In short, suppose that you have created an accounting app on the desktop, and that you need to create PDF invoices and PDF reports from this app. Suppose that you use an AGPL PDF library to create the PDF.

To make a long story short: your conclusion was wrong, because modification has a broader meaning than you initially assumed.

Update in answer to comments asking for actual proof of my claims:

DISCLAIMER:

I have quite some firsthand experience in matters like these, and it's quite disappointing to see how many developers are interpreting the law in a way that best suits the open source user, going against the rights of the open source producer. I would expect developers to stand up for the rights for their fellow open source developers. In my experience however, decision makers at a company have a better understanding than the company's developers. Management usually makes much less of a problem of the dual license system that allows their company to use software under a commercial license in cases where the company can't comply with the copyleft license.

An example: suppose that you have created an accounting app on the desktop, and that you need to create PDF invoices and PDF reports from this app. Suppose that you use an AGPL PDF library to create the PDF.

To make a long story short: your conclusion was wrong, because modification has a broader meaning than you initially assumed.

Update in answer to comments asking for proof of my claims:

Personal background:

I have quite some firsthand experience in matters like these, and it's quite disappointing to see how many developers are interpreting the law in a way that best suits the open source user, going against the rights of the open source producer. I would expect developers to stand up for each other, and respect the rights of their fellow open source developers. In my experience however, decision makers at a company have a better understanding than the company's developers. Management usually makes much less of a problem of the dual license system that allows their company to use software under a commercial license in cases where the company can't comply with the copyleft license.

If a counter-party tells the judge that the AGPL doesn't apply when you access an AGPL library from a closed source system through a service that is open sourced under the AGPL, feel free to use the "murder" comparison in court to explain why the judge should discard the technical details of such a workaround, and you increase your chances at winning your case. Actually, the infringing party might get a more severe sentence because of the intentional attempt to avoid compliance with the open source license.

Note: most of the comments I refer to were removed by their author.

If a counter-party tells the judge that the AGPL doesn't apply when you access an AGPL library from a closed source system through a service that is open sourced under the AGPL, feel free to use the "murder" comparison in court to explain why the judge should discard the technical details of such a workaround. You'll increase your chances at winning your case. Actually, the infringing party might get a more severe sentence because of the intentional attempt to avoid compliance with the open source license.

Update

This update is rather off-topic, but I'm adding it to stop the flow of comments by 3D1T0R who, for some unknown reason, doubts my expertise in these matters.

The infringing party stalled, and came back with false arguments (similar to the arguments made by 3D1T0R in the comments).

In his comments, 3D1T0R claims that this isn't an illustrative example as creating invoices isn't an essential part of accounting software. Landsgericht Köln didn't agree with that point of view. All software using the infringing module was taken from the market because the infringing party didn't want to open source it. Notice the word v e r b o t e n in the rulling. It was forbidden to distribute software that uses iText as a whole or in part without permission of the copyright owners of iText, given the fact that the copyleft of the AGPL wasn't respected.

Actually, "being an essential part" isn't a criterium. In Artifex versus Hancom, the South-Korean company Hancom used Artifex's Ghostscript to create PDF output from documents created with their office suite. Creating PDF was an optional feature that wasn't essential for the rest of the office functionality to work. However, Artifex went to court, and a Californian judge issued a ruling that the GPL could indeed be enforced even in cases where the GPL code is only used for a small feature that isn't essential in the context of the complete work.

In his comments, 3D1T0R claims that there are workarounds, but that's wishful thinking. A judge usually takes into account the technical effect rather than the technical implementation.

If you don't understand the difference between technical effect and technical implementation, consider this: suppose that person A wants to murder person X, but he hires person B to kill person X, then person A isn't technically the murderer of person X, but the effect is the same: person X is killed because of person A, and for a judge person A will be guilty all the same (and usually get a higher sentence than person B).

Use this comparison to a judge in court to explain why he should discard the technical details of a workaround, and you'll win your case. Actually, the infringing party might get a more severe sentence because of the intentional attempt to avoid compliance with the open source license.

If you do not agree with the above, please document with existing cases that went to court. Please be aware of the fact that allegations that have not been proven in court have no value whatsoever.

I have quite some experience in both cases, but unfortunately, I can't disclose more information than what is disclosed in this update. However, I think this is already more information than what is provided by 3D1T0R to support his (or her) claims.

It's quite disappointing to see how many developers are interpreting the law in a way that best suits the open source user, going against the rights of the open source producer.

Update in answer to comments asking for actual proof of my claims:

The infringing party stalled, and came back with false arguments (similar to some arguments made in the comments).

In the comments, I read claims that this isn't an illustrative example because creating invoices isn't an essential part of accounting software. Landsgericht Köln didn't agree with that point of view. All software using the infringing module was taken from the market because the infringing party didn't want to open source it. Notice the word v e r b o t e n in the rulling. It was forbidden to distribute software that uses iText as a whole or in part without permission of the copyright owners of iText, given the fact that the copyleft of the AGPL wasn't respected.

Actually, being an essential part isn't even a criterium. In Artifex versus Hancom, the South-Korean company Hancom used Artifex's Ghostscript to create PDF output from documents created with their office suite. Creating PDF was an optional feature that wasn't essential for the rest of the office functionality to work. However, Artifex went to court, and a Californian judge issued a ruling that the GPL could indeed be enforced even in cases where the GPL code is only used for a small feature that isn't essential in the context of the complete work.

Often you read about thought experiments that tend to work around the (A)GPL, but such thought experiments are usually based on wishful thinking. In my (real-world) experience, a judge takes into account the technical effect rather than the technical implementation.

If you don't understand the difference between technical effect and technical implementation, consider this: suppose that person A wants to murder person X, but he hires person B to kill person X, then person A isn't technically the murderer of person X, but the effect is the same: person X is murdered because of person A, and for a judge person A will be guilty all the same (and usually get a higher sentence than person B).

If a counter-party tells the judge that the AGPL doesn't apply when you access an AGPL library from a closed source system through a service that is open sourced under the AGPL, feel free to use the "murder" comparison in court to explain why the judge should discard the technical details of such a workaround, and you increase your chances at winning your case. Actually, the infringing party might get a more severe sentence because of the intentional attempt to avoid compliance with the open source license.

If you do not agree with the above, please document with existing cases that went to court. Please be aware of the fact that allegations that have not been proven in court have no value whatsoever.

I have quite some firsthand experience in matters like these, and it's quite disappointing to see how many developers are interpreting the law in a way that best suits the open source user, going against the rights of the open source producer. I would expect developers to stand up for the rights for their fellow open source developers. In my experience however, decision makers at a company have a better understanding than the company's developers. Management usually makes much less of a problem of the dual license system that allows their company to use software under a commercial license in cases where the company can't comply with the copyleft license.