diff --git a/Sources/AnyLanguageModel/Models/OpenAILanguageModel.swift b/Sources/AnyLanguageModel/Models/OpenAILanguageModel.swift
index bd3d8c1..6299ce0 100644
--- a/Sources/AnyLanguageModel/Models/OpenAILanguageModel.swift
+++ b/Sources/AnyLanguageModel/Models/OpenAILanguageModel.swift
@@ -514,9 +514,12 @@ public struct OpenAILanguageModel: LanguageModel {
                     for invocation in invocations {
                         let output = invocation.output
                         entries.append(.toolOutput(output))
-                        let toolSegments: [Transcript.Segment] = output.segments
-                        let blocks = convertSegmentsToOpenAIBlocks(toolSegments)
-                        messages.append(OpenAIMessage(role: .tool(id: invocation.call.id), content: .blocks(blocks)))
+                        messages.append(
+                            OpenAIMessage(
+                                role: .tool(id: invocation.call.id),
+                                content: .text(convertSegmentsToToolContentString(output.segments))
+                            )
+                        )
                     }
                     continue
                 }
@@ -579,9 +582,12 @@ public struct OpenAILanguageModel: LanguageModel {
                     for invocation in invocations {
                         let output = invocation.output
                         entries.append(.toolOutput(output))
-                        let toolSegments: [Transcript.Segment] = output.segments
-                        let blocks = convertSegmentsToOpenAIBlocks(toolSegments)
-                        messages.append(OpenAIMessage(role: .tool(id: invocation.call.id), content: .blocks(blocks)))
+                        messages.append(
+                            OpenAIMessage(
+                                role: .tool(id: invocation.call.id),
+                                content: .text(convertSegmentsToToolContentString(output.segments))
+                            )
+                        )
                     }
                     continue
                 }
@@ -1168,7 +1174,7 @@ extension Transcript {
                 messages.append(
                     .init(
                         role: .tool(id: toolOutput.id),
-                        content: .blocks(convertSegmentsToOpenAIBlocks(toolOutput.segments))
+                        content: .text(convertSegmentsToToolContentString(toolOutput.segments))
                     )
                 )
             }
@@ -1308,6 +1314,26 @@ private func convertSegmentsToOpenAIBlocks(_ segments: [Transcript.Segment]) ->
     return blocks
 }
 
+/// Converts a list of transcript segments to a string representation suitable for tool message content.
+private func convertSegmentsToToolContentString(_ segments: [Transcript.Segment]) -> String {
+    // Tool message content must be string (or text-only parts) per API spec.
+    // String is used for broad provider compatibility.
+    // Image segments are intentionally omitted because tool outputs only support text.
+    segments.compactMap { segment in
+        switch segment {
+        case .text(let textSegment):
+            return textSegment.content
+        case .structure(let structuredSegment):
+            switch structuredSegment.content.kind {
+            case .string(let text): return text
+            default: return structuredSegment.content.jsonString
+            }
+        case .image:
+            return nil
+        }
+    }.joined(separator: "\n")
+}
+
 private struct OpenAITool: Hashable, Codable, Sendable {
     let type: String
     let function: OpenAIFunction