diff --git a/CHANGELOG.md b/CHANGELOG.md
index d17c12761..c5f0e860d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,16 @@
 # Changelog
 
+## 2026.1.12-2
+
+### Changes
+- Subagents: add config to set default sub-agent model (`agents.defaults.subagents.model` + per-agent override); still overridden by `sessions_spawn.model`.
+
+## 2026.1.12-1
+
+### Changes
+- Heartbeat: raise default `ackMaxChars` to 300 so any `HEARTBEAT_OK` replies with short padding stay internal (fewer noisy heartbeat posts on providers).
+- Onboarding: normalize API key inputs (strip `export KEY=...` wrappers) so shell-style entries paste cleanly.
+
 ## 2026.1.12
 
 ### Highlights
diff --git a/docs/gateway/configuration.md b/docs/gateway/configuration.md
index f56339647..322647bd7 100644
--- a/docs/gateway/configuration.md
+++ b/docs/gateway/configuration.md
@@ -1276,6 +1276,7 @@ Example:
       },
       maxConcurrent: 3,
       subagents: {
+        model: "minimax/MiniMax-M2.1",
         maxConcurrent: 1,
         archiveAfterMinutes: 60
       },
@@ -1478,6 +1479,7 @@ Note: `applyPatch` is only under `tools.exec` (no `tools.bash` alias).
 Legacy: `tools.bash` is still accepted as an alias.
 
 `agents.defaults.subagents` configures sub-agent defaults:
+- `model`: default model for spawned sub-agents (string or `{ primary, fallbacks }`). If omitted, sub-agents inherit the caller’s model unless overridden per agent or per call.
 - `maxConcurrent`: max concurrent sub-agent runs (default 1)
 - `archiveAfterMinutes`: auto-archive sub-agent sessions after N minutes (default 60; set `0` to disable)
 - Per-subagent tool policy: `tools.subagents.tools.allow` / `tools.subagents.tools.deny` (deny wins)
diff --git a/docs/tools/subagents.md b/docs/tools/subagents.md
index e39d20ab7..ce0cbb15f 100644
--- a/docs/tools/subagents.md
+++ b/docs/tools/subagents.md
@@ -20,6 +20,7 @@ Primary goals:
 Use `sessions_spawn`:
 - Starts a sub-agent run (`deliver: false`, global lane: `subagent`)
 - Then runs an announce step and posts the announce reply to the requester chat provider
+- Default model: inherits the caller unless you set `agents.defaults.subagents.model` (or per-agent `agents.list[].subagents.model`); an explicit `sessions_spawn.model` still wins.
 
 Tool params:
 - `task` (required)
diff --git a/src/agents/clawdbot-tools.subagents.test.ts b/src/agents/clawdbot-tools.subagents.test.ts
index d8c99cbfc..405ad64bc 100644
--- a/src/agents/clawdbot-tools.subagents.test.ts
+++ b/src/agents/clawdbot-tools.subagents.test.ts
@@ -687,6 +687,91 @@ describe("subagents", () => {
     });
   });
 
+  it("sessions_spawn applies default subagent model from defaults config", async () => {
+    resetSubagentRegistryForTests();
+    callGatewayMock.mockReset();
+    configOverride = {
+      session: { mainKey: "main", scope: "per-sender" },
+      agents: { defaults: { subagents: { model: "minimax/MiniMax-M2.1" } } },
+    };
+    const calls: Array<{ method?: string; params?: unknown }> = [];
+
+    callGatewayMock.mockImplementation(async (opts: unknown) => {
+      const request = opts as { method?: string; params?: unknown };
+      calls.push(request);
+      if (request.method === "sessions.patch") {
+        return { ok: true };
+      }
+      if (request.method === "agent") {
+        return { runId: "run-default-model", status: "accepted" };
+      }
+      return {};
+    });
+
+    const tool = createClawdbotTools({
+      agentSessionKey: "agent:main:main",
+      agentProvider: "discord",
+    }).find((candidate) => candidate.name === "sessions_spawn");
+    if (!tool) throw new Error("missing sessions_spawn tool");
+
+    const result = await tool.execute("call-default-model", {
+      task: "do thing",
+    });
+    expect(result.details).toMatchObject({
+      status: "accepted",
+      modelApplied: true,
+    });
+
+    const patchCall = calls.find((call) => call.method === "sessions.patch");
+    expect(patchCall?.params).toMatchObject({
+      model: "minimax/MiniMax-M2.1",
+    });
+  });
+
+  it("sessions_spawn prefers per-agent subagent model over defaults", async () => {
+    resetSubagentRegistryForTests();
+    callGatewayMock.mockReset();
+    configOverride = {
+      session: { mainKey: "main", scope: "per-sender" },
+      agents: {
+        defaults: { subagents: { model: "minimax/MiniMax-M2.1" } },
+        list: [{ id: "research", subagents: { model: "opencode/claude" } }],
+      },
+    };
+    const calls: Array<{ method?: string; params?: unknown }> = [];
+
+    callGatewayMock.mockImplementation(async (opts: unknown) => {
+      const request = opts as { method?: string; params?: unknown };
+      calls.push(request);
+      if (request.method === "sessions.patch") {
+        return { ok: true };
+      }
+      if (request.method === "agent") {
+        return { runId: "run-agent-model", status: "accepted" };
+      }
+      return {};
+    });
+
+    const tool = createClawdbotTools({
+      agentSessionKey: "agent:research:main",
+      agentProvider: "discord",
+    }).find((candidate) => candidate.name === "sessions_spawn");
+    if (!tool) throw new Error("missing sessions_spawn tool");
+
+    const result = await tool.execute("call-agent-model", {
+      task: "do thing",
+    });
+    expect(result.details).toMatchObject({
+      status: "accepted",
+      modelApplied: true,
+    });
+
+    const patchCall = calls.find((call) => call.method === "sessions.patch");
+    expect(patchCall?.params).toMatchObject({
+      model: "opencode/claude",
+    });
+  });
+
   it("sessions_spawn skips invalid model overrides and continues", async () => {
     resetSubagentRegistryForTests();
     callGatewayMock.mockReset();
diff --git a/src/agents/tools/sessions-spawn-tool.ts b/src/agents/tools/sessions-spawn-tool.ts
index 1a0eb5565..b0abee9f0 100644
--- a/src/agents/tools/sessions-spawn-tool.ts
+++ b/src/agents/tools/sessions-spawn-tool.ts
@@ -35,6 +35,17 @@ const SessionsSpawnToolSchema = Type.Object({
   ),
 });
 
+function normalizeModelSelection(value: unknown): string | undefined {
+  if (typeof value === "string") {
+    const trimmed = value.trim();
+    return trimmed || undefined;
+  }
+  if (!value || typeof value !== "object") return undefined;
+  const primary = (value as { primary?: unknown }).primary;
+  if (typeof primary === "string" && primary.trim()) return primary.trim();
+  return undefined;
+}
+
 export function createSessionsSpawnTool(opts?: {
   agentSessionKey?: string;
   agentProvider?: GatewayMessageProvider;
@@ -51,7 +62,7 @@ export function createSessionsSpawnTool(opts?: {
       const task = readStringParam(params, "task", { required: true });
       const label = typeof params.label === "string" ? params.label.trim() : "";
       const requestedAgentId = readStringParam(params, "agentId");
-      const model = readStringParam(params, "model");
+      const modelOverride = readStringParam(params, "model");
       const cleanup =
         params.cleanup === "keep" || params.cleanup === "delete"
           ? (params.cleanup as "keep" | "delete")
@@ -129,11 +140,16 @@ export function createSessionsSpawnTool(opts?: {
       }
       const childSessionKey = `agent:${targetAgentId}:subagent:${crypto.randomUUID()}`;
       const shouldPatchSpawnedBy = opts?.sandboxed === true;
-      if (model) {
+      const targetAgentConfig = resolveAgentConfig(cfg, targetAgentId);
+      const resolvedModel =
+        normalizeModelSelection(modelOverride) ??
+        normalizeModelSelection(targetAgentConfig?.subagents?.model) ??
+        normalizeModelSelection(cfg.agents?.defaults?.subagents?.model);
+      if (resolvedModel) {
         try {
           await callGateway({
             method: "sessions.patch",
-            params: { key: childSessionKey, model },
+            params: { key: childSessionKey, model: resolvedModel },
             timeoutMs: 10_000,
           });
           modelApplied = true;
@@ -218,7 +234,7 @@ export function createSessionsSpawnTool(opts?: {
         status: "accepted",
         childSessionKey,
         runId: childRunId,
-        modelApplied: model ? modelApplied : undefined,
+        modelApplied: resolvedModel ? modelApplied : undefined,
         warning: modelWarning,
       });
     },
diff --git a/src/config/types.ts b/src/config/types.ts
index 1c723122e..d1d5e6af6 100644
--- a/src/config/types.ts
+++ b/src/config/types.ts
@@ -1090,6 +1090,8 @@ export type ToolsConfig = {
   };
   /** Sub-agent tool policy defaults (deny wins). */
   subagents?: {
+    /** Default model selection for spawned sub-agents (string or {primary,fallbacks}). */
+    model?: string | { primary?: string; fallbacks?: string[] };
     tools?: {
       allow?: string[];
       deny?: string[];
@@ -1119,6 +1121,8 @@ export type AgentConfig = {
   subagents?: {
     /** Allow spawning sub-agents under other agent ids. Use "*" to allow any. */
     allowAgents?: string[];
+    /** Per-agent default model for spawned sub-agents (string or {primary,fallbacks}). */
+    model?: string | { primary?: string; fallbacks?: string[] };
   };
   sandbox?: {
     mode?: "off" | "non-main" | "all";
diff --git a/src/config/zod-schema.ts b/src/config/zod-schema.ts
index af8fd547a..5cf9662bd 100644
--- a/src/config/zod-schema.ts
+++ b/src/config/zod-schema.ts
@@ -924,6 +924,15 @@ const AgentEntrySchema = z.object({
   subagents: z
     .object({
       allowAgents: z.array(z.string()).optional(),
+      model: z
+        .union([
+          z.string(),
+          z.object({
+            primary: z.string().optional(),
+            fallbacks: z.array(z.string()).optional(),
+          }),
+        ])
+        .optional(),
     })
     .optional(),
   sandbox: AgentSandboxSchema,
@@ -1227,6 +1236,15 @@ const AgentDefaultsSchema = z
       .object({
         maxConcurrent: z.number().int().positive().optional(),
         archiveAfterMinutes: z.number().int().positive().optional(),
+        model: z
+          .union([
+            z.string(),
+            z.object({
+              primary: z.string().optional(),
+              fallbacks: z.array(z.string()).optional(),
+            }),
+          ])
+          .optional(),
       })
       .optional(),
     sandbox: z