diff --git a/FirebaseAI/Sources/Chat.swift b/FirebaseAI/Sources/Chat.swift
index 1aa2c3490c7..01d0b4a0056 100644
--- a/FirebaseAI/Sources/Chat.swift
+++ b/FirebaseAI/Sources/Chat.swift
@@ -146,11 +146,13 @@ public final class Chat: Sendable {
     for aggregate in chunks {
       // Loop through all the parts, aggregating the text and adding the images.
       for part in aggregate.parts {
-        switch part {
-        case let textPart as TextPart:
-          combinedText += textPart.text
+        guard !part.isThought else {
+          continue
+        }
 
-        default:
+        if let textPart = part as? TextPart {
+          combinedText += textPart.text
+        } else {
           // Don't combine it, just add to the content. If there's any text pending, add that as
           // a part.
           if !combinedText.isEmpty {
diff --git a/FirebaseAI/Sources/GenerateContentResponse.swift b/FirebaseAI/Sources/GenerateContentResponse.swift
index b0e348d2192..19e4d797419 100644
--- a/FirebaseAI/Sources/GenerateContentResponse.swift
+++ b/FirebaseAI/Sources/GenerateContentResponse.swift
@@ -66,12 +66,10 @@ public struct GenerateContentResponse: Sendable {
       return nil
     }
     let textValues: [String] = candidate.content.parts.compactMap { part in
-      switch part {
-      case let textPart as TextPart:
-        return textPart.text
-      default:
+      guard let textPart = part as? TextPart, !part.isThought else {
         return nil
       }
+      return textPart.text
     }
     guard textValues.count > 0 else {
       AILog.error(
@@ -89,12 +87,10 @@ public struct GenerateContentResponse: Sendable {
       return []
     }
     return candidate.content.parts.compactMap { part in
-      switch part {
-      case let functionCallPart as FunctionCallPart:
-        return functionCallPart
-      default:
+      guard let functionCallPart = part as? FunctionCallPart, !part.isThought else {
         return nil
       }
+      return functionCallPart
     }
   }
 
@@ -107,7 +103,12 @@ public struct GenerateContentResponse: Sendable {
       """)
       return []
     }
-    return candidate.content.parts.compactMap { $0 as? InlineDataPart }
+    return candidate.content.parts.compactMap { part in
+      guard let inlineDataPart = part as? InlineDataPart, !part.isThought else {
+        return nil
+      }
+      return inlineDataPart
+    }
   }
 
   /// Initializer for SwiftUI previews or tests.
diff --git a/FirebaseAI/Sources/ModelContent.swift b/FirebaseAI/Sources/ModelContent.swift
index 7d82bd76445..7fdd7d428c0 100644
--- a/FirebaseAI/Sources/ModelContent.swift
+++ b/FirebaseAI/Sources/ModelContent.swift
@@ -31,19 +31,31 @@ extension [ModelContent] {
   }
 }
 
-/// A type describing data in media formats interpretable by an AI model. Each generative AI
-/// request or response contains an `Array` of ``ModelContent``s, and each ``ModelContent`` value
-/// may comprise multiple heterogeneous ``Part``s.
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
-public struct ModelContent: Equatable, Sendable {
-  enum InternalPart: Equatable, Sendable {
+struct InternalPart: Equatable, Sendable {
+  enum OneOfData: Equatable, Sendable {
     case text(String)
-    case inlineData(mimetype: String, Data)
-    case fileData(mimetype: String, uri: String)
+    case inlineData(InlineData)
+    case fileData(FileData)
     case functionCall(FunctionCall)
     case functionResponse(FunctionResponse)
   }
 
+  let data: OneOfData
+
+  let isThought: Bool?
+
+  init(_ data: OneOfData, isThought: Bool?) {
+    self.data = data
+    self.isThought = isThought
+  }
+}
+
+/// A type describing data in media formats interpretable by an AI model. Each generative AI
+/// request or response contains an `Array` of ``ModelContent``s, and each ``ModelContent`` value
+/// may comprise multiple heterogeneous ``Part``s.
+@available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
+public struct ModelContent: Equatable, Sendable {
   /// The role of the entity creating the ``ModelContent``. For user-generated client requests,
   /// for example, the role is `user`.
   public let role: String?
@@ -52,17 +64,17 @@ public struct ModelContent: Equatable, Sendable {
   public var parts: [any Part] {
     var convertedParts = [any Part]()
     for part in internalParts {
-      switch part {
+      switch part.data {
       case let .text(text):
-        convertedParts.append(TextPart(text))
-      case let .inlineData(mimetype, data):
-        convertedParts.append(InlineDataPart(data: data, mimeType: mimetype))
-      case let .fileData(mimetype, uri):
-        convertedParts.append(FileDataPart(uri: uri, mimeType: mimetype))
+        convertedParts.append(TextPart(text, isThought: part.isThought))
+      case let .inlineData(inlineData):
+        convertedParts.append(InlineDataPart(inlineData, isThought: part.isThought))
+      case let .fileData(fileData):
+        convertedParts.append(FileDataPart(fileData, isThought: part.isThought))
       case let .functionCall(functionCall):
-        convertedParts.append(FunctionCallPart(functionCall))
+        convertedParts.append(FunctionCallPart(functionCall, isThought: part.isThought))
       case let .functionResponse(functionResponse):
-        convertedParts.append(FunctionResponsePart(functionResponse))
+        convertedParts.append(FunctionResponsePart(functionResponse, isThought: part.isThought))
       }
     }
     return convertedParts
@@ -78,17 +90,28 @@ public struct ModelContent: Equatable, Sendable {
     for part in parts {
       switch part {
       case let textPart as TextPart:
-        convertedParts.append(.text(textPart.text))
+        convertedParts.append(InternalPart(.text(textPart.text), isThought: textPart._isThought))
       case let inlineDataPart as InlineDataPart:
-        let inlineData = inlineDataPart.inlineData
-        convertedParts.append(.inlineData(mimetype: inlineData.mimeType, inlineData.data))
+        convertedParts.append(
+          InternalPart(.inlineData(inlineDataPart.inlineData), isThought: inlineDataPart._isThought)
+        )
       case let fileDataPart as FileDataPart:
-        let fileData = fileDataPart.fileData
-        convertedParts.append(.fileData(mimetype: fileData.mimeType, uri: fileData.fileURI))
+        convertedParts.append(
+          InternalPart(.fileData(fileDataPart.fileData), isThought: fileDataPart._isThought)
+        )
       case let functionCallPart as FunctionCallPart:
-        convertedParts.append(.functionCall(functionCallPart.functionCall))
+        convertedParts.append(
+          InternalPart(
+            .functionCall(functionCallPart.functionCall), isThought: functionCallPart._isThought
+          )
+        )
       case let functionResponsePart as FunctionResponsePart:
-        convertedParts.append(.functionResponse(functionResponsePart.functionResponse))
+        convertedParts.append(
+          InternalPart(
+            .functionResponse(functionResponsePart.functionResponse),
+            isThought: functionResponsePart._isThought
+          )
+        )
       default:
         fatalError()
       }
@@ -121,7 +144,26 @@ extension ModelContent: Codable {
 }
 
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
-extension ModelContent.InternalPart: Codable {
+extension InternalPart: Codable {
+  enum CodingKeys: String, CodingKey {
+    case isThought = "thought"
+  }
+
+  public func encode(to encoder: Encoder) throws {
+    try data.encode(to: encoder)
+    var container = encoder.container(keyedBy: CodingKeys.self)
+    try container.encodeIfPresent(isThought, forKey: .isThought)
+  }
+
+  public init(from decoder: Decoder) throws {
+    data = try OneOfData(from: decoder)
+    let container = try decoder.container(keyedBy: CodingKeys.self)
+    isThought = try container.decodeIfPresent(Bool.self, forKey: .isThought)
+  }
+}
+
+@available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
+extension InternalPart.OneOfData: Codable {
   enum CodingKeys: String, CodingKey {
     case text
     case inlineData
@@ -135,10 +177,10 @@ extension ModelContent.InternalPart: Codable {
     switch self {
     case let .text(text):
       try container.encode(text, forKey: .text)
-    case let .inlineData(mimetype, bytes):
-      try container.encode(InlineData(data: bytes, mimeType: mimetype), forKey: .inlineData)
-    case let .fileData(mimetype: mimetype, url):
-      try container.encode(FileData(fileURI: url, mimeType: mimetype), forKey: .fileData)
+    case let .inlineData(inlineData):
+      try container.encode(inlineData, forKey: .inlineData)
+    case let .fileData(fileData):
+      try container.encode(fileData, forKey: .fileData)
     case let .functionCall(functionCall):
       try container.encode(functionCall, forKey: .functionCall)
     case let .functionResponse(functionResponse):
@@ -151,11 +193,9 @@ extension ModelContent.InternalPart: Codable {
     if values.contains(.text) {
       self = try .text(values.decode(String.self, forKey: .text))
     } else if values.contains(.inlineData) {
-      let inlineData = try values.decode(InlineData.self, forKey: .inlineData)
-      self = .inlineData(mimetype: inlineData.mimeType, inlineData.data)
+      self = try .inlineData(values.decode(InlineData.self, forKey: .inlineData))
     } else if values.contains(.fileData) {
-      let fileData = try values.decode(FileData.self, forKey: .fileData)
-      self = .fileData(mimetype: fileData.mimeType, uri: fileData.fileURI)
+      self = try .fileData(values.decode(FileData.self, forKey: .fileData))
     } else if values.contains(.functionCall) {
       self = try .functionCall(values.decode(FunctionCall.self, forKey: .functionCall))
     } else if values.contains(.functionResponse) {
diff --git a/FirebaseAI/Sources/Types/Internal/InternalPart.swift b/FirebaseAI/Sources/Types/Internal/InternalPart.swift
index d543fb80f38..062ae8aa93d 100644
--- a/FirebaseAI/Sources/Types/Internal/InternalPart.swift
+++ b/FirebaseAI/Sources/Types/Internal/InternalPart.swift
@@ -67,6 +67,8 @@ struct FunctionResponse: Codable, Equatable, Sendable {
 struct ErrorPart: Part, Error {
   let error: Error
 
+  let isThought = false
+
   init(_ error: Error) {
     self.error = error
   }
diff --git a/FirebaseAI/Sources/Types/Public/Part.swift b/FirebaseAI/Sources/Types/Public/Part.swift
index 4890b725f4d..4fea490a7bf 100644
--- a/FirebaseAI/Sources/Types/Public/Part.swift
+++ b/FirebaseAI/Sources/Types/Public/Part.swift
@@ -18,7 +18,9 @@ import Foundation
 ///
 /// Within a single value of ``Part``, different data types may not mix.
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
-public protocol Part: PartsRepresentable, Codable, Sendable, Equatable {}
+public protocol Part: PartsRepresentable, Codable, Sendable, Equatable {
+  var isThought: Bool { get }
+}
 
 /// A text part containing a string value.
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
@@ -26,8 +28,17 @@ public struct TextPart: Part {
   /// Text value.
   public let text: String
 
+  public var isThought: Bool { _isThought ?? false }
+
+  let _isThought: Bool?
+
   public init(_ text: String) {
+    self.init(text, isThought: nil)
+  }
+
+  init(_ text: String, isThought: Bool?) {
     self.text = text
+    _isThought = isThought
   }
 }
 
@@ -45,6 +56,7 @@ public struct TextPart: Part {
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
 public struct InlineDataPart: Part {
   let inlineData: InlineData
+  let _isThought: Bool?
 
   /// The data provided in the inline data part.
   public var data: Data { inlineData.data }
@@ -52,6 +64,8 @@ public struct InlineDataPart: Part {
   /// The IANA standard MIME type of the data.
   public var mimeType: String { inlineData.mimeType }
 
+  public var isThought: Bool { _isThought ?? false }
+
   /// Creates an inline data part from data and a MIME type.
   ///
   /// > Important: Supported input types depend on the model on the model being used; see [input
@@ -67,11 +81,12 @@ public struct InlineDataPart: Part {
   ///     requirements](https://firebase.google.com/docs/vertex-ai/input-file-requirements) for
   ///     supported values.
   public init(data: Data, mimeType: String) {
-    self.init(InlineData(data: data, mimeType: mimeType))
+    self.init(InlineData(data: data, mimeType: mimeType), isThought: nil)
   }
 
-  init(_ inlineData: InlineData) {
+  init(_ inlineData: InlineData, isThought: Bool?) {
     self.inlineData = inlineData
+    _isThought = isThought
   }
 }
 
@@ -79,9 +94,11 @@ public struct InlineDataPart: Part {
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
 public struct FileDataPart: Part {
   let fileData: FileData
+  let _isThought: Bool?
 
   public var uri: String { fileData.fileURI }
   public var mimeType: String { fileData.mimeType }
+  public var isThought: Bool { _isThought ?? false }
 
   /// Constructs a new file data part.
   ///
@@ -93,11 +110,12 @@ public struct FileDataPart: Part {
   ///     requirements](https://firebase.google.com/docs/vertex-ai/input-file-requirements) for
   ///     supported values.
   public init(uri: String, mimeType: String) {
-    self.init(FileData(fileURI: uri, mimeType: mimeType))
+    self.init(FileData(fileURI: uri, mimeType: mimeType), isThought: nil)
   }
 
-  init(_ fileData: FileData) {
+  init(_ fileData: FileData, isThought: Bool?) {
     self.fileData = fileData
+    _isThought = isThought
   }
 }
 
@@ -105,6 +123,7 @@ public struct FileDataPart: Part {
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
 public struct FunctionCallPart: Part {
   let functionCall: FunctionCall
+  let _isThought: Bool?
 
   /// The name of the function to call.
   public var name: String { functionCall.name }
@@ -112,6 +131,8 @@ public struct FunctionCallPart: Part {
   /// The function parameters and values.
   public var args: JSONObject { functionCall.args }
 
+  public var isThought: Bool { _isThought ?? false }
+
   /// Constructs a new function call part.
   ///
   /// > Note: A `FunctionCallPart` is typically received from the model, rather than created
@@ -121,11 +142,12 @@ public struct FunctionCallPart: Part {
   ///   - name: The name of the function to call.
   ///   - args: The function parameters and values.
   public init(name: String, args: JSONObject) {
-    self.init(FunctionCall(name: name, args: args))
+    self.init(FunctionCall(name: name, args: args), isThought: nil)
   }
 
-  init(_ functionCall: FunctionCall) {
+  init(_ functionCall: FunctionCall, isThought: Bool?) {
     self.functionCall = functionCall
+    _isThought = isThought
   }
 }
 
@@ -137,6 +159,7 @@ public struct FunctionCallPart: Part {
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
 public struct FunctionResponsePart: Part {
   let functionResponse: FunctionResponse
+  let _isThought: Bool?
 
   /// The name of the function that was called.
   public var name: String { functionResponse.name }
@@ -144,16 +167,19 @@ public struct FunctionResponsePart: Part {
   /// The function's response or return value.
   public var response: JSONObject { functionResponse.response }
 
+  public var isThought: Bool { _isThought ?? false }
+
   /// Constructs a new `FunctionResponse`.
   ///
   /// - Parameters:
   ///   - name: The name of the function that was called.
   ///   - response: The function's response.
   public init(name: String, response: JSONObject) {
-    self.init(FunctionResponse(name: name, response: response))
+    self.init(FunctionResponse(name: name, response: response), isThought: nil)
   }
 
-  init(_ functionResponse: FunctionResponse) {
+  init(_ functionResponse: FunctionResponse, isThought: Bool?) {
     self.functionResponse = functionResponse
+    _isThought = isThought
   }
 }
diff --git a/FirebaseAI/Sources/Types/Public/ThinkingConfig.swift b/FirebaseAI/Sources/Types/Public/ThinkingConfig.swift
index c0e8f31465b..c9e2cc43f4e 100644
--- a/FirebaseAI/Sources/Types/Public/ThinkingConfig.swift
+++ b/FirebaseAI/Sources/Types/Public/ThinkingConfig.swift
@@ -37,12 +37,15 @@ public struct ThinkingConfig: Sendable {
   /// feature or if the specified budget is not within the model's supported range.
   let thinkingBudget: Int?
 
+  let includeThoughts: Bool?
+
   /// Initializes a new `ThinkingConfig`.
   ///
   /// - Parameters:
   ///   - thinkingBudget: The maximum number of tokens to be used for the model's thinking process.
-  public init(thinkingBudget: Int? = nil) {
+  public init(thinkingBudget: Int? = nil, includeThoughts: Bool? = nil) {
     self.thinkingBudget = thinkingBudget
+    self.includeThoughts = includeThoughts
   }
 }
 
diff --git a/FirebaseAI/Tests/TestApp/Tests/Integration/GenerateContentIntegrationTests.swift b/FirebaseAI/Tests/TestApp/Tests/Integration/GenerateContentIntegrationTests.swift
index da77cea9df7..0092efe33ce 100644
--- a/FirebaseAI/Tests/TestApp/Tests/Integration/GenerateContentIntegrationTests.swift
+++ b/FirebaseAI/Tests/TestApp/Tests/Integration/GenerateContentIntegrationTests.swift
@@ -134,47 +134,83 @@ struct GenerateContentIntegrationTests {
     #expect(candidatesTokensDetails.tokenCount == usageMetadata.candidatesTokenCount)
   }
 
-  @Test(arguments: [
-    (InstanceConfig.vertexAI_v1beta, ModelNames.gemini2_5_Flash, 0),
-    (InstanceConfig.vertexAI_v1beta, ModelNames.gemini2_5_Flash, 24576),
-    (InstanceConfig.vertexAI_v1beta_global, ModelNames.gemini2_5_Pro, 128),
-    (InstanceConfig.vertexAI_v1beta_global, ModelNames.gemini2_5_Pro, 32768),
-    (InstanceConfig.googleAI_v1beta, ModelNames.gemini2_5_Flash, 0),
-    (InstanceConfig.googleAI_v1beta, ModelNames.gemini2_5_Flash, 24576),
-    (InstanceConfig.googleAI_v1beta, ModelNames.gemini2_5_Pro, 128),
-    (InstanceConfig.googleAI_v1beta, ModelNames.gemini2_5_Pro, 32768),
-    (InstanceConfig.googleAI_v1beta_freeTier, ModelNames.gemini2_5_Flash, 0),
-    (InstanceConfig.googleAI_v1beta_freeTier, ModelNames.gemini2_5_Flash, 24576),
-  ])
+  @Test(
+    arguments: [
+      (.vertexAI_v1beta, ModelNames.gemini2_5_Flash, ThinkingConfig(thinkingBudget: 0)),
+      (.vertexAI_v1beta, ModelNames.gemini2_5_Flash, ThinkingConfig(thinkingBudget: 24576)),
+      (.vertexAI_v1beta, ModelNames.gemini2_5_Flash, ThinkingConfig(
+        thinkingBudget: 24576, includeThoughts: true
+      )),
+      (.vertexAI_v1beta_global, ModelNames.gemini2_5_Pro, ThinkingConfig(thinkingBudget: 128)),
+      (.vertexAI_v1beta_global, ModelNames.gemini2_5_Pro, ThinkingConfig(thinkingBudget: 32768)),
+      (.vertexAI_v1beta_global, ModelNames.gemini2_5_Pro, ThinkingConfig(
+        thinkingBudget: 32768, includeThoughts: true
+      )),
+      (.googleAI_v1beta, ModelNames.gemini2_5_Flash, ThinkingConfig(thinkingBudget: 0)),
+      (.googleAI_v1beta, ModelNames.gemini2_5_Flash, ThinkingConfig(thinkingBudget: 24576)),
+      (.googleAI_v1beta, ModelNames.gemini2_5_Flash, ThinkingConfig(
+        thinkingBudget: 24576, includeThoughts: true
+      )),
+      (.googleAI_v1beta, ModelNames.gemini2_5_Pro, ThinkingConfig(thinkingBudget: 128)),
+      (.googleAI_v1beta, ModelNames.gemini2_5_Pro, ThinkingConfig(thinkingBudget: 32768)),
+      (.googleAI_v1beta, ModelNames.gemini2_5_Pro, ThinkingConfig(
+        thinkingBudget: 32768, includeThoughts: true
+      )),
+      (.googleAI_v1beta_freeTier, ModelNames.gemini2_5_Flash, ThinkingConfig(thinkingBudget: 0)),
+      (
+        .googleAI_v1beta_freeTier,
+        ModelNames.gemini2_5_Flash,
+        ThinkingConfig(thinkingBudget: 24576)
+      ),
+      (.googleAI_v1beta_freeTier, ModelNames.gemini2_5_Flash, ThinkingConfig(
+        thinkingBudget: 24576, includeThoughts: true
+      )),
+      (.googleAI_v1beta_freeTier_bypassProxy, ModelNames.gemini2_5_Flash, ThinkingConfig(
+        thinkingBudget: 0
+      )),
+      (.googleAI_v1beta_freeTier_bypassProxy, ModelNames.gemini2_5_Flash, ThinkingConfig(
+        thinkingBudget: 24576
+      )),
+      (.googleAI_v1beta_freeTier_bypassProxy, ModelNames.gemini2_5_Flash, ThinkingConfig(
+        thinkingBudget: 24576, includeThoughts: true
+      )),
+    ] as [(InstanceConfig, String, ThinkingConfig)]
+  )
   func generateContentThinking(_ config: InstanceConfig, modelName: String,
-                               thinkingBudget: Int) async throws {
+                               thinkingConfig: ThinkingConfig) async throws {
     let model = FirebaseAI.componentInstance(config).generativeModel(
       modelName: modelName,
       generationConfig: GenerationConfig(
         temperature: 0.0,
         topP: 0.0,
         topK: 1,
-        thinkingConfig: ThinkingConfig(thinkingBudget: thinkingBudget)
+        thinkingConfig: thinkingConfig
       ),
       safetySettings: safetySettings
     )
+    let chat = model.startChat()
     let prompt = "Where is Google headquarters located? Answer with the city name only."
 
-    let response = try await model.generateContent(prompt)
+    let response = try await chat.sendMessage(prompt)
 
     let text = try #require(response.text).trimmingCharacters(in: .whitespacesAndNewlines)
     #expect(text == "Mountain View")
 
+    let candidate = try #require(response.candidates.first)
+    let thoughtParts = candidate.content.parts.compactMap { $0.isThought ? $0 : nil }
+    #expect(thoughtParts.isEmpty != thinkingConfig.includeThoughts)
+
     let usageMetadata = try #require(response.usageMetadata)
     #expect(usageMetadata.promptTokenCount.isEqual(to: 13, accuracy: tokenCountAccuracy))
     #expect(usageMetadata.promptTokensDetails.count == 1)
     let promptTokensDetails = try #require(usageMetadata.promptTokensDetails.first)
     #expect(promptTokensDetails.modality == .text)
     #expect(promptTokensDetails.tokenCount == usageMetadata.promptTokenCount)
-    if thinkingBudget == 0 {
-      #expect(usageMetadata.thoughtsTokenCount == 0)
-    } else {
+    if let thinkingBudget = thinkingConfig.thinkingBudget, thinkingBudget > 0 {
+      #expect(usageMetadata.thoughtsTokenCount > 0)
       #expect(usageMetadata.thoughtsTokenCount <= thinkingBudget)
+    } else {
+      #expect(usageMetadata.thoughtsTokenCount == 0)
     }
     #expect(usageMetadata.candidatesTokenCount.isEqual(to: 3, accuracy: tokenCountAccuracy))
     // The `candidatesTokensDetails` field is erroneously omitted when using the Google AI (Gemini