diff --git a/CHANGELOG.md b/CHANGELOG.md
index 069c369..31a6a80 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## 0.7.1 (2026-04-22)
+
+### Changed (behavioral, follow-up to v0.7.0)
+
+- **`Step::Base#run_once` no longer swallows adapter-phase `ArgumentError` as `:input_error`.** The previous blanket `rescue ArgumentError` was there to convert DSL misconfiguration (e.g. missing `prompt`) into an `:input_error` Result. Side effect: programmer bugs in adapter code that raised `ArgumentError` (wrong arity, bad config argument) were silently coerced into `:input_error` and retried as if the user had given bad input. Now the rescue is narrowed to the Runner-construction phase only — DSL configuration errors still produce `:input_error` (the `prompt has not been set` case is regression-tested), but `ArgumentError` raised from adapter code during `Runner#call` propagates to the caller. Input-type validation failures continue to produce `:input_error` through `InputValidator`'s own scoped rescue, unchanged.
+
 ## 0.7.0 (2026-04-21)
 
 ### Breaking changes
diff --git a/Gemfile.lock b/Gemfile.lock
index 68c1daa..f87ef61 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    ruby_llm-contract (0.7.0)
+    ruby_llm-contract (0.7.1)
       dry-types (~> 1.7)
       ruby_llm (~> 1.0)
       ruby_llm-schema (~> 0.3)
@@ -258,7 +258,7 @@ CHECKSUMS
   rubocop-ast (1.49.1) sha256=4412f3ee70f6fe4546cc489548e0f6fcf76cafcfa80fa03af67098ffed755035
   ruby-progressbar (1.13.0) sha256=80fc9c47a9b640d6834e0dc7b3c94c9df37f08cb072b7761e4a71e22cff29b33
   ruby_llm (1.14.0) sha256=57c6f7034fc4a44504ea137d70f853b07824f1c1cdbe774ab3ab3522e7098deb
-  ruby_llm-contract (0.7.0)
+  ruby_llm-contract (0.7.1)
   ruby_llm-schema (0.3.0) sha256=a591edc5ca1b7f0304f0e2261de61ba4b3bea17be09f5cf7558153adfda3dec6
   ruby_parser (3.22.0) sha256=1eb4937cd9eb220aa2d194e352a24dba90aef00751e24c8dfffdb14000f15d23
   rubycritic (4.12.0) sha256=024fed90fe656fa939f6ea80aab17569699ac3863d0b52fd72cb99892247abc8
diff --git a/README.md b/README.md
index 320ed37..8dcf422 100644
--- a/README.md
+++ b/README.md
@@ -350,7 +350,9 @@ Full procedure with examples: **[Optimizing retry_policy](docs/guide/optimizing_
 
 ## Roadmap
 
-**v0.7 (current):** Sharpened retry semantics. `DEFAULT_RETRY_ON` now targets LLM output variance only (`:validation_failed`, `:parse_error`); transport errors are delegated to ruby_llm's Faraday retry. `AdapterCaller` narrowed to let programmer errors propagate instead of masking them as retries. Breaking change — see [CHANGELOG](CHANGELOG.md) for migration.
+**v0.7.1 (current):** Follow-up — `Step::Base#run_once` no longer masks adapter-phase `ArgumentError` as `:input_error`. Programmer bugs in adapter code now propagate; DSL misconfiguration still becomes `:input_error` via narrower rescue.
+
+**v0.7.0:** Sharpened retry semantics. `DEFAULT_RETRY_ON` now targets LLM output variance only (`:validation_failed`, `:parse_error`); transport errors are delegated to ruby_llm's Faraday retry. `AdapterCaller` narrowed to let programmer errors propagate instead of masking them as retries. Breaking change — see [CHANGELOG](CHANGELOG.md) for migration.
 
 **v0.6:** "What should I do?" — `Step.recommend` returns optimal model, reasoning effort, and retry chain. Per-attempt `reasoning_effort` in retry policies.
 
diff --git a/lib/ruby_llm/contract/step/base.rb b/lib/ruby_llm/contract/step/base.rb
index 149d786..85b7a5b 100644
--- a/lib/ruby_llm/contract/step/base.rb
+++ b/lib/ruby_llm/contract/step/base.rb
@@ -186,20 +186,36 @@ def resolve_adapter(context)
                                             "{ |c| c.default_adapter = ... } or pass context: { adapter: ... }"
           end
 
+          # ADR-0021 deliverable 2: narrow ArgumentError rescue to DSL-setup phase only.
+          #
+          # DSL misconfiguration (e.g. `prompt has not been set`, missing required
+          # attributes) surfaces as ArgumentError when constructing Runner. We catch
+          # that and return :input_error — these are contract-definition issues the
+          # caller can handle as "bad input to the step definition".
+          #
+          # Runner#call itself does NOT get a blanket rescue: input-type validation
+          # failures return :input_error from within InputValidator; adapter/runtime
+          # programmer bugs (NoMethodError, adapter-code ArgumentError) must propagate
+          # instead of being silently masked as :input_error.
           def run_once(input, adapter:, model:, context_temperature: nil, extra_options: {})
             effective_temp = context_temperature || temperature
-            Runner.new(
-              input_type: input_type, output_type: output_type,
-              prompt_block: prompt, contract_definition: effective_contract,
-              adapter: adapter, model: model, output_schema: output_schema,
-              max_output: max_output, max_input: max_input, max_cost: max_cost,
-              on_unknown_pricing: on_unknown_pricing,
-              temperature: effective_temp, extra_options: extra_options,
-              observers: class_observers
-            ).call(input)
-          rescue ArgumentError => e
-            Result.new(status: :input_error, raw_output: nil, parsed_output: nil,
-                       validation_errors: [e.message])
+            runner =
+              begin
+                Runner.new(
+                  input_type: input_type, output_type: output_type,
+                  prompt_block: prompt, contract_definition: effective_contract,
+                  adapter: adapter, model: model, output_schema: output_schema,
+                  max_output: max_output, max_input: max_input, max_cost: max_cost,
+                  on_unknown_pricing: on_unknown_pricing,
+                  temperature: effective_temp, extra_options: extra_options,
+                  observers: class_observers
+                )
+              rescue ArgumentError => e
+                return Result.new(status: :input_error, raw_output: nil, parsed_output: nil,
+                                  validation_errors: [e.message])
+              end
+
+            runner.call(input)
           end
 
           def log_result(result)
diff --git a/lib/ruby_llm/contract/version.rb b/lib/ruby_llm/contract/version.rb
index 044e181..fc8fbde 100644
--- a/lib/ruby_llm/contract/version.rb
+++ b/lib/ruby_llm/contract/version.rb
@@ -2,6 +2,6 @@
 
 module RubyLLM
   module Contract
-    VERSION = "0.7.0"
+    VERSION = "0.7.1"
   end
 end
diff --git a/spec/ruby_llm/contract/step/retry_integration_spec.rb b/spec/ruby_llm/contract/step/retry_integration_spec.rb
index 1ba4582..df8c154 100644
--- a/spec/ruby_llm/contract/step/retry_integration_spec.rb
+++ b/spec/ruby_llm/contract/step/retry_integration_spec.rb
@@ -513,5 +513,31 @@
 
       expect { step.run("hi", context: { adapter: adapter }) }.to raise_error(NoMethodError)
     end
+
+    # ADR-0021 deliverable 2: programmer ArgumentError from adapter code must
+    # propagate, not be silently coerced into :input_error by Step::Base#run_once.
+    # Before the fix, a blanket `rescue ArgumentError` around the whole runner
+    # chain masked adapter bugs as "bad user input".
+    it "propagates ArgumentError from adapter code (programmer bug, not bad input)" do
+      adapter = Object.new
+      adapter.define_singleton_method(:call) do |**_opts|
+        raise ArgumentError, "adapter called with wrong arity — this is a bug"
+      end
+
+      step = Class.new(RubyLLM::Contract::Step::Base) { prompt "{input}" }
+
+      expect { step.run("hi", context: { adapter: adapter }) }
+        .to raise_error(ArgumentError, /adapter called with wrong arity/)
+    end
+
+    it "still converts DSL misconfiguration ArgumentError to :input_error (prompt missing)" do
+      adapter = RubyLLM::Contract::Adapters::Test.new(response: "ok")
+      step = Class.new(RubyLLM::Contract::Step::Base) { output_type String }
+
+      result = step.run("hi", context: { adapter: adapter })
+
+      expect(result.status).to eq(:input_error)
+      expect(result.validation_errors.first).to include("prompt has not been set")
+    end
   end
 end