diff --git a/.github/scripts/previous-release-tag.sh b/.github/scripts/previous-release-tag.sh
new file mode 100755
index 0000000..47a4715
--- /dev/null
+++ b/.github/scripts/previous-release-tag.sh
@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+# Resolves the nearest reachable prior SemVer release tag from
+# <current_tag> (FR-011). Prints the previous tag on stdout if one exists;
+# prints nothing and exits 0 if this is the first release (FR-009's
+# fallback path decides what to do with an empty result -- this script
+# only reports the fact).
+set -euo pipefail
+
+usage() {
+  echo "usage: $0 <current_tag>" >&2
+  exit 2
+}
+
+[ $# -ge 1 ] || usage
+current_tag="$1"
+
+if ! git rev-parse -q --verify "refs/tags/${current_tag}" >/dev/null; then
+  echo "error: tag '$current_tag' does not exist in this repository" >&2
+  exit 1
+fi
+
+# git's --match is a shell glob, not a regex: "v[0-9]*" also matches
+# non-release tags like "v1.1.0-rc1" (any suffix after a leading digit).
+# Walk back past any such match until we find one that's strict SemVer
+# (vMAJOR.MINOR.PATCH, no suffix), or run out of tags entirely.
+ref="${current_tag}^"
+while candidate="$(git describe --tags --match "v[0-9]*" --abbrev=0 "$ref" 2>/dev/null)"; do
+  if [[ "$candidate" =~ ^v[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
+    echo "$candidate"
+    exit 0
+  fi
+  ref="${candidate}^"
+done
diff --git a/.github/scripts/tests/test-previous-release-tag.sh b/.github/scripts/tests/test-previous-release-tag.sh
new file mode 100755
index 0000000..d50db24
--- /dev/null
+++ b/.github/scripts/tests/test-previous-release-tag.sh
@@ -0,0 +1,79 @@
+#!/usr/bin/env bash
+# Fixture-based tests for previous-release-tag.sh (T005). Builds a throwaway
+# git repo with a known tag history rather than touching the real repo's
+# tags. Run manually:
+#   bash .github/scripts/tests/test-previous-release-tag.sh
+set -euo pipefail
+
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+script="$script_dir/../previous-release-tag.sh"
+
+tmp="$(mktemp -d)"
+trap 'rm -rf "$tmp"' EXIT
+
+pass=0
+fail=0
+
+check_eq() {
+  local desc="$1" expected="$2" actual="$3"
+  if [ "$expected" == "$actual" ]; then
+    pass=$((pass + 1))
+  else
+    fail=$((fail + 1))
+    echo "FAIL: $desc (expected '$expected', got '$actual')"
+  fi
+}
+
+assert_fail() {
+  local desc="$1"; shift
+  if "$@" >/dev/null 2>&1; then
+    fail=$((fail + 1))
+    echo "FAIL (expected failure but succeeded): $desc"
+  else
+    pass=$((pass + 1))
+  fi
+}
+
+(
+  cd "$tmp"
+  git init -q
+  git config user.email "test@example.com"
+  git config user.name "Test"
+  # Lightweight, unsigned tags regardless of the caller's global git config
+  # (e.g. tag.gpgsign=true would otherwise force an annotated/signed tag
+  # here, which fails without an interactive editor or GPG key).
+  git config tag.gpgsign false
+  tag() { git -c tag.gpgsign=false tag "$1"; }
+
+  echo "one" >file.txt
+  git add file.txt
+  git commit -q -m "first"
+  tag v1.0.0
+
+  echo "two" >file.txt
+  git commit -q -am "second"
+  tag v1.1.0
+
+  echo "three" >file.txt
+  git commit -q -am "third"
+  tag v1.1.0-rc1
+
+  echo "four" >file.txt
+  git commit -q -am "fourth"
+  tag v2.0.0
+)
+
+out="$(cd "$tmp" && bash "$script" "v1.1.0")"
+check_eq "previous tag for v1.1.0 is v1.0.0" "v1.0.0" "$out"
+
+out="$(cd "$tmp" && bash "$script" "v2.0.0")"
+check_eq "previous tag for v2.0.0 is v1.1.0, not the intervening v1.1.0-rc1 prerelease" "v1.1.0" "$out"
+
+out="$(cd "$tmp" && bash "$script" "v1.0.0")"
+check_eq "first release has no previous tag (empty output, FR-009 fallback)" "" "$out"
+
+assert_fail "nonexistent tag fails" bash -c "cd '$tmp' && bash '$script' v9.9.9"
+
+echo "----"
+echo "pass=$pass fail=$fail"
+[ "$fail" -eq 0 ]
diff --git a/.github/scripts/tests/test-validate-release-tag.sh b/.github/scripts/tests/test-validate-release-tag.sh
new file mode 100755
index 0000000..fed0759
--- /dev/null
+++ b/.github/scripts/tests/test-validate-release-tag.sh
@@ -0,0 +1,71 @@
+#!/usr/bin/env bash
+# Fixture-based tests for validate-release-tag.sh (T005, Constitution I:
+# written before/alongside the script, not after). Run manually:
+#   bash .github/scripts/tests/test-validate-release-tag.sh
+set -euo pipefail
+
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+script="$script_dir/../validate-release-tag.sh"
+
+tmp="$(mktemp -d)"
+trap 'rm -rf "$tmp"' EXIT
+
+pass=0
+fail=0
+
+assert_ok() {
+  local desc="$1"; shift
+  local capture; capture="$(mktemp "$tmp/out.XXXXXX")"
+  if "$@" >"$capture" 2>&1; then
+    pass=$((pass + 1))
+  else
+    fail=$((fail + 1))
+    echo "FAIL (expected success): $desc"
+    cat "$capture"
+  fi
+  rm -f "$capture"
+}
+
+assert_fail() {
+  local desc="$1"; shift
+  local capture; capture="$(mktemp "$tmp/out.XXXXXX")"
+  if "$@" >"$capture" 2>&1; then
+    fail=$((fail + 1))
+    echo "FAIL (expected failure but succeeded): $desc"
+    cat "$capture"
+  else
+    pass=$((pass + 1))
+  fi
+  rm -f "$capture"
+}
+
+cargo_toml="$tmp/Cargo.toml"
+cat >"$cargo_toml" <<'EOF'
+[workspace]
+members = ["crates/*"]
+
+[workspace.package]
+version = "1.2.3"
+edition = "2021"
+EOF
+
+assert_ok "matching tag passes" bash "$script" "v1.2.3" "$cargo_toml"
+assert_fail "mismatched patch fails" bash "$script" "v1.2.4" "$cargo_toml"
+assert_fail "mismatched minor fails" bash "$script" "v1.3.3" "$cargo_toml"
+assert_fail "missing v prefix fails" bash "$script" "1.2.3" "$cargo_toml"
+assert_fail "non-semver tag fails" bash "$script" "v1.2" "$cargo_toml"
+assert_fail "pre-release suffix fails (out of scope per spec.md Assumptions)" bash "$script" "v1.2.3-rc1" "$cargo_toml"
+assert_fail "missing Cargo.toml fails" bash "$script" "v1.2.3" "$tmp/does-not-exist.toml"
+
+# Mismatch error message must name both values (SC-006).
+out="$(bash "$script" "v9.9.9" "$cargo_toml" 2>&1 || true)"
+if [[ "$out" == *"v9.9.9"* && "$out" == *"1.2.3"* ]]; then
+  pass=$((pass + 1))
+else
+  fail=$((fail + 1))
+  echo "FAIL: mismatch error must name both tag and workspace version, got: $out"
+fi
+
+echo "----"
+echo "pass=$pass fail=$fail"
+[ "$fail" -eq 0 ]
diff --git a/.github/scripts/validate-release-tag.sh b/.github/scripts/validate-release-tag.sh
new file mode 100755
index 0000000..84d7b31
--- /dev/null
+++ b/.github/scripts/validate-release-tag.sh
@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+# Validates a release tag against Cargo.toml's canonical workspace version
+# (FR-010). Previous-release-tag resolution (FR-011) is a separate concern,
+# implemented in previous-release-tag.sh. See
+# specs/005-ci-release-versioning/spec.md.
+set -euo pipefail
+
+usage() {
+  echo "usage: $0 <tag> [cargo_toml_path]" >&2
+  exit 2
+}
+
+[ $# -ge 1 ] || usage
+tag="$1"
+cargo_toml="${2:-Cargo.toml}"
+
+if [ ! -f "$cargo_toml" ]; then
+  echo "error: $cargo_toml not found" >&2
+  exit 1
+fi
+
+if ! [[ "$tag" =~ ^v[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
+  echo "error: tag '$tag' is not a valid SemVer release tag (expected vMAJOR.MINOR.PATCH)" >&2
+  exit 1
+fi
+
+workspace_version="$(awk '
+  /^[[:space:]]*\[workspace\.package\]/ { in_section = 1; next }
+  /^[[:space:]]*\[/ { in_section = 0 }
+  in_section && /^[[:space:]]*version[[:space:]]*=/ {
+    match($0, /"[^"]+"/)
+    print substr($0, RSTART + 1, RLENGTH - 2)
+    exit
+  }
+' "$cargo_toml")"
+
+if [ -z "$workspace_version" ]; then
+  echo "error: could not find [workspace.package].version in $cargo_toml" >&2
+  exit 1
+fi
+
+expected_tag="v${workspace_version}"
+
+if [ "$tag" != "$expected_tag" ]; then
+  echo "error: tag/version mismatch: tag is '$tag', but [workspace.package].version is '$workspace_version' (expected tag '$expected_tag')" >&2
+  exit 1
+fi
+
+echo "ok: tag '$tag' matches workspace version '$workspace_version'"
diff --git a/specs/005-ci-release-versioning/tasks.md b/specs/005-ci-release-versioning/tasks.md
index 4b57293..5f871b1 100644
--- a/specs/005-ci-release-versioning/tasks.md
+++ b/specs/005-ci-release-versioning/tasks.md
@@ -86,7 +86,7 @@ shared by both release publishing (US2) and versioning/notes (US3).
 
 **CRITICAL**: No US2/US3 task starts before this phase is complete.
 
-- [ ] **T005** [Test-first, per Constitution I] Write a fixture-based test
+- [x] **T005** [Test-first, per Constitution I] Write a fixture-based test
   for version/tag validation *before* implementing it: asserts a
   `v<tag>`/`Cargo.toml` mismatch fails with an error naming both values
   (SC-006), and that re-publishing an existing tag is rejected. Then
@@ -96,6 +96,15 @@ shared by both release publishing (US2) and versioning/notes (US3).
   Also implements `previous_release_tag` selection (FR-011): nearest
   reachable prior SemVer tag via `git describe --tags --match "v[0-9]*"
   --abbrev=0 <current_tag>^`.
+  **Done 2026-07-30**: `.github/scripts/validate-release-tag.sh` (tag
+  format + version match, SC-006 error message) and
+  `.github/scripts/previous-release-tag.sh` (FR-011), each with fixture
+  tests under `.github/scripts/tests/` run against a throwaway git repo
+  fixture (not the real repo's tags). Re-publishing-an-existing-tag
+  rejection is **not** in these scripts — it needs to check the real
+  GitHub Release API (does a release already exist for this tag?), which
+  is T012's concern in `release.yml`, not something these offline,
+  git-only scripts can determine.
 - [ ] **T006** Implement build-identifier computation: deterministic
   `GITHUB_RUN_NUMBER.GITHUB_RUN_ATTEMPT` (FR-005), evaluated numerically as
   `(GITHUB_RUN_NUMBER, GITHUB_RUN_ATTEMPT)` for "strictly increasing" checks