model=model)

    with ThreadPoolExecutor(max_workers=max_workers) as executor:
        futures = [executor.submit(_fetch_one, batch) for batch in message_batches]
        return [f.result() for f in futures]
```

With 3 parallel workers × 15 motions per batch = 45 motions per chunk. This yielded **2.0x speedup** (0.5 → 1.0 motions/sec). After fixing the parameter-passing bug (see "Critical gotcha" below), 5 workers × 20 motions = 100 motions per chunk achieved **3.4x speedup** (1.1 → 3.7 motions/sec).

### 3. Model selection for language quality

Tested three models on the same problematic motions:

| Model | Quality | Speed | Issues |
|-------|---------|-------|--------|
| `google/gemma-4-26b-a4b-it` | Good | Slow (~25s/batch) | Occasional English words |
| `mistralai/mistral-small-2603` | **Excellent** | Medium | None observed |
| `mistralai/mistral-small-3.2-24b-instruct` | Good | Medium | One blank output |

**Recommendation**: `mistralai/mistral-small-2603` for Dutch-language tasks. Test on a representative sample (20-50 items) before committing to a full backfill.

### 4. Post-processing pipeline

Always add a post-processing step to catch model failures:

```python
def _post_process_summary(self, text: str) -> str:
    # 1. Remove lines that are mostly non-Latin (Chinese, Arabic, etc.)
    # 2. Replace known English words with Dutch equivalents
    # 3. Fix common typos (e.g., "lageinkomen" → "laag inkomen")
    # 4. Reject if >10 common English words remain
    # 5. Remove metadata fragments like "(45-102)"
    # 6. Normalize whitespace and punctuation
```

This caught: Arabic script hallucinations, English words like "filthy", Dutch typos like "formuliernoten", and metadata leaking from titles.

### 5. Adaptive backfill script

Create a backfill script that monitors throughput and adjusts parameters dynamically:

```python
# Start conservative
api_batch_size = 15
parallel_batches = 3
chunk_size = api_batch_size * parallel_batches  # 45

# After each chunk, measure time
if chunk_time < avg_chunk_time * 0.8:
    api_batch_size += 1  # API is fast, increase batch
elif chunk_time > avg_chunk_time * 1.5:
    api_batch_size -= 1  # API is slow, decrease batch

    # On repeated failures, back off
    if failures_in_row >= 2:
        api_batch_size -= 2
        delay_between_chunks += 1.0
```

### Critical gotcha: pass parallelism config all the way through

When adding `parallel_batches` to the orchestration layer, make sure it actually reaches the API call. A common bug is adding the parameter to the outer script but leaving hardcoded defaults in the inner method:

```python
# summarizer.py — WRONG (hardcoded defaults)
def generate_layman_explanations_batch_parallel(
    self, motions, model=None, parallel_batches=3, sub_batch_size=15
):
    ...

def update_motion_summaries(
    self, ..., api_batch_size=15, parallel_batches=3  # still hardcoded!
):
    ...
    summaries = self.generate_layman_explanations_batch_parallel(
        motions_for_api,
        model=config.QWEN_MODEL,
        parallel_batches=3,        # <-- BUG: ignores parameter
        sub_batch_size=15,         # <-- BUG: ignores parameter
    )
```

**Fix**: pass the parameters through and compute `sub_batch_size` dynamically:

```python
# summarizer.py — CORRECT
def update_motion_summaries(
    self, ..., api_batch_size=15, parallel_batches=3
):
    ...
    summaries = self.generate_layman_explanations_batch_parallel(
        motions_for_api,
        model=config.QWEN_MODEL,
        parallel_batches=parallel_batches,
        sub_batch_size=api_batch_size // parallel_batches,
    )
```

**Impact of this bug**: With `parallel_batches=3` hardcoded, increasing backfill.py to 5 workers had zero effect. After fixing: speed jumped from **1.1 → 3.7 motions/sec** (3.4x).

### 6. Accurate cost tracking

Update cost estimates when switching models. The old estimate assumed Qwen at $0.60/M tokens, but Mistral Small is much cheaper:

```python
# Mistral Small 2603 pricing (OpenRouter)
TOKEN_PRICE_PER_MILLION = 0.07  # blended input+output
TOKENS_PER_MOTION = 480         # ~400 input + ~80 output
COST_PER_MOTION = (TOKENS_PER_MOTION / 1_000_000) * TOKEN_PRICE_PER_MILLION
# ≈ $0.000034 per motion
```

## Why This Matters

- **Speed**: From ~700/hour (single) to ~13,320/hour (parallel batch, 5 workers) — **19x faster**
- **Cost**: From ~$15-20 estimated to ~$1-2 actual for 30,000 motions
- **Quality**: Model testing on edge cases prevents garbage-in-garbage-out at scale
- **Reliability**: Post-processing catches ~5% of outputs that would degrade UX

## When to Apply

- Processing 1,000+ items through any LLM API
- API charges per-request (not per-token) or has high latency per request
- Output quality is critical and model hallucinations are unacceptable
- Running overnight/background backfills where throughput matters more than latency

## Examples

### Before (single-motion API calls)
```python
for motion in motions:
    summary = ai.chat_completion(build_prompt(motion))
    # 50 motions = 50 API calls, ~250s total
```

### After (parallel batching)
```python
# Split into 3 batches of 15
batches = [motions[i:i+15] for i in range(0, 50, 15)]
message_batches = [build_batch_prompt(batch) for batch in batches]
results = chat_completion_json_parallel(message_batches, max_workers=3)
# 50 motions = 3 API calls, ~25s total
```

## Related

- `ai_provider.py` — `chat_completion_json_parallel()` implementation
- `summarizer.py` — `MotionSummarizer` with batch and parallel methods
- `backfill.py` — Adaptive backfill script with dynamic parameter tuning
