diff --git a/src/loop-guard.ts b/src/loop-guard.ts
new file mode 100644
index 0000000..7f784f2
Binary files /dev/null and b/src/loop-guard.ts differ
diff --git a/src/sdk.ts b/src/sdk.ts
index dfa9a80..0ada81a 100644
--- a/src/sdk.ts
+++ b/src/sdk.ts
@@ -23,6 +23,7 @@ import type {
 	SandboxOptions,
 } from "./sdk-types.js";
 import { CostTracker } from "./cost-tracker.js";
+import { createLoopGuard } from "./loop-guard.js";
 import { context as otelContext } from "@opentelemetry/api";
 import {
 	wrapToolWithOtel,
@@ -294,11 +295,19 @@ export function query(options: QueryOptions): Query {
 			if (c.top_k !== undefined) modelOptions.topK = c.top_k;
 		}
 
-		if (options.maxTurns !== undefined) {
-			modelOptions.maxTurns = options.maxTurns;
-		}
+		// Resolve the tool-use turn cap. pi-agent-core does not enforce a bound
+		// itself, so gitagent must — otherwise a provider that re-requests the
+		// same tool call loops forever (issue #58). Precedence: explicit option
+		// > agent manifest runtime.max_turns > safe default.
+		const DEFAULT_MAX_TURNS = 50;
+		const maxTurns =
+			options.maxTurns ?? loaded.manifest.runtime?.max_turns ?? DEFAULT_MAX_TURNS;
+		// Reflect the effective cap in agent state so telemetry/inspection shows
+		// the limit the loop guard actually enforces, not just when it was passed.
+		modelOptions.maxTurns = maxTurns;
 
 		// 8. Create Agent
+		const loopGuard = createLoopGuard({ maxTurns });
 		const agent = new Agent({
 			initialState: {
 				systemPrompt,
@@ -306,6 +315,7 @@ export function query(options: QueryOptions): Query {
 				tools,
 				...modelOptions,
 			},
+			afterToolCall: loopGuard.afterToolCall,
 		});
 
 		// 9. Subscribe to events and map to GCMessage
diff --git a/test/loop-guard.test.ts b/test/loop-guard.test.ts
new file mode 100644
index 0000000..03beec2
--- /dev/null
+++ b/test/loop-guard.test.ts
@@ -0,0 +1,84 @@
+import { describe, it, before } from "node:test";
+import assert from "node:assert/strict";
+
+let createLoopGuard: typeof import("../dist/loop-guard.js").createLoopGuard;
+
+before(async () => {
+	({ createLoopGuard } = await import("../dist/loop-guard.js"));
+});
+
+// Build a minimal AfterToolCallContext. The guard only reads assistantMessage
+// (by reference), toolCall.name, args, and result.content.
+function ctx(turn: object, name: string, args: unknown, details: unknown = undefined) {
+	return {
+		assistantMessage: turn as any,
+		toolCall: { type: "toolCall", id: "x", name, arguments: args as any },
+		args,
+		result: { content: [{ type: "text", text: "ok" }], details },
+		isError: false,
+		context: {} as any,
+	};
+}
+
+describe("createLoopGuard", () => {
+	it("does not terminate under normal, varied tool use", async () => {
+		const guard = createLoopGuard({ maxTurns: 50 });
+		for (let i = 0; i < 10; i++) {
+			const r = await guard.afterToolCall(ctx({}, "read", { path: `f${i}` }));
+			assert.equal(r, undefined);
+		}
+	});
+
+	it("terminates when the same tool call repeats maxRepeats times", async () => {
+		const guard = createLoopGuard({ maxTurns: 1000, maxRepeats: 5 });
+		let terminatedAt = -1;
+		for (let i = 0; i < 10; i++) {
+			// Distinct turn each time, but identical name+args → stuck loop
+			const r = await guard.afterToolCall(ctx({}, "write", { path: "a", content: "" }));
+			if (r?.terminate) {
+				terminatedAt = i;
+				break;
+			}
+		}
+		assert.equal(terminatedAt, 4, "should terminate on the 5th identical call");
+	});
+
+	it("resets the repeat counter when arguments change", async () => {
+		const guard = createLoopGuard({ maxTurns: 1000, maxRepeats: 3 });
+		// 2 identical, then a different one, then 2 identical again → never 3 in a row
+		assert.equal((await guard.afterToolCall(ctx({}, "write", { p: 1 })))?.terminate, undefined);
+		assert.equal((await guard.afterToolCall(ctx({}, "write", { p: 1 })))?.terminate, undefined);
+		assert.equal((await guard.afterToolCall(ctx({}, "write", { p: 2 })))?.terminate, undefined);
+		assert.equal((await guard.afterToolCall(ctx({}, "write", { p: 2 })))?.terminate, undefined);
+	});
+
+	it("terminates when maxTurns distinct turns is exceeded", async () => {
+		const guard = createLoopGuard({ maxTurns: 3, maxRepeats: 100 });
+		// Each call is a new turn object with varied args (no repeat trip).
+		assert.equal((await guard.afterToolCall(ctx({}, "read", { p: 1 })))?.terminate, undefined);
+		assert.equal((await guard.afterToolCall(ctx({}, "read", { p: 2 })))?.terminate, undefined);
+		const third = await guard.afterToolCall(ctx({}, "read", { p: 3 }));
+		assert.equal(third?.terminate, true, "3rd distinct turn hits maxTurns=3");
+	});
+
+	it("counts one turn per batch (shared assistantMessage reference)", async () => {
+		const guard = createLoopGuard({ maxTurns: 2, maxRepeats: 100 });
+		const turnA = {};
+		// Two calls in the SAME batch → one turn, must not trip maxTurns=2 yet.
+		assert.equal((await guard.afterToolCall(ctx(turnA, "read", { p: 1 })))?.terminate, undefined);
+		assert.equal((await guard.afterToolCall(ctx(turnA, "read", { p: 2 })))?.terminate, undefined);
+		// New batch → second turn → hits maxTurns=2.
+		const turnB = {};
+		const r = await guard.afterToolCall(ctx(turnB, "read", { p: 3 }));
+		assert.equal(r?.terminate, true);
+	});
+
+	it("preserves original tool output (content + details) when terminating", async () => {
+		const guard = createLoopGuard({ maxTurns: 1, maxRepeats: 100 });
+		const r = await guard.afterToolCall(ctx({}, "read", { p: 1 }, { rows: 3 }));
+		assert.equal(r?.terminate, true);
+		assert.equal(r?.content?.[0]?.text, "ok");
+		assert.match(r?.content?.[1]?.text ?? "", /loop-guard/);
+		assert.deepEqual(r?.details, { rows: 3 });
+	});
+});