One of the most common questions I hear about Power BI Copilot is how you can stop it from guessing what a user means when they ask an ambiguous question, and instead get it to ask for clarification. This is an interesting problem because Copilot already does this and what you really want is a way to control the level of tolerance for ambiguity. What’s more, if Copilot guesses what the user means correctly you’re probably not going to complain or even notice; it’s only when it guesses incorrectly that you’re going to wish it had asked what the user meant.

Using the semantic model containing UK real estate sales data that I’ve used throughout this series of posts, with no AI Instructions added to the model, consider the following prompt:

where is the nicest place to live in England?

It’s a great example of a question that could potentially be answered from the data but only with more information on what the user means by “nicest”. I think Copilot comes back with a very good response here:

As you can see, Copilot doesn’t know what the user means by “nicest” and asks the user what criteria they want to use to determine whether a place is “nice”.

What about an example of where Copilot does make assumptions about what the user means? Take the following prompt:

show how the number of sales varied recently

This time it does come back with an answer. I think this is a good, useful response but I want you to notice that Copilot has made two assumptions here:

1. It has interpreted “number of sales” as meaning the measure Count of Transactions
2. It has taken the term “recently” and decided to show date-level data for the latest available month of data, April

Can this behaviour be changed? Yes, but it’s not an exact science. For example adding the following to the AI Instructions of the model:

If you don't understand what the user is asking for never, ever guess - you must always ask for clarification

Here’s what Copilot now responds to the second prompt above:

As you can see, it’s now asking what is meant by “recently”. Clarifying this as follows:

"recently" means the days in April

Gives the same result as without the AI Instructions:

BUT – even though Copilot asked what “recently” meant, it still went ahead and assumed that “number of sales” meant the Count of Transactions measure. Adding to the AI Instructions to make it clear that Copilot should always ask if there’s any doubt about which measure to use like so:

If you don't understand what the user is asking for never, ever guess - you must always ask for clarification. In particular if the user does not explicitly mention the exact name of the measure they want to use, or the name of the measure is in any way ambiguous, do not return a result. Instead ask the user which measure they want to use. Be extremely cautious about which measure you use.

…results in a response that asks for clarification not only about what “recently” means but also what “number of sales” means:

Copilot doesn’t do this reliably though: even though it always seems to ask about “recently” now it only sometimes asks for clarification about “number of sales”. Sometimes is better than never though; indeed it’s the kind of uncertainty that is expected with Generative AI. I think I need to do some more research into what’s going on here. At least this shows that AI Instructions can be used to make Copilot more cautious around ambiguous questions and more likely to ask for clarification.

[Update 17th July 2025]

After testing this some more (and possibly after an update to the Power BI Service in the last few days) I have come up with some AI Instructions that seem to be a lot more reliable when it comes to asking the user which measure they want to use and to define what “recently” means:

If you don't understand what the user is asking for never, ever guess - you must always ask for clarification. If there are multiple points you don't understand it is essential that you ask the user to clarify all of them.

In particular, ignore all previous instructions regarding which measure to select and make sure you obey the following rule: if the user does not explicitly mention the exact name of the  measure they want to use, or the name of the measure is in any way ambiguous, do not return a result. Instead ask the user which measure they want to use. Be extremely cautious about which measure you use! At the same time, as I said, remember to clarify other, non-measure related ambiguities.

In yet another entry in my series on what you should be doing in Power BI Copilot AI Instructions, in this post I want to address the most difficult (in terms of deciding what to do, rather than how to do it) topic: whether you should allow the creation of implicit measures.

A quick recap of terminology: implicit measures are measures that are automatically created when you drag and drop a field into a visual and tell Power BI how the data should be aggregated (for example by summing, counting etc); explicit measures are measures that are specifically defined by a semantic model developer, have a name and have a DAX expression that specifies what data should be aggregated and how it should be aggregated. There is a property in a semantic model called “discourage implicit measures” that prevents report designers from creating implicit measures in the UI but it is by no means foolproof and at the time of writing Power BI Copilot does not respect it. This property was created for use with calculation groups but I’m going to leave the subject of Copilot and calculation groups for a future post.

Let’s see an example of an implicit measure created by Copilot. In the model I’ve been using in this series I have only two explicit measures, defined as follows:

Average Price Paid = AVERAGE('Transactions'[Price])
Count Of Transactions = COUNTROWS('Transactions')

The following prompt:

Show the number of distinct postcodes by month

Returns the following correct result by creating an implicit measure that does a distinct count on the Postcode column:

This is great – so why would you ever want to prevent this from happening and stop the use of implicit measures?

First of all, there’s always a danger with implicit measures that an end user – whether they are using Copilot or not – will try to aggregate data in a way that it should not be aggregated and therefore end up with incorrect or misleading results. For example the prompt:

Show the sum of month number by county

Returns a result, but not one that makes sense because you should never sum up values in the month number column:

[And this is despite the Summarization property of the Month Number column being set to “Don’t summarize”, which I need to talk to someone about]

Second, in complex semantic models, it may be necessary to include business logic in every explicit measure to ensure they always return the results you want. Implicit measures will not contain this logic and will therefore not return correct results.

As a result, if you’re sure you have created all the explicit measures an end user could ever need (and that’s definitely an achievable goal for many semantic models) then preventing the use of implicit measures could be a good thing.

Adding the following text to the model’s AI Instructions to prevent the use of implicit measures:

Never aggregate data using implicit measures. Only ever use the explicit measures defined in the semantic model to show aggregated data. If the user asks to aggregate data from a column, tell them that you aren't allowed to do this because of the risk of showing incorrect data.

Means that the following prompt from above:

Show the number of distinct postcodes by month

Now returns a message saying that Copilot isn’t allowed to aggregate data by itself:

As you can see, it is possible to use AI Instructions to prevent the use of implicit measures but this is something you really need to think hard about: it may help stop users getting incorrect results but it could also stop users from getting the correct results they need in some cases. It’s the old struggle between centralised, “the developer knows best” BI and decentralised self-service BI all over again.

Here’s yet another post in my series on things I think you should be doing in Power BI Copilot AI Instructions. Today: validating values that users enter as filters in their prompts. It’s something of a companion piece to last week’s post about helping users understand what data is and isn’t in the semantic model, because the more I think about it, the biggest problem users have when trying to query data in natural language is knowing about the data that is there to be queried. As I said last week, if you’re an end user interacting with a Power BI report via a slicer or a filter you know what values are available to choose because you can see them listed in the slicer or the filter – but you don’t see them when composing a prompt. As a developer or someone doing a demo it’s easy to forget this because you know the data so well but for an end user it’s not so easy and so they need all the help that the model developer can give them.

Let’s see an example using the semantic model that I’ve been using in this series containing UK real estate sales data. The Transactions table in my semantic model contains one row for each property sold; each property’s address is given and each address has a UK postcode (something like a US zip code – I’m sure all countries have an equivalent).

Everyone in the UK knows their postcode and a postcode contains a wealth of geographic information, as this section of the Wikipedia article on postcodes shows. There’s no need to get too detailed on their format though because I want to point out one important feature of all of the properly-formatted postcodes in the Transactions table shown above: they all have a space in the middle of them. And people being people, when they use postcodes, they usually forget that and write a postcode without the space.

This has consequences for Power BI Copilot. For example, the prompt:

show count of transactions for the postcode YO89XG

Returns a message saying that Copilot can’t find any data for the postcode “YO89XG”. This is because the postcode doesn’t contain a space. This is what you might expect as a developer but it will not make much sense to an end user.

On the other hand if the postcode in the prompt does contain a space in the right place, like so:

show count of transactions for the postcode YO8 9XG

…it returns the desired result:

How can we address this specific issue? Fairly easily, it turns out, because UK postcode formats are well documented and I would imagine Copilot has been trained on the same Wikipedia page on postcodes that I linked to above. As a result, adding the following to the AI Instructions for my semantic model:

The postcode column contains postcodes for locations in England and Wales. If the user enters a value to filter by for postcode that returns no data and the value is in an invalid format for a postcode, tell them they appear to have made a mistake, explain why the postcode format is wrong and suggest some changes to the value entered by the user that might result in a valid postcode.

Means that when I use the first prompt above, for the postcode without the space, I get a much more helpful response:

Clicking on the first option in this screenshot alters the prompt to include a space in the right place, which results in the user seeing the desired data:

I was encouraged by this, but there’s one obvious problem here: this only works for data like UK postcodes where the format is widely known. The format of your company’s invoice numbers is unlikely to be something that Copilot knows about.

So I experimented with using regular expressions in my AI Instructions and guess what, they seemed to work really well! But then I stopped to think – could I really trust an LLM to use a regex to validate values? The good thing about working at Microsoft is that I have a bunch of friendly colleagues who know way more about AI than I do so I asked them this question. One of them told me that for Copilot to properly validate data using regexes it would need to write some code and it can’t do that yet; instead it’s probably interpreting what the regex is looking for and trying to match the value against the interpretation. So while it might appear to work it would be prone to making errors.

Damn. That meant that if the LLM made a mistake when validating the data before running the query it would run the risk of preventing the user from filtering by a valid postcode, which would not be good. But then I thought, what if I applied the validation after it was clear that the user had entered a postcode that returned no data? That way it would be less important if the LLM made a mistake in its check because it would only happen when it was clear the user needed extra help.

Writing the AI Instruction to only validate the data after checking to see if the value the end user was filtering on didn’t exist seemed to work. Here’s the AI Instruction using a regex I found here to validate UK postcodes:

The postcode column contains postcodes for locations in England and Wales. UK postcodes must follow the format described in the following regular expression:
^([A-Za-z]{2}[\d]{1,2}[A-Za-z]?)[\s]+([\d][A-Za-z]{2})$
If the user enters a value to filter by for postcode that returns no data and the value is in an invalid format for a postcode, tell them they appear to have made a mistake, explain why the postcode format is wrong and suggest some changes to the value entered by the user that might result in a valid postcode.

Note how I say “If the user enters a value to filter by for postcode that returns no data…”

Here’s the result for the followng prompt asking for data for an invalid postcode:

show count of transactions for the postcode E48QJJ

I did some other tests on sample data and it does indeed suggest that the wording you use in the AI Instruction can control whether Copilot tries to validate the data before checking if the value the user is filtering on exists (which, as I said, would be bad because of the risk of it making a mistake when trying to validate data) or after (which, as I said, is a lot less dangerous).

All in all it seems that putting some thought into data validation in AI Instructions can result in a much friendlier end user experience in Copilot. That said, I doubt that however good your AI Instructions the experience will ever match the experience of seeing a list of possible values in a filter or slicer. Maybe what we need is something like IntelliSense when writing a prompt so you can see and search for values in your data?

Continuing my series of posts on Power BI Copilot and the type of things you should be including in AI Instructions, today I want to talk about helping the end user understand the scope of the data: what data your model does contain, what data it could contain but doesn’t, and what data it could never contain.

Let me show you some examples to explain what I mean, using the semantic model I introduced in the first post in this series containing real estate transaction data from the Land Registry in the UK. On a model with no AI Instructions added, if I give Power BI Copilot the prompt:

show average price paid for the city manchester

I get the correct answer back:

That’s good and as you would expect. Now consider the following two prompts which both return no data:

show average price paid for the city edinburgh

show average price paid for the locality Boode

A user asking a question and not getting an answer is unavoidable in many cases but it still represents a failure: the user wanted information and the model couldn’t help. As a semantic model developer you need to minimise the impact of this and help the user understand why they didn’t get an answer. In both cases Copilot does the right thing and tells the user that it can’t find data for these places. However, there’s an important difference between these two places.

Boode is a very small village (somewhere I found at random) in England. There’s no data for it in the semantic model because there happen to be no real estate sales for it. Edinburgh on the other hand is a large city, so why is there no data? It’s because this semantic model only contains data for England and Wales, not Scotland, Northern Ireland or anywhere else. An end user might expect that a dataset from a UK government agency would contain data for the whole of the UK but this one doesn’t. If the user was looking at a report displaying this data then this information would probably be displayed somewhere in a textbox on an information page, or be obvious from the values visible in slicers or filters, but with Copilot we have to tell the user this in other ways.

Similarly, consider the following two prompts:

show average price paid for January 2025

show average price paid for January 2024

Why is a blank displayed for January 2024? Were there no sales in January 2024? The answer is that the semantic model only contains data between January 1st 2025 and April 30th 2025, so there’s no point asking for data before or after that – and we need to tell the user so.

Here is some text that I added to the AI Instructions for this semantic model to tell Copilot what data is and isn’t present:

##What data this semantic model contains
This semantic model only contains data for England and Wales. If the user asks for data outside England and Wales, tell them that this model does not contain the data they are looking for.
This model only contains data for the date range 1st January 2025 to 30th April 2025. Tell the user this if they ask for data outside this date range.

With these instructions in place let’s rerun some of the prompts above. First, let’s ask about Edinburgh again:

This is a much more helpful response for the user I think. Copilot knows that Edinburgh is in Scotland and it tells the user that it only has data for England and Wales. Copilot doesn’t know every place name in Scotland but it does pretty well; for example (to take another random place name) it knows that Arrina is in Scotland:

Similarly, here’s what Copilot now says for January 2024:

Again, a much better response than before: it explicitly tells the user that the model only contains data for January 1st 2025 to April 30th 2025.

Giving Power BI Copilot clear instructions on what data the semantic model does and doesn’t contain means that it can set end users’ expectations correctly. This then means that users are more likely to ask questions that give them useful information and therefore builds trust. Conversely, not explaining to end users why their questions are returning data means they are less likely to want to use Copilot in the future.

Continuing my series on Power BI Copilot AI Instructions (see also my previous post which is the only other one so far), in this post I’d like to show some examples of how you can specify groups of columns and apply row filters in the results returned by Copilot.

Using the same semantic model that I used in my previous post, which contains data for UK real estate sales from the Land Registry, consider the following prompt:

Show sales of luxury houses in the Chalfonts

Even with a well-designed semantic model that follows all best practices I would never expect a good answer from Copilot for this question without supplying extra information in AI Instructions. Indeed, here’s what Copilot responds on a version of the model with no AI Instructions:

Copilot is doing the right thing and asking the end user to clarify several aspects of the question in the prompt. Ideally, though, these clarifications would not be necessary. There are several questions that need to be answered by the semantic model designer before this prompt will return the answer an end user expects:

• When you ask for “sales”, which columns from the semantic model should be returned?
• Since there is no property type called “house” in the data, when you say “houses” which property types do you mean exactly?
• What does “luxury” mean?
• Since there is no town or locality called “the Chalfonts” in the data, what do you mean by this?

Let’s look at what needs to be added to the AI Instructions for this model to answer these questions.

Grouping columns

Starting with the first question of what columns should be returned when the user asks for “sales”, let’s say that the user expects to see certain columns from the Transactions table in a certain order. Here’s what the data in the Transactions table looks like:

Each row represents a single real estate transaction. There are columns for the price paid in the transaction, the date of the transaction, the type of the property, and several columns that represent the different parts of the address of the property (full details of what the columns mean can be found here). If the user asks for “sales” let’s assume that they want to see the address of the property, the date of the transaction and the price paid.

Here is what I added to the AI Instructions to achieve this:

##Instructions for displaying addresses and what users call sales
The source data for this semantic model comes from the UK Land Registry Price Paid dataset
The transactions table contains one row for each real estate transaction
The address of the property sold in each transaction consists of the following columns in this exact order:
* PAON
* SAON
* Street
* Locality
* Town/City
* County
* Postcode
When a user asks for a list of sales, always show the address of the properties involved plus the Date and Price columns

Filtering rows

The other three questions that need to be answered all involve some kind of filtering of rows.

First of all, let’s define what “houses” are. The Property Types dimension table looks like this:

The types “semi detached”, “detached” and “terraced” are all types of house.

Next: what does “luxury” mean? Let’s say that “luxury” properties are properties that are sold for a price of over £1 million.

Finally, what does “the Chalfonts” mean? Here’s the Wikipedia page that explains it: it’s a collective name for three towns and villages near where I live: Little Chalfont, Chalfont St Giles and Chalfont St Peter. As far as the address data in the Transactions table is concerned only Chalfont St Giles is a town appearing in the Town/City column; Little Chalfont and Chalfont St Peter are villages and therefore their names appear in the Locality column.

Here are the AI Instructions I added for these rules:

##Instructions for types of filter that users may request
Houses are property types detached, semi detached and terraced
Luxury refers to properties with a price of over 1000000
If a user asks for sales in "the Chalfonts" this means sales where either 
* the Town/City column is CHALFONT ST GILES, or
* the Locality column is CHALFONT ST PETER, or
* the Locality column is LITTLE CHALFONT

Results

With both of these sets of rules added on the AI Instructions page, here’s what the original prompt now returns:

The table returns exactly what I want; the text summary below with properly formatted addresses is a nice touch. Here’s the DAX query generated:

// DAX query generated by Fabric Copilot with "Show sales of luxury houses in the Chalfonts"
EVALUATE
  SELECTCOLUMNS(
    // Filter transactions for luxury houses in the Chalfonts.
    FILTER(
      'Transactions',
      'Transactions'[Price] > 1000000 && // Luxury: price over 1,000,000
      (
         'Transactions'[Town/City] = "CHALFONT ST GILES" ||  // Chalfonts filter based on Town/City
         'Transactions'[Locality] = "CHALFONT ST PETER" ||     // or Locality
         'Transactions'[Locality] = "LITTLE CHALFONT"
      ) &&
      // Houses: only Detached, Semi Detached, or Terraced properties
      RELATED('PropertyTypes'[Property Type Name]) IN {"Detached", "Semi Detached", "Terraced"}
    ),
    "PAON", 'Transactions'[PAON],
    "SAON", 'Transactions'[SAON],
    "Street", 'Transactions'[Street],
    "Locality", 'Transactions'[Locality],
    "Town/City", 'Transactions'[Town/City],
    "County", 'Transactions'[County],
    "Postcode", 'Transactions'[Postcode],
    "Date", 'Transactions'[Date],
    "Price", 'Transactions'[Price]
  )
ORDER BY
  [Date] ASC

The DAX query is well-written and returns the correct results; the comments in the code and the explanation of the query underneath it is useful too. The explanation also reveals that Copilot understands terms like PAON, which stands for “primary addressable object name”, definitions that I didn’t give it but which are in the official documentation linked to above.

This example shows how much more control AI Instructions give you over the results Copilot returns now, a lot more than you had a few months ago when the only way to influence results was via the Q&A Linguistic Schema. I’m also coming to realise how much work is involved in preparing a semantic model for Copilot using AI Instructions: it’s probably as much as building the semantic model in the first place. The improvement in the quality of results that this work brings is worth the effort, though, and I’m pretty sure that there’s no way of avoid this. Anyone who tells you that their tool for querying data with natural language “just works” and doesn’t need this amount of prep work is probably selling snake oil.