diff --git a/docs/implementation-plans/linear-static-3d-euler-beam-implementation-report.md b/docs/implementation-plans/linear-static-3d-euler-beam-implementation-report.md
index d457e4a..5f0139f 100644
--- a/docs/implementation-plans/linear-static-3d-euler-beam-implementation-report.md
+++ b/docs/implementation-plans/linear-static-3d-euler-beam-implementation-report.md
@@ -45,3 +45,43 @@
 - handoff: `fesa_solver`, `fesa_unit_tests`, `fesa_tests`, normalized
   `Fesa::MKL`, `Fesa::TBB`, `Fesa::HDF5`, and `solverVersion()` are available
   to Step 8.
+
+## Step 8 — core-diagnostics
+
+- task_id: `TASK-08`
+- status: `completed`
+- changed_files: `include/fesa/core/source_identity.hpp`,
+  `include/fesa/core/diagnostic.hpp`, `include/fesa/core/status.hpp`,
+  `src/fesa/core/diagnostic.cpp`, `src/fesa/core/status.cpp`,
+  `src/fesa/CMakeLists.txt`, `tests/CMakeLists.txt`,
+  `tests/unit/core/source_identity_test.cpp`,
+  `tests/unit/core/diagnostic_test.cpp`, `tests/unit/core/status_test.cpp`,
+  `docs/implementation-plans/linear-static-3d-euler-beam-implementation-report.md`
+- requirement_ids: `FESA-REQ-LS3DEB-033`, `FESA-REQ-LS3DEB-034`
+- test_ids: `T08-CORE-001`, `T08-CORE-002`, `T08-CORE-003`
+
+| stage | exact command | exit_code | expected_or_observed_result | evidence_tail |
+| --- | --- | ---: | --- | --- |
+| RED | `cmake --build .harness/build --config Debug --target fesa_tests` | 1 | Tests are registered before production and fail for the three missing core headers | MSVC C1083 for `fesa/core/diagnostic.hpp`, `fesa/core/source_identity.hpp`, and `fesa/core/status.hpp` |
+| GREEN-build | `cmake --build .harness/build --config Debug --target fesa_tests` | 0 | Minimal core implementation and all test translation units build | `diagnostic.cpp`, `status.cpp`, and three core tests compile; `fesa_unit_tests.exe` links |
+| GREEN-test | `ctest --test-dir .harness/build -C Debug -R CoreDiagnostics --output-on-failure` | 0 | Identity, deterministic ordering, and Result exclusivity pass | 3/3 `CoreDiagnostics` tests passed |
+| VERIFY-configure | `cmake -S . -B .harness/build -A x64 -DFESA_GTEST_SOURCE_DIR=C:/git/googletest "-DMKL_DIR=C:/Program Files (x86)/Intel/oneAPI/mkl/2026.1/lib/cmake/mkl" "-DTBB_DIR=C:/Program Files (x86)/Intel/oneAPI/tbb/2023.1/lib/cmake/tbb" "-DHDF5_DIR=C:/Program Files/HDF_Group/HDF5/2.1.1/cmake"` | 0 | Approved MSVC x64 build tree regenerates with normalized dependencies | Configure and generate completed; MKL 2026.1 resolved |
+| VERIFY-build | `cmake --build .harness/build --config Debug` | 0 | Full Debug build passes without a new FESA warning | `fesa_solver.lib` and `fesa_unit_tests.exe` built under `/W4 /WX` |
+| VERIFY-targeted | `ctest --test-dir .harness/build -C Debug -R CoreDiagnostics --output-on-failure` | 0 | Focused Step 8 suite remains green | 3/3 tests passed |
+| VERIFY-discovery | `ctest --test-dir .harness/build -C Debug --show-only=json-v1` | 0 | CTest discovers the existing and new named tests | 5 tests discovered with feature and unit labels |
+| VERIFY-full | `ctest --test-dir .harness/build -C Debug --output-on-failure` | 0 | Full accumulated C++ suite has zero failures | 5/5 tests passed |
+| VERIFY-harness | `uv run --with pytest python -m pytest -v -rs` | 0 | Harness regression suite remains green | 4/4 tests passed |
+
+- contract_checks: `SourceEntityId` preserves numeric and raw label forms;
+  `sortDiagnostics()` applies the fixed file/line/keyword/entity/code tuple with
+  stable discovery-order ties; `Status` owns sorted diagnostics and optional
+  failure category; `Result<T>` owns either a value or failed status and rejects
+  both failed value access and construction from an OK status. Core public
+  headers include only FESA core or C++ standard-library headers; no
+  MKL/TBB/HDF5 names are present.
+- generated_evidence: `.harness/build/src/fesa/Debug/fesa_solver.lib`,
+  `.harness/build/tests/Debug/fesa_unit_tests.exe`
+- reference_diff: unchanged
+- handoff: `SourceLocation`, `SourceEntityId`, `Diagnostic`, deterministic
+  diagnostic sorting, `FailureCategory`, `Status`, and `Result<T>` are available
+  to Step 9 and later parser/model/solver tasks.
diff --git a/include/fesa/core/diagnostic.hpp b/include/fesa/core/diagnostic.hpp
new file mode 100644
index 0000000..df68cc7
--- /dev/null
+++ b/include/fesa/core/diagnostic.hpp
@@ -0,0 +1,30 @@
+#pragma once
+
+#include "fesa/core/source_identity.hpp"
+
+#include <string>
+#include <vector>
+
+namespace fesa {
+
+// Distinguishes recoverable warnings from errors that stop the current operation.
+enum class Severity {
+    warning,
+    error
+};
+
+// Carries a structured, backend-independent diagnostic record.
+struct Diagnostic {
+    Severity severity;
+    std::string code;
+    SourceLocation location;
+    std::string keyword;
+    std::string entityIdentity;
+    std::string message;
+};
+
+// Orders diagnostics by their externally visible source tuple while retaining
+// discovery order for records with identical keys.
+void sortDiagnostics(std::vector<Diagnostic>& diagnostics);
+
+} // namespace fesa
diff --git a/include/fesa/core/source_identity.hpp b/include/fesa/core/source_identity.hpp
new file mode 100644
index 0000000..61732ac
--- /dev/null
+++ b/include/fesa/core/source_identity.hpp
@@ -0,0 +1,23 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <filesystem>
+#include <string>
+
+namespace fesa {
+
+// Identifies the physical input location that produced a model item or diagnostic.
+struct SourceLocation {
+    std::filesystem::path file;
+    std::size_t line;
+};
+
+// Preserves both semantic and raw-text forms of an input entity identity.
+struct SourceEntityId {
+    std::string instanceName;
+    std::int64_t sourceLabel;
+    std::string sourceLabelText;
+};
+
+} // namespace fesa
diff --git a/include/fesa/core/status.hpp b/include/fesa/core/status.hpp
new file mode 100644
index 0000000..ee8313b
--- /dev/null
+++ b/include/fesa/core/status.hpp
@@ -0,0 +1,94 @@
+#pragma once
+
+#include "fesa/core/diagnostic.hpp"
+
+#include <optional>
+#include <stdexcept>
+#include <utility>
+#include <vector>
+
+namespace fesa {
+
+// Maps a failure to the stable command-line exit-code classes defined by V0.
+enum class FailureCategory {
+    input,
+    model,
+    solver,
+    output
+};
+
+// Transports success or structured diagnostics without exposing backend errors.
+class Status {
+public:
+    static Status ok();
+    static Status failure(std::vector<Diagnostic> diagnostics);
+    static Status failure(
+        FailureCategory category, std::vector<Diagnostic> diagnostics);
+
+    bool isOk() const noexcept;
+    std::optional<FailureCategory> failureCategory() const noexcept;
+    const std::vector<Diagnostic>& diagnostics() const noexcept;
+
+private:
+    Status(
+        bool isOk,
+        std::optional<FailureCategory> category,
+        std::vector<Diagnostic> diagnostics);
+
+    bool isOk_;
+    std::optional<FailureCategory> category_;
+    std::vector<Diagnostic> diagnostics_;
+};
+
+// Owns exactly one successful value or one failed Status.
+template<class T>
+class Result {
+public:
+    static Result success(T value) {
+        return Result{SuccessTag{}, std::move(value)};
+    }
+
+    static Result failure(Status status) {
+        if (status.isOk()) {
+            throw std::invalid_argument{"A failed Result requires a failed Status."};
+        }
+        return Result{FailureTag{}, std::move(status)};
+    }
+
+    bool hasValue() const noexcept {
+        return value_.has_value();
+    }
+
+    T& value() {
+        if (!value_) {
+            throw std::logic_error{"Result has no value."};
+        }
+        return *value_;
+    }
+
+    const T& value() const {
+        if (!value_) {
+            throw std::logic_error{"Result has no value."};
+        }
+        return *value_;
+    }
+
+    const Status& status() const noexcept {
+        return status_;
+    }
+
+private:
+    struct SuccessTag {};
+    struct FailureTag {};
+
+    Result(SuccessTag, T value)
+        : value_{std::move(value)}, status_{Status::ok()} {}
+
+    Result(FailureTag, Status status)
+        : value_{std::nullopt}, status_{std::move(status)} {}
+
+    std::optional<T> value_;
+    Status status_;
+};
+
+} // namespace fesa
diff --git a/src/fesa/CMakeLists.txt b/src/fesa/CMakeLists.txt
index 6a78be9..a21a90a 100644
--- a/src/fesa/CMakeLists.txt
+++ b/src/fesa/CMakeLists.txt
@@ -2,6 +2,8 @@ add_library(
     fesa_solver
     STATIC
     build_info.cpp
+    core/diagnostic.cpp
+    core/status.cpp
 )
 
 target_include_directories(
diff --git a/src/fesa/core/diagnostic.cpp b/src/fesa/core/diagnostic.cpp
new file mode 100644
index 0000000..06e355d
--- /dev/null
+++ b/src/fesa/core/diagnostic.cpp
@@ -0,0 +1,30 @@
+#include "fesa/core/diagnostic.hpp"
+
+#include <algorithm>
+#include <tuple>
+
+namespace fesa {
+
+void sortDiagnostics(std::vector<Diagnostic>& diagnostics) {
+    // stable_sort makes discovery order the final tie-breaker without storing it
+    // in the externally visible Diagnostic record.
+    std::stable_sort(
+        diagnostics.begin(),
+        diagnostics.end(),
+        [](const Diagnostic& left, const Diagnostic& right) {
+            return std::tie(
+                       left.location.file,
+                       left.location.line,
+                       left.keyword,
+                       left.entityIdentity,
+                       left.code) <
+                   std::tie(
+                       right.location.file,
+                       right.location.line,
+                       right.keyword,
+                       right.entityIdentity,
+                       right.code);
+        });
+}
+
+} // namespace fesa
diff --git a/src/fesa/core/status.cpp b/src/fesa/core/status.cpp
new file mode 100644
index 0000000..6563352
--- /dev/null
+++ b/src/fesa/core/status.cpp
@@ -0,0 +1,42 @@
+#include "fesa/core/status.hpp"
+
+#include <utility>
+
+namespace fesa {
+
+Status Status::ok() {
+    return Status{true, std::nullopt, {}};
+}
+
+Status Status::failure(std::vector<Diagnostic> diagnostics) {
+    sortDiagnostics(diagnostics);
+    return Status{false, std::nullopt, std::move(diagnostics)};
+}
+
+Status Status::failure(
+    FailureCategory category, std::vector<Diagnostic> diagnostics) {
+    sortDiagnostics(diagnostics);
+    return Status{false, category, std::move(diagnostics)};
+}
+
+bool Status::isOk() const noexcept {
+    return isOk_;
+}
+
+std::optional<FailureCategory> Status::failureCategory() const noexcept {
+    return category_;
+}
+
+const std::vector<Diagnostic>& Status::diagnostics() const noexcept {
+    return diagnostics_;
+}
+
+Status::Status(
+    bool isOk,
+    std::optional<FailureCategory> category,
+    std::vector<Diagnostic> diagnostics)
+    : isOk_{isOk},
+      category_{category},
+      diagnostics_{std::move(diagnostics)} {}
+
+} // namespace fesa
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 30ee1f3..9337a44 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -3,6 +3,9 @@ include(GoogleTest)
 add_executable(
     fesa_unit_tests
     unit/build_info_test.cpp
+    unit/core/diagnostic_test.cpp
+    unit/core/source_identity_test.cpp
+    unit/core/status_test.cpp
 )
 
 target_link_libraries(
diff --git a/tests/unit/core/diagnostic_test.cpp b/tests/unit/core/diagnostic_test.cpp
new file mode 100644
index 0000000..840e2c2
--- /dev/null
+++ b/tests/unit/core/diagnostic_test.cpp
@@ -0,0 +1,59 @@
+#include "fesa/core/diagnostic.hpp"
+
+#include <gtest/gtest.h>
+
+#include <filesystem>
+#include <string>
+#include <utility>
+#include <vector>
+
+namespace {
+
+fesa::Diagnostic makeDiagnostic(
+    std::string file,
+    std::size_t line,
+    std::string keyword,
+    std::string entityIdentity,
+    std::string code,
+    std::string message) {
+    return fesa::Diagnostic{
+        fesa::Severity::error,
+        std::move(code),
+        {std::filesystem::path{std::move(file)}, line},
+        std::move(keyword),
+        std::move(entityIdentity),
+        std::move(message)};
+}
+
+} // namespace
+
+TEST(CoreDiagnostics, DiagnosticsSortDeterministically) {
+    std::vector<fesa::Diagnostic> diagnostics{
+        makeDiagnostic("b.inp", 1U, "*NODE", "I.1", "a", "file-b"),
+        makeDiagnostic("a.inp", 3U, "*NODE", "I.1", "z", "code-z"),
+        makeDiagnostic("a.inp", 3U, "*NODE", "I.1", "a", "first-equal"),
+        makeDiagnostic("a.inp", 3U, "*NODE", "I.1", "a", "second-equal"),
+        makeDiagnostic("a.inp", 3U, "*NODE", "I.2", "a", "entity-2"),
+        makeDiagnostic("a.inp", 3U, "*BOUNDARY", "I.1", "a", "keyword"),
+        makeDiagnostic("a.inp", 2U, "*NODE", "I.1", "a", "line")};
+
+    fesa::sortDiagnostics(diagnostics);
+
+    ASSERT_EQ(diagnostics.size(), 7U);
+    EXPECT_EQ(diagnostics[0].message, "line");
+    EXPECT_EQ(diagnostics[1].message, "keyword");
+    EXPECT_EQ(diagnostics[2].message, "first-equal");
+    EXPECT_EQ(diagnostics[3].message, "second-equal");
+    EXPECT_EQ(diagnostics[4].message, "code-z");
+    EXPECT_EQ(diagnostics[5].message, "entity-2");
+    EXPECT_EQ(diagnostics[6].message, "file-b");
+
+    const fesa::Diagnostic& exact = diagnostics[2];
+    EXPECT_EQ(exact.severity, fesa::Severity::error);
+    EXPECT_EQ(exact.code, "a");
+    EXPECT_EQ(exact.location.file, std::filesystem::path{"a.inp"});
+    EXPECT_EQ(exact.location.line, 3U);
+    EXPECT_EQ(exact.keyword, "*NODE");
+    EXPECT_EQ(exact.entityIdentity, "I.1");
+    EXPECT_EQ(exact.message, "first-equal");
+}
diff --git a/tests/unit/core/source_identity_test.cpp b/tests/unit/core/source_identity_test.cpp
new file mode 100644
index 0000000..236a91f
--- /dev/null
+++ b/tests/unit/core/source_identity_test.cpp
@@ -0,0 +1,20 @@
+#include "fesa/core/source_identity.hpp"
+
+#include <gtest/gtest.h>
+
+#include <cstdint>
+#include <filesystem>
+#include <string>
+
+TEST(CoreDiagnostics, SourceIdentityPreservesRawIdentity) {
+    const fesa::SourceLocation location{
+        std::filesystem::path{"models/My Beam.inp"}, 27U};
+    const fesa::SourceEntityId identity{
+        "Beam-Instance_A", std::int64_t{42}, "00042"};
+
+    EXPECT_EQ(location.file, std::filesystem::path{"models/My Beam.inp"});
+    EXPECT_EQ(location.line, 27U);
+    EXPECT_EQ(identity.instanceName, "Beam-Instance_A");
+    EXPECT_EQ(identity.sourceLabel, 42);
+    EXPECT_EQ(identity.sourceLabelText, "00042");
+}
diff --git a/tests/unit/core/status_test.cpp b/tests/unit/core/status_test.cpp
new file mode 100644
index 0000000..8798a05
--- /dev/null
+++ b/tests/unit/core/status_test.cpp
@@ -0,0 +1,66 @@
+#include "fesa/core/status.hpp"
+
+#include <gtest/gtest.h>
+
+#include <filesystem>
+#include <stdexcept>
+#include <string>
+#include <utility>
+#include <vector>
+
+namespace {
+
+fesa::Diagnostic modelDiagnostic() {
+    return fesa::Diagnostic{
+        fesa::Severity::error,
+        "invalid-beam-length",
+        {std::filesystem::path{"beam.inp"}, 12U},
+        "*ELEMENT",
+        "Beam-1.10",
+        "Beam length must be positive."};
+}
+
+} // namespace
+
+TEST(CoreDiagnostics, ResultEnforcesValueErrorExclusivity) {
+    const fesa::Status ok = fesa::Status::ok();
+    EXPECT_TRUE(ok.isOk());
+    EXPECT_FALSE(ok.failureCategory().has_value());
+    EXPECT_TRUE(ok.diagnostics().empty());
+
+    const fesa::Status uncategorized =
+        fesa::Status::failure(std::vector<fesa::Diagnostic>{modelDiagnostic()});
+    EXPECT_FALSE(uncategorized.isOk());
+    EXPECT_FALSE(uncategorized.failureCategory().has_value());
+    ASSERT_EQ(uncategorized.diagnostics().size(), 1U);
+    EXPECT_EQ(uncategorized.diagnostics()[0].code, "invalid-beam-length");
+
+    const fesa::Status categorized = fesa::Status::failure(
+        fesa::FailureCategory::model,
+        std::vector<fesa::Diagnostic>{modelDiagnostic()});
+    EXPECT_FALSE(categorized.isOk());
+    ASSERT_TRUE(categorized.failureCategory().has_value());
+    EXPECT_EQ(*categorized.failureCategory(), fesa::FailureCategory::model);
+
+    const auto success = fesa::Result<std::string>::success("solved");
+    EXPECT_TRUE(success.hasValue());
+    EXPECT_TRUE(success.status().isOk());
+    EXPECT_EQ(success.value(), "solved");
+
+    auto copied = success;
+    EXPECT_EQ(copied.value(), "solved");
+    auto moved = std::move(copied);
+    EXPECT_EQ(moved.value(), "solved");
+
+    auto failure = fesa::Result<std::string>::failure(categorized);
+    EXPECT_FALSE(failure.hasValue());
+    EXPECT_FALSE(failure.status().isOk());
+    EXPECT_EQ(failure.status().failureCategory(), fesa::FailureCategory::model);
+    EXPECT_THROW(failure.value(), std::logic_error);
+
+    const auto& constFailure = failure;
+    EXPECT_THROW(constFailure.value(), std::logic_error);
+    EXPECT_THROW(
+        (void)fesa::Result<std::string>::failure(fesa::Status::ok()),
+        std::invalid_argument);
+}