diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5f6630ef3..787d6fa62 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 - WhatsApp: refactor vCard parsing helper and improve empty contact card summaries. (#624) — thanks @steipete
 - WhatsApp: include phone numbers when multiple contacts are shared. (#625) — thanks @mahmoudashraf93
+- Agents: warn on small context windows (<32k) and block unusable ones (<16k). — thanks @steipete
 - Pairing: cap pending DM pairing requests at 3 per provider and avoid pairing replies for outbound DMs. — thanks @steipete
 - macOS: replace relay smoke test with version check in packaging script. (#615) — thanks @YuriNachos
 - macOS: avoid clearing Launch at Login during app initialization. (#607) — thanks @wes-davis
diff --git a/src/agents/context-window-guard.test.ts b/src/agents/context-window-guard.test.ts
new file mode 100644
index 000000000..83315a660
--- /dev/null
+++ b/src/agents/context-window-guard.test.ts
@@ -0,0 +1,135 @@
+import { describe, expect, it } from "vitest";
+
+import type { ClawdbotConfig } from "../config/config.js";
+import {
+  CONTEXT_WINDOW_HARD_MIN_TOKENS,
+  CONTEXT_WINDOW_WARN_BELOW_TOKENS,
+  evaluateContextWindowGuard,
+  resolveContextWindowInfo,
+} from "./context-window-guard.js";
+
+describe("context-window-guard", () => {
+  it("blocks below 16k (model metadata)", () => {
+    const info = resolveContextWindowInfo({
+      cfg: undefined,
+      provider: "openrouter",
+      modelId: "tiny",
+      modelContextWindow: 8000,
+      defaultTokens: 200_000,
+    });
+    const guard = evaluateContextWindowGuard({ info });
+    expect(guard.source).toBe("model");
+    expect(guard.tokens).toBe(8000);
+    expect(guard.shouldWarn).toBe(true);
+    expect(guard.shouldBlock).toBe(true);
+  });
+
+  it("warns below 32k but does not block at 16k+", () => {
+    const info = resolveContextWindowInfo({
+      cfg: undefined,
+      provider: "openai",
+      modelId: "small",
+      modelContextWindow: 24_000,
+      defaultTokens: 200_000,
+    });
+    const guard = evaluateContextWindowGuard({ info });
+    expect(guard.tokens).toBe(24_000);
+    expect(guard.shouldWarn).toBe(true);
+    expect(guard.shouldBlock).toBe(false);
+  });
+
+  it("does not warn at 32k+ (model metadata)", () => {
+    const info = resolveContextWindowInfo({
+      cfg: undefined,
+      provider: "openai",
+      modelId: "ok",
+      modelContextWindow: 64_000,
+      defaultTokens: 200_000,
+    });
+    const guard = evaluateContextWindowGuard({ info });
+    expect(guard.shouldWarn).toBe(false);
+    expect(guard.shouldBlock).toBe(false);
+  });
+
+  it("uses models.providers.*.models[].contextWindow when present", () => {
+    const cfg = {
+      models: {
+        providers: {
+          openrouter: {
+            baseUrl: "http://localhost",
+            apiKey: "x",
+            models: [
+              {
+                id: "tiny",
+                name: "tiny",
+                reasoning: false,
+                input: ["text"],
+                cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+                contextWindow: 12_000,
+                maxTokens: 256,
+              },
+            ],
+          },
+        },
+      },
+    } satisfies ClawdbotConfig;
+
+    const info = resolveContextWindowInfo({
+      cfg,
+      provider: "openrouter",
+      modelId: "tiny",
+      modelContextWindow: undefined,
+      defaultTokens: 200_000,
+    });
+    const guard = evaluateContextWindowGuard({ info });
+    expect(info.source).toBe("modelsConfig");
+    expect(guard.shouldBlock).toBe(true);
+  });
+
+  it("falls back to agents.defaults.contextTokens", () => {
+    const cfg = {
+      agents: { defaults: { contextTokens: 20_000 } },
+    } satisfies ClawdbotConfig;
+    const info = resolveContextWindowInfo({
+      cfg,
+      provider: "anthropic",
+      modelId: "whatever",
+      modelContextWindow: undefined,
+      defaultTokens: 200_000,
+    });
+    const guard = evaluateContextWindowGuard({ info });
+    expect(info.source).toBe("agentContextTokens");
+    expect(guard.shouldWarn).toBe(true);
+    expect(guard.shouldBlock).toBe(false);
+  });
+
+  it("uses default when nothing else is available", () => {
+    const info = resolveContextWindowInfo({
+      cfg: undefined,
+      provider: "anthropic",
+      modelId: "unknown",
+      modelContextWindow: undefined,
+      defaultTokens: 200_000,
+    });
+    const guard = evaluateContextWindowGuard({ info });
+    expect(info.source).toBe("default");
+    expect(guard.shouldWarn).toBe(false);
+    expect(guard.shouldBlock).toBe(false);
+  });
+
+  it("allows overriding thresholds", () => {
+    const info = { tokens: 10_000, source: "model" as const };
+    const guard = evaluateContextWindowGuard({
+      info,
+      warnBelowTokens: 12_000,
+      hardMinTokens: 9_000,
+    });
+    expect(guard.shouldWarn).toBe(true);
+    expect(guard.shouldBlock).toBe(false);
+  });
+
+  it("exports thresholds as expected", () => {
+    expect(CONTEXT_WINDOW_HARD_MIN_TOKENS).toBe(16_000);
+    expect(CONTEXT_WINDOW_WARN_BELOW_TOKENS).toBe(32_000);
+  });
+});
diff --git a/src/agents/context-window-guard.ts b/src/agents/context-window-guard.ts
new file mode 100644
index 000000000..952d6a4d5
--- /dev/null
+++ b/src/agents/context-window-guard.ts
@@ -0,0 +1,84 @@
+import type { ClawdbotConfig } from "../config/config.js";
+
+export const CONTEXT_WINDOW_HARD_MIN_TOKENS = 16_000;
+export const CONTEXT_WINDOW_WARN_BELOW_TOKENS = 32_000;
+
+export type ContextWindowSource =
+  | "model"
+  | "modelsConfig"
+  | "agentContextTokens"
+  | "default";
+
+export type ContextWindowInfo = {
+  tokens: number;
+  source: ContextWindowSource;
+};
+
+function normalizePositiveInt(value: unknown): number | null {
+  if (typeof value !== "number" || !Number.isFinite(value)) return null;
+  const int = Math.floor(value);
+  return int > 0 ? int : null;
+}
+
+export function resolveContextWindowInfo(params: {
+  cfg: ClawdbotConfig | undefined;
+  provider: string;
+  modelId: string;
+  modelContextWindow?: number;
+  defaultTokens: number;
+}): ContextWindowInfo {
+  const fromModel = normalizePositiveInt(params.modelContextWindow);
+  if (fromModel) return { tokens: fromModel, source: "model" };
+
+  const fromModelsConfig = (() => {
+    const providers = params.cfg?.models?.providers as
+      | Record<
+          string,
+          { models?: Array<{ id?: string; contextWindow?: number }> }
+        >
+      | undefined;
+    const providerEntry = providers?.[params.provider];
+    const models = Array.isArray(providerEntry?.models)
+      ? providerEntry.models
+      : [];
+    const match = models.find((m) => m?.id === params.modelId);
+    return normalizePositiveInt(match?.contextWindow);
+  })();
+  if (fromModelsConfig)
+    return { tokens: fromModelsConfig, source: "modelsConfig" };
+
+  const fromAgentConfig = normalizePositiveInt(
+    params.cfg?.agents?.defaults?.contextTokens,
+  );
+  if (fromAgentConfig)
+    return { tokens: fromAgentConfig, source: "agentContextTokens" };
+
+  return { tokens: Math.floor(params.defaultTokens), source: "default" };
+}
+
+export type ContextWindowGuardResult = ContextWindowInfo & {
+  shouldWarn: boolean;
+  shouldBlock: boolean;
+};
+
+export function evaluateContextWindowGuard(params: {
+  info: ContextWindowInfo;
+  warnBelowTokens?: number;
+  hardMinTokens?: number;
+}): ContextWindowGuardResult {
+  const warnBelow = Math.max(
+    1,
+    Math.floor(params.warnBelowTokens ?? CONTEXT_WINDOW_WARN_BELOW_TOKENS),
+  );
+  const hardMin = Math.max(
+    1,
+    Math.floor(params.hardMinTokens ?? CONTEXT_WINDOW_HARD_MIN_TOKENS),
+  );
+  const tokens = Math.max(0, Math.floor(params.info.tokens));
+  return {
+    ...params.info,
+    tokens,
+    shouldWarn: tokens > 0 && tokens < warnBelow,
+    shouldBlock: tokens > 0 && tokens < hardMin,
+  };
+}
diff --git a/src/agents/pi-embedded-runner.ts b/src/agents/pi-embedded-runner.ts
index 230c75b25..a95d5f38d 100644
--- a/src/agents/pi-embedded-runner.ts
+++ b/src/agents/pi-embedded-runner.ts
@@ -42,6 +42,12 @@ import {
   markAuthProfileUsed,
 } from "./auth-profiles.js";
 import type { BashElevatedDefaults } from "./bash-tools.js";
+import {
+  CONTEXT_WINDOW_HARD_MIN_TOKENS,
+  CONTEXT_WINDOW_WARN_BELOW_TOKENS,
+  evaluateContextWindowGuard,
+  resolveContextWindowInfo,
+} from "./context-window-guard.js";
 import {
   DEFAULT_CONTEXT_TOKENS,
   DEFAULT_MODEL,
@@ -183,41 +189,13 @@ function resolveContextWindowTokens(params: {
   modelId: string;
   model: Model<Api> | undefined;
 }): number {
-  const fromModel =
-    typeof params.model?.contextWindow === "number" &&
-    Number.isFinite(params.model.contextWindow) &&
-    params.model.contextWindow > 0
-      ? params.model.contextWindow
-      : undefined;
-  if (fromModel) return fromModel;
-
-  const fromModelsConfig = (() => {
-    const providers = params.cfg?.models?.providers as
-      | Record<
-          string,
-          { models?: Array<{ id?: string; contextWindow?: number }> }
-        >
-      | undefined;
-    const providerEntry = providers?.[params.provider];
-    const models = Array.isArray(providerEntry?.models)
-      ? providerEntry.models
-      : [];
-    const match = models.find((m) => m?.id === params.modelId);
-    return typeof match?.contextWindow === "number" && match.contextWindow > 0
-      ? match.contextWindow
-      : undefined;
-  })();
-  if (fromModelsConfig) return fromModelsConfig;
-
-  const fromAgentConfig =
-    typeof params.cfg?.agents?.defaults?.contextTokens === "number" &&
-    Number.isFinite(params.cfg.agents.defaults.contextTokens) &&
-    params.cfg.agents.defaults.contextTokens > 0
-      ? Math.floor(params.cfg.agents.defaults.contextTokens)
-      : undefined;
-  if (fromAgentConfig) return fromAgentConfig;
-
-  return DEFAULT_CONTEXT_TOKENS;
+  return resolveContextWindowInfo({
+    cfg: params.cfg,
+    provider: params.provider,
+    modelId: params.modelId,
+    modelContextWindow: params.model?.contextWindow,
+    defaultTokens: DEFAULT_CONTEXT_TOKENS,
+  }).tokens;
 }
 
 function buildContextPruningExtension(params: {
@@ -1086,6 +1064,33 @@ export async function runEmbeddedPiAgent(params: {
       if (!model) {
         throw new Error(error ?? `Unknown model: ${provider}/${modelId}`);
       }
+
+      const ctxInfo = resolveContextWindowInfo({
+        cfg: params.config,
+        provider,
+        modelId,
+        modelContextWindow: model.contextWindow,
+        defaultTokens: DEFAULT_CONTEXT_TOKENS,
+      });
+      const ctxGuard = evaluateContextWindowGuard({
+        info: ctxInfo,
+        warnBelowTokens: CONTEXT_WINDOW_WARN_BELOW_TOKENS,
+        hardMinTokens: CONTEXT_WINDOW_HARD_MIN_TOKENS,
+      });
+      if (ctxGuard.shouldWarn) {
+        log.warn(
+          `low context window: ${provider}/${modelId} ctx=${ctxGuard.tokens} (warn<${CONTEXT_WINDOW_WARN_BELOW_TOKENS}) source=${ctxGuard.source}`,
+        );
+      }
+      if (ctxGuard.shouldBlock) {
+        log.error(
+          `blocked model (context window too small): ${provider}/${modelId} ctx=${ctxGuard.tokens} (min=${CONTEXT_WINDOW_HARD_MIN_TOKENS}) source=${ctxGuard.source}`,
+        );
+        throw new FailoverError(
+          `Model context window too small (${ctxGuard.tokens} tokens). Minimum is ${CONTEXT_WINDOW_HARD_MIN_TOKENS}.`,
+          { reason: "unknown", provider, model: modelId },
+        );
+      }
       const authStore = ensureAuthProfileStore(agentDir);
       const explicitProfileId = params.authProfileId?.trim();
       const profileOrder = resolveAuthProfileOrder({