diff --git a/packages/chat/src/routes.ts b/packages/chat/src/routes.ts
index 49a1d53a5..d632c8dc2 100644
--- a/packages/chat/src/routes.ts
+++ b/packages/chat/src/routes.ts
@@ -409,6 +409,19 @@ export function createChatRoutes(deps: CreateChatRoutesDeps): Hono<TenantEnv> {
       // same launch-and-join core `POST .../invite` uses, run inline
       // here so the chat comes back from this single call already
       // carrying its one agent participant.
+      //
+      // The agent launch runs after the host, tenant, and settings are
+      // all live. Any failure here leaves a half-built channel that must
+      // be rolled back: the tenant and settings this handler minted are
+      // compensated and deleted exactly as the host-launch path above
+      // compensates a tenant whose host never came up, so a retry starts
+      // clean rather than reusing an orphaned tenant.
+      //
+      // InferenceResolutionError (no model requirements / no inference
+      // source) is a caller-correctable config problem → 409. Other launch
+      // failures (too many @mentions, transient platform errors) → 422.
+      // Compensation failures are logged loudly and never swallow the
+      // original launch error.
       try {
         const joined = await launchAndJoinAgent(
           { store: deps.store, platform: deps.platform, publish },
@@ -443,13 +456,40 @@ export function createChatRoutes(deps: CreateChatRoutesDeps): Hono<TenantEnv> {
           201,
         );
       } catch (err) {
+        log.error(
+          "Agent launch failed for channel {channelId} after the host " +
+            "launched and settings were written; compensating the channel " +
+            "tenant and deleting its settings",
+          { channelId, tenantId: channelTenant.tenantId, err },
+        );
+        try {
+          await deps.tenancy.compensateChannelTenant(channelTenant.tenantId);
+          await deps.store.deleteChannelSettings(tenant.id, channelId);
+        } catch (compensationErr) {
+          log.error(
+            "Compensation failed after agent launch failure for channel " +
+              "{channelId}; the orphaned tenant {tenantId} and/or its " +
+              "settings require manual cleanup",
+            {
+              channelId,
+              tenantId: channelTenant.tenantId,
+              compensationErr,
+            },
+          );
+        }
         if (err instanceof InferenceResolutionError) {
           return c.json(
             ErrorEnvelope("not_launchable", err.resolutionMessage),
             409,
           );
         }
-        throw err;
+        return c.json(
+          ErrorEnvelope(
+            "agent_launch_failed",
+            err instanceof Error ? err.message : String(err),
+          ),
+          422,
+        );
       }
     },
   );
diff --git a/packages/chat/src/store.ts b/packages/chat/src/store.ts
index 5371e50a3..3e3493462 100644
--- a/packages/chat/src/store.ts
+++ b/packages/chat/src/store.ts
@@ -90,6 +90,14 @@ export interface ChatStore {
     tenantId: string,
     channelId: string,
   ): Promise<ChannelSettingsRow | undefined>;
+  /**
+   * Removes a channel's settings row. Used only to compensate a channel
+   * whose creation a downstream step (the agent launch) failed to
+   * complete — the channel host, tenant, and settings were all written
+   * before the launch failed, so rolling the channel back means deleting
+   * each of them in turn (see `routes.ts`'s create handler).
+   */
+  deleteChannelSettings(tenantId: string, channelId: string): Promise<void>;
   listChannelSettings(
     tenantId: string,
     kind?: string,
@@ -156,6 +164,17 @@ export function createDrizzleChatStore<TSchema extends Record<string, unknown>>(
       return selected as ChannelSettingsRow | undefined;
     },
 
+    async deleteChannelSettings(tenantId, channelId) {
+      await db
+        .delete(channelSettings)
+        .where(
+          and(
+            eq(channelSettings.tenantId, tenantId),
+            eq(channelSettings.channelId, channelId),
+          ),
+        );
+    },
+
     async listChannelSettings(tenantId, kind) {
       const rows = await db
         .select()
@@ -312,6 +331,10 @@ export function createInMemoryChatStore(): ChatStore {
       return settingsByKey.get(settingsKey(tenantId, channelId));
     },
 
+    async deleteChannelSettings(tenantId, channelId) {
+      settingsByKey.delete(settingsKey(tenantId, channelId));
+    },
+
     async listChannelSettings(tenantId, kind) {
       const rows = [...settingsByKey.values()].filter(
         (row) => row.tenantId === tenantId,
diff --git a/packages/chat/test/routes.test.ts b/packages/chat/test/routes.test.ts
index 51ede1d77..a7b7fcb6a 100644
--- a/packages/chat/test/routes.test.ts
+++ b/packages/chat/test/routes.test.ts
@@ -180,6 +180,39 @@ describe("POST /channels", () => {
       "This definition declares no model requirements",
     );
   });
+
+  test("agent launch failure returns 422, not 500, and compensates the channel", async () => {
+    const deps = buildDeps({
+      platform: fakePlatform({
+        invitable: [{ id: "wfd_echo", name: "Echo" }],
+        launchInvite: () =>
+          Promise.reject(new Error("blocked: too many @mentions; max 5")),
+      }),
+    });
+    const app = mountAs(createChatRoutes(deps), "prn_alice");
+
+    const response = await app.request("/channels", {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({ kind: "chat", definitionId: "wfd_echo" }),
+    });
+
+    expect(response.status).toBe(422);
+    const body = (await response.json()) as {
+      error: { code: string; message: string };
+    };
+    expect(body.error.code).toBe("agent_launch_failed");
+    expect(body.error.message).toContain("too many @mentions");
+
+    // The half-built channel is rolled back: its settings row is gone
+    // and its minted tenant is compensated, so a retry starts clean.
+    const tenancy = deps.tenancy as ReturnType<
+      typeof createInMemoryChannelTenancyStore
+    >;
+    const channels = await deps.store.listChannelSettings(TENANT.id);
+    expect(channels).toHaveLength(0);
+    expect(await tenancy.listChildChannelTenancies(TENANT.id)).toHaveLength(0);
+  });
 });
 
 describe("POST /channels/:id/invite", () => {