diff --git a/providers/openai/language_model_hooks.go b/providers/openai/language_model_hooks.go
index c4ee8efaf..d5169cddd 100644
--- a/providers/openai/language_model_hooks.go
+++ b/providers/openai/language_model_hooks.go
@@ -297,7 +297,13 @@ func DefaultStreamProviderMetadataFunc(choice openai.ChatCompletionChoice, metad
 func DefaultToPrompt(prompt fantasy.Prompt, _, _ string) ([]openai.ChatCompletionMessageParamUnion, []fantasy.CallWarning) {
 	var messages []openai.ChatCompletionMessageParamUnion
 	var warnings []fantasy.CallWarning
+	// All tool replies in a batch must precede synthetic user media.
+	var pendingToolMedia []openai.ChatCompletionMessageParamUnion
 	for _, msg := range prompt {
+		if msg.Role != fantasy.MessageRoleTool {
+			messages = append(messages, pendingToolMedia...)
+			pendingToolMedia = nil
+		}
 		switch msg.Role {
 		case fantasy.MessageRoleSystem:
 			var systemPromptParts []string
@@ -585,7 +591,8 @@ func DefaultToPrompt(prompt fantasy.Prompt, _, _ string) ([]openai.ChatCompletio
 					// OpenAI Chat Completions tool messages cannot carry image
 					// or audio content directly; see ToolResultMediaMessages.
 					mediaMessages, mediaWarnings := ToolResultMediaMessages(output, toolResultPart.ToolCallID)
-					messages = append(messages, mediaMessages...)
+					messages = append(messages, mediaMessages[0])
+					pendingToolMedia = append(pendingToolMedia, mediaMessages[1:]...)
 					warnings = append(warnings, mediaWarnings...)
 				default:
 					warnings = append(warnings, fantasy.CallWarning{
@@ -596,7 +603,7 @@ func DefaultToPrompt(prompt fantasy.Prompt, _, _ string) ([]openai.ChatCompletio
 			}
 		}
 	}
-	return messages, warnings
+	return append(messages, pendingToolMedia...), warnings
 }
 
 // ToolResultMediaMessages maps a tool-result media output to the chat
diff --git a/providers/openai/tool_result_media_order_test.go b/providers/openai/tool_result_media_order_test.go
new file mode 100644
index 000000000..ebd501eeb
--- /dev/null
+++ b/providers/openai/tool_result_media_order_test.go
@@ -0,0 +1,104 @@
+package openai_test
+
+import (
+	"testing"
+
+	"charm.land/fantasy"
+	"charm.land/fantasy/providers/openai"
+	"charm.land/fantasy/providers/openaicompat"
+	openaisdk "github.com/openai/openai-go/v3"
+	"github.com/stretchr/testify/require"
+)
+
+func TestToolMediaFollowsToolResultBatch(t *testing.T) {
+	t.Parallel()
+
+	for _, provider := range []struct {
+		name     string
+		toPrompt func(fantasy.Prompt, string, string) ([]openaisdk.ChatCompletionMessageParamUnion, []fantasy.CallWarning)
+	}{
+		{name: "OpenAI", toPrompt: openai.DefaultToPrompt},
+		{name: "OpenAICompatible", toPrompt: openaicompat.ToPromptFunc},
+	} {
+		t.Run(provider.name, func(t *testing.T) {
+			t.Parallel()
+			for _, tc := range []struct {
+				name             string
+				separateMessages bool
+				secondMedia      bool
+				followup         bool
+			}{
+				{name: "SeparateMessagesMixed", separateMessages: true},
+				{name: "SeparateMessagesTwoMedia", separateMessages: true, secondMedia: true},
+				{name: "SameMessageMixed"},
+				{name: "SameMessageTwoMedia", secondMedia: true},
+				{name: "BeforeFollowingMessages", separateMessages: true, secondMedia: true, followup: true},
+			} {
+				t.Run(tc.name, func(t *testing.T) {
+					t.Parallel()
+					media := fantasy.ToolResultOutputContentMedia{Data: "AAEC", MediaType: "image/png", Text: "first result"}
+					results := []fantasy.MessagePart{
+						fantasy.ToolResultPart{ToolCallID: "call-a", Output: media},
+						fantasy.ToolResultPart{ToolCallID: "call-b", Output: fantasy.ToolResultOutputContentText{Text: "second result"}},
+					}
+					imageURLs := []string{"data:image/png;base64,AAEC"}
+					if tc.secondMedia {
+						media.Data, media.Text = "AwQF", "second result"
+						results[1] = fantasy.ToolResultPart{ToolCallID: "call-b", Output: media}
+						imageURLs = append(imageURLs, "data:image/png;base64,AwQF")
+					}
+					prompt := fantasy.Prompt{{
+						Role: fantasy.MessageRoleAssistant,
+						Content: []fantasy.MessagePart{
+							fantasy.ToolCallPart{ToolCallID: "call-a", ToolName: "first", Input: "{}"},
+							fantasy.ToolCallPart{ToolCallID: "call-b", ToolName: "second", Input: "{}"},
+						},
+					}}
+					if tc.separateMessages {
+						for _, result := range results {
+							prompt = append(prompt, fantasy.Message{Role: fantasy.MessageRoleTool, Content: []fantasy.MessagePart{result}})
+						}
+					} else {
+						prompt = append(prompt, fantasy.Message{Role: fantasy.MessageRoleTool, Content: results})
+					}
+					if tc.followup {
+						prompt = append(prompt,
+							fantasy.Message{Role: fantasy.MessageRoleAssistant, Content: []fantasy.MessagePart{fantasy.TextPart{Text: "done"}}},
+							fantasy.Message{Role: fantasy.MessageRoleUser, Content: []fantasy.MessagePart{fantasy.TextPart{Text: "thanks"}}},
+						)
+					}
+
+					messages, warnings := provider.toPrompt(prompt, "", "")
+					require.Empty(t, warnings)
+					wantLen := 3 + len(imageURLs)
+					if tc.followup {
+						wantLen += 2
+					}
+					require.Len(t, messages, wantLen)
+					require.NotNil(t, messages[0].OfAssistant)
+					require.Len(t, messages[0].OfAssistant.ToolCalls, 2)
+					for i, want := range []struct{ id, text string }{{"call-a", "first result"}, {"call-b", "second result"}} {
+						tool := messages[i+1].OfTool
+						require.NotNil(t, tool, "all tool results must precede synthetic user media")
+						require.Equal(t, want.id, tool.ToolCallID)
+						require.Equal(t, want.text, tool.Content.OfString.Value)
+					}
+					for i, url := range imageURLs {
+						user := messages[3+i].OfUser
+						require.NotNil(t, user)
+						require.Len(t, user.Content.OfArrayOfContentParts, 1)
+						image := user.Content.OfArrayOfContentParts[0].OfImageURL
+						require.NotNil(t, image)
+						require.Equal(t, url, image.ImageURL.URL)
+					}
+					if tc.followup {
+						require.NotNil(t, messages[wantLen-2].OfAssistant)
+						require.Equal(t, "done", messages[wantLen-2].OfAssistant.Content.OfString.Value)
+						require.NotNil(t, messages[wantLen-1].OfUser)
+						require.Equal(t, "thanks", messages[wantLen-1].OfUser.Content.OfString.Value)
+					}
+				})
+			}
+		})
+	}
+}
diff --git a/providers/openaicompat/language_model_hooks.go b/providers/openaicompat/language_model_hooks.go
index b7d7ff5d1..daa616d5a 100644
--- a/providers/openaicompat/language_model_hooks.go
+++ b/providers/openaicompat/language_model_hooks.go
@@ -160,8 +160,14 @@ func ToPromptFunc(prompt fantasy.Prompt, _, _ string) ([]openaisdk.ChatCompletio
 	var messages []openaisdk.ChatCompletionMessageParamUnion
 	var warnings []fantasy.CallWarning
 	hasReasoning := false
+	// All tool replies in a batch must precede synthetic user media.
+	var pendingToolMedia []openaisdk.ChatCompletionMessageParamUnion
 
 	for _, msg := range prompt {
+		if msg.Role != fantasy.MessageRoleTool {
+			messages = append(messages, pendingToolMedia...)
+			pendingToolMedia = nil
+		}
 		switch msg.Role {
 		case fantasy.MessageRoleSystem:
 			var blocks []openaisdk.ChatCompletionContentPartTextParam
@@ -502,7 +508,8 @@ func ToPromptFunc(prompt fantasy.Prompt, _, _ string) ([]openaisdk.ChatCompletio
 					// helper, which emits a text tool message plus a synthetic
 					// user message holding the media.
 					mediaMessages, mediaWarnings := openai.ToolResultMediaMessages(output, toolResultPart.ToolCallID)
-					messages = append(messages, mediaMessages...)
+					messages = append(messages, mediaMessages[0])
+					pendingToolMedia = append(pendingToolMedia, mediaMessages[1:]...)
 					warnings = append(warnings, mediaWarnings...)
 				default:
 					warnings = append(warnings, fantasy.CallWarning{
@@ -513,7 +520,7 @@ func ToPromptFunc(prompt fantasy.Prompt, _, _ string) ([]openaisdk.ChatCompletio
 			}
 		}
 	}
-	return messages, warnings
+	return append(messages, pendingToolMedia...), warnings
 }
 
 // toolResultMediaUserPart maps a tool-result media output to an OpenAI chat