quarkusio/quarkus · error · IllegalStateException

Only one of runtimeValue, function or assistantFunction is a

Error message

Only one of runtimeValue, function or assistantFunction is allowed

What it means

BuildTimeActionBuildItem.ActionBuilder.function() throws this when a runtimeValue or assistantFunction was already set. An action can only have exactly one backing implementation, so setters are mutually exclusive.

Source

Thrown at extensions/devui/deployment-spi/src/main/java/io/quarkus/devui/spi/buildtime/BuildTimeActionBuildItem.java:169

            return this;
        }

        public ActionBuilder enableMcpFunctionByDefault() {
            this.mcpEnabledByDefault = true;
            return this;
        }

        /**
         * @deprecated Use {@link #enableMcpFunctionByDefault()} instead.
         */
        @Deprecated(forRemoval = true)
        public ActionBuilder enableMcpFuctionByDefault() {
            return enableMcpFunctionByDefault();
        }

        public <T> ActionBuilder function(Function<Map<String, String>, T> function) {
            if (this.runtimeValue != null || this.assistantFunction != null)
                throw new IllegalStateException("Only one of runtimeValue, function or assistantFunction is allowed");
            this.function = function;
            return this;
        }

        public <T> ActionBuilder assistantFunction(BiFunction<Object, Map<String, String>, ?> assistantFunction) {
            if (this.function != null || this.runtimeValue != null)
                throw new IllegalStateException("Only one of runtimeValue, function or assistantFunction is allowed");
            this.assistantFunction = assistantFunction;
            return this;
        }

        public ActionBuilder runtime(RuntimeValue<?> runtimeValue) {
            if (this.function != null || this.assistantFunction != null)
                throw new IllegalStateException("Only one of runtimeValue, function or assistantFunction is allowed");
            this.runtimeValue = runtimeValue;
            return this;
        }

View on GitHub (pinned to e1c734241f)

Solutions

Remove the earlier runtime(...) or assistantFunction(...) call so only function(...) remains
If you want both behaviors, combine them in a single Function instead of setting two

Example fix

// before
builder.runtime(myRuntimeValue).function(this::myFunction);
// after
builder.function(this::myFunction);

Defensive patterns

Strategy: validation

Validate before calling

// pick exactly one implementation before building
if (useRuntimeValue) builder.runtime(rv); else builder.function(f);

Try / catch

try {
    builder.function(f);
} catch (IllegalStateException e) {
    if (e.getMessage().contains("Only one of")) { /* remove conflicting setter */ }
    else throw e;
}

Prevention

Choose the implementation style first (runtime vs function vs assistantFunction) and set only one
Avoid chaining multiple implementation setters in the same expression
Centralize action creation in one helper class

When it happens

Trigger: Chaining e.g. builder.runtime(rv).function(f) or calling function() twice on the same ActionBuilder.

Common situations: Refactoring from a runtime-value-backed action to a function-backed one without removing the runtime() call.

Related errors

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/d47ed91505eaf56d. Report an issue: GitHub.