diff --git a/harnesses/composites/code-airouter-repo/v1/harness.yaml b/harnesses/composites/code-airouter-repo/v1/harness.yaml
index a48aca2..c471304 100644
--- a/harnesses/composites/code-airouter-repo/v1/harness.yaml
+++ b/harnesses/composites/code-airouter-repo/v1/harness.yaml
@@ -3,6 +3,10 @@ name: code-airouter-repo
 version: 1
 description: "Code agent with Airouter Qwen3.6 + repo clone via SSH"
 
+# Inherits airouter context's label gate — only the airouter dispatcher
+# advertises this composite (see dispatcher/poller._collect_supported_harnesses).
+requires_labels: [airouter]
+
 layers:
   - context: qwen-code-methodology/v1
   - context: best-practices/v1
diff --git a/harnesses/composites/code-airouter-tdd-repo/v1/harness.yaml b/harnesses/composites/code-airouter-tdd-repo/v1/harness.yaml
index 69b31cc..714a47d 100644
--- a/harnesses/composites/code-airouter-tdd-repo/v1/harness.yaml
+++ b/harnesses/composites/code-airouter-tdd-repo/v1/harness.yaml
@@ -3,6 +3,10 @@ name: code-airouter-tdd-repo
 version: 1
 description: "Airouter Qwen3.6 code agent with TDD enforcement — tests locked read-only, must pass before finish"
 
+# Inherits airouter context's label gate — only the airouter dispatcher
+# advertises this composite (see dispatcher/poller._collect_supported_harnesses).
+requires_labels: [airouter]
+
 layers:
   - context: qwen-code-methodology/v1
   - context: best-practices/v1
diff --git a/harnesses/contexts/agent-repo/v1/finalize.sh b/harnesses/contexts/agent-repo/v1/finalize.sh
index cf7e99d..1fbc69a 100644
--- a/harnesses/contexts/agent-repo/v1/finalize.sh
+++ b/harnesses/contexts/agent-repo/v1/finalize.sh
@@ -260,6 +260,68 @@ if [ -n "${AGENT_EXPECTED_OUTPUT:-}" ]; then
     fi
 fi
 
+# AR-21 (2026-05-08): Diff-against-upstream verification.
+# AGENT_EXPECTED_CHANGED_FILES: comma-separated list of paths that MUST appear
+#   in the working-tree diff vs upstream main. Catches the failure mode where
+#   an agent reports "succeeded" but produced no actual change to the target
+#   file (real incident: 2026-05-08 dogfood batch, gotchas item 30 — three
+#   "succeeded" tasks merged nothing actionable).
+# AGENT_FORBIDDEN_CHANGED_FILES: comma-separated list of paths that MUST NOT
+#   appear in the diff. Catches the inverse: an agent silently destroying or
+#   refactoring files outside the task scope (real incident: 2026-05-08 task
+#   4a2f2988 — agent stripped 9 unrelated functions; gotchas item 17/21).
+DIFF_VERIFIED=true
+DIFF_MISMATCH=""
+DIFF_SUMMARY=""
+if [ -d /workspace/reference/main/.git ]; then
+    # All files actually changed in the agent's commit (vs upstream HEAD).
+    REF_HEAD=$(git -C /workspace/reference/main rev-parse HEAD 2>/dev/null || echo "")
+    if [ -n "$REF_HEAD" ]; then
+        # Files modified/added/deleted by this commit.
+        DIFF_SUMMARY=$(git diff --name-only "$REF_HEAD"..HEAD 2>/dev/null | tr '\n' ',' | sed 's/,$//')
+
+        # Required files must each appear in DIFF_SUMMARY.
+        if [ -n "${AGENT_EXPECTED_CHANGED_FILES:-}" ]; then
+            echo "Verifying required changed files: $AGENT_EXPECTED_CHANGED_FILES"
+            IFS=',' read -ra _REQUIRED <<< "$AGENT_EXPECTED_CHANGED_FILES"
+            for required in "${_REQUIRED[@]}"; do
+                required="${required#"${required%%[![:space:]]*}"}"
+                required="${required%"${required##*[![:space:]]}"}"
+                [ -z "$required" ] && continue
+                if ! echo ",$DIFF_SUMMARY," | grep -qF ",$required,"; then
+                    echo "ERROR: Required change to '$required' missing from agent's diff"
+                    DIFF_VERIFIED=false
+                    DIFF_MISMATCH="$DIFF_MISMATCH missing:$required"
+                fi
+            done
+        fi
+
+        # Forbidden files must NOT appear in DIFF_SUMMARY.
+        if [ -n "${AGENT_FORBIDDEN_CHANGED_FILES:-}" ]; then
+            echo "Verifying forbidden files unchanged: $AGENT_FORBIDDEN_CHANGED_FILES"
+            IFS=',' read -ra _FORBIDDEN <<< "$AGENT_FORBIDDEN_CHANGED_FILES"
+            for forbidden in "${_FORBIDDEN[@]}"; do
+                forbidden="${forbidden#"${forbidden%%[![:space:]]*}"}"
+                forbidden="${forbidden%"${forbidden##*[![:space:]]}"}"
+                [ -z "$forbidden" ] && continue
+                if echo ",$DIFF_SUMMARY," | grep -qF ",$forbidden,"; then
+                    echo "ERROR: Forbidden file '$forbidden' was modified by agent"
+                    DIFF_VERIFIED=false
+                    DIFF_MISMATCH="$DIFF_MISMATCH forbidden:$forbidden"
+                fi
+            done
+        fi
+
+        if [ "$DIFF_VERIFIED" = "true" ] && [ -n "${AGENT_EXPECTED_CHANGED_FILES:-}${AGENT_FORBIDDEN_CHANGED_FILES:-}" ]; then
+            echo "Diff verification passed (changed: $DIFF_SUMMARY)"
+        fi
+    else
+        echo "WARNING: /workspace/reference/main has no commits; skipping diff verification"
+    fi
+else
+    echo "Note: /workspace/reference/main not present; skipping AR-21 diff verification"
+fi
+
 # AR-19: Push with retry — attempt up to PUSH_RETRIES+1 times (default 2: initial + 1 retry)
 echo "Pushing branch $BRANCH to $REPO_URL..."
 PUSHED=false
@@ -311,6 +373,12 @@ meta['diff_kb'] = float('$DIFF_KB') if '$DIFF_KB' else 0.0
 meta['output_validated'] = $( [ '$OUTPUT_VALIDATED' = 'true' ] && echo 'True' || echo 'False' )
 if '$OUTPUT_MISSING':
     meta['output_missing'] = '$OUTPUT_MISSING'
+# AR-21: diff verification metadata
+meta['diff_verified'] = $( [ '$DIFF_VERIFIED' = 'true' ] && echo 'True' || echo 'False' )
+if '$DIFF_MISMATCH'.strip():
+    meta['diff_mismatch'] = '$DIFF_MISMATCH'.strip()
+if '$DIFF_SUMMARY':
+    meta['diff_changed_files'] = [f for f in '$DIFF_SUMMARY'.split(',') if f]
 with open(out, 'w') as f:
     json.dump(meta, f)
 print('Wrote ci_metadata.json')
@@ -320,4 +388,13 @@ if [ "$PUSHED" = "false" ]; then
     exit 1
 fi
 
+# AR-21: fail the task if diff verification didn't pass — even if push succeeded.
+# Branch is preserved on the agent repo for forensics, but the task ends as
+# failed so the operator + CP know it shouldn't be merged.
+if [ "$DIFF_VERIFIED" = "false" ]; then
+    echo "ERROR: Diff verification failed:$DIFF_MISMATCH" >&2
+    echo "Branch $BRANCH is pushed for forensics, but the task is being marked failed." >&2
+    exit 1
+fi
+
 echo "=== agent-repo/v1 finalize.sh complete ==="
\ No newline at end of file
diff --git a/harnesses/contexts/agent-repo/v1/init.sh b/harnesses/contexts/agent-repo/v1/init.sh
index ed83133..61ee9f9 100644
--- a/harnesses/contexts/agent-repo/v1/init.sh
+++ b/harnesses/contexts/agent-repo/v1/init.sh
@@ -38,17 +38,57 @@ for ref in refs:
 "
 fi
 
-# Clone agent repo working branch (AR-14)
+# Clone agent repo working branch (AR-14).
+#
+# AR-14a (2026-05-08): seed fresh task branches from /workspace/reference/main/
+# rather than the agent-repo fork's main, so a stale fork (e.g. periodic
+# "Fork cleanup" PRs that reset main) doesn't poison every fresh task with
+# old project state. Existing AGENT_BRANCH cherry-picks remain unchanged
+# (continuing prior work). Pre-existing operator workaround in
+# memory/gotchas-airouter.md item 27.
 if [ -n "${AGENT_REPO_URL:-}" ] && [ -n "${AGENT_BRANCH:-}" ]; then
     echo "Cloning agent working repo: $AGENT_REPO_URL (branch: $AGENT_BRANCH)"
     if git clone --depth 1 --branch "$AGENT_BRANCH" "$AGENT_REPO_URL" /workspace/project 2>/dev/null; then
-        echo "Cloned existing branch $AGENT_BRANCH"
+        echo "Cloned existing branch $AGENT_BRANCH (continuing prior work)"
     else
-        echo "Branch $AGENT_BRANCH does not exist, creating fresh clone..."
-        # Clone default branch, then checkout new branch
-        if git clone --depth 1 "$AGENT_REPO_URL" /workspace/project; then
+        echo "Branch $AGENT_BRANCH does not exist — seeding fresh branch from upstream reference"
+        # Full clone (not --depth 1) so we get a working remote for finalize.sh push.
+        if git clone "$AGENT_REPO_URL" /workspace/project; then
             cd /workspace/project
-            git checkout -b "$AGENT_BRANCH"
+            REF_REPO=/workspace/reference/main
+            if [ -d "$REF_REPO/.git" ]; then
+                # Seed working tree from the upstream reference clone — AR-14a.
+                # Reference is read-only (chmod a-w), but git can still read it
+                # as a local-path remote for fetch + reset.
+                #
+                # Use a temporary remote name so we don't collide with 'origin'.
+                git remote add upstream-ref "$REF_REPO"
+                git fetch upstream-ref --depth 1 2>&1 | head -3 || {
+                    echo "WARNING: failed to fetch from upstream reference; falling back to fork main" >&2
+                    git remote remove upstream-ref 2>/dev/null
+                    git checkout -b "$AGENT_BRANCH"
+                }
+                if git rev-parse upstream-ref/HEAD >/dev/null 2>&1; then
+                    UPSTREAM_REF="upstream-ref/HEAD"
+                elif git rev-parse upstream-ref/main >/dev/null 2>&1; then
+                    UPSTREAM_REF="upstream-ref/main"
+                else
+                    UPSTREAM_REF=""
+                fi
+                if [ -n "$UPSTREAM_REF" ]; then
+                    git checkout -b "$AGENT_BRANCH" "$UPSTREAM_REF"
+                    git remote remove upstream-ref
+                    echo "Seeded $AGENT_BRANCH from $REF_REPO ($(git log --oneline -1)) — AR-14a"
+                else
+                    git remote remove upstream-ref 2>/dev/null
+                    git checkout -b "$AGENT_BRANCH"
+                    echo "WARNING: upstream-ref had no resolvable HEAD; using fork main (may be stale)" >&2
+                fi
+            else
+                # No reference clone available — fall back to fork main.
+                git checkout -b "$AGENT_BRANCH"
+                echo "WARNING: $REF_REPO/.git not found; using fork main (may be stale)" >&2
+            fi
         else
             echo "ERROR: Failed to clone agent repo $AGENT_REPO_URL" >&2
             exit 1
diff --git a/harnesses/contexts/airouter/v1/harness.yaml b/harnesses/contexts/airouter/v1/harness.yaml
index c67042d..c5efb9d 100644
--- a/harnesses/contexts/airouter/v1/harness.yaml
+++ b/harnesses/contexts/airouter/v1/harness.yaml
@@ -5,6 +5,12 @@ description: "Airouter.ch Qwen3.6 — OpenAI-compatible agentic runner"
 requires: []
 provides: [agentic-runner]
 
+# Label-gated capability: only dispatchers with the `airouter` label have the
+# ESO mount + Ollama setup needed to run this context. Filter prevents the
+# main dispatcher from claiming airouter tasks (real incident: 2026-05-08
+# dogfood batch, gotchas-airouter.md item 29).
+requires_labels: [airouter]
+
 env:
   OPENAI_BASE_URL: "https://api.airouter.ch/v1"
   # Agentic runner reads the api key from this file at request time.