diff --git a/.github/workflows/nightly-install.yml b/.github/workflows/nightly-install.yml
index 6c5b2f5de..9fedbc088 100644
--- a/.github/workflows/nightly-install.yml
+++ b/.github/workflows/nightly-install.yml
@@ -72,11 +72,22 @@ jobs:
         # (~/.kiro, ~/.claude, ~/.codex, ~/.config/opencode). With a fresh HOME
         # there is no Python 3.12 either, so this also exercises the script's
         # `uv python install` fallback every night.
+        #
+        # The script refuses, by design, when no supported AI tool is present
+        # ("No supported AI tools detected"), and the runner has none: no
+        # harness binary on PATH and nothing in the fresh HOME. So the fixture
+        # plants the four harness markers `detect_available_agent_tools()`
+        # reads without a binary -- ~/.kiro, ~/.claude, ~/.pi, ~/.grok -- and
+        # the install-agents step then exercises all four link sets for real.
+        # (Without this the job failed every night from 2026-09-15, before its
+        # verify steps ever ran; pinned by test_ci_workflow_contract.py.)
         env:
           HOME: ${{ runner.temp }}/home
           UV_TOOL_DIR: ${{ runner.temp }}/tools
           UV_TOOL_BIN_DIR: ${{ runner.temp }}/bin
-        run: mkdir -p "$HOME" && ./scripts/install.sh --non-interactive --no-smoke
+        run: |
+          mkdir -p "$HOME/.kiro" "$HOME/.claude" "$HOME/.pi" "$HOME/.grok"
+          ./scripts/install.sh --non-interactive --no-smoke
 
       - name: Verify installed CLI entry points
         env:
@@ -84,3 +95,17 @@ jobs:
         run: |
           "$UV_TOOL_BIN_DIR/studyloop" --version
           "$UV_TOOL_BIN_DIR/session-export" --help
+
+      - name: Verify installed agent definitions
+        # One artefact per planted harness, each a path `install agents`
+        # writes for that tool (installers.py _TOOL_LINKS / _configure_*).
+        env:
+          HOME: ${{ runner.temp }}/home
+        run: |
+          test -e "$HOME/.kiro/agents/study-mentor.json"
+          test -e "$HOME/.kiro/agents/study-plan-architect.json"
+          test -e "$HOME/.claude/agents/socratic-mentor.md"
+          test -e "$HOME/.claude/agents/study-plan-architect.md"
+          test -e "$HOME/.pi/agent/AGENTS.md"
+          test -e "$HOME/.grok/hooks/studyloop.json"
+          test -e "$HOME/.grok/rules/session-db.md"
diff --git a/openspec/changes/plan-integration-followons/tasks.md b/openspec/changes/plan-integration-followons/tasks.md
index a8f912e72..8f3b6d44b 100644
--- a/openspec/changes/plan-integration-followons/tasks.md
+++ b/openspec/changes/plan-integration-followons/tasks.md
@@ -228,7 +228,11 @@ writer, through the existing gate, closes that. Kept out of item 3 so item 3's f
       byte-equal afterwards; single `main` on both sides. (3) closeout comments — done: posted on #8–#15 from the
       re-verified draft (`receipts/issue-closeout-draft-2026-09-16.md`, status paragraph records the shas rewritten
       and the rows overtaken); #8, #9, #11, #12, #13, #14 closed as completed; #10 and #15 open (row 3b); #7
-      reopened (auto-closed by PR #20's body) with the parent mapping. PR #20's body left as merged. **Open:**
-      (4) the two item-6 issues (after T6.4) and #21's closing comment or decision; (5) the local tag
+      reopened (auto-closed by PR #20's body) with the parent mapping. PR #20's body left as merged. (4) the two
+      item-6 issues — done 2026-09-19: **#25** (D-D derived plan bias) and **#26** (D-E nudge + retire/snooze), both
+      `ready-for-agent`, bodies drawn from the proposals with every cited test id, symbol and path checked against
+      `main` `a03fc9bd`; #21 given a status comment from the harness receipt and **left open** (its DoD names OpenCode
+      in core; OpenCode stays preview on the `opencode.db` exporter and the no-completed-reply gaps). **Open:**
+      (5) the local tag
       `archive/feat-clean-start-2026-09-15` — owner: push or discard; (6) the GitHub Support ticket text — owner;
       (7) revoke both tokens and delete `~/tmp/.env` — owner (D-J).
diff --git a/packages/studyloop/tests/test_ci_workflow_contract.py b/packages/studyloop/tests/test_ci_workflow_contract.py
index ebd150b8b..be7101fcd 100644
--- a/packages/studyloop/tests/test_ci_workflow_contract.py
+++ b/packages/studyloop/tests/test_ci_workflow_contract.py
@@ -236,3 +236,57 @@ def test_sast_and_pre_commit_bandit_skip_lists_agree() -> None:
     )
 
     assert sast_skipped == precommit_skipped == ci_standards_skipped
+
+
+NIGHTLY_WORKFLOW = WORKFLOW_DIR / "nightly-install.yml"
+
+
+def _nightly_workflow() -> dict[str, Any]:
+    return yaml.safe_load(NIGHTLY_WORKFLOW.read_text(encoding="utf-8"))
+
+
+def test_nightly_installer_job_plants_a_harness_before_running_install_sh() -> None:
+    """The nightly `installer` job (A4, added 2026-09-14) isolates HOME so
+    `studyloop install agents` writes into scratch -- and had never passed:
+    an empty HOME has no harness, `detect_available_agent_tools()` finds
+    nothing, and `install.sh` exits 1 at "Installing agent definitions"
+    exactly as the README says it should when no supported AI tool exists.
+    The job died five nights running before its verify step ever ran.
+
+    The fixture must supply the precondition the script documents: at least
+    one harness marker directory that the detector reads without a binary
+    (`~/.kiro`, `~/.claude`, `~/.pi`, `~/.grok`), created in the isolated
+    HOME by the same step that runs the script. And the job must then check
+    what `install agents` wrote, or the isolation buys nothing.
+    """
+    data = _nightly_workflow()
+    installer = data["jobs"]["installer"]
+    steps = installer["steps"]
+    run_step = next(step for step in steps if step.get("name") == "Run scripts/install.sh")
+
+    assert run_step["env"]["HOME"] == "${{ runner.temp }}/home", (
+        "HOME isolation is the point of the job; it must stay"
+    )
+    run = run_step["run"]
+    assert "./scripts/install.sh --non-interactive --no-smoke" in run
+
+    planted = [
+        marker
+        for marker in ("$HOME/.kiro", "$HOME/.claude", "$HOME/.pi", "$HOME/.grok")
+        if marker in run
+    ]
+    assert planted, (
+        "the installer job runs install.sh in an empty HOME; plant at least one "
+        "directory-detected harness marker first or `install agents` exits 1"
+    )
+    plant_at = min(run.index(marker) for marker in planted)
+    assert plant_at < run.index("./scripts/install.sh"), "plant the marker BEFORE the script runs"
+
+    verify = next(
+        (step for step in steps if step.get("name") == "Verify installed agent definitions"),
+        None,
+    )
+    assert verify is not None, "the job must verify what `install agents` wrote into HOME"
+    assert verify["env"]["HOME"] == "${{ runner.temp }}/home"
+    for planted_marker in planted:
+        assert planted_marker in verify["run"], f"verify step does not look inside {planted_marker}"