"Resolved before it was meant to be"

The error reads roughly:

Cannot mutate the dependencies of configuration ':<module>:<someVariant>'
after the configuration's child configuration ':<module>:runtimeClasspath'
was resolved. After a configuration has been observed, it should not be modified.

You get it from a line as ordinary as dependencies { someVariant("group:artifact:1.2.3") }. The build worked yesterday; the only change is that one line, or one newly applied plugin.

What is actually happening

A plugin creates extra configurations (per variant, per architecture, per flavour) and chains them into the main graph with implementation.extendsFrom(variantConfig). Somewhere during apply(), something resolves runtimeClasspath. Resolution freezes that configuration and every ancestor it extends — which now includes your variant configuration. Your dependency line then arrives too late.

The usual way a plugin resolves at apply time is a task registered with constructor arguments: Gradle must instantiate the task eagerly to pass them, the task's init block runs during apply, and something on that path walks the runtime classpath (typically to locate a jar task's output).

Traps

The message names the wrong culprit. It names the configuration you mutated and the child that was already observed. Neither is the plugin that did the resolving. There is no field in that message for "who resolved it", so the natural reading — "my dependency line is wrong" — sends you into rewriting syntax, which is exactly the loop to avoid.

These are documented dead ends. Do not re-run them. Every one was tried against this failure and changed nothing:

• moving the per-variant dependencies into their own earlier dependencies { } block;
• configurations.named("someVariant") { dependencies.add(...) } instead of the DSL accessor;
• direct Maven coordinates instead of a plugin-provided accessor;
• disabling one of the two suspect plugins;
• changing or removing the toolchain declaration — different message, same family.

The pattern in the first three: variations on how you write the mutation — and the mutation was never the problem. The last two probe elsewhere and still change nothing, which is what points the finger away from your build file entirely.

Note what is not in that list. Disabling both suspects together was written down as the next experiment and never run, so nothing here says the failure needs both out at once — that stayed an open question. What shipped instead was plugin application order plus a sibling configuration, both below.

Bisect plugins, not syntax. Start from a minimal template that is known to work — the packaging or framework vendor almost always publishes one — and add your plugins back one at a time until the error appears. Two published templates worked here; diffing their plugin lists against the failing module pointed straight at two extra plugins that neither template had, both of which inspect the runtime classpath eagerly. Treat that diff as a strong hint at the culprit — it is what makes the bisection short, not a finished diagnosis, and here the diagnosis was never actually closed.

# adapted — bisection loop, one plugin per iteration
git checkout -b bisect-plugins
# edit <module>/build.gradle.kts: comment out every plugin except the minimal set
./gradlew :<module>:tasks --no-configuration-cache   # configuration phase alone reproduces it
# uncomment one plugin, re-run, repeat until it fails

Configuration-phase-only tasks (tasks, help, dependencies) reproduce this without compiling anything, so each iteration is seconds rather than minutes.

Plugin application order can be the entire fix. Some plugins do their task creation at apply time and only succeed when an artifact-producing task already exists. Applying the packaging plugin after the plugin that provides a plain jar task, but before a plugin that would later replace that task with a target-specific one, is a real and load-bearing ordering:

// adapted — one further plugin dropped, comments rewritten
plugins {
    alias(libs.plugins.compose.multiplatform)   // provides `jar`
    alias(libs.plugins.conveyor)                // must see `jar` at apply time
    alias(libs.plugins.compose.compiler)
    alias(libs.plugins.kotlin.multiplatform)    // would otherwise replace `jar`
}

Reordering the plugins { } block is cheap and worth trying before anything structural.

A sibling configuration is the escape hatch when order is not enough. Create your own configuration that extends the runtime classpath, and let the eager consumer resolve that instead; the lock then lands on a configuration nobody needs to mutate afterwards:

// adapted
configurations.create("desktopRuntimeClasspath") {
    extendsFrom(configurations.getByName("jvmRuntimeClasspath"))
}

Keep the reproduction. A one-module repro against the vendor's minimal template is what you send upstream, and it is also what tells you a later plugin release fixed the problem — without it you will re-derive the whole chain on the next version bump. Record the non-fixes with it; the list is worth more than the fix, because it is what stops the next person burning an afternoon.

Related family: strict input/output validation. Modern Gradle also aborts when a plugin produces a file that a consuming task uses without declaring it as an input. Same shape — the error names the consuming task, not the producing plugin. Fix by pinning the producing plugin back to a version that declares it and adding an explicit dependsOn from the consumer as defence in depth.

Verifying it

Run the configuration phase only (./gradlew :<module>:tasks) — if it fails there, nothing about compilation or your source is involved, which already rules out half of what people check first.

Verify a fix with --no-configuration-cache, and treat a green run with the configuration cache as no evidence at all until you have seen it miss: on a cache hit Gradle replays a stored configuration result instead of re-running the phase that was failing, so the run can be green while the problem is untouched. Watch the build output for "Configuration cache entry reused" versus "stored", and force a re-run before drawing a conclusion.