diff --git a/src/auto-detect-model.ts b/src/auto-detect-model.ts
new file mode 100644
index 0000000..3f57b6a
--- /dev/null
+++ b/src/auto-detect-model.ts
@@ -0,0 +1,47 @@
+// Zero-config model auto-detection (issue #14).
+//
+// When no model is configured via --model, agent.yaml, or env config override,
+// check known provider API keys (via pi-ai's getEnvApiKey, so Bedrock / Vertex /
+// Azure / Copilot ambient credentials are picked up too, not just a hardcoded
+// env var list) and fall back to a sensible default model for the first
+// provider that has credentials available.
+
+import { getEnvApiKey } from "@mariozechner/pi-ai";
+
+// Checked in this order; first provider with credentials available wins.
+// Versionless aliases only, so this never goes stale as new point releases ship.
+//
+// Direct API-key providers first (explicit, deliberate signal); ambient/
+// enterprise credential sources (Bedrock, Vertex, Azure, Copilot) checked
+// after, since those can be present incidentally (e.g. a leftover AWS_PROFILE
+// or an active gcloud login unrelated to this agent).
+const DEFAULT_MODEL_BY_PROVIDER: Array<{ provider: string; model: string }> = [
+	{ provider: "anthropic", model: "claude-sonnet-4-6" },
+	{ provider: "openai", model: "gpt-4o" },
+	{ provider: "google", model: "gemini-2.0-flash" },
+	{ provider: "xai", model: "grok-4-fast" },
+	{ provider: "groq", model: "llama-3.3-70b-versatile" },
+	{ provider: "mistral", model: "mistral-large-latest" },
+	{ provider: "amazon-bedrock", model: "anthropic.claude-sonnet-4-6" },
+	{ provider: "google-vertex", model: "gemini-2.0-flash" },
+	{ provider: "azure-openai-responses", model: "gpt-4o" },
+	{ provider: "github-copilot", model: "claude-sonnet-4.6" },
+];
+
+export interface AutoDetectedModel {
+	modelStr: string;
+	provider: string;
+}
+
+/**
+ * Find the first provider with a usable API key/credential in the environment
+ * and return its default model, or undefined if none are configured.
+ */
+export function autoDetectModel(): AutoDetectedModel | undefined {
+	for (const { provider, model } of DEFAULT_MODEL_BY_PROVIDER) {
+		if (getEnvApiKey(provider)) {
+			return { modelStr: `${provider}:${model}`, provider };
+		}
+	}
+	return undefined;
+}
diff --git a/src/loader.ts b/src/loader.ts
index b8d193e..b548635 100644
--- a/src/loader.ts
+++ b/src/loader.ts
@@ -19,6 +19,7 @@ import { loadExamples, formatExamplesForPrompt } from "./examples.js";
 import type { ExampleEntry } from "./examples.js";
 import { validateCompliance, loadComplianceContext, formatComplianceWarnings } from "./compliance.js";
 import type { ComplianceWarning } from "./compliance.js";
+import { autoDetectModel } from "./auto-detect-model.js";
 import { discoverAndLoadPlugins } from "./plugins.js";
 import type { LoadedPlugin } from "./plugin-types.js";
 import type { PluginConfig } from "./plugin-types.js";
@@ -33,7 +34,7 @@ export interface AgentManifest {
 	tags?: string[];
 	metadata?: Record<string, string | number | boolean>;
 	model: {
-		preferred: string;
+		preferred?: string;
 		fallback: string[];
 		constraints?: {
 			temperature?: number;
@@ -241,6 +242,9 @@ export async function loadAgent(
 	// Parse agent.yaml
 	const manifestRaw = await readFile(join(agentDir, "agent.yaml"), "utf-8");
 	let manifest = yaml.load(manifestRaw) as AgentManifest;
+	// model: is optional in agent.yaml (auto-detection can fill it in later) —
+	// normalize here so downstream code can always assume manifest.model exists.
+	manifest.model ??= { fallback: [] };
 
 	// Load environment config
 	const envConfig = await loadEnvConfig(agentDir, envFlag);
@@ -379,11 +383,18 @@ Do NOT track trivial single-command tasks (e.g. "what time is it"). But DO check
 
 	const systemPrompt = parts.join("\n\n");
 
-	// Resolve model — env config model_override > CLI flag > manifest preferred
-	const modelStr = envConfig.model_override || modelFlag || manifest.model.preferred;
+	// Resolve model — env config model_override > CLI flag > manifest preferred > auto-detected from API key
+	let modelStr = envConfig.model_override || modelFlag || manifest.model?.preferred;
+	if (!modelStr) {
+		const detected = autoDetectModel();
+		if (detected) {
+			console.log(`Using ${detected.modelStr} (auto-detected from ${detected.provider} credentials)`);
+			modelStr = detected.modelStr;
+		}
+	}
 	if (!modelStr) {
 		throw new Error(
-			'No model configured. Either:\n  - Set model.preferred in agent.yaml (e.g., "anthropic:claude-sonnet-4-5-20250929")\n  - Pass --model provider:model on the command line',
+			'No model configured. Either:\n  - Set model.preferred in agent.yaml (e.g., "anthropic:claude-sonnet-4-5-20250929")\n  - Pass --model provider:model on the command line\n  - Set an API key for a known provider (e.g. ANTHROPIC_API_KEY) to auto-select a default model',
 		);
 	}