diff --git a/packages/core/src/agent.ts b/packages/core/src/agent.ts
index 72f9f9c..f58e25c 100644
--- a/packages/core/src/agent.ts
+++ b/packages/core/src/agent.ts
@@ -155,10 +155,11 @@ export function effectiveMaxContextLength(configured: number, contextWindow: unk
  * Output cap for meta requests (title generation / vision describing): these carry their own
  * small hardcoded budget, tightened further by the entry's per-model `max_tokens` when that is
  * smaller — a cap the user pinned below the budget must bind every request to that model. The
- * budget is never raised.
+ * budget is never raised. A non-positive cap (-1 = uncapped) never tightens: Math.min against
+ * it would send max_tokens -1 on the wire, and meta requests must keep their small budget.
  */
 export function metaMaxTokens(budget: number, modelCap: number | undefined): number {
-  return modelCap !== undefined ? Math.min(budget, modelCap) : budget;
+  return modelCap !== undefined && modelCap > 0 ? Math.min(budget, modelCap) : budget;
 }
 
 /** Create or load an Agent. */
diff --git a/packages/core/src/engine/context-engine.ts b/packages/core/src/engine/context-engine.ts
index 6cef147..dbb4a77 100644
--- a/packages/core/src/engine/context-engine.ts
+++ b/packages/core/src/engine/context-engine.ts
@@ -125,7 +125,7 @@ export interface ContextEngineDeps {
   trace?: TraceSink;
   /** Engine initial state (derived by replaying Trace on Session resumption). */
   initialState?: EngineInitialState;
-  /** Maximum LLM turns for a single Task. Defaults to 100. */
+  /** Maximum LLM turns for a single Task. Defaults to 100; -1 removes the cap. */
   maxTurns?: number;
   /** Maximum automatic retries for LLM timeout/reconnect within a single run. Defaults to 2. */
   maxReconnects?: number;
@@ -298,8 +298,11 @@ export class ContextEngine {
     let nextInput: OmniMessage[] = input;
 
     for (;;) {
-      // max_turns guard: emit a length notice and stop once exceeded.
-      if (turnCount >= this.maxTurns) {
+      // max_turns guard: emit a length notice and stop once exceeded. A non-positive cap
+      // (-1 per the config contract "must be > 0 or -1") disables the guard entirely —
+      // same convention as maxSessionTurns in shouldCompact (issue #55: -1 used to trip
+      // `0 >= -1` and stop before the first turn).
+      if (this.maxTurns > 0 && turnCount >= this.maxTurns) {
         // This turn's pending input (usually the previous turn's tool outputs) was never
         // submitted to the LLM: hold it as carry-over, to be resent merged with new input on
         // the next `run` (same as interruption-cleanup case A) — the previous turn's assistant
diff --git a/packages/core/src/interfaces.ts b/packages/core/src/interfaces.ts
index 486fa5c..d36e3c2 100644
--- a/packages/core/src/interfaces.ts
+++ b/packages/core/src/interfaces.ts
@@ -101,6 +101,7 @@ export interface GenerativeModelConfig {
   /** Full system Prompt after placeholder substitution in the system_config.system_prompt template. */
   systemPrompt?: string;
   contextWindow?: number;
+  /** Output token cap per Request; non-positive (-1) means no explicit cap (omitted from the request). */
   maxTokens?: number;
   thinkingLevel?: ThinkingLevelName;
   /** LLM Request timeout (ms): from system_config.model.timeoutMs; <=0 disables it. Defaults to 120000. */
diff --git a/packages/core/src/llm/generative-model.ts b/packages/core/src/llm/generative-model.ts
index fa6661f..0751f0c 100644
--- a/packages/core/src/llm/generative-model.ts
+++ b/packages/core/src/llm/generative-model.ts
@@ -1113,7 +1113,10 @@ export function buildUniConfig(config: GenerativeModelConfig): UniConfig {
   if (config.systemPrompt !== undefined) {
     uniConfig.system_prompt = config.systemPrompt;
   }
-  if (config.maxTokens !== undefined) {
+  // Non-positive (-1 per the config contract) means "no explicit cap": the key is left off
+  // the wire so the provider default applies — sent literally, every provider rejects a
+  // negative max_tokens with a 400 (issue #55's sibling).
+  if (config.maxTokens !== undefined && config.maxTokens > 0) {
     uniConfig.max_tokens = config.maxTokens;
   }
   const thinking = mapThinkingLevel(config.thinkingLevel);
diff --git a/packages/core/src/session.ts b/packages/core/src/session.ts
index f273ac6..192ef24 100644
--- a/packages/core/src/session.ts
+++ b/packages/core/src/session.ts
@@ -38,6 +38,7 @@ export interface SessionConfig {
   llm: LLMInterface;
   environment: EnvironmentInterface;
   trace?: TraceSink;
+  /** Maximum LLM turns per Task (default 100; -1 removes the cap). */
   maxTurns?: number;
   /** Creates a new LLM object after compaction (carries over the Session's accumulated Token count); context compaction is unavailable if not provided. */
   createLLM?: (sessionTokens: TokenCounts) => LLMInterface;
diff --git a/packages/core/test/agent.test.ts b/packages/core/test/agent.test.ts
index 6852087..1310809 100644
--- a/packages/core/test/agent.test.ts
+++ b/packages/core/test/agent.test.ts
@@ -78,6 +78,7 @@ describe("metaMaxTokens (meta-request budget tightened by the per-model cap)", (
     expect(metaMaxTokens(300, 8000)).toBe(300); // ample cap: the small budget stays
     expect(metaMaxTokens(300, 128)).toBe(128); // pinned below the budget: the cap binds
     expect(metaMaxTokens(2048, 1024)).toBe(1024); // vision-describer budget, same rule
+    expect(metaMaxTokens(300, -1)).toBe(300); // -1 = uncapped: never tightens to -1 (issue #55 sibling)
   });
 });
 
diff --git a/packages/core/test/engine.test.ts b/packages/core/test/engine.test.ts
index d54407f..dd7e712 100644
--- a/packages/core/test/engine.test.ts
+++ b/packages/core/test/engine.test.ts
@@ -331,6 +331,47 @@ describe("ContextEngine ReAct loop (mock LLM, approve callback)", () => {
     ).toBe(true);
   });
 
+  it("maxTurns -1 removes the cap instead of stopping before the first turn (issue #55)", async () => {
+    // Two tool-call turns followed by a final text turn: with the old `0 >= -1` guard the
+    // engine emitted the stop note without ever calling the LLM.
+    let calls = 0;
+    const llm: LLMInterface = {
+      async *streamGenerate() {
+        calls += 1;
+        if (calls <= 2) {
+          yield toolCall({
+            name: "exec_command",
+            arguments: JSON.stringify({ cmd: "true" }),
+            toolCallId: `c${calls}`,
+            stopReason: "completed",
+          });
+        } else {
+          yield assistantText("Done");
+        }
+        yield tokenUsage(emptyTokenCounts(), {
+          cache_read: 0,
+          cache_write: 0,
+          output: 1,
+          total: 1,
+        });
+        return { status: "completed" };
+      },
+    };
+    const environment = new Environment({
+      workspaceDir: workspace,
+      toolConfig: execCommandToolConfig(),
+    });
+    const engine = new ContextEngine({ llm, environment, maxTurns: -1 });
+
+    const all = await collectRun(engine, [userText("go")], allowAll);
+    expect(calls).toBe(3);
+    const texts = all
+      .filter((m) => isCompleteModelMessage(m) && m.payload.type === "text")
+      .map((m) => (m.payload as TextPayload).text);
+    expect(texts.some((t) => t.includes("reached max turns"))).toBe(false);
+    expect(texts.some((t) => t === "Done")).toBe(true);
+  });
+
   it("max turns with pending tool outputs carries them over so the next run pairs the tool_call (issue #33)", async () => {
     const received: OmniMessage[][] = [];
     const llm: LLMInterface = {
diff --git a/packages/core/test/llm.test.ts b/packages/core/test/llm.test.ts
index ed3225a..a639221 100644
--- a/packages/core/test/llm.test.ts
+++ b/packages/core/test/llm.test.ts
@@ -1052,6 +1052,11 @@ describe("config helpers", () => {
     expect("system_prompt" in minimal).toBe(false);
     expect("max_tokens" in minimal).toBe(false);
     expect("thinking_level" in minimal).toBe(false);
+
+    // max_tokens -1 (the config's "no cap" sentinel) stays OFF the wire — sent literally,
+    // providers reject a negative max_tokens with a 400.
+    const uncapped = buildUniConfig({ modelId: "m", tools: [], maxTokens: -1 });
+    expect("max_tokens" in uncapped).toBe(false);
   });
 
   it("omits tools when empty and never sets tool_choice (strict endpoints reject both)", () => {
diff --git a/packages/docs/content/configuration.en.md b/packages/docs/content/configuration.en.md
index 6089331..e3d2e3d 100644
--- a/packages/docs/content/configuration.en.md
+++ b/packages/docs/content/configuration.en.md
@@ -93,8 +93,8 @@ Edit this file via the CLI (`penguin config model …`) or the Web Models page 
 | `description` | — | Agent description |
 | `version` | `1` | Agent State version (a natural number), incremented on each successful optimization |
 | `system_prompt` | built-in template | Required; the only template with placeholder substitution |
-| `max_turns` | `100` | Maximum LLM turns per Task |
-| `model.max_tokens` | `32000` | Output Token limit per Request |
+| `max_turns` | `100` | Maximum LLM turns per Task (-1 removes the cap) |
+| `model.max_tokens` | `32000` | Output Token limit per Request (-1 = no cap, provider default) |
 | `model.thinking_level` | `medium` | `none` / `low` / `medium` / `high` / `xhigh` |
 | `model.timeoutMs` | `120000` | Per-Request timeout (milliseconds) |
 | `compaction.max_context_length` | `128000` | Context Token threshold that triggers compaction |
diff --git a/packages/docs/content/configuration.zh.md b/packages/docs/content/configuration.zh.md
index aefea0f..15b8da7 100644
--- a/packages/docs/content/configuration.zh.md
+++ b/packages/docs/content/configuration.zh.md
@@ -93,8 +93,8 @@ output = 0.857143
 | `description` | — | Agent 描述 |
 | `version` | `1` | Agent State 版本号（自然数），每次成功优化自增 |
 | `system_prompt` | 内置模板 | 必填；唯一进行占位符替换的模板 |
-| `max_turns` | `100` | 单个 Task 的最大 LLM 轮数 |
-| `model.max_tokens` | `32000` | 单次输出 Token 上限 |
+| `max_turns` | `100` | 单个 Task 的最大 LLM 轮数（-1 不限制） |
+| `model.max_tokens` | `32000` | 单次输出 Token 上限（-1 不设上限，用服务商默认） |
 | `model.thinking_level` | `medium` | `none` / `low` / `medium` / `high` / `xhigh` |
 | `model.timeoutMs` | `120000` | 单次 Request 超时（毫秒） |
 | `compaction.max_context_length` | `128000` | 触发压缩的上下文 Token 阈值 |