diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2bea539..d1c03d6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,39 @@ changed, why it matters, and which user-visible guarantee got stronger. Use it a
 the fastest factual tour of Goal Mode's evolution from prompt discipline into a
 guard-enforced workflow.
 
+## Unreleased
+
+### Fix: switching to Build no longer hijacks the session back to Goal (issue #428)
+
+After starting `/goal`, interrupting, switching to **Build**, and prompting it,
+the Build response would finish and Goal Mode would *automatically switch the
+main agent back to `goal`* and start running review subagents — exactly the
+opposite of what an agent switch is supposed to do.
+
+**Root cause.** When you switch a session off the `goal` agent, `chat.params` /
+`chat.message` correctly set `state.active = false`. But the idle handler's
+"is this still a goal session?" check OR'd in two terms that ignored that
+deactivation:
+
+- `Boolean(state.contract)` — the contract persists from the prior goal, so a
+  stale contract re-activated the session; and
+- `goalSessionActiveForAgent("goal", state)` — which is *tautological*: it
+  returns `true` unconditionally when its first argument is the primary `"goal"`
+  agent, so once a session had ever been a goal it was treated as one forever.
+
+The result: on the Build turn's idle, the guard launched a programmatic review
+cycle, and the reviewer batch (pinned to the goal primary) switched the user's
+main agent back to Goal — the reported "automatically switching to Build".
+
+**Fix.** The idle path now requires `state.active` for a session to be eligible
+for programmatic review or auto-continue. A stale contract (or any other
+persisted goal state) can no longer re-activate a session you explicitly paused;
+only switching back to the `goal` agent (or running `/goal`) re-activates it, and
+reviews then resume with all of your contract, evidence, and review-cycle state
+intact. As defense-in-depth, `launchReviewBatch` itself now refuses to run on a
+`state.active === false` session, so a future regression in the eligibility check
+can never switch the user's main agent back to Goal.
+
 ## v0.7.0
 
 ### Feature: YOLO mode + a fully customizable guard
diff --git a/README.md b/README.md
index 37d8bb2..20897d6 100644
--- a/README.md
+++ b/README.md
@@ -346,9 +346,15 @@ on this tool.
   of pausing.
 - **Goal Mode stopped after switching agents.** Switching the session off the `goal`
   agent — to Build or Plan, or through an action that cycles the agent — intentionally
-  pauses Goal Mode; the guard shows a toast and stops treating that turn as a goal. Your
-  contract, reviews, and evidence are preserved. Switch back to the `goal` agent, or run
-  `/goal`, to resume with all state intact.
+  pauses Goal Mode; the guard shows a toast and stops treating that turn as a goal.
+  While paused, **no** Goal activity fires on that session: the sidebar hides, the
+  completion claim is no longer rewritten, auto-continue sends no "keep going" prompt,
+  and **programmatic reviews are not launched** (so the guard will never switch your
+  main agent back to `goal` on its own — see
+  [issue #428](https://github.com/devinoldenburg/opencode-goal-mode/issues/428)). Your
+  contract, reviews, and evidence are preserved. Switch back to the `goal` agent, or
+  run `/goal`, to resume with all state intact and reviews will run again on the next
+  idle.
 - **A safe command was blocked.** Inspect how the analyzer reads it with
   `node benchmarks/external.mjs --json`, allow it for that project with
   `allowCommands`, and please
diff --git a/plugins/goal-guard/guard.js b/plugins/goal-guard/guard.js
index e9d08fa..7738714 100644
--- a/plugins/goal-guard/guard.js
+++ b/plugins/goal-guard/guard.js
@@ -287,10 +287,25 @@ export function createGuard(input = {}, options = {}, overrides = {}) {
         state.verificationSeen ||
         (Array.isArray(state.evidence) && state.evidence.length > 0) ||
         (Array.isArray(state.changedFiles) && state.changedFiles.length > 0);
-      const goalSession =
-        state.active ||
-        Boolean(state.contract) ||
-        goalSessionActiveForAgent("goal", state);
+      // The user can explicitly switch a goal session off the goal primary — to
+      // Build/Plan, or via an action that cycles the agent — and `chat.params` /
+      // `chat.message` faithfully set `state.active=false` when that happens. The
+      // guard MUST honor that deactivation here: a stale contract (or any other
+      // persisted goal state) MUST NOT re-activate the session, or the guard would
+      // launch programmatic reviews on the user's Build turn, and the reviewer
+      // batch (pinned to the goal primary in review-runner.js) would switch the
+      // main agent back to Goal against the user's explicit choice — the
+      // "automatically switching to Build" report in issue #428.
+      //
+      // The previous expression additionally OR'd in `Boolean(state.contract)` and
+      // `goalSessionActiveForAgent("goal", state)`, but the latter is TAUTOLOGICAL:
+      // `goalSessionActiveForAgent` returns `true` unconditionally when its first
+      // argument is the primary `"goal"` agent (see agents.js), so once a session
+      // had ever been a goal the idle path treated it as a goal forever, no matter
+      // how many times the user switched away. Only an ACTUALLY active goal session
+      // is eligible for review / auto-continue; switching back to the goal agent
+      // (or running /goal) re-activates it and reviews resume with all state intact.
+      const goalSession = state.active;
       const outstandingGates = missingGates(state, config).length;
       const anchoredContract = Boolean(state.contract) && !state.contract?.auto;
       const canProgrammaticReview =
diff --git a/plugins/goal-guard/review-runner.js b/plugins/goal-guard/review-runner.js
index 2ed66e2..a287c60 100644
--- a/plugins/goal-guard/review-runner.js
+++ b/plugins/goal-guard/review-runner.js
@@ -199,6 +199,20 @@ function freshRecordedVerdict(state, agent, beforeSeq) {
 /** Launch every reviewer in one batched subtask prompt (parallel on the host). */
 async function launchReviewBatch(client, sessionID, agents, state, model, { sleep, idleDeferMs }) {
   if (!agents.length) return;
+  // Defense-in-depth for issue #428: the only legitimate caller is the idle path
+  // of an ACTIVE goal session, but a future regression in the caller's `goalSession`
+  // check must NEVER be able to launch a reviewer batch on a session the user has
+  // explicitly switched away from (Build/Plan/etc.). Doing so would pin the batch to
+  // the goal primary and silently switch the user's main agent back to Goal — the
+  // exact symptom in the bug report. Bail out (treated as SessionBusy so the outer
+  // retry loop defers instead of marking every reviewer failed) when the session is
+  // not actually active. `state` is the live store record, so a fresh agent switch
+  // is reflected here even if it happened after the caller's eligibility check.
+  if (state && state.active === false) {
+    const err = new Error("Goal Guard: session is not an active goal session — refusing to launch programmatic review");
+    err.name = "SessionBusyError";
+    throw err;
+  }
   if (idleDeferMs > 0) await sleep(idleDeferMs);
   await promptSubtaskWithRetry(
     client,
@@ -208,6 +222,9 @@ async function launchReviewBatch(client, sessionID, agents, state, model, { slee
       // goal session (the same programmatic path as task/command subtasks), never on
       // a Build/Plan session. Reference PRIMARY_AGENT, not a bare literal, so a
       // rename of the primary agent id cannot desync this from the rest of the guard.
+      // (Safe now that the eligibility check above guarantees the session is active:
+      // when active, the session's current agent is already the goal primary or a
+      // goal worker — so this pin is a no-op for the user's main agent.)
       agent: PRIMARY_AGENT,
       ...(model ? { model } : {}),
       parts: agents.map((agent) => ({
diff --git a/tests/plugin.test.mjs b/tests/plugin.test.mjs
index a38b9b7..f6200a4 100644
--- a/tests/plugin.test.mjs
+++ b/tests/plugin.test.mjs
@@ -513,6 +513,117 @@ test("a Build (non-goal) idle session is never auto-continued", async () => {
   assert.equal(sent.length, 0);
 });
 
+test("REGRESSION [issue #428]: a goal session switched to Build is NOT re-activated on idle (no reviews, no agent switch)", async () => {
+  // The reporter's flow: /goal → interrupt → switch to Build → prompt Build → after
+  // Build's response the guard switched the main agent back to Goal and ran reviews.
+  // Root cause: resolveIdleSession's `goalSession` check OR'd in a tautological
+  // goalSessionActiveForAgent("goal", state) (always true) plus a stale contract, so
+  // an explicitly deactivated session was still treated as a goal on idle. The fix is
+  // to require state.active; this test pins the regression.
+  const launched = [];
+  const prompts = [];
+  const client = {
+    app: { log: async () => {} },
+    tui: { showToast: async () => {} },
+    session: {
+      promptAsync: async ({ path, body }) => {
+        prompts.push({ id: path.id, agent: body?.agent, body });
+        for (const part of body?.parts || []) {
+          if (part.type === "subtask") launched.push(part.agent);
+        }
+      },
+      messages: async () => ({ data: [] }),
+    },
+  };
+  const guard = __test.createGuard(
+    { client },
+    { abortGraceMs: 0, reviewPollMs: 2, reviewTimeoutMs: 100, reviewIdleDeferMs: 0 },
+    { persistence: noopPersistence, clock: () => 1, syncIdle: true },
+  );
+
+  // 1) Start a goal and anchor work (contract auto-seeded, edit recorded).
+  await guard.hooks["chat.params"]({ sessionID: "s", agent: "goal" }, {});
+  await guard.hooks["chat.message"]({ sessionID: "s", agent: "goal" }, { parts: [{ type: "text", text: "implement the feature" }] });
+  await guard.hooks["tool.execute.after"]({ tool: "edit", sessionID: "s", callID: "c", args: {} }, { output: "", title: "", metadata: {} });
+  assert.equal(guard.store.stateFor("s").active, true);
+  assert.ok(guard.store.stateFor("s").contract, "contract anchored");
+
+  // 2) User switches the SAME session to Build.
+  await guard.hooks["chat.params"]({ sessionID: "s", agent: "build" }, {});
+  assert.equal(guard.store.stateFor("s").active, false, "Build deactivates the goal session");
+  assert.equal(guard.store.stateFor("s").currentAgent, "build");
+
+  // 3) User prompts Build (a genuine new user turn on the build agent).
+  await guard.hooks["chat.message"]({ sessionID: "s", agent: "build" }, { parts: [{ type: "text", text: "just check it compiles" }] });
+
+  // 4) Build's turn finishes → idle fires.
+  await guard.hooks.event({ event: { type: "session.idle", properties: { sessionID: "s" } } });
+
+  // 5) The guard must NOT have launched reviewers or switched the agent to goal.
+  assert.equal(launched.length, 0, `BUG #428: reviewers must not launch on a Build session (got ${launched.length})`);
+  const switchedToGoal = prompts.some((p) => p.agent === "goal");
+  assert.equal(switchedToGoal, false, "BUG #428: the guard must NOT switch the main agent back to goal");
+  // Contract + work are preserved so the user can resume by switching back.
+  assert.ok(guard.store.stateFor("s").contract, "goal state is preserved across the Build excursion");
+  assert.ok((guard.store.stateFor("s").lastEditSeq || 0) > 0, "edit history is preserved");
+});
+
+test("REGRESSION [issue #428] companion: switching back to Goal re-activates and programmatic review resumes", async () => {
+  // The fix must NOT over-fire — switching back to the goal agent (or running /goal)
+  // re-activates the session and programmatic reviews run with all prior state intact.
+  const launched = [];
+  const prompts = [];
+  let currentAgent = null;
+  const client = {
+    app: { log: async () => {} },
+    tui: { showToast: async () => {} },
+    session: {
+      promptAsync: async ({ path, body }) => {
+        prompts.push({ id: path.id, agent: body?.agent, body });
+        for (const part of body?.parts || []) {
+          if (part.type === "subtask") {
+            currentAgent = part.agent;
+            launched.push(part.agent);
+          }
+        }
+      },
+      messages: async () => ({
+        data: [
+          {
+            info: { role: "assistant", agent: currentAgent },
+            parts: launched.map((agent) => ({
+              type: "tool",
+              tool: "task",
+              state: { status: "completed", input: { subagent_type: agent }, output: "Reviewed.\nVerdict: PASS" },
+            })),
+          },
+        ],
+      }),
+    },
+  };
+  const guard = __test.createGuard(
+    { client },
+    { abortGraceMs: 0, reviewPollMs: 2, reviewTimeoutMs: 500, reviewIdleDeferMs: 0 },
+    { persistence: noopPersistence, clock: () => 1, syncIdle: true },
+  );
+
+  // Goal → Build (idle is a no-op for reviews) → back to Goal (idle runs reviews).
+  await guard.hooks["chat.params"]({ sessionID: "s", agent: "goal" }, {});
+  await guard.hooks["chat.message"]({ sessionID: "s", agent: "goal" }, { parts: [{ type: "text", text: "implement the feature" }] });
+  await guard.hooks["tool.execute.after"]({ tool: "edit", sessionID: "s", callID: "c", args: {} }, { output: "", title: "", metadata: {} });
+  await guard.hooks["chat.params"]({ sessionID: "s", agent: "build" }, {});
+  await guard.hooks.event({ event: { type: "session.idle", properties: { sessionID: "s" } } });
+  assert.equal(launched.length, 0, "Build idle: no reviews");
+
+  // Switch back to Goal and prompt it; idle now runs the programmatic review cycle.
+  await guard.hooks["chat.params"]({ sessionID: "s", agent: "goal" }, {});
+  await guard.hooks["chat.message"]({ sessionID: "s", agent: "goal" }, { parts: [{ type: "text", text: "resume" }] });
+  assert.equal(guard.store.stateFor("s").active, true, "switching back to Goal re-activates the session");
+  await guard.hooks.event({ event: { type: "session.idle", properties: { sessionID: "s" } } });
+
+  assert.ok(launched.length >= 5, `programmatic reviews resume after re-activation (got ${launched.length})`);
+});
+
 test("auto-continue can be disabled via config", async () => {
   const sent = [];
   const guard = __test.createGuard(
diff --git a/tests/programmatic-review.test.mjs b/tests/programmatic-review.test.mjs
index 7fbaceb..1c36cfc 100644
--- a/tests/programmatic-review.test.mjs
+++ b/tests/programmatic-review.test.mjs
@@ -236,3 +236,24 @@ test("patch/str_replace edits count as work for programmatic review", async () =
   await guard.hooks.event({ event: { type: "session.idle", properties: { sessionID: "g" } } });
   assert.ok(launched.length >= 5, `patch tool must mark work and trigger review (got ${launched.length})`);
 });
+
+test("REGRESSION [issue #428]: a deactivated session never launches a reviewer batch (defense-in-depth in launchReviewBatch)", async () => {
+  // The idle path now blocks this upstream, but launchReviewBatch is the very call
+  // that pins the batch to the goal primary and would switch the user's main agent.
+  // A future regression in the eligibility check must NOT be able to reach it for a
+  // session the user has switched to Build — so launchReviewBatch refuses outright
+  // when state.active is false, surfacing as a SessionBusy deferral (not a hard
+  // failure that marks every reviewer failed).
+  const { guard, launched } = makeReviewingGuard("PASS");
+  await startGoalWithWork(guard.hooks);
+  // User switches the active goal session to Build.
+  await guard.hooks["chat.params"]({ sessionID: "g", agent: "build" }, {});
+  assert.equal(guard.store.stateFor("g").active, false);
+  await guard.hooks.event({ event: { type: "session.idle", properties: { sessionID: "g" } } });
+  assert.equal(launched.length, 0, "no reviewer subtask may launch on a deactivated (Build) session");
+  // Re-activate and confirm reviews then run normally — the deferral did not wedge.
+  await guard.hooks["chat.params"]({ sessionID: "g", agent: "goal", model: MODEL }, {});
+  await guard.hooks["chat.message"]({ sessionID: "g", agent: "goal", model: MODEL }, { parts: [{ type: "text", text: "resume" }] });
+  await guard.hooks.event({ event: { type: "session.idle", properties: { sessionID: "g" } } });
+  assert.ok(launched.length >= 5, `reviews run normally after re-activation (got ${launched.length})`);
+});