OpenFeign/feign · error · UnsupportedOperationException

Streaming Decoding is not supported.

Error message

Streaming Decoding is not supported.

What it means

The reactive Feign builder overrides doNotCloseAfterDecode() to throw UnsupportedOperationException, because streaming (keep-alive after decode) semantics conflict with the reactive model where the publisher itself controls consumption and connection release. Calling this builder option on the reactive builder always fails immediately.

Solutions

1. Remove the doNotCloseAfterDecode() call from the reactive builder configuration
2. Obtain streaming behavior through the reactive publisher itself (e.g. Flux<T> with backpressure) instead
3. If streaming decode is truly required, use the non-reactive Feign builder that supports it

Example fix

// before
ReactorFeign.builder().doNotCloseAfterDecode().target(Api.class, url);
// after
ReactorFeign.builder().target(Api.class, url);

if (builder instanceof feign.reactive.ReactiveFeign.Builder) {
  // do not call doNotCloseAfterDecode()
}

When it happens

Trigger: Calling builder.doNotCloseAfterDecode() on ReactiveFeign.Builder / ReactorFeign.Builder / RxJavaFeign.Builder while configuring the client.

Common situations: Porting configuration code from a streaming-capable Feign builder to the reactive builder; enabling streaming decode globally in shared builder utility code.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Invocation Handler Factory overrides are not supported.
• Invocation Handler Factory overrides are not supported.
• Empty targets don't have URLs
• at least one decoder is required
• at least one encoder is required

AI-assisted analysis of OpenFeign/feign@e2a1e27560 (2026-09-10). Data as JSON: /api/errors/41af5d4bcd47ae92. Report an issue: GitHub.

Appendix: source

Thrown at reactive/src/main/java/feign/reactive/ReactiveFeign.java:57

    /**
     * Build the Feign instance.
     *
     * @return a new Feign Instance.
     */
    @Override
    public Feign internalBuild() {
      if (!(this.contract instanceof ReactiveDelegatingContract)) {
        super.contract(new ReactiveDelegatingContract(this.contract));
      } else {
        super.contract(this.contract);
      }
      return super.internalBuild();
    }

    @Override
    public Feign.Builder doNotCloseAfterDecode() {
      throw new UnsupportedOperationException("Streaming Decoding is not supported.");
    }
  }
}

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