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2026-04-19 | 🔧 Fixing the Misplaced H2 🤖

🐛 The Bug

🔍 A subtle character-level splitting bug caused the H2 heading marker to separate from the changes link in daily reflection pages.

🪞 Each reflection page ends with a changes link formatted as an H2 heading, like this: the text starts with ”## ” followed by a wikilink to the changes page.

📐 When a blog series section needed to be inserted above the changes link, the code searched for the position of the changes link prefix to know where to split the content. 🎯 But the prefix it searched for was just the bare wikilink portion, not the full H2 heading.

💥 This meant the split happened three characters too late, right between the ”## ” and the wikilink. 🧩 After reassembly, the ”## ” sat orphaned on its own line, and the changes link appeared below without its heading prefix.

🔬 Root Cause

🧮 The function findFirstSectionIndex uses text breakpoints to locate where trailing sections begin. 📏 It calculates a character index, then splits the content at that position.

🔑 The changesLinkPrefix constant was defined as the bare wikilink start, without the H2 marker. 📍 So the breakpoint landed inside the heading line rather than at its start.

🗂️ The trailingSectionHeaders list included this prefix, and insertNewSection relied on it to position new blog series sections before the changes link. 💔 Every time a new section was inserted, the heading got torn apart.

✅ The Fix

🎯 The fix is a single-line change: changesLinkPrefix now includes the H2 marker. 📐 This ensures findFirstSectionIndex finds the correct position at the start of the full heading, so the split never separates the marker from the link.

🧹 While fixing the bug, a duplication was also eliminated. 🔗 Both buildReflectionContent and ensureChangesLinkInReflection independently constructed the same changes link string. 📦 Now a shared changesLink function lives in DailyReflection, and both callers use it. 🛡️ This prevents the two construction sites from diverging in the future.

🔴🟢 Test-Driven Development

🧪 Following TDD discipline, a failing test was written first. 📝 The test creates a reflection with an H2 changes link at the bottom, then inserts a blog series section. 🔍 It verifies that the changes link retains its H2 prefix and that no orphaned ”## ” appears on a separate line. ❌ The test failed before the fix, confirming the bug. ✅ After the one-line change, it passed, along with all 1963 existing tests.

💡 Lesson Learned

🧩 When a search pattern is used to locate a split point in text, the pattern must match the full unit that should stay together. 📏 A prefix that starts mid-line will produce a mid-line split, silently breaking the structure. 🔑 This is especially easy to miss when the prefix was originally defined for detection purposes and later reused for positioning, since detection only needs a substring match while positioning needs a boundary-aware match.

🏷️ Domain Types Over Primitives

🔤 The original changesLink function accepted a Text parameter for the date, even though a Day type already existed in the codebase. 📐 The AGENTS.md rules are explicit: never pass date strings between functions, and when extracting a pure function, always introduce proper domain types in the same change.

🔄 Fixing this meant cascading the Day type through several functions: buildReflectionContent, ensureDailyReflection, updateDailyReflection, and ensureChangesLinkInReflection. 📦 Each function now accepts Day and calls formatDay internally only at the boundaries where text is truly needed, like constructing file paths, generating frontmatter, or writing log messages.

🧬 This pattern of pushing formatting to the edges and keeping domain types in the core is exactly the functional core, imperative shell architecture that AGENTS.md prescribes. 🛡️ It prevents an entire class of bugs where two functions independently format the same Day but produce subtly different text.

🏗️ Toward Structural Content Editing

🔍 This bug points to a deeper fragility in how the codebase edits markdown files. 📏 The current approach treats content as a flat text string: it searches for substrings to find section positions, calculates character indices, and splits the string at those offsets. 💥 This approach is fragile because any shift in the content, whether from an earlier insertion, a changed prefix, or even an extra newline, can throw off the character offset and corrupt the structure.

🌳 A more robust approach would be to parse markdown into a structured representation, such as an abstract syntax tree, before performing edits. 🧩 With an AST, inserting a section becomes finding the right node in the tree, adding a sibling node, and rendering the tree back to text. 📐 The tree structure means edits are always well-formed: you cannot accidentally split a heading from its content because they are bound together as a single node.

🔧 Several Haskell libraries exist for markdown parsing and rendering, such as commonmark and pandoc. 🎯 A lightweight approach would parse the reflection content into a list of section blocks, each carrying its heading and body. 📦 Insertions and reorderings would operate on this list, and the final rendering step would produce valid markdown.

⚖️ The tradeoff is complexity. 🔍 Parsing and rendering markdown introduces a round-trip that must preserve the exact formatting of the original content, including emoji, wikilinks, and frontmatter. 🧪 Any formatting differences would show up as unwanted diffs. 🎯 A more pragmatic middle ground might be a line-oriented approach: split the content into lines, identify section boundaries by line-level pattern matching, and manipulate whole line groups rather than character offsets.

📋 A follow-up issue has been created to explore this structural approach without blocking the current bug fix.

🤖 Why AI Agents Keep Violating AGENTS.md

🔍 After reviewing the AGENTS.md rules and reflecting on the violations in this PR, here are some hypotheses for why AI coding agents repeatedly break explicitly documented rules.

🧠 Hypothesis one: context window limitations. 📏 AGENTS.md is one of many inputs competing for attention in the context window. 🎯 When the agent is focused on solving the immediate problem, like finding the right substring prefix, the architectural rules from AGENTS.md can fade from active consideration. 💡 Corrective action: the custom instructions system prompt could include a mandatory checklist step that requires the agent to verify each new function signature against the domain types rules before committing.

🔄 Hypothesis two: pattern mimicry over principle application. 🪞 AI agents tend to match patterns from surrounding code rather than applying rules from documentation. 📐 When the existing codebase uses Text for dates throughout (buildReflectionContent, ensureDailyReflection, and all their callers), the agent naturally follows that pattern even though AGENTS.md explicitly says not to. 💡 Corrective action: prioritize refactoring existing code to follow the rules, so that the patterns the agent mimics are already correct.

📦 Hypothesis three: scope minimization bias. 🎯 The agent is trained to make minimal changes, which conflicts with rules like vertical slices that require cascading type changes through callers. 📐 Changing changesLink to Day felt like scope creep when the bug was a simple string prefix issue. 💡 Corrective action: make the AGENTS.md rules more explicit about when cascading changes are mandatory rather than optional, and perhaps include concrete examples of what a vertical slice looks like.

🔑 Hypothesis four: rules conflict with each other. 📋 The instruction to make the smallest possible changes directly conflicts with no dead code and vertical slices which demand comprehensive cleanup. 🧩 The agent resolves this conflict by defaulting to the smallest change, since that is typically reinforced more strongly in its training. 💡 Corrective action: explicitly rank the rules by priority, so the agent knows that domain types over primitives outranks minimal changes when they conflict.

📚 Book Recommendations

📖 Similar

• A Philosophy of Software Design by John Ousterhout is relevant because it emphasizes how small design decisions, like choosing the right abstraction boundary, prevent entire classes of bugs from arising.
• The Pragmatic Programmer by David Thomas and Andrew Hunt is relevant because its principle of not repeating yourself directly applies to the duplication that was eliminated in this fix.

↔️ Contrasting

• Move Fast and Break Things by Jonathan Taplin offers a contrasting philosophy where speed is prioritized over careful boundary design, which is exactly what led to this bug in the first place.

🔗 Related

• Refactoring: Improving the Design of Existing Code by Martin Fowler explores how to improve code structure incrementally, which mirrors the approach of fixing the bug while also extracting the shared function.