From 33ec5502c38ac6841a0754577ebff2483fa10cef Mon Sep 17 00:00:00 2001
From: Luis Pater <webmaster@idotorg.org>
Date: Sun, 4 Oct 2026 05:50:40 +0800
Subject: [PATCH] fix(claude): refine model version parsing for assistant
 prefill rejection

- Strip provider namespaces and prefix before evaluating model family
- Parse major and minor version numbers dynamically while ignoring snapshot dates
- Correctly identify unsupported assistant prefill models for Fable, Opus 5+, and Sonnet 4.6+

Closes: #5483
---
 .../claude_openai-responses_request.go        | 37 ++++++++++++++++---
 .../claude_openai-responses_request_test.go   | 26 +++++++++++++
 2 files changed, 58 insertions(+), 5 deletions(-)

diff --git a/internal/translator/claude/openai/responses/claude_openai-responses_request.go b/internal/translator/claude/openai/responses/claude_openai-responses_request.go
index 9c880b7ca..d4251a627 100644
--- a/internal/translator/claude/openai/responses/claude_openai-responses_request.go
+++ b/internal/translator/claude/openai/responses/claude_openai-responses_request.go
@@ -727,12 +727,39 @@ func stripTrailingClaudeThinkingBlocks(messages [][]byte) [][]byte {
 // disallows trailing assistant prefill in its conversation history.
 func claudeModelRejectsAssistantPrefill(modelName string) bool {
 	normalized := strings.ToLower(strings.TrimSpace(modelName))
-	for _, family := range []string{"fable", "opus-5", "sonnet-4-6"} {
-		if strings.Contains(normalized, family) {
-			return true
-		}
+	// Provider namespaces are not part of the model family.
+	if index := strings.LastIndexByte(normalized, '/'); index >= 0 {
+		normalized = normalized[index+1:]
 	}
-	return false
+	normalized = strings.TrimPrefix(normalized, "claude-")
+	tokens := strings.Split(strings.ReplaceAll(normalized, ".", "-"), "-")
+	if tokens[0] == "fable" {
+		return true
+	}
+	if len(tokens) < 2 || (tokens[0] != "opus" && tokens[0] != "sonnet") {
+		return false
+	}
+	parseVersion := func(token string) int {
+		// Eight-digit snapshot dates must never be treated as versions.
+		if token == "" || len(token) >= 8 {
+			return -1
+		}
+		for _, digit := range token {
+			if digit < '0' || digit > '9' {
+				return -1
+			}
+		}
+		version, errAtoi := strconv.Atoi(token)
+		if errAtoi != nil {
+			return -1
+		}
+		return version
+	}
+	major := parseVersion(tokens[1])
+	if major >= 5 {
+		return true
+	}
+	return tokens[0] == "sonnet" && major == 4 && len(tokens) > 2 && parseVersion(tokens[2]) >= 6
 }
 
 // responsesSystemUnsupportedBlock represents a system-level content part that
diff --git a/internal/translator/claude/openai/responses/claude_openai-responses_request_test.go b/internal/translator/claude/openai/responses/claude_openai-responses_request_test.go
index 655adac67..d95d83727 100644
--- a/internal/translator/claude/openai/responses/claude_openai-responses_request_test.go
+++ b/internal/translator/claude/openai/responses/claude_openai-responses_request_test.go
@@ -2036,6 +2036,18 @@ func TestConvertOpenAIResponsesRequestToClaude_UnsupportedPrefillModelsStripTrai
 		"claude-fable-5",
 		"claude-opus-5",
 		"claude-sonnet-4-6",
+		"fable",
+		"opus-5",
+		"sonnet-4.6",
+		"anthropic/claude-opus-5-thinking",
+		"claude-sonnet-4.6",
+		"claude-sonnet-4-7",
+		"claude-sonnet-4-10",
+		"claude-sonnet-5",
+		"claude-opus-6",
+		"claude-opus-5.1",
+		"claude-sonnet-4-6-20260217",
+		" ANTHROPIC/CLAUDE-OPUS-5-THINKING ",
 	}
 	for _, model := range unsupportedModels {
 		t.Run(model, func(t *testing.T) {
@@ -2065,6 +2077,20 @@ func TestConvertOpenAIResponsesRequestToClaude_SupportedPrefillModelsPreserveAss
 	supportedModels := []string{
 		"claude-sonnet-4-5",
 		"claude-haiku-4-5",
+		"claude-3-opus-20240229",
+		"claude-opus-20240229",
+		"claude-sonnet-4-20260217",
+		"claude-sonnet-4.5",
+		"not-a-fable-model",
+		"my-custom-opus-5-wrapper",
+		"my-sonnet-4-6-wrapper",
+		"claude-fabled-5",
+		"claude-opus-5foo",
+		"claude-sonnet-4-6foo",
+		"fable/gpt-4o",
+		"opus",
+		"sonnet",
+		"",
 	}
 	for _, model := range supportedModels {
 		t.Run(model, func(t *testing.T) {