fix(ai): skip unsupported parts in ModelContent instead of crashing

Addresses review feedback: replace the fatalError() in ModelContent.init(role:parts:) for unrecognized Part types with an AILog error and skip the part. Part is a public protocol that apps can conform to, and the internal ErrorPart (returned for failed image conversions) also reached this branch, so both are runtime-reachable paths. ErrorPart is handled explicitly with its own message code.
paulb777 · Oct 2, 2026 · Oct 1, 2026 · Oct 1, 2026 · Oct 1, 2026 · Oct 2, 2026
commit 94168bf577a0b0a659cc883eef97f527d569b583
diff --git a/FirebaseAI/CHANGELOG.md b/FirebaseAI/CHANGELOG.md
@@ -1,7 +1,9 @@
 # Unreleased
 - [fixed] Fixed crashes when a chat session's history contained parts with
-  unrecognized data or code execution parts received from the server, and when
-  decoding a malformed duration from the server. (#16728)
+  unrecognized data or code execution parts received from the server, when
+  creating a `ModelContent` with an unsupported `Part` type or an image that
+  failed to convert, and when decoding a malformed duration from the server.
+  (#16728)
 
 # 13.0.0
 - [changed] **Breaking Change**: Renamed `Tool` to `GenerativeModel.Tool` to

diff --git a/FirebaseAI/Sources/AILog.swift b/FirebaseAI/Sources/AILog.swift
@@ -83,6 +83,8 @@ enum AILog {
     case invalidWebsocketURL = 4004
     case duplicateLiveSessionSetupComplete = 4005
     case malformedURL = 4006
+    case modelContentUnsupportedPartType = 4007
+    case modelContentPartConversionFailed = 4008
 
     // SDK Debugging
     case loadRequestStreamResponseLine = 5000

diff --git a/FirebaseAI/Sources/ModelContent.swift b/FirebaseAI/Sources/ModelContent.swift
@@ -160,8 +160,16 @@ public struct ModelContent: Equatable, Sendable {
           isThought: codeExecutionResultPart._isThought,
           thoughtSignature: codeExecutionResultPart.thoughtSignature
         ))
+      case let errorPart as ErrorPart:
+        AILog.error(
+          code: .modelContentPartConversionFailed,
+          "Skipping a part that failed to convert to model content: \(errorPart.error)"
+        )
       default:
-        fatalError()
+        AILog.error(
+          code: .modelContentUnsupportedPartType,
+          "Skipping a part with unsupported type: \(type(of: part))"
+        )
       }
     }
     internalParts = convertedParts

diff --git a/FirebaseAI/Tests/Unit/PartTests.swift b/FirebaseAI/Tests/Unit/PartTests.swift
@@ -296,8 +296,46 @@ final class PartTests: XCTestCase {
     """)
   }
 
+  // MARK: - ModelContent Conversion
+
+  func testModelContent_unsupportedPartType_isSkippedAndLogged() throws {
+    var loggedCodes: [AILog.MessageCode] = []
+    AILog.logInterceptor = { _, code, _ in
+      loggedCodes.append(code)
+    }
+    defer { AILog.logInterceptor = nil }
+
+    let content = ModelContent(parts: [TextPart("Hello"), CustomPart(), TextPart("world")])
+
+    XCTAssertEqual(content.parts.count, 2)
+    XCTAssertEqual(content.parts.compactMap { ($0 as? TextPart)?.text }, ["Hello", "world"])
+    XCTAssertEqual(loggedCodes, [.modelContentUnsupportedPartType])
+  }
+
+  func testModelContent_errorPart_isSkippedAndLogged() throws {
+    var loggedCodes: [AILog.MessageCode] = []
+    AILog.logInterceptor = { _, code, _ in
+      loggedCodes.append(code)
+    }
+    defer { AILog.logInterceptor = nil }
+
+    let content = ModelContent(parts: [
+      TextPart("Hello"),
+      ErrorPart(ImageConversionError.couldNotConvertToJPEG),
+    ])
+
+    XCTAssertEqual(content.parts.count, 1)
+    XCTAssertEqual((content.parts.first as? TextPart)?.text, "Hello")
+    XCTAssertEqual(loggedCodes, [.modelContentPartConversionFailed])
+  }
+
   // MARK: - Helpers
 
+  /// A `Part` conformance defined outside of the SDK.
+  private struct CustomPart: Part {
+    var isThought: Bool { false }
+  }
+
   private static func bundle() -> Bundle {
     #if SWIFT_PACKAGE
       return Bundle.module