diff --git a/docs/internal/overview.md b/docs/internal/overview.md
index 0e4e120..bfbbb2b 100644
--- a/docs/internal/overview.md
+++ b/docs/internal/overview.md
@@ -46,7 +46,7 @@ these implemented packages:
 | `internal/tempdir` | Disposable system temporary storage outside source worktrees. |
 | `internal/process` | Cancellable child-process execution with per-command environment overlays and removals. |
 | `internal/preparation` | Coordination for expected-commit tag checkout and Go module dependency preparation. |
-| `internal/validation` | Shared default Go validation for captured candidates; `internal/candidate` retains capture, callback sequencing, and local-state rechecks. |
+| `internal/validation` | Shared default Go validation and structured check failures for captured candidates; `internal/candidate` retains capture, callback sequencing, and local-state rechecks. |
 | `internal/publish` | Publication guards, one validation run, annotated tag creation, push, and recovery state. |
 | `internal/runtimeinfo` | Private version lookup used by the runtime facade. |
 | `internal/versioncontract` | Shared stable-version rules and linker-symbol identity. |
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index 2384b26..b7ae2cf 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -1,6 +1,6 @@
 # Validation model implementation plan
 
-Status: In progress; the shared validation executor is complete.
+Status: In progress; the shared validation executor and internal diagnostics are complete.
 
 This plan implements the enabling work in [model.md](model.md), guided by
 [ADR 0003](../adr/0003-declarative-validation.md). It is written for a Luna
@@ -206,6 +206,10 @@ inventory to reflect the implemented executor.
 
 ## Stage 2: Introduce internal validation diagnostics
 
+**Completion:** Complete. Shared Go setup and test failures carry the internal
+`go-test` identity and preserve their underlying errors, including cancellation
+and subprocess exit errors.
+
 **Outcome:** The executor returns a small structured failure with an underlying
 cause, independently of public error conversion.
 
diff --git a/internal/candidate/check_test.go b/internal/candidate/check_test.go
index e49098c..1167ac8 100644
--- a/internal/candidate/check_test.go
+++ b/internal/candidate/check_test.go
@@ -9,6 +9,8 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+
+	"gitea.maximumdirect.net/eric/ReleaseKit/internal/validation"
 )
 
 const testVersion = "v1.2.3"
@@ -123,6 +125,14 @@ func TestDefaultValidationDoesNotInheritGOFLAGSRunFilter(t *testing.T) {
 			if err == nil || !strings.Contains(output.String(), "TestExpectedFailure") {
 				t.Fatalf("Check() error = %v, output = %q; wanted the failing standard test to run", err, output.String())
 			}
+			var failure *validation.Failure
+			if !errors.As(err, &failure) || failure.Check != validation.GoTest {
+				t.Fatalf("Check() error = %v; want a go-test validation failure", err)
+			}
+			var exitErr *exec.ExitError
+			if !errors.As(err, &exitErr) {
+				t.Fatalf("Check() error = %v; want the Go process exit error preserved", err)
+			}
 		})
 	}
 }
diff --git a/internal/validation/diagnostic.go b/internal/validation/diagnostic.go
new file mode 100644
index 0000000..516f8cd
--- /dev/null
+++ b/internal/validation/diagnostic.go
@@ -0,0 +1,92 @@
+package validation
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Check identifies one shared validation check.
+type Check string
+
+const (
+	// GoTest identifies the base Go test suite.
+	GoTest Check = "go-test"
+)
+
+// Finding describes a validation observation at an optional source location.
+type Finding struct {
+	Path      string
+	Line      int
+	Column    int
+	Message   string
+	Target    string
+	Directive string
+}
+
+// FileSizeMeasurement describes a measured file size and its selected limit.
+type FileSizeMeasurement struct {
+	Bytes      int64
+	LimitBytes int64
+}
+
+// Failure describes a failed shared validation check and preserves its cause.
+type Failure struct {
+	Check       Check
+	Findings    []Finding
+	Measurement *FileSizeMeasurement
+	Cause       error
+}
+
+// Error returns a useful summary of the failed check.
+func (failure *Failure) Error() string {
+	return formatFailure(failure)
+}
+
+// Unwrap returns the underlying validation error.
+func (failure *Failure) Unwrap() error {
+	if failure == nil {
+		return nil
+	}
+	return failure.Cause
+}
+
+func formatFailure(failure *Failure) string {
+	if failure == nil {
+		return "<nil>"
+	}
+	var summary strings.Builder
+	if failure.Check == "" {
+		summary.WriteString("validation failed")
+	} else {
+		fmt.Fprintf(&summary, "%s validation failed", failure.Check)
+	}
+	for _, finding := range failure.Findings {
+		summary.WriteString(": ")
+		if finding.Path != "" {
+			summary.WriteString(finding.Path)
+			if finding.Line > 0 {
+				fmt.Fprintf(&summary, ":%d", finding.Line)
+				if finding.Column > 0 {
+					fmt.Fprintf(&summary, ":%d", finding.Column)
+				}
+			}
+			summary.WriteString(": ")
+		}
+		summary.WriteString(finding.Message)
+	}
+	if failure.Measurement != nil {
+		fmt.Fprintf(&summary, ": measured %d bytes against a %d byte limit", failure.Measurement.Bytes, failure.Measurement.LimitBytes)
+	}
+	if failure.Cause != nil {
+		if len(failure.Findings) == 0 && failure.Measurement == nil {
+			summary.WriteString(": ")
+		} else {
+			summary.WriteString("; ")
+		}
+		summary.WriteString(failure.Cause.Error())
+	}
+	return summary.String()
+}
+
+var _ error = (*Failure)(nil)
+var _ interface{ Unwrap() error } = (*Failure)(nil)
diff --git a/internal/validation/diagnostic_test.go b/internal/validation/diagnostic_test.go
new file mode 100644
index 0000000..57d3cdd
--- /dev/null
+++ b/internal/validation/diagnostic_test.go
@@ -0,0 +1,49 @@
+package validation
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+)
+
+func TestRunPreservesCanceledGoSetup(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	err := Run(ctx, Request{RepositoryDir: t.TempDir()})
+	var failure *Failure
+	if !errors.As(err, &failure) {
+		t.Fatalf("Run() error = %v, want a validation failure", err)
+	}
+	if failure.Check != GoTest {
+		t.Fatalf("failure check = %q, want %q", failure.Check, GoTest)
+	}
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("Run() error = %v, want context cancellation preserved", err)
+	}
+}
+
+func TestFailureFormatsDiagnosticsAndUnwrapsCause(t *testing.T) {
+	wantCause := errors.New("underlying check error")
+	failure := &Failure{
+		Check: GoTest,
+		Findings: []Finding{{
+			Path:    "README.md",
+			Line:    4,
+			Column:  2,
+			Message: "broken destination",
+		}},
+		Measurement: &FileSizeMeasurement{Bytes: 12, LimitBytes: 10},
+		Cause:       wantCause,
+	}
+
+	if !errors.Is(failure, wantCause) {
+		t.Fatalf("Failure does not unwrap its cause")
+	}
+	for _, part := range []string{"go-test", "README.md:4:2", "broken destination", "12 bytes", "10 byte limit", wantCause.Error()} {
+		if !strings.Contains(failure.Error(), part) {
+			t.Errorf("Failure.Error() = %q; want it to include %q", failure.Error(), part)
+		}
+	}
+}
diff --git a/internal/validation/validation.go b/internal/validation/validation.go
index 0b1ee6b..890ca70 100644
--- a/internal/validation/validation.go
+++ b/internal/validation/validation.go
@@ -3,7 +3,6 @@ package validation
 
 import (
 	"context"
-	"fmt"
 	"io"
 
 	"gitea.maximumdirect.net/eric/ReleaseKit/internal/gotool"
@@ -25,9 +24,12 @@ type Request struct {
 func Run(ctx context.Context, request Request) error {
 	tool, err := gotool.New(ctx, request.RepositoryDir)
 	if err != nil {
-		return fmt.Errorf("prepare Go toolchain: %w", err)
+		return &Failure{Check: GoTest, Cause: err}
 	}
 	_, err = tool.Run(ctx, request.RepositoryDir, gotool.Native, gotool.Target{}, request.Output,
 		"test", "-mod=readonly", "-count=1", "./...")
-	return err
+	if err != nil {
+		return &Failure{Check: GoTest, Cause: err}
+	}
+	return nil
 }