diff --git a/README.md b/README.md
index 62296fb..e29c810 100644
--- a/README.md
+++ b/README.md
@@ -35,7 +35,8 @@ This workspace is registered as a child-local planning workspace. Registration d
 |       |-- CTO-CASE-CANDIDATE-BACKEND-ISSUES.md
 |       |-- CTO-HARNESS-EVIDENCE-INTERFACE-CONTRACT.md
 |       |-- CTO-CASE-SOURCE-ADMISSION-RECORD.md
-|       `-- CTO-CASE-ADAPTER-CONTRACT.md
+|       |-- CTO-CASE-ADAPTER-CONTRACT.md
+|       `-- CTO-CASE-FAILURE-FIXTURE-MATRIX.md
 `-- tools/
     `-- validate_cto_child.py
 ```
diff --git a/WORKBOARD.yaml b/WORKBOARD.yaml
index 84852e3..4c2e705 100644
--- a/WORKBOARD.yaml
+++ b/WORKBOARD.yaml
@@ -31,8 +31,8 @@ items:
     owner: jp
   - id: CTO-WORK-007
     title: Case Failure Fixture Matrix
-    status: candidate
-    source: sot/03-PROTOCOLS/CTO-CASE-CANDIDATE-BACKEND-ISSUES.md
+    status: validated
+    source: sot/03-PROTOCOLS/CTO-CASE-FAILURE-FIXTURE-MATRIX.md
     owner: jp
   - id: CTO-WORK-008
     title: Staged Proof Gate Records
diff --git a/sot/03-PROTOCOLS/CTO-CASE-FAILURE-FIXTURE-MATRIX.md b/sot/03-PROTOCOLS/CTO-CASE-FAILURE-FIXTURE-MATRIX.md
new file mode 100644
index 0000000..52cefae
--- /dev/null
+++ b/sot/03-PROTOCOLS/CTO-CASE-FAILURE-FIXTURE-MATRIX.md
@@ -0,0 +1,75 @@
+---
+name: cto-case-failure-fixture-matrix
+tier: local
+status: draft
+owner: jp
+source: sot/03-PROTOCOLS/CTO-CASE-CANDIDATE-BACKEND-PRD.md
+created: 2026-05-31
+last_reviewed: 2026-05-31
+lifecycle_classification: planning
+core_promotion_status: not-promoted
+description: Child-local failure fixture matrix for later Case adapter fail-closed testing.
+---
+
+# CTO Case Failure Fixture Matrix
+
+Local planning SOT only. Not a Core Protocol. Not active Core authority.
+
+## Purpose
+
+Define the required failure fixtures for later Case adapter testing before any executable Case integration exists.
+
+This matrix turns failure-mode language into concrete expected evidence. It prevents the future adapter from proving only the happy path.
+
+## Non-Authority Notice
+
+This matrix does not authorize Runtime behavior, WebUI Product behavior, Case execution, real-repo mutation, merge, deploy, push, close, vendor-source mutation, external developer repository mutation, or Core promotion.
+
+## Matrix Rules
+
+- Each row maps to the CTO Harness Evidence Interface.
+- Each row must produce `report.json`, `events.normalized.jsonl`, `patch.diff`, `test.log`, and `trace.jsonl` when implemented.
+- Each row must produce `artifact_digests` and freshness proof when implemented.
+- Each row must record a blocker reason in `report.json`.
+- Each row must emit a normalized event when the failure class has one.
+- Each row must fail closed with `status` set to `fail` or `blocked`.
+- Each row must use nonzero exit behavior when the adapter contract marks the failure executable.
+- This matrix is planning-only and does not run Case.
+
+## Required Failure Fixtures
+
+| Fixture | Trigger | Expected blocker reason | Normalized event | Report status | Exit behavior | Evidence mapping |
+| --- | --- | --- | --- | --- | --- | --- |
+| no-diff | Backend reports success but no source diff exists when a diff is required. | `no_diff` | `git.diff.checked` | `fail` | nonzero | `patch.diff` exists and is empty; `changed_files` is empty; `blockers` includes `no_diff`. |
+| disallowed-file | Backend changes a path outside `allowed_paths`. | `disallowed_file_change` | `git.diff.checked` | `fail` | nonzero | `changed_files` lists offending path; `allowed_writes_passed` is `false`; `patch.diff` captures the change. |
+| failed-tests | Verification command exits nonzero. | `verification_failed` | `verification.completed` | `fail` | nonzero | `test.log` captures failing command output; `verification` records command and nonzero exit. |
+| missing-test-command | Task contract requires verification but no command is available. | `missing_test_command` | `verification.completed` | `blocked` | nonzero | `test.log` states no command was available; `verification` records missing command. |
+| missing-event | Required normalized event is absent or out of order. | `missing_required_event` | `run.completed` | `fail` | nonzero | `events.normalized.jsonl` is present; validator identifies missing or out-of-order event. |
+| reviewer-reject | Case reviewer rejects the result. | `reviewer_rejected` | `verification.completed` | `blocked` | nonzero | backend raw logs record reviewer rejection; normalized evidence records blocked result without approving mutation. |
+| approval-denied | Human approval is required and denied. | `approval_denied` | `approval.denied` | `blocked` | nonzero | approval event records actor, scope, mutation mode, timestamp, and denial. |
+| timeout | Backend or verification exceeds configured timeout. | `timeout` | `run.completed` | `blocked` | nonzero | `trace.jsonl` records timeout point; `report.json` records timeout blocker. |
+| dirty-starting-tree | Target repository is dirty before backend work starts when clean start is required. | `dirty_starting_tree` | `task.contract.created` | `blocked` | nonzero | `trace.jsonl` records preflight failure; no mutation occurs. |
+| dirty-ending-tree | Run leaves untracked or unintended dirty state after checks. | `dirty_ending_tree` | `run.completed` | `fail` | nonzero | `trace.jsonl` records ending state check; `report.json` names dirty paths. |
+| artifact-write-failure | Required artifact cannot be written. | `artifact_write_failure` | `run.completed` | `fail` | nonzero | `report.json` may be partial only if write failure happens after report creation; trace records failed artifact path. |
+| provider-unavailable | Backend provider, model, CLI, or dependency is unavailable. | `provider_unavailable` | `run.completed` | `blocked` | nonzero | backend raw logs record provider failure; no source mutation occurs. |
+
+## Acceptance Rule
+
+A future Case adapter cannot progress to staged proof until these fixture classes have expected evidence defined in the adapter validator.
+
+The first executable implementation may start with a subset only if the staged proof gate explicitly marks the remaining rows as blocked and not accepted for default candidacy.
+
+## Validator Expectations
+
+Current validation is planning-only and checks this matrix exists with required rows.
+
+Later adapter validation must execute or simulate each row and verify:
+
+- blocker reason;
+- normalized event;
+- report status;
+- exit behavior;
+- required artifact presence;
+- digest presence;
+- freshness proof;
+- no hidden source mutation.
diff --git a/tools/validate_cto_child.py b/tools/validate_cto_child.py
index 6fc4a66..c3af202 100644
--- a/tools/validate_cto_child.py
+++ b/tools/validate_cto_child.py
@@ -21,6 +21,7 @@ REQUIRED_FILES = [
     "sot/03-PROTOCOLS/CTO-HARNESS-EVIDENCE-INTERFACE-CONTRACT.md",
     "sot/03-PROTOCOLS/CTO-CASE-SOURCE-ADMISSION-RECORD.md",
     "sot/03-PROTOCOLS/CTO-CASE-ADAPTER-CONTRACT.md",
+    "sot/03-PROTOCOLS/CTO-CASE-FAILURE-FIXTURE-MATRIX.md",
 ]
 
 REQUIRED_BRIEF_PHRASES = [
@@ -119,6 +120,43 @@ REQUIRED_ADAPTER_CONTRACT_PHRASES = [
     "This contract deliberately does not create `case-engine.sh`.",
 ]
 
+REQUIRED_FAILURE_FIXTURE_PHRASES = [
+    "Local planning SOT only. Not a Core Protocol. Not active Core authority.",
+    "This matrix does not authorize Runtime behavior",
+    "This matrix is planning-only and does not run Case.",
+    "no-diff",
+    "disallowed-file",
+    "failed-tests",
+    "missing-test-command",
+    "missing-event",
+    "reviewer-reject",
+    "approval-denied",
+    "timeout",
+    "dirty-starting-tree",
+    "dirty-ending-tree",
+    "artifact-write-failure",
+    "provider-unavailable",
+    "`no_diff`",
+    "`disallowed_file_change`",
+    "`verification_failed`",
+    "`missing_test_command`",
+    "`missing_required_event`",
+    "`reviewer_rejected`",
+    "`approval_denied`",
+    "`dirty_starting_tree`",
+    "`dirty_ending_tree`",
+    "`artifact_write_failure`",
+    "`provider_unavailable`",
+    "report.json",
+    "events.normalized.jsonl",
+    "patch.diff",
+    "test.log",
+    "trace.jsonl",
+    "artifact_digests",
+    "freshness proof",
+    "fail closed",
+]
+
 
 def main() -> int:
     checked: list[str] = []
@@ -196,6 +234,16 @@ def main() -> int:
             if phrase not in text:
                 errors.append(f"missing_adapter_contract_phrase:{phrase}")
 
+    failure_matrix = ROOT / "sot/03-PROTOCOLS/CTO-CASE-FAILURE-FIXTURE-MATRIX.md"
+    if failure_matrix.is_file():
+        text = failure_matrix.read_text(encoding="utf-8")
+        if "core_promotion_status: not-promoted" not in text:
+            errors.append("failure_matrix_missing_not_promoted_frontmatter")
+        for phrase in REQUIRED_FAILURE_FIXTURE_PHRASES:
+            checked.append(f"failure_matrix_phrase:{phrase}")
+            if phrase not in text:
+                errors.append(f"missing_failure_matrix_phrase:{phrase}")
+
     board = ROOT / "WORKBOARD.yaml"
     if board.is_file():
         text = board.read_text(encoding="utf-8")
@@ -211,6 +259,8 @@ def main() -> int:
             errors.append("workboard_missing_source_admission_record_source")
         if "CTO-CASE-ADAPTER-CONTRACT.md" not in text:
             errors.append("workboard_missing_adapter_contract_source")
+        if "CTO-CASE-FAILURE-FIXTURE-MATRIX.md" not in text:
+            errors.append("workboard_missing_failure_matrix_source")
 
     payload = {
         "ok": not errors,