diff --git a/README.md b/README.md
index 3788bc93..3e7c3efb 100644
--- a/README.md
+++ b/README.md
@@ -101,6 +101,24 @@ scripts/test-release.sh                       # release selftests
 
 A failed health check never activates; the previously active release remains deployed. Updating the software therefore cannot corrupt the running installation: package beside, gate, then flip. Verified by the update/refusal/rollback drills in BUILD-LOG Phase 7.
 
+## Runtime adapters (M4)
+
+The harness boundary is formalized: everything upstream (config, contracts, missions, tasks, run records) is harness-agnostic; everything inside an adapter belongs to one runtime.
+
+```text
+adapters/<name>/adapter.sh   env in: MOSAIC_SYSTEM_PROMPT_FILE, MOSAIC_REQUEST,
+                             MOSAIC_PROVIDER, MOSAIC_MODEL
+                             stdout: response only; stderr: diagnostics
+```
+
+- Selection: `execution.adapter` in config.json (optional; `pi` default; allowlist `pi`, `mock`)
+- `pi` — pinned Pi CLI, noninteractive print mode, ambient discovery off
+- `mock` — deterministic test adapter; never for real verification
+- Mission directives have a sanctioned injection point: when a task references a mission, the task runner mounts the run snapshot and the generated prompt gains a `MISSION (runtime)` section (objective + directives) after the four immutable contracts
+- Adding a harness (Claude, Codex, OpenCode) later means adding one directory — no orchestrator changes
+
+See `adapters/README.md` for the full contract.
+
 See `docs/plans/2026-09-02_atomic-mosaic-foundation.md` for the full plan.
 
 Inside the container:
@@ -137,7 +155,7 @@ scripts/verify.sh      # full gated test; exit 0 only on exact MOSAIC_HELLO_OK
 scripts/run-task.sh    # run a mission/task file (see Missions & tasks)
 scripts/release.sh     # package / activate / rollback / status (see Release model)
 scripts/test-config.sh # fast config-layer selftests (no Docker)
-scripts/test-task.sh   # mission/task selftests (schema + live runs)
+scripts/test-task.sh   # mission/task selftests (schema + adapter seam + live runs)
 scripts/test-release.sh # release selftests
 scripts/reset.sh       # delete the configured data root (safety-checked)
 ```
diff --git a/scripts/test-config.sh b/scripts/test-config.sh
index b1b5d5a1..6aaba50d 100755
--- a/scripts/test-config.sh
+++ b/scripts/test-config.sh
@@ -12,6 +12,10 @@ trap 'rm -rf "$SANDBOX"' EXIT
 PASS=0
 FAIL=0
 
+check() {
+    if [ "$2" = "0" ]; then PASS=$((PASS+1)); echo "ok   $1"; else FAIL=$((FAIL+1)); echo "FAIL $1"; fi
+}
+
 # expect_exit NAME EXPECTED_RC -- command...
 expect_exit() {
     local name="$1" expected="$2"
diff --git a/scripts/test-task.sh b/scripts/test-task.sh
index c6e421b4..d5506f3d 100755
--- a/scripts/test-task.sh
+++ b/scripts/test-task.sh
@@ -30,6 +30,16 @@ check() {
     if [ "$2" = "0" ]; then PASS=$((PASS+1)); echo "ok   $1"; else FAIL=$((FAIL+1)); echo "FAIL $1"; fi
 }
 
+latest_reason() {
+    local latest
+    latest="$(ls -dt "$SANDBOX/data/runs"/r-* 2>/dev/null | head -1)"
+    [ -n "$latest" ] && node -e 'try{const r=JSON.parse(require("fs").readFileSync(process.argv[1],"utf8"));console.log(r.reason??"")}catch{console.log("")}' "$latest/result.json" 2>/dev/null
+}
+
+check() {
+    if [ "$2" = "0" ]; then PASS=$((PASS+1)); echo "ok   $1"; else FAIL=$((FAIL+1)); echo "FAIL $1"; fi
+}
+
 CONFIG="$SANDBOX/config.json"
 DATA_ROOT="$SANDBOX/data"
 mkdir -p "$DATA_ROOT"
@@ -92,6 +102,50 @@ $TASK validate "$SANDBOX/ok.json" >/dev/null 2>&1
 M2=$(stat -c %Y "$SANDBOX/ok.json")
 [ "$M1" = "$M2" ] && check "validation does not modify the task file" 0 || check "validation does not modify the task file" 1
 
+# ---------- adapter seam: deterministic mock cases (Docker, no provider) ----------
+if docker info >/dev/null 2>&1; then
+    good_task "$SANDBOX/ok.json"
+    mock_config() { # file adapter
+        cat > "$SANDBOX/$1" <<EOF
+{"configVersion":1,"environment":"development","dataRoot":"$SANDBOX/data","execution":{"backend":"docker","provider":"zai","model":"m","adapter":"$2"}}
+EOF
+    }
+
+    mock_config mock-adapters.json mock
+    expect_exit "mock adapter: gate passes on matching mock response" 0 -- \
+        env MOSAIC_CONFIG="$SANDBOX/mock-adapters.json" MOSAIC_MOCK_RESPONSE=MOSAIC_HELLO_OK \
+        scripts/run-task.sh run "$SANDBOX/ok.json"
+
+    expect_exit "mock adapter: expect-mismatch recorded" 1 -- \
+        env MOSAIC_CONFIG="$SANDBOX/mock-adapters.json" MOSAIC_MOCK_RESPONSE=SOMETHING_ELSE \
+        scripts/run-task.sh run "$SANDBOX/ok.json"
+    [ "$(latest_reason)" = "expect-mismatch" ] \
+        && check "mismatch reason recorded" 0 || check "mismatch reason recorded" 1
+
+    mock_config bad-adapter.json nonexistent
+    # The config layer rejects unknown adapters first, so run-task.sh fails
+    # closed (exit 1) before any container or run record exists.
+    expect_exit "unknown adapter fails closed" 1 -- \
+        env MOSAIC_CONFIG="$SANDBOX/bad-adapter.json" scripts/run-task.sh run "$SANDBOX/ok.json"
+
+    good_mission "$SANDBOX/m-ok.json"
+    printf '{"taskVersion":1,"id":"t-mission","prompt":"ignored by mock","mission":"m-ok.json"}' > "$SANDBOX/mission-task.json"
+    # Dedicated mission with distinctive directives for the injection assertion.
+    cat > "$SANDBOX/m-seam.json" <<'EOF'
+{"missionVersion":1,"id":"m-seam","objective":"Prove the mission injection point.","directives":["Seam directive A.","Seam directive B."]}
+EOF
+    printf '{"taskVersion":1,"id":"t-mission-seam","prompt":"ignored by mock","mission":"m-seam.json"}' > "$SANDBOX/seam-task.json"
+    expect_exit "mission task runs via mock adapter" 0 -- \
+        env MOSAIC_CONFIG="$SANDBOX/mock-adapters.json" MOSAIC_MOCK_RESPONSE=MOCKED \
+        scripts/run-task.sh run "$SANDBOX/seam-task.json"
+    grep -q 'MISSION (runtime)' "$SANDBOX/data/system-prompt.md" \
+        && grep -q 'Seam directive A.' "$SANDBOX/data/system-prompt.md" \
+        && check "mission section injected into generated prompt" 0 \
+        || check "mission section injected into generated prompt" 1
+else
+    echo "skip adapter seam cases (docker daemon unavailable)"
+fi
+
 # ---------- live: real runs (Docker + credentials required) ----------
 # On failure, surface the run record + agent stderr BEFORE the sandbox
 # cleanup destroys them. Never let a wrong-exit mask the real reason.
@@ -115,14 +169,15 @@ latest_reason() {
 }
 
 if docker info >/dev/null 2>&1; then
+    RUNS1=$(ls "$DATA_ROOT/runs" 2>/dev/null | wc -l)
     if scripts/run-task.sh run "$SANDBOX/ok.json" >/dev/null 2>&1; then
         PASS=$((PASS+1)); echo "ok   live hello task succeeds with exact marker"
     else
         FAIL=$((FAIL+1)); echo "FAIL live hello task succeeds with exact marker" >&2
         dump_latest_run
     fi
-    [ -f "$DATA_ROOT/runs" ] && RUNS1=$(ls "$DATA_ROOT/runs" | wc -l)
-    R1="$(ls "$DATA_ROOT/runs" | head -1)"
+
+    R1="$(ls -t "$DATA_ROOT/runs" | head -1)"   # newest = the live run above
     [ -f "$DATA_ROOT/runs/$R1/result.json" ] && check "result.json written in run dir" 0 || check "result.json written in run dir" 1
     node -e '
 const r = JSON.parse(require("fs").readFileSync(process.argv[1], "utf8"));