Subject line testing is one of the highest-leverage moves in B2B email marketing — a 5-point lift in open rate on a 10,000-contact list compounds across every send. But most teams test ad hoc, without a consistent hypothesis framework, and results live in a spreadsheet that nobody references when the next email goes out. The institutional knowledge never builds. This stack systematizes the process: pull actual Marketo open rate data by segment, run it through Claude's variant generator, get a set of hypothesis-backed subject line variants for each audience, and build a test log that actually compounds over time.
The Stack
The Prompt
This stack is built around the Ad Copy Variant Generator Prompt. Here's the abbreviated version — the full prompt with all variables and usage notes is on its own page.
You are a B2B email specialist generating subject line test variants. Review the Marketo email performance data below, segmented by audience type. For each audience segment, identify the subject line patterns that produced the highest open rates. Generate 5 subject line variants for the next send — each with a distinct approach: urgency, curiosity, specificity, social proof, and personalization. For each variant, state the hypothesis and the expected open rate vs. current average.[ ... continued — see full prompt ]
The Workflow
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Export Marketo email performance data
Pull open rate, click rate, and subject line text by campaign for the last 6 months. Include send date and audience segment so Claude can identify patterns by send context, not just subject line content.
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Segment data by audience type
Split the export into cold, MQL, and customer segments before running the analysis. Subject line patterns that work for customers almost never transfer to cold lists — keep the learning separate.
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Paste segmented data into the Ad Copy Variant Generator prompt
Use subject line testing framing. Claude analyzes open rate patterns by angle type and generates 5 hypothesis-backed variants per segment with the reasoning behind each approach.
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Review the variants and select tests to run
Pick 2–3 variants per send for a standard A/B or multivariate test in Marketo. Prioritize variants that test a meaningfully different hypothesis, not just different wording for the same angle.
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Log results in Notion and build the pattern library
After each test, enter the result in your Notion test log alongside the hypothesis and the winning angle. Over 6 months, you'll have a pattern library that tells you what actually works for each audience — and Claude's next batch will be better for it.
What This Replaces
- Subject line testing done on gut feel with no framework for what angle is being tested
- Test results that live in email platform reports nobody checks after the campaign closes
- Subject line libraries that don't distinguish between what works for cold audiences vs. engaged lists
Related Stacks
New stacks drop weekly.
Each one includes the tools, the Claude prompt, and the workflow logic. Free — built for in-house B2B demand gen managers.