[AI] Add support for Thinking Levels

Gemini 2.5 series models and newer utilize a thinking process before
generating a response. Thinking levels are an alternative to thinking
budgets as they are preset values that Gemini 3 models can use to
limit how many tokens are dedicated to the thinking process.

The [thinking
documentation](https://firebase.google.com/docs/ai-logic/thinking)
gives more details.

Author: rlazo

firebase-ai/src/main/kotlin/com/google/firebase/ai/type/ThinkingConfig.kt

@@ -19,11 +19,17 @@ package com.google.firebase.ai.type
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
-/** Configuration parameters for thinking features. */
+/**
+ * Gemini 2.5 series models and newer utilize a thinking process before generating a response. This
+ * allows them to reason through complex problems and plan a more coherent and accurate answer. See
+ * the [thinking documentation](https://firebase.google.com/docs/ai-logic/thinking) for more
+ * details.
+ */
 public class ThinkingConfig
 private constructor(
   internal val thinkingBudget: Int? = null,
-  internal val includeThoughts: Boolean? = null
+  internal val includeThoughts: Boolean? = null,
+  internal val thinkingLevel: ThinkingLevel? = null
 ) {
 
   public class Builder() {
@@ -35,11 +41,19 @@ private constructor(
     @set:JvmSynthetic // hide void setter from Java
     public var includeThoughts: Boolean? = null
 
+    @JvmField
+    @set:JvmSynthetic // hide void setter from Java
+    public var thinkingLevel: ThinkingLevel? = null
+
     /**
-     * Indicates the thinking budget in tokens. `0` is disabled. `-1` is dynamic. The default values
-     * and allowed ranges are model dependent.
+     * Indicates the thinking budget in tokens.
+     *
+     * Use `0` for disabled, and `-1` for dynamic. The range of
+     * [supported thinking budget values](https://firebase.google.com/docs/ai-logic/thinking#supported-thinking-budget-values)
+     * depends on the model.
      */
     public fun setThinkingBudget(thinkingBudget: Int): Builder = apply {
+      assert(thinkingLevel == null) { "Cannot set both `thinkingBudget` and `thinkingLevel`" }
       this.thinkingBudget = thinkingBudget
     }
 
@@ -55,16 +69,27 @@ private constructor(
       this.includeThoughts = includeThoughts
     }
 
+    /** Indicates the thinking budget based in Levels. */
+    public fun setThinkingLevel(thinkingLevel: ThinkingLevel): Builder = apply {
+      assert(thinkingBudget == null) { "Cannot set both `thinkingBudget` and `thinkingLevel`" }
+      this.thinkingLevel = thinkingLevel
+    }
+
     public fun build(): ThinkingConfig =
-      ThinkingConfig(thinkingBudget = thinkingBudget, includeThoughts = includeThoughts)
+      ThinkingConfig(
+        thinkingBudget = thinkingBudget,
+        includeThoughts = includeThoughts,
+        thinkingLevel = thinkingLevel
+      )
   }
 
-  internal fun toInternal() = Internal(thinkingBudget, includeThoughts)
+  internal fun toInternal() = Internal(thinkingBudget, includeThoughts, thinkingLevel?.toInternal())
 
   @Serializable
   internal data class Internal(
     @SerialName("thinking_budget") val thinkingBudget: Int? = null,
-    val includeThoughts: Boolean? = null
+    val includeThoughts: Boolean? = null,
+    @SerialName("thinking_level") val thinkingLevel: ThinkingLevel.Internal? = null,
   )
 }
 

firebase-ai/src/main/kotlin/com/google/firebase/ai/type/ThinkingLevel.kt

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.firebase.ai.type
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/** Specifies the quality of the thinking response. */
+public class ThinkingLevel private constructor(public val ordinal: Int) {
+  internal fun toInternal() =
+    when (this) {
+      LOW -> Internal.LOW
+      HIGH -> Internal.HIGH
+      else -> throw makeMissingCaseException("ThinkingLevel", ordinal)
+    }
+
+  @Serializable
+  internal enum class Internal {
+    @SerialName("THINKING_LEVEL_UNSPECIFIED") UNSPECIFIED,
+    LOW,
+    HIGH,
+  }
+  public companion object {
+    /** A lower quality thinking response, which provides low latency. */
+    @JvmField public val LOW: ThinkingLevel = ThinkingLevel(0)
+
+    /** A higher quality thinking response, which may increase latency. */
+    @JvmField public val HIGH: ThinkingLevel = ThinkingLevel(1)
+  }
+}