diff --git a/.circleci/config.yml b/.circleci/config.yml
index 6a13a208..f5d4b34b 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -3,10 +3,18 @@ version: 2
 jobs:
   test-crystal: &test-template
     docker:
-      - image: crystallang/crystal:0.29.0
+      - image: crystallang/crystal:0.31.1
     steps:
       - checkout
-  test:
+  test-crystal-0.30.1:
+    <<: *test-template
+    docker:
+      - image: crystallang/crystal:0.30.1
+    steps:
+      - checkout
+      - run: shards
+      - run: SPEC_VERBOSE=1 ./bin/test
+  test-crystal-0.31.1:
     <<: *test-template
     steps:
       - checkout
@@ -29,7 +37,8 @@ workflows:
   version: 2
   ci:
     jobs:
-      - test
+      - test-crystal-0.31.1
+      - test-crystal-0.30.1
   nightly:
     triggers:
       - schedule:
@@ -38,5 +47,6 @@ workflows:
             branches:
               only: master
     jobs:
-      - test
+      - test-crystal-0.31.1
+      - test-crystal-0.30.1
       - test-mutations
diff --git a/CHANGELOG.md b/CHANGELOG.md
index bab29ce7..d915c2c8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased] - Unknown
 
+- Crystal 0.31.x compatibility (no changes were needed for crystal 0.30.x). Be careful
+  with crystal 0.31.0 and 0.31.1 because there is a bug that might cause your CI job
+  to pass even with failing tests. See [#8420](https://github.com/crystal-lang/crystal/issues/8420).
+
 ## [6.0.0] - 2019-06-30
 
 ### Changed
diff --git a/shard.yml b/shard.yml
index 7c9c8c4a..f36b36b8 100644
--- a/shard.yml
+++ b/shard.yml
@@ -4,7 +4,7 @@ version: 6.0.0
 authors:
   - Hannes Käufler <hannes.kaeufler@gmail.com>
 
-crystal: 0.29.0
+crystal: 0.31.1
 
 license: MIT
 
diff --git a/spec/cli_options_spec.cr b/spec/cli_options_spec.cr
index 863d57ff..cdd28153 100644
--- a/spec/cli_options_spec.cr
+++ b/spec/cli_options_spec.cr
@@ -101,7 +101,11 @@ module Crytic
       it "defaults to a fail fast preamble" do
         opts = cli_options_parser.parse([] of String)
 
-        opts.preamble.should eq Generator::Generator::DEFAULT_PREAMBLE
+        {% if Crystal::VERSION == "0.31.0" || Crystal::VERSION == "0.31.1" %}
+          opts.preamble.should eq "require \"spec\"\n\n"
+        {% else %}
+          opts.preamble.should eq Generator::Generator::DEFAULT_PREAMBLE
+        {% end %}
       end
 
       {% for flag in ["-m", "--min-msi"] %}
diff --git a/spec/fake_generator.cr b/spec/fake_generator.cr
index 89e842ef..696fd6f3 100644
--- a/spec/fake_generator.cr
+++ b/spec/fake_generator.cr
@@ -6,7 +6,7 @@ class FakeGenerator < Crytic::Generator::Generator
                  @neutral = FakeMutation.new.as(Crytic::Mutation::Mutation))
   end
 
-  def mutations_for(source : Array(Crytic::Subject), specs : Array(String))
+  def mutations_for(source : Array(Crytic::Subject), specs : Array(String)) : Array(Crytic::Generator::MutationSet)
     [Crytic::Generator::MutationSet.new(
       neutral: @neutral,
       mutated: @mutations)]
diff --git a/spec/fake_mutation.cr b/spec/fake_mutation.cr
index c9f4c763..8a986eb6 100644
--- a/spec/fake_mutation.cr
+++ b/spec/fake_mutation.cr
@@ -16,7 +16,7 @@ class FakeMutation < Crytic::Mutation::Mutation
   def initialize(@reported_status = Crytic::Mutation::Status::Uncovered)
   end
 
-  def run
+  def run : Crytic::Mutation::Result
     @run_call_count += 1
     Crytic::Mutation::Result.new(@reported_status, irrelevant_mutant, "", "")
   end
diff --git a/spec/integration_spec.cr b/spec/integration_spec.cr
index 08a03e5e..301bcc05 100644
--- a/spec/integration_spec.cr
+++ b/spec/integration_spec.cr
@@ -1,96 +1,96 @@
 require "./spec_helper"
 
 {% unless flag?("skip-integration") %}
-describe Crytic do
-  describe "--help/-h" do
-    it "prints usage info" do
-      result = run_crytic("--help")
-      result.output.should contain("Usage: crytic [arguments]")
-      result.exit_code.should eq 0
-      result = run_crytic("-h")
-      result.output.should contain("Usage: crytic [arguments]")
-      result.exit_code.should eq 0
+  describe Crytic do
+    describe "--help/-h" do
+      it "prints usage info" do
+        result = run_crytic("--help")
+        result.output.should contain("Usage: crytic [arguments]")
+        result.exit_code.should eq 0
+        result = run_crytic("-h")
+        result.output.should contain("Usage: crytic [arguments]")
+        result.exit_code.should eq 0
+      end
     end
-  end
 
-  describe "--preamble/-p" do
-    it "injects the given custom preamble, failing the neutral mutant" do
-      result = run_crytic("-s ./fixtures/conditionals/fully_covered.cr ./fixtures/conditionals/uncovered_spec.cr -p 'exit 1'")
-      result.output.should contain("unmodified subject")
-      result.output.should_not contain("ConditionFlip")
-      result.exit_code.should eq 1
+    describe "--preamble/-p" do
+      it "injects the given custom preamble, failing the neutral mutant" do
+        result = run_crytic("-s ./fixtures/conditionals/fully_covered.cr ./fixtures/conditionals/uncovered_spec.cr -p 'exit 1'")
+        result.output.should contain("unmodified subject")
+        result.output.should_not contain("ConditionFlip")
+        result.exit_code.should eq 1
+      end
     end
-  end
 
-  describe "with a fully covered subject" do
-    it "passes the mutation specs" do
-      result = run_crytic("-s ./fixtures/conditionals/fully_covered.cr ./fixtures/conditionals/fully_covered_spec.cr")
-      result.output.should contain("✅ ConditionFlip")
-      result.output.should contain("✅ BoolLiteralFlip")
-      result.output.should contain("3 covered")
-      result.exit_code.should eq 0
+    describe "with a fully covered subject" do
+      it "passes the mutation specs" do
+        result = run_crytic("-s ./fixtures/conditionals/fully_covered.cr ./fixtures/conditionals/fully_covered_spec.cr")
+        result.output.should contain("✅ ConditionFlip")
+        result.output.should contain("✅ BoolLiteralFlip")
+        result.output.should contain("3 covered")
+        result.exit_code.should eq 0
+      end
     end
-  end
 
-  describe "with an insufficiently covered subject" do
-    it "fails the mutation specs" do
-      result = run_crytic("-s ./fixtures/conditionals/fully_covered.cr ./fixtures/conditionals/uncovered_spec.cr")
-      result.output.should contain("❌ ConditionFlip")
-      result.output.should contain("❌ BoolLiteralFlip")
-      result.output.should contain("3 uncovered")
-      result.exit_code.should be > 0
-    end
+    describe "with an insufficiently covered subject" do
+      it "fails the mutation specs" do
+        result = run_crytic("-s ./fixtures/conditionals/fully_covered.cr ./fixtures/conditionals/uncovered_spec.cr")
+        result.output.should contain("❌ ConditionFlip")
+        result.output.should contain("❌ BoolLiteralFlip")
+        result.output.should contain("3 uncovered")
+        result.exit_code.should be > 0
+      end
 
-    it "exits successfully when the msi threshold is set sufficiently" do
-      result = run_crytic("--min-msi=0.0 -s ./fixtures/conditionals/fully_covered.cr ./fixtures/conditionals/uncovered_spec.cr")
-      result.exit_code.should eq 0
+      it "exits successfully when the msi threshold is set sufficiently" do
+        result = run_crytic("--min-msi=0.0 -s ./fixtures/conditionals/fully_covered.cr ./fixtures/conditionals/uncovered_spec.cr")
+        result.exit_code.should eq 0
+      end
     end
-  end
 
-  describe "without passing a subject or tests" do
-    it "mutates all sources and runs all tests" do
-      result = run_crytic_in_dir("./fixtures/autofind")
-      result.output.should contain("✅ ConditionFlip")
-      result.output.should contain("✅ BoolLiteralFlip")
-      result.output.should contain("✅ NumberLiteralSignFlip")
-      result.output.should contain("✅ NumberLiteralChange")
-      result.output.should contain("❌ NumberLiteralSignFlip")
-      result.output.should contain("❌ NumberLiteralChange")
-      result.output.should contain("2 uncovered")
-      result.exit_code.should be > 0
+    describe "without passing a subject or tests" do
+      it "mutates all sources and runs all tests" do
+        result = run_crytic_in_dir("./fixtures/autofind")
+        result.output.should contain("✅ ConditionFlip")
+        result.output.should contain("✅ BoolLiteralFlip")
+        result.output.should contain("✅ NumberLiteralSignFlip")
+        result.output.should contain("✅ NumberLiteralChange")
+        result.output.should contain("❌ NumberLiteralSignFlip")
+        result.output.should contain("❌ NumberLiteralChange")
+        result.output.should contain("2 uncovered")
+        result.exit_code.should be > 0
+      end
     end
-  end
 
-  describe "subject without any coverage" do
-    it "fails all mutants" do
-      result = run_crytic("-s ./fixtures/uncovered/without.cr ./fixtures/uncovered/without_spec.cr")
-      result.output.should contain("❌ BoolLiteralFlip")
-      result.output.should contain("❌ ConditionFlip")
-      result.output.should contain("❌ NumberLiteralSignFlip")
-      result.output.should contain("❌ NumberLiteralChange")
-      result.output.should contain("9 uncovered")
-      result.exit_code.should be > 0
+    describe "subject without any coverage" do
+      it "fails all mutants" do
+        result = run_crytic("-s ./fixtures/uncovered/without.cr ./fixtures/uncovered/without_spec.cr")
+        result.output.should contain("❌ BoolLiteralFlip")
+        result.output.should contain("❌ ConditionFlip")
+        result.output.should contain("❌ NumberLiteralSignFlip")
+        result.output.should contain("❌ NumberLiteralChange")
+        result.output.should contain("9 uncovered")
+        result.exit_code.should be > 0
+      end
     end
-  end
 
-  describe "a failing initial test suite" do
-    it "reports initial failure" do
-      result = run_crytic("-s ./fixtures/uncovered/without.cr ./fixtures/failing/failing_spec.cr")
-      result.output.should contain "❌ Original test suite failed.\n"
-      result.output.should contain "no overload matches"
-      result.exit_code.should be > 0
+    describe "a failing initial test suite" do
+      it "reports initial failure" do
+        result = run_crytic("-s ./fixtures/uncovered/without.cr ./fixtures/failing/failing_spec.cr")
+        result.output.should contain "❌ Original test suite failed.\n"
+        result.output.should contain "no overload matches"
+        result.exit_code.should be > 0
+      end
     end
-  end
 
-  describe "a subject that is mutated into an endless loop" do
-    it "finishes and reports a timed out spec" do
-      result = run_crytic("-s ./fixtures/timeout/timeout.cr ./fixtures/timeout/timeout_spec.cr")
-      result.output.should contain "✅ Original test suite passed.\n"
-      result.output.should contain "1 timeout"
-      result.exit_code.should be > 0
+    describe "a subject that is mutated into an endless loop" do
+      it "finishes and reports a timed out spec" do
+        result = run_crytic("-s ./fixtures/timeout/timeout.cr ./fixtures/timeout/timeout_spec.cr")
+        result.output.should contain "✅ Original test suite passed.\n"
+        result.output.should contain "1 timeout"
+        result.exit_code.should be > 0
+      end
     end
   end
-end
 {% end %}
 
 def run_crytic_in_dir(dir : String)
diff --git a/src/crytic/cli_options.cr b/src/crytic/cli_options.cr
index d07fe82d..88370df7 100644
--- a/src/crytic/cli_options.cr
+++ b/src/crytic/cli_options.cr
@@ -2,6 +2,7 @@ require "./generator/generator"
 require "./mutant/possibilities"
 require "./reporter/*"
 require "./side_effects"
+require "colorize"
 require "option_parser"
 
 module Crytic
@@ -9,8 +10,8 @@ module Crytic
     DEFAULT_SPEC_FILES_GLOB = "./spec/**/*_spec.cr"
     getter msi_threshold = 100.0
     getter mutants : Array(Mutant::Possibilities) = Generator::Generator::ALL_MUTANTS
-    getter preamble = Generator::Generator::DEFAULT_PREAMBLE
     getter reporters = [] of Reporter::Reporter
+    @preamble = Generator::Generator::DEFAULT_PREAMBLE
     @spec_files = [] of String
     @subject = [] of String
 
@@ -87,6 +88,24 @@ module Crytic
       end.map { |path| Subject.from_filepath(path) }
     end
 
+    def preamble
+      {% if Crystal::VERSION == "0.31.1" || Crystal::VERSION == "0.31.0" %}
+        if @preamble =~ /fail_fast/
+          puts(<<-MSG.colorize(:yellow))
+
+          ⚠️  You specified a preamble that uses Spec#fail_fast. Due to a bug
+             in crystal 0.31.0 and 0.31.1, the Spec#fail_fast mode cannot be
+             used in crytic. We disabled it for you. See
+             https://github.com/crystal-lang/crystal/issues/8420 for details.
+
+          MSG
+          return @preamble.gsub(/Spec\.fail_fast\s*=\s*true/, "")
+        end
+      {% end %}
+
+      @preamble
+    end
+
     private def console_reporter
       Reporter::IoReporter.new(@side_effects.std_out)
     end
diff --git a/src/crytic/generator/in_memory_generator.cr b/src/crytic/generator/in_memory_generator.cr
index 768e468b..18539509 100644
--- a/src/crytic/generator/in_memory_generator.cr
+++ b/src/crytic/generator/in_memory_generator.cr
@@ -15,7 +15,7 @@ module Crytic::Generator
     )
     end
 
-    def mutations_for(sources : Array(Subject), specs : Array(String))
+    def mutations_for(sources : Array(Subject), specs : Array(String)) : Array(MutationSet)
       sources
         .map do |src|
           MutationSet.new(
diff --git a/src/crytic/mutation/isolated_mutation.cr b/src/crytic/mutation/isolated_mutation.cr
index 2cfb6bb7..78f3f1d4 100644
--- a/src/crytic/mutation/isolated_mutation.cr
+++ b/src/crytic/mutation/isolated_mutation.cr
@@ -14,10 +14,10 @@ module Crytic::Mutation
 
     # Compiles the mutated source code into a binary and runs this binary,
     # recording exit code, stderr and stdout output.
-    def run
+    def run : Result
       mutated = @environment.perform_mutation
       process_result = run(mutated)
-      success_messages_in_output = /Finished/ =~ process_result[:output]
+      success_messages_in_output = /F/ =~ process_result[:output]
       status = if process_result[:exit_code] == ProcessRunner::SUCCESS
                  Status::Uncovered
                elsif process_result[:exit_code] == ProcessRunner::TIMEOUT
diff --git a/src/crytic/reporter/io_reporter.cr b/src/crytic/reporter/io_reporter.cr
index dfcb9770..182c07ed 100644
--- a/src/crytic/reporter/io_reporter.cr
+++ b/src/crytic/reporter/io_reporter.cr
@@ -8,7 +8,7 @@ module Crytic::Reporter
   class IoReporter < Reporter
     INDENT = "    "
 
-    def initialize(@io : IO, @start_time = Time.now)
+    def initialize(@io : IO, @start_time = Time.utc)
     end
 
     def report_original_result(original_result)
@@ -77,7 +77,7 @@ module Crytic::Reporter
     end
 
     private def elapsed_time
-      Time.now - @start_time
+      Time.utc - @start_time
     end
   end
 end