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A Power Query Formula.Firewall error normally has one of two causes: sources with privacy levels that cannot be combined in the same partition, or a partition that both references another query and directly accesses a source. Read the complete diagnostic, verify each source’s classification, then rebuild the query structure where necessary. Disabling privacy checks should be a last-resort security decision, not the default fix.
What the Formula.Firewall error is protecting
Power Query’s Data Privacy Firewall controls how data can move between different sources. It is designed to reduce accidental disclosure when a query combines databases, files, web requests, and other connections. The risk is not limited to the visible merge step: query folding can translate Power Query transformations into a native query sent back to a source, creating a path for information from one source to influence another.
The firewall divides evaluation into partitions. A partition can generally access compatible sources or reference other partitions, but it cannot normally do both at once. Incompatible sources in the same partition can also be blocked. The exact rule that applies depends on the source privacy levels and the way the M queries are chained.
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Identify which Formula.Firewall message you have
Open the error pane and record the reason, message, and detail, including the query and step names. Microsoft’s Power Query error guidance recommends capturing all three because the short banner alone can hide the useful distinction.
| Message pattern | What it indicates | First thing to inspect |
|---|---|---|
Formula.Firewall: Query 'Query1' (step 'Source') is accessing data sources that have privacy levels which cannot be used together. Please rebuild this data combination. |
Sources directly accessed by the same partition have incompatible privacy classifications, or the combination crosses a boundary the firewall will not permit. | List every source in that partition and compare its assigned level with the data’s actual sensitivity. |
Formula.Firewall: Query 'Query1' (step 'Source') references other queries or steps, so it may not directly access a data source. Please rebuild this data combination. |
The partition both imports data from another query or partition and directly opens a source. | Trace the referenced query and the direct source call, then redesign that boundary. |
These are Microsoft’s exact messages from the Data Privacy Firewall documentation. A merge can produce either pattern, so do not assume every Formula.Firewall failure is a credential problem.
Understand the three privacy levels before changing anything
Assign a level according to who is allowed to see the data and the trust boundary around the system. A label chosen merely to make refresh succeed can permit a flow that your organization did not intend.
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| Level | Use it for | Documented combination behavior |
|---|---|---|
| Public | Information intended to be broadly available, such as public web data. | Public/Public is one of the combinations Microsoft documents as allowing bidirectional sharing in the relevant case. |
| Organizational | Data available inside a trusted organizational boundary but not to the general public. | Organizational data can fold into Private and other Organizational sources, but not into Public sources. Organizational/Organizational is documented as allowing bidirectional sharing in the relevant case. |
| Private | Sensitive or confidential information that should remain isolated. | Private data cannot fold into other sources in the documented privacy model. A Private source is therefore often the boundary that exposes an incompatible combination. |
Microsoft’s Privacy levels in Power Query explains these categories and their effects on folding. Do not change a confidential source to Public simply because that removes a block; the new label would no longer represent the data.
Follow this safe diagnostic sequence
- Capture the complete diagnostic. Note the named query, step, reason, message, and detail. If the named step is
Source, inspect the source expression and any functions or referenced queries it calls rather than only the final table. - Inventory every input. Write down each file, database, API, workbook, folder, or other connector used by the failing query and by queries it references. Include indirect inputs: a staging query may hide the connection that matters.
- Verify classifications. For each input, decide whether it is Public, Organizational, or Private based on its real sensitivity and permitted audience. Microsoft specifically advises using Private for highly sensitive or confidential sources.
- Match the message to the structure. The “privacy levels … cannot be used together” wording points to source compatibility in one partition. The “references other queries or steps” wording points to a partition that mixes a reference with direct source access.
- Correct the smallest necessary boundary. Fix an inaccurate privacy assignment, or restructure the M queries so source retrieval and references occur in a firewall-compatible arrangement. Keep confidential inputs isolated unless your governance rules explicitly permit the combination.
- Refresh and recheck the diagnostic. Test the affected query after clearing or updating permissions as appropriate. If the message changes, use the new query and step names to continue tracing rather than applying a blanket bypass.
Fix incompatible privacy levels
Check which sources are accessed together
Start with the failing step and follow its dependencies. If one partition directly opens a private database and a public web endpoint, the firewall may reject the combination even when the merge itself is logically valid. The same applies when a source was incorrectly marked Private or Organizational after a connection was recreated.
Open the host’s source-permission interface, inspect each connection, and correct only classifications that are inaccurate. A source that contains payroll, customer, health, financial, or other confidential records should not be downgraded to Public for convenience. If the sources truly belong to different trust boundaries, stage or transform them in a design that preserves that separation instead of weakening the labels.
Account for query folding
Folding can improve performance by translating transformations into a native query at the source. It can also allow values from one source to influence a request to another, which is why the firewall evaluates privacy levels before allowing the operation. A change that appears harmless in the editor can therefore alter where filtering or joins execute. Treat a successful refresh after changing levels as a security result to review, not merely a technical success.
Fix the “references other queries or steps” pattern
This message means the failing partition is doing two things that the firewall cannot safely govern together: it references another query or partition and also accesses a data source directly. A common shape is a query that retrieves a table from a staging query and then calls a web or database connector in a later step.
Trace the dependency chain
- Open the failing query and identify every step that references another query, function, or parameterized table.
- Identify every step that directly invokes a connector, such as a file reader, database connector, or web request.
- Determine which operations are in the same partition and which are evaluated through referenced partitions.
- Check that the sources involved have compatible privacy classifications before changing the shape.
Restructure the retrieval boundary
Microsoft’s firewall documentation illustrates moving the referenced data retrieval and the subsequent source request into a single query when the sources have compatible privacy levels. In practical terms, avoid a partition that first consumes another query and then reaches outside that partition. Instead, place the related retrieval steps under one controlled query boundary, or separate the work into stages that do not mix a reference and a direct source call in the same partition.
There is no universal rewrite: the right arrangement depends on whether the referenced result is a parameter, a lookup table, a function result, or a full dataset. Preserve the intended data boundary and validate that folding has not changed where sensitive filtering occurs.
Where to set privacy levels
Power Query Desktop and Power BI Desktop
In Power Query Desktop, use Data source settings, select the connection, choose Edit Permissions, and set Privacy Levels. File-level permissions are also available through the current file’s Data source settings. Power BI Desktop provides global and current-file privacy options; the exact labels can vary slightly by release.
After changing a permission, return to the query and refresh the affected preview or load. If the connection was created more than once, check each matching entry: changing one credential record does not necessarily change another connection with a different path or server name.
Power Query Online
Power Query Online exposes privacy through connection settings or the project’s managed connections. Project options can also allow combining data from multiple sources. Microsoft warns that enabling this combination may expose sensitive or confidential data, so use it only when the project’s security requirements permit the flow. Navigation depends on the host and connection type.
Should you ignore privacy levels?
Ignoring privacy levels can remove a block, but it can also allow sensitive data to move between sources without the firewall’s protection. Microsoft describes this as a trade-off involving both functionality and security, not a harmless troubleshooting switch. Decide with the data owner or security team, document the reason, and confirm the setting’s scope before using it.
Host and model limitations matter. The documented Desktop option does not work for semantic models, and it does not become a privacy override for reports published to the Power BI service. Settings in Power Query Online and project connections have their own scopes. A workaround that succeeds in a local workbook may therefore fail after publication or may not apply to the service refresh path.
Before considering an override, confirm that the source classifications are accurate, the query partitions are intentional, and the required combination is approved. If performance is the motivation, first examine folding, staging, and source-side filtering rather than removing the control wholesale.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common pitfalls and recovery paths
Changing a sensitive source to Public
This may make a combination appear to work while misrepresenting the source’s sensitivity. Restore the correct level and redesign the combination or obtain an explicit security decision; do not leave the inaccurate label in place.
Fixing only the visible merge
The named step may be a source or an earlier dependency, not the merge operation. Trace referenced queries and functions so that indirect connections are included in the inventory.
Assuming every host uses the same switch
Desktop, Power BI Desktop, and Power Query Online expose different settings and scopes. Check the host-specific path and test the environment where the final refresh will run.
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After restructuring, verify which filters and joins execute at the source and whether confidential values can influence another connector request. A refresh that completes is not proof that the resulting data flow is acceptable.
Clearing permissions without recording them
Credential resets can remove a saved privacy choice and make a previously stable query fail again. Record the connection identity, privacy level, and host scope when you correct the issue so the fix can be reproduced safely.
When the error is not a privacy-level mismatch
If all source classifications are accurate and compatible, the partition-reference message is the stronger clue. Revisit the dependency graph and look for a direct connector call after a referenced query or step. Conversely, if the message names incompatible privacy levels, changing the query shape without correcting an inaccurate classification may leave the underlying boundary unresolved.
Also verify that you are testing the same environment used for refresh. A local preview, a published semantic model, and a Power Query Online project can apply different privacy settings and connection scopes. The error’s query and step names, together with the host’s permission page, provide the evidence needed to select the next change.
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FAQ
Does Formula.Firewall mean my credentials are invalid?
No. The message concerns privacy boundaries or partition structure. Credential errors usually identify authentication or authorization directly; Formula.Firewall identifies a data-combination rule.
Can Public and Private sources ever be merged?
Not automatically under the documented firewall model. Whether a design can be made safe depends on the partition arrangement, folding behavior, and approved data boundary; changing Private to Public is not an appropriate shortcut.
Why did a workbook fix fail after publishing?
Desktop privacy settings do not carry through as a universal override for reports published to the Power BI service, and semantic models do not use the documented Desktop ignore setting. Recheck the service connections and model-specific configuration.
What should I give an administrator when escalating?
Provide the full reason, message, and detail; the named query and step; a source inventory with intended privacy levels; the host and refresh path; and a diagram or list showing which queries reference others and which steps directly access sources.
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Key takeaway
Treat Formula.Firewall as a data-flow design signal. Classify each source honestly, distinguish incompatible source levels from a partition that mixes references with direct access, and then rebuild only the boundary that violates the rule. Use an ignore-privacy option only when its security impact, host scope, and publication behavior are understood and approved.
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