diff --git a/instana.gemspec b/instana.gemspec
index 0277f4f6..b637e66a 100644
--- a/instana.gemspec
+++ b/instana.gemspec
@@ -40,6 +40,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "fakefs"
   spec.add_development_dependency "irb"
   spec.add_development_dependency "benchmark"
+  spec.add_development_dependency "rack"
 
   spec.add_runtime_dependency('base64', '>= 0.1')
   spec.add_runtime_dependency('logger')
diff --git a/test/trace/otel_context_leak_test.rb b/test/trace/otel_context_leak_test.rb
new file mode 100644
index 00000000..ff391320
--- /dev/null
+++ b/test/trace/otel_context_leak_test.rb
@@ -0,0 +1,241 @@
+# (c) Copyright IBM Corp. 2026
+
+require 'test_helper'
+require 'rack/mock_request'
+
+# Regression tests for the OTel context-stack corruption bugs where certain
+# instrumentation paths left a non-SpanContext object as the `context` of a
+# non-recording span, causing `NoMethodError` on the next `start_span` call.
+#
+# Each scenario maps directly to a root-cause analysis documented in the
+# companion file `test_app_scenarios.rb`.
+class OtelContextLeakTest < Minitest::Test
+  # -----------------------------------------------------------------------
+  # Setup / Teardown
+  # -----------------------------------------------------------------------
+
+  def setup
+    @rack_app = Rack::Builder.new do
+      use Instana::Rack
+      run ->(_env) { [200, { 'content-type' => 'text/plain' }, ['ok']] }
+    end
+    @mock_request = Rack::MockRequest.new(@rack_app)
+  end
+
+  def teardown
+    # Reset the OTel context stack to ROOT unconditionally. attach+detach is
+    # a no-op here because detach restores the *previous* value (the leaked
+    # frame), not ROOT. Context.clear resets the thread-local directly.
+    OpenTelemetry::Context.clear
+
+    ::Instana.tracer.clear!
+    Instana.agent.define_singleton_method(:ready?) { true }
+    ::Instana.config[:allow_exit_as_root] = false
+    clear_all!
+  end
+
+  # -----------------------------------------------------------------------
+  # Shared helpers
+  # -----------------------------------------------------------------------
+
+  # Builds and attaches the corrupted OTel context frame that the unpatched
+  # Rack middleware would leave behind — a non_recording_span whose `.context`
+  # returns an OpenTelemetry::Context instead of a SpanContext.
+  #
+  # Yields the attached token; always detaches on exit regardless of outcome.
+  def with_corrupted_frame(trace_id: 'abc123', span_id: 'def456')
+    span_ctx       = Instana::SpanContext.new(trace_id: trace_id, span_id: span_id)
+    nrs            = OpenTelemetry::Trace.non_recording_span(span_ctx)
+    corrupted_ctx  = Instana::Trace.context_with_span(nrs) # OTel Context, not SpanContext
+    corrupted_span = OpenTelemetry::Trace.non_recording_span(corrupted_ctx) # .context => OTel Context
+    leaked_frame   = OpenTelemetry::Trace.context_with_span(corrupted_span)
+    token          = OpenTelemetry::Context.attach(leaked_frame)
+    yield token
+  ensure
+    OpenTelemetry::Context.detach(token)
+  end
+
+  # Assert that the span returned by start_span is a live, recording Instana span.
+  def assert_recording_span(span, msg = nil)
+    assert_instance_of Instana::Span, span, msg || 'Expected an Instana::Span'
+    assert span.recording?, "#{msg || 'Span'} must be recording"
+  end
+
+  # -----------------------------------------------------------------------
+  # RACK — context-stack detach (agent down + upstream headers)
+  #
+  # When the agent is down and a request arrives with upstream Instana trace
+  # headers, Rack builds a parent_context (an OTel Context), attaches it, and
+  # — before the fix — never detached it because the `detach` call was inside
+  # the `tracing?` guard.  The poisoned frame survived to the next request,
+  # which crashed with NoMethodError in tracer_provider / samplers.
+  #
+  # Fix: rack.rb ensure block unconditionally calls `Context.detach(@trace_token)`.
+  # -----------------------------------------------------------------------
+
+  def test_rack_detaches_context_token_when_agent_is_down
+    Instana.agent.define_singleton_method(:ready?) { false }
+
+    res = @mock_request.get(
+      '/',
+      'HTTP_X_INSTANA_T' => 'abc123',
+      'HTTP_X_INSTANA_S' => 'def456',
+      'HTTP_X_INSTANA_L' => '1'
+    )
+
+    assert_equal 200, res.status
+    assert_equal OpenTelemetry::Context::ROOT, OpenTelemetry::Context.current,
+                 'Rack must detach its context token even when the agent is down'
+  end
+
+  def test_rack_recovers_cleanly_after_upstream_headers_arrive_while_agent_is_down
+    Instana.agent.define_singleton_method(:ready?) { false }
+    @mock_request.get(
+      '/',
+      'HTTP_X_INSTANA_T' => 'abc123',
+      'HTTP_X_INSTANA_S' => 'def456',
+      'HTTP_X_INSTANA_L' => '1'
+    )
+
+    Instana.agent.define_singleton_method(:ready?) { true }
+
+    res = @mock_request.get('/')
+    assert_equal 200, res.status
+    # The context stack must be clean again after a successful traced request.
+    assert_equal OpenTelemetry::Context::ROOT, OpenTelemetry::Context.current,
+                 'Rack must restore context to ROOT after a normal traced request'
+  end
+
+  # -----------------------------------------------------------------------
+  # REST-CLIENT — explicit with_parent on poisoned stack
+  #
+  # rest-client.rb forwards `OpenTelemetry::Context.current` as `with_parent`
+  # to `start_span`.  When the thread's context stack holds a corrupted frame,
+  # this bypasses the `||=` fallback guard and routes the bad context directly
+  # into `internal_start_span`, hitting the crash in samplers.rb.
+  #
+  # Fix: samplers.rb guards `.tracestate` with `respond_to?`.
+  # -----------------------------------------------------------------------
+
+  def test_rest_client_start_span_tolerates_poisoned_otel_context_on_stack
+    with_corrupted_frame(trace_id: 'abc123', span_id: 'def456') do
+      span = ::Instana.tracer.start_span(:'rest-client', with_parent: OpenTelemetry::Context.current)
+      assert_recording_span span, 'rest-client span'
+      span.finish
+    end
+  end
+
+  # -----------------------------------------------------------------------
+  # NET-HTTP — implicit Context.current fallback on poisoned stack
+  #
+  # net-http.rb calls `start_span(:'net-http')` without `with_parent`, so
+  # tracer.rb falls back to `OpenTelemetry::Context.current`.
+  #
+  # When allow_exit_as_root=false (default): `tracing?` returns false when
+  # `current_span` is nil, so `skip_instrumentation?` short-circuits and
+  # `start_span` is never reached — no crash, span not recorded (expected).
+  #
+  # When allow_exit_as_root=true: `tracing?` returns true, the guard is
+  # bypassed, and `start_span` is called with the poisoned context.
+  #
+  # Fix: same as REST-CLIENT — samplers.rb guards `.tracestate`.
+  # -----------------------------------------------------------------------
+
+  def test_net_http_skips_instrumentation_when_not_tracing_and_exit_as_root_is_off
+    with_corrupted_frame(trace_id: 'aaa111', span_id: 'bbb222') do
+      # Reproduce the skip_instrumentation? logic from net-http.rb line 70-73.
+      # `started?` is an instance method of Net::HTTP and is not testable here
+      # without a live connection, so we omit it — it is not relevant to the
+      # poisoned-context path being exercised.
+      dnt_spans = [:dynamodb, :sqs, :sns, :s3]
+      skip = !Instana.tracer.tracing? ||
+             !Instana.config[:nethttp][:enabled] ||
+             (!::Instana.tracer.current_span.nil? &&
+               dnt_spans.include?(::Instana.tracer.current_span.name))
+
+      assert skip, 'net-http must skip instrumentation when tracing? is false'
+      refute Instana.tracer.tracing?,
+             'tracing? must be false when current_span is nil and allow_exit_as_root is off'
+    end
+  end
+
+  def test_net_http_start_span_tolerates_poisoned_otel_context_when_exit_as_root_is_on
+    ::Instana.config[:allow_exit_as_root] = true
+
+    with_corrupted_frame(trace_id: 'aaa111', span_id: 'bbb222') do
+      assert Instana.tracer.tracing?, 'tracing? must be true when allow_exit_as_root is on'
+
+      span = ::Instana.tracer.start_span(:'net-http')
+      assert_recording_span span, 'net-http span'
+      span.finish
+    end
+  end
+
+  # -----------------------------------------------------------------------
+  # GRPC — corrupted parent_context passed directly as with_parent
+  #
+  # grpc.rb builds a parent_context via `non_recording_span` +
+  # `Trace.context_with_span`, producing an OTel Context (not a SpanContext),
+  # then passes it directly as `with_parent: parent_context`.  With the agent
+  # up this reaches `internal_start_span` → samplers.rb → crash.
+  #
+  # Fix: same as REST-CLIENT — samplers.rb guards `.tracestate`.
+  # -----------------------------------------------------------------------
+
+  def test_grpc_start_span_tolerates_context_wrapping_otel_context_as_parent
+    # Replicate exactly what GRPCServerInstrumentation#handle_* does when
+    # upstream Instana headers are present.
+    incoming_context = Instana::SpanContext.new(
+      trace_id: ::Instana::Util.header_to_id('deadbeef'),
+      span_id: ::Instana::Util.header_to_id('cafebabe')
+    )
+    nrs            = OpenTelemetry::Trace.non_recording_span(incoming_context)
+    parent_context = Instana::Trace.context_with_span(nrs) # OTel Context — the corrupted form
+
+    span = ::Instana.tracer.start_span(:'rpc-server', with_parent: parent_context)
+    assert_recording_span span, 'rpc-server span'
+    span.finish
+  end
+
+  # -----------------------------------------------------------------------
+  # ACTION CABLE — context stored from Rack-poisoned thread
+  #
+  # When the WebSocket upgrade request runs on a Rack-poisoned thread (agent
+  # was down, upstream headers present), `OpenTelemetry::Context.current`
+  # returns the corrupted frame.  ActionCable stores that as `@instana_trace_context`.
+  # A later transmit/dispatch_action wraps it in `non_recording_span`, then
+  # calls `start_span` via `in_span`, which fires the crash.
+  #
+  # Fix: rack.rb detach fix ensures the poison is gone before ActionCable ever
+  #      reads `Context.current`; samplers.rb fix provides a second line of
+  #      defence if any other path produces the same corrupted shape.
+  # -----------------------------------------------------------------------
+
+  def test_action_cable_start_span_tolerates_context_stored_from_rack_poisoned_thread
+    # Step 1: Rack-poison the thread (agent down + upstream headers).
+    Instana.agent.define_singleton_method(:ready?) { false }
+    @mock_request.get(
+      '/',
+      'HTTP_X_INSTANA_T' => 'dead01',
+      'HTTP_X_INSTANA_S' => 'beef02',
+      'HTTP_X_INSTANA_L' => '1'
+    )
+
+    # Step 2: Agent recovers.  ActionCable#process reads Context.current to
+    # store the trace context for the WebSocket session.
+    Instana.agent.define_singleton_method(:ready?) { true }
+    stored_ctx = if ::Instana.tracer.tracing?
+                   ::Instana.tracer.current_span.context
+                 else
+                   OpenTelemetry::Context.current
+                 end
+
+    # Step 3: A later transmit/dispatch_action wraps stored_ctx and starts a span.
+    nrs_cable = OpenTelemetry::Trace.non_recording_span(stored_ctx)
+    Instana::Trace.with_span(nrs_cable) do
+      span = ::Instana.tracer.start_span(:'rpc-server')
+      assert_recording_span span, 'rpc-server span in ActionCable dispatch'
+      span.finish
+    end
+  end
+end