diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c611389e3..fbd18ec98 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -112,6 +112,40 @@ jobs:
             echo "No shell scripts found — skipping"
           fi
 
+  # Informational self-validation of THIS repo's canonical AGENTS.md against the
+  # standalone structural linter (scripts/agents-md-lint.sh, Phase 2 of epic
+  # #642). Phase 3 (#646) runs it on every PR in the linter's default
+  # INFORMATIONAL mode: each structural finding surfaces as a GitHub ::warning::
+  # annotation, but the job never fails the build. This job is deliberately NOT a
+  # branch-protection required status check — promotion to a blocking/required
+  # check (--mode failing + branch protection) is deferred to Phase 4. Do NOT add
+  # it to branch protection or pass --mode failing while it remains informational.
+  # See test/workflows/ci/agents-md-selfcheck.bats.
+  agents-md-selfcheck:
+    name: AGENTS.md Self-Check (informational)
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+
+      - name: Lint canonical AGENTS.md (informational)
+        # Default (informational) mode always exits 0 — structural findings
+        # surface as ::warning:: annotations without failing the build, mirroring
+        # run-bats.sh's non-fatal-signal convention. Do NOT pass --mode failing:
+        # promoting this to a blocking/required check is Phase 4 (see job header).
+        run: |
+          bash scripts/agents-md-lint.sh AGENTS.md \
+            | while IFS=$'\t' read -r severity rule line message; do
+                [ -n "$rule" ] || continue
+                if [ "$line" = "-" ] || [ -z "$line" ]; then
+                  echo "::warning file=AGENTS.md::[${severity}] ${rule}: ${message}"
+                else
+                  echo "::warning file=AGENTS.md,line=${line}::[${severity}] ${rule}: ${message}"
+                fi
+              done
+
   agent-security:
     name: Agent Security Scan
     runs-on: ubuntu-latest
diff --git a/test/workflows/ci/agents-md-selfcheck.bats b/test/workflows/ci/agents-md-selfcheck.bats
new file mode 100644
index 000000000..b8dee7abb
--- /dev/null
+++ b/test/workflows/ci/agents-md-selfcheck.bats
@@ -0,0 +1,102 @@
+#!/usr/bin/env bats
+# Regression guard for the informational AGENTS.md self-check wired into
+# .github/workflows/ci.yml (issue #646, Phase 3 of epic #642).
+#
+# Phase 3 runs the standalone structural linter (scripts/agents-md-lint.sh)
+# against THIS repo's own canonical AGENTS.md on every pull request, in
+# INFORMATIONAL mode: it annotates each structural finding with a GitHub
+# ::warning:: but must never fail the build (promotion to a failing/required
+# check is deferred to Phase 4). These tests pin that contract so a future edit
+# cannot silently (a) drop the self-check, (b) point it at the wrong file, or
+# (c) flip it to failing mode and turn it into a de-facto blocking gate before
+# Phase 4 sign-off.
+
+load 'helpers/setup'
+
+# Emit the `run:` step block whose text contains the given marker substring.
+# Steps in ci.yml start at a 6-space "- " (the `steps:` child indent); every
+# deeper-indented line belongs to that step. (Mirrors secret-scan-config.bats.)
+step_block_with() {
+  local marker="$1"
+  local block="" out="" line=""
+  while IFS= read -r line || [[ -n "$line" ]]; do
+    line="${line%$'\r'}"
+    if [[ "$line" == "      - "* ]]; then
+      if [[ "$block" == "      - "* ]]; then
+        [[ "$block" == *"$marker"* ]] && out="$block"
+      fi
+      block="$line"$'\n'
+    elif [[ "$line" =~ ^[[:space:]]{8} ]] || [[ "$line" =~ ^[[:space:]]*$ ]]; then
+      if [[ -n "$block" ]]; then
+        block+="$line"$'\n'
+      fi
+    else
+      if [[ "$block" == "      - "* ]]; then
+        [[ "$block" == *"$marker"* ]] && out="$block"
+      fi
+      block=""
+    fi
+  done < "$TT_WORKFLOW"
+  if [[ "$block" == "      - "* ]]; then
+    [[ "$block" == *"$marker"* ]] && out="$block"
+  fi
+  printf '%s' "$out"
+}
+
+@test "agents-md-selfcheck: the ci workflow exists" {
+  [[ -f "$TT_WORKFLOW" ]]
+}
+
+@test "agents-md-selfcheck: the linter script it invokes exists and is executable" {
+  # The self-check runs the Phase-2 standalone linter; if the path drifts the
+  # step would fail with file-not-found and red an otherwise-informational job.
+  [[ -x "${TT_REPO_ROOT}/scripts/agents-md-lint.sh" ]]
+}
+
+@test "agents-md-selfcheck: ci runs the structural linter against this repo's AGENTS.md" {
+  local block code
+  block="$(step_block_with 'scripts/agents-md-lint.sh')"
+  [[ -n "$block" ]]
+  # Strip comments so a comment mentioning AGENTS.md cannot satisfy the assertion.
+  code="$(printf '%s\n' "$block" | grep -vE '^[[:space:]]*#')"
+  [[ "$code" == *"AGENTS.md"* ]]
+}
+
+@test "agents-md-selfcheck: the self-check stays informational (no --mode failing)" {
+  # Informational default mode always exits 0; passing --mode failing would turn
+  # a REQUIRED-level finding into a build failure — that promotion is Phase 4.
+  # Strip comment lines first: a header comment saying "do NOT pass --mode
+  # failing" is good documentation and must not trip this assertion, which is
+  # about the actual invocation.
+  local block code
+  block="$(step_block_with 'scripts/agents-md-lint.sh')"
+  [[ -n "$block" ]]
+  code="$(printf '%s\n' "$block" | grep -vE '^[[:space:]]*#')"
+  # Verify the linter is invoked with AGENTS.md as its argument, not just mentioned.
+  [[ "$code" == *"scripts/agents-md-lint.sh"*"AGENTS.md"* ]]
+  [[ "$code" != *"--mode failing"* ]]
+  [[ "$code" != *"--mode=failing"* ]]
+}
+
+@test "agents-md-selfcheck: findings surface as ::warning:: annotations" {
+  # Mirror run-bats.sh's non-fatal-signal convention: findings annotate without
+  # failing the build.
+  local block
+  block="$(step_block_with 'scripts/agents-md-lint.sh')"
+  [[ -n "$block" ]]
+  [[ "$block" == *"::warning"* ]]
+}
+
+@test "agents-md-selfcheck: the self-check itself documents its informational, deferred-to-Phase-4 status" {
+  # AC #3: no new branch-protection required check while informational. The
+  # self-check's OWN step block must document that it is informational and that
+  # promotion to a blocking/required check is deferred to Phase 4, so a reviewer
+  # editing it later knows the non-blocking behaviour is deliberate. Scoping the
+  # assertion to the step block (not the whole file) keeps it from passing on an
+  # unrelated "informational" comment elsewhere in ci.yml.
+  local block
+  block="$(step_block_with 'scripts/agents-md-lint.sh')"
+  [[ -n "$block" ]]
+  [[ "$block" == *"informational"* ]]
+  [[ "$block" == *"Phase 4"* ]]
+}