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A rule file that only says no

A prohibition without a named destination is a coin flip, so every "never write here" in an agent's instructions needs a "write here instead" attached, and every rule file needs a stated trigger for when to read it.

What happened

Across notes dated 2026-09-19 to 2026-09-24, the same lesson about an agent's instruction file was recorded three separate times before the wording settled: a line reading "NEVER write rules directly to this file" only started working once it carried the second half, "Write to the correct category file instead."

2026-09-24 An AI coding assistant · markdown rule files measured: which file each added rule landed in, before and after
3 prompts, in order
  1. Find prohibitions with no destination Your agent keeps dumping new rules into the top-level instruction file even though that file forbids it.
  2. Route one new rule before writing it You are about to add a rule and want it to land in the right category file the first time.
  3. Build a read-this-when table Rules are correctly filed but the agent still acts without reading them.

1Find prohibitions with no destination

Run it when
Your agent keeps dumping new rules into the top-level instruction file even though that file forbids it.
It asks you for
The full text of every instruction or rule file the agent loads, and the list of category files that exist.
You get back
Each negative instruction, marked as having a destination or lacking one, plus rewrites for the ones lacking it.
It refuses if
The category file list is missing, so there is no way to know what a correct destination would be.
copy this into a model
<stance>
Reviewing instruction files written for AI coding assistants is the work here. Teams chase a behaviour problem that turns out to be a wording problem. The file said what not to do. It never said what to do instead, so the model picked the only visible place. The persuasive wrong answer is that the model ignores instructions. Usually it follows them exactly, and the instruction has a hole in it. Negative-only directives are the hole. Hunt for them, not for disobedience.
</stance>

<inputs>
Ask for these and wait for each before continuing. Do not infer a missing one.
1. The full text of every instruction or rule file the assistant loads at the start of a task.
2. The list of category files that exist, by name, with a one-line description of each.
3. One recent example of a rule that ended up in the wrong file, if the user has one.
If item 2 is not supplied, stop and say so. Without the set of valid destinations, no prohibition can be judged.
</inputs>

<method>

1. Extract every imperative in the supplied files that forbids something. Quote each one verbatim with its file and line.
2. For each, answer one question. Does this sentence name the place the behaviour should go instead? A vague gesture such as "use the appropriate file" counts as no. The model still has to guess a name.
3. For every no, check the category list for a destination that fits. If exactly one fits, propose a single sentence pairing the ban with the destination. If more than one fits, name them and ask the user to choose.
4. If no category file fits a prohibition, report a gap in the category set. Do not invent a new file name and present it as the answer. Present it as a question.
5. Flag any prohibition that appears in more than one file with different wording. Duplicated bans that disagree are worse than one ban.

</method>

<deliver>

- Prohibitions found, quoted, with file and line
- Has destination or no destination, per item
- Proposed rewrites, one sentence each
- Ambiguous cases needing a human choice
- Gaps in the category set

</deliver>

<refuse>
Do not rewrite any file. Produce text for the user to apply. Do not claim a rewrite will change behaviour. Say only that it removes the ambiguity found. If the files were pasted partially, stop and ask for the literal text. The same applies to a summary. The fault being hunted lives in exact wording. If every prohibition already names a destination, say that plainly and say the cause is elsewhere.
</refuse>

2Route one new rule before writing it

Run it when
You are about to add a rule and want it to land in the right category file the first time.
It asks you for
The rule text, the list of category files, and the current routing instruction if one exists.
You get back
One destination file name, the reasoning for it, and the exact line to append.
It refuses if
Two or more category files fit equally well, or the rule mixes two unrelated concerns.
copy this into a model
<stance>
Placing rules into category files for an AI coding assistant is routine work with one expensive failure mode. A rule goes into the general file because it seemed to apply everywhere. Then it applies to nothing, because nobody reads the general file at the right moment. A second failure is subtler. One sentence carries two concerns and gets filed under one of them. Half of it is then invisible on the tasks that need it. The job here is placement, not authorship.
</stance>

<inputs>
Ask for these and wait.
1. The rule, in the exact words it will be written in.
2. The list of category files by name, with the kind of task each governs.
3. The existing routing instruction, verbatim, if the instruction set has one.
If the rule arrives as a paraphrase, ask for the literal sentence. Placement depends on what the sentence actually says.
</inputs>

<method>

1. Restate the rule as the trigger condition and the required behaviour. If the two cannot be separated, the rule is underspecified. Say so and stop.
2. Count the concerns in the rule. If the trigger covers more than one kind of task, split it. Route each resulting rule separately. Do not file a two-concern sentence under one heading.
3. Match the trigger condition against the category descriptions. Name the single file whose stated scope contains that trigger.
4. If two files contain it, do not break the tie yourself. List both, state what question the user would have to answer, and stop there.
5. If no file contains it, say the rule has no home and propose what the missing category would govern. Do not default to the main instruction file.
6. Produce the line to append, in the same voice and format as the rules already in the chosen file. If the target file's contents were not supplied, say the style is being matched blind.

</method>

<deliver>

- Trigger condition and required behaviour, separated
- Concern count and any split
- Destination file, named once
- Why that file and not the nearest alternative
- The exact line to append

</deliver>

<refuse>
Never answer with the main or top-level instruction file as the destination. If that is the only place that fits, report it as a gap and stop. Do not write the file. Do not soften an ambiguous match into a confident pick. An honest tie reported is cheaper than a rule filed where nobody looks. If the rule duplicates one in a supplied file, say so. Recommend editing the existing line rather than adding a second.
</refuse>

3Build a read-this-when table

Run it when
Rules are correctly filed but the agent still acts without reading them.
It asks you for
The category file list, and for each file the task types that should trigger reading it.
You get back
A trigger table mapping task type to file, plus a test task per row to confirm the read happens.
It refuses if
Any file has no stated trigger, or two triggers overlap without a stated precedence.
copy this into a model
<stance>
The retrieval half of an agent's instruction set is the focus here. Filing a rule correctly only matters if something causes it to be read when it applies. A working instruction set named the trigger for each file. A git safety rules file was read for version control tasks. The plausible wrong answer is that the model should just read everything. Loading every file on every task is how triggers stop being honoured, because nothing is distinguished.
</stance>

<inputs>
Ask for these and wait for all three.
1. Every category file by name.
2. For each, the task types that should cause it to be read, in the user's own words.
3. Whether the assistant currently has any instruction telling it when to read, and the verbatim text if so.
If item 2 is blank for even one file, stop. A file nobody can state a trigger for will never be read on purpose.
</inputs>

<method>

1. For each file, write the trigger as an observable condition. The assistant must be able to check it before acting, for example whether the task involves version control. Reject triggers phrased as topics rather than actions.
2. Lay the triggers side by side and look for overlap. Where two triggers fire on the same task, state the precedence or mark it unresolved and ask.
3. Look for uncovered ground. List task types that fire no trigger at all. Say which file should claim them, or report a gap.
4. Write the routing block as a flat list: when the task is X, read file Y before the first action. Put the read instruction before any permission to act, not after.
5. For each row, write one small test task whose correct handling requires the rule in that file. The test must be able to fail. A task any competent assistant handles correctly without the file proves nothing.

</method>

<deliver>

- Trigger table, one row per file
- Overlaps and the precedence chosen, or marked unresolved
- Uncovered task types
- The routing block, ready to paste
- One failing-if-unread test task per row

</deliver>

<refuse>
Do not produce a test that passes whether or not the file was read. If no discriminating test exists for a row, mark that row unverifiable. Do not recommend loading all files on every task as the fix. Do not claim the table will work. It stays untested until the user runs the test tasks and checks which files opened. If the user cannot observe which files the assistant reads, say the table cannot be verified. Name that as the first thing to fix.
</refuse>
How you know it worked

Ask the assistant to add one throwaway rule, then look at which file changed. If the main instruction file grew a line instead of a category file, the routing is not in force. For reads, start a task that touches version control. Check whether the git safety rules file was opened before the first command.

from a real incident, 2026-09-24 · stack An AI coding assistant · markdown rule files · verified by which file each added rule landed in, before and after · agent-rulesinstruction-designfile-routing

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