diff --git a/backend/internal/pkg/apicompat/chatcompletions_anthropic_bridge.go b/backend/internal/pkg/apicompat/chatcompletions_anthropic_bridge.go
index b434180573..1890cccf44 100644
--- a/backend/internal/pkg/apicompat/chatcompletions_anthropic_bridge.go
+++ b/backend/internal/pkg/apicompat/chatcompletions_anthropic_bridge.go
@@ -3,6 +3,7 @@ package apicompat
 import (
 	"encoding/json"
 	"fmt"
+	"sort"
 	"strings"
 	"time"
 )
@@ -79,13 +80,23 @@ func AnthropicToChatCompletionsRequest(req *AnthropicRequest) (*ChatCompletionsR
 	}
 
 	// tool_choice is only forwarded when tools survived the conversion
-	// (upstream rejects tool_choice without tools).
+	// (upstream rejects tool_choice without tools), and a named choice only when
+	// it points at a declared tool — mirroring responsesToolChoiceToChatToolChoice,
+	// chat upstreams 400 on tool_choice referencing an unknown tool.
 	if len(out.Tools) > 0 && len(req.ToolChoice) > 0 {
-		tc, err := convertAnthropicToolChoiceToChat(req.ToolChoice)
+		declared := make(map[string]bool, len(out.Tools))
+		for _, tool := range out.Tools {
+			if tool.Function != nil {
+				declared[tool.Function.Name] = true
+			}
+		}
+		tc, err := convertAnthropicToolChoiceToChat(req.ToolChoice, declared)
 		if err != nil {
 			return nil, fmt.Errorf("convert tool_choice: %w", err)
 		}
-		out.ToolChoice = tc
+		if len(tc) > 0 {
+			out.ToolChoice = tc
+		}
 	}
 
 	// Reasoning effort: output_config.effort maps 1:1 (max→xhigh). thinking.type
@@ -189,16 +200,24 @@ func anthropicUserToChatMessages(raw json.RawMessage) ([]ChatMessage, error) {
 		}
 	}
 
-	// Remaining text + image blocks → user message with content parts.
+	// Remaining text + image blocks → user message. The double-conversion path
+	// (responsesContentPartsToChatContent) folds text-only content into a single
+	// string joined with "\n\n" and only uses the parts-array form when an image
+	// is present — strict chat upstreams reject array content — so the direct
+	// bridge preserves that folding.
+	var textParts []string
 	var parts []ChatContentPart
+	hasImage := false
 	for _, b := range blocks {
 		switch b.Type {
 		case "text":
 			if b.Text != "" {
+				textParts = append(textParts, b.Text)
 				parts = append(parts, ChatContentPart{Type: "text", Text: b.Text})
 			}
 		case "image":
 			if uri := anthropicImageToDataURI(b.Source); uri != "" {
+				hasImage = true
 				parts = append(parts, ChatContentPart{
 					Type:     "image_url",
 					ImageURL: &ChatImageURL{URL: uri},
@@ -206,19 +225,25 @@ func anthropicUserToChatMessages(raw json.RawMessage) ([]ChatMessage, error) {
 			}
 		}
 	}
-	parts = append(parts, toolResultImageParts...)
-
-	if len(parts) > 0 {
-		// Mixed/structured content → array form; single text → string form
-		// (normalizeChatMessages will collapse a single-text-part array to a
-		// plain string if the upstream prefers it).
-		content, err := json.Marshal(parts)
-		if err != nil {
-			return nil, err
-		}
-		out = append(out, ChatMessage{Role: "user", Content: content})
+	if len(toolResultImageParts) > 0 {
+		hasImage = true
+		parts = append(parts, toolResultImageParts...)
 	}
 
+	if !hasImage {
+		if len(textParts) > 0 {
+			content, _ := json.Marshal(strings.Join(textParts, "\n\n"))
+			out = append(out, ChatMessage{Role: "user", Content: content})
+		}
+		return out, nil
+	}
+
+	content, err := json.Marshal(parts)
+	if err != nil {
+		return nil, err
+	}
+	out = append(out, ChatMessage{Role: "user", Content: content})
+
 	return out, nil
 }
 
@@ -290,13 +315,16 @@ func anthropicToolsToChatTools(tools []AnthropicTool) []ChatTool {
 }
 
 // convertAnthropicToolChoiceToChat maps Anthropic tool_choice to Chat
-// Completions tool_choice.
+// Completions tool_choice. A nil result means the choice is dropped: like the
+// double-conversion path (responsesToolChoiceToChatToolChoice), a named choice
+// pointing at an undeclared tool or an unknown choice type is not forwarded,
+// because chat upstreams reject it.
 //
 //	{"type":"auto"}            → "auto"
 //	{"type":"any"}             → "required"
 //	{"type":"none"}            → "none"
-//	{"type":"tool","name":"X"} → {"type":"function","function":{"name":"X"}}
-func convertAnthropicToolChoiceToChat(raw json.RawMessage) (json.RawMessage, error) {
+//	{"type":"tool","name":"X"} → {"type":"function","function":{"name":"X"}} (X declared)
+func convertAnthropicToolChoiceToChat(raw json.RawMessage, declared map[string]bool) (json.RawMessage, error) {
 	var tc struct {
 		Type string `json:"type"`
 		Name string `json:"name"`
@@ -313,12 +341,15 @@ func convertAnthropicToolChoiceToChat(raw json.RawMessage) (json.RawMessage, err
 	case "none":
 		return json.Marshal("none")
 	case "tool":
+		if tc.Name == "" || !declared[tc.Name] {
+			return nil, nil
+		}
 		return json.Marshal(map[string]any{
 			"type":     "function",
 			"function": map[string]string{"name": tc.Name},
 		})
 	default:
-		return raw, nil
+		return nil, nil
 	}
 }
 
@@ -351,9 +382,7 @@ func ChatCompletionsResponseToAnthropic(resp *ChatCompletionsResponse, model str
 	}
 
 	if resp != nil {
-		if out.ID == "" {
-			out.ID = resp.ID
-		}
+		out.ID = resp.ID
 		if out.Model == "" {
 			out.Model = resp.Model
 		}
@@ -373,6 +402,11 @@ func ChatCompletionsResponseToAnthropic(resp *ChatCompletionsResponse, model str
 	if len(out.Content) == 0 {
 		out.Content = []AnthropicContentBlock{{Type: "text", Text: ""}}
 	}
+	// The double-conversion path generates a response id when the upstream
+	// omits one (ChatCompletionsResponseToResponses); clients treat it as required.
+	if out.ID == "" {
+		out.ID = generateResponsesID()
+	}
 
 	return out
 }
@@ -420,23 +454,23 @@ func chatMessageToAnthropicBlocks(message ChatMessage) []AnthropicContentBlock {
 // chatFinishReasonToAnthropicStopReason maps Chat Completions finish_reason to
 // Anthropic stop_reason.
 //
-//	"stop"           → "end_turn" (or "tool_use" if tool_use blocks present)
-//	"length"         → "max_tokens"
-//	"tool_calls"     → "tool_use"
-//	"content_filter" → "end_turn"
+//	"length"     → "max_tokens"
+//	"tool_calls" → "tool_use"
+//	other        → "end_turn" (or "tool_use" if tool_use blocks present)
+//
+// "stop", "content_filter", and unknown reasons all map to a completed response
+// in the double-conversion path, which then derives stop_reason from the blocks.
 func chatFinishReasonToAnthropicStopReason(reason string, blocks []AnthropicContentBlock) string {
 	switch reason {
 	case "length":
 		return "max_tokens"
 	case "tool_calls":
 		return "tool_use"
-	case "stop":
+	default:
 		if containsAnthropicToolUseBlock(blocks) {
 			return "tool_use"
 		}
 		return "end_turn"
-	default:
-		return "end_turn"
 	}
 }
 
@@ -451,8 +485,14 @@ func chatUsageToAnthropicUsage(usage *ChatUsage) AnthropicUsage {
 	cacheCreationTokens := 0
 	if usage.PromptTokensDetails != nil {
 		cachedTokens = usage.PromptTokensDetails.CachedTokens
-		cacheCreationTokens = usage.PromptTokensDetails.CacheCreationTokens +
-			usage.PromptTokensDetails.CacheWriteTokens
+		// cache_write_tokens and cache_creation_tokens are alternate spellings of
+		// the same quantity, not additive; the double-conversion path
+		// (ChatUsageToResponsesUsage) prefers write and falls back to creation.
+		if usage.PromptTokensDetails.CacheWriteTokens > 0 {
+			cacheCreationTokens = usage.PromptTokensDetails.CacheWriteTokens
+		} else {
+			cacheCreationTokens = usage.PromptTokensDetails.CacheCreationTokens
+		}
 	}
 
 	inputTokens := usage.PromptTokens - cachedTokens - cacheCreationTokens
@@ -485,17 +525,20 @@ type ChatCompletionsToAnthropicStreamState struct {
 	ContentBlockOpen    bool
 	CurrentBlockType    string // "text" | "thinking" | "tool_use"
 	CurrentToolName     string
-	CurrentToolArgs     string
 	CurrentToolHadDelta bool
 	HasToolCall         bool
 
 	// Tool calls keyed by the upstream tool_call index. The Anthropic block
-	// index assigned at content_block_start time is stored so later argument
-	// deltas for the same tool land on the right block.
+	// index is assigned when the tool block is announced (content_block_start),
+	// which is deferred until the tool's name has arrived. Argument fragments
+	// and the call ID seen before the name are buffered and flushed with the
+	// announcement; tools whose name never arrives are announced with an empty
+	// name at finalize so their arguments are not lost.
 	toolBlockIndex    map[int]int
 	toolAnnounced     map[int]bool
 	toolName          map[int]string
-	pendingToolCallID map[int]string // call ID received before the name (deferred announce)
+	pendingToolCallID map[int]string
+	pendingToolArgs   map[int]string
 
 	// Reasoning (DeepSeek-style): reasoning_content streamed before content.
 	// No separate reasoning block index — it uses ContentBlockIndex like the
@@ -524,6 +567,7 @@ func NewChatCompletionsToAnthropicStreamState(model string) *ChatCompletionsToAn
 		toolAnnounced:     make(map[int]bool),
 		toolName:          make(map[int]string),
 		pendingToolCallID: make(map[int]string),
+		pendingToolArgs:   make(map[int]string),
 	}
 }
 
@@ -601,6 +645,24 @@ func FinalizeChatCompletionsAnthropicStream(state *ChatCompletionsToAnthropicStr
 	if !state.MessageStartSent {
 		events = append(events, ensureCCAnthropicMessageStart(state)...)
 	}
+
+	// Announce tools whose name never arrived so their buffered arguments are
+	// not silently dropped. The double-conversion path announced these
+	// immediately with an empty name; the deferred announcement keeps that data
+	// preservation while still delivering correct names when they do arrive.
+	if len(state.pendingToolCallID) > 0 {
+		idxs := make([]int, 0, len(state.pendingToolCallID))
+		for idx := range state.pendingToolCallID {
+			idxs = append(idxs, idx)
+		}
+		sort.Ints(idxs)
+		for _, idx := range idxs {
+			callID := state.pendingToolCallID[idx]
+			events = append(events, closeCCAnthropicBlock(state)...)
+			events = append(events, announceCCAnthropicToolBlock(state, idx, callID, "")...)
+		}
+	}
+
 	events = append(events, closeCCAnthropicBlock(state)...)
 
 	stopReason := ccFinishReasonToAnthropicStopReason(state.FinishReason, state.HasToolCall)
@@ -683,9 +745,11 @@ func ensureCCAnthropicTextBlock(state *ChatCompletionsToAnthropicStreamState) []
 	return events
 }
 
-// handleCCAnthropicToolCall processes one upstream tool_call delta. A new index
-// opens a tool_use block (deferred if the name hasn't arrived yet); argument
-// fragments emit input_json_delta on the tool's block.
+// handleCCAnthropicToolCall processes one upstream tool_call delta. The
+// content_block_start for a tool is deferred until its name has arrived (some
+// upstreams stream id/arguments before the name); argument fragments seen
+// before the announcement are buffered and flushed with it, later fragments
+// stream as input_json_delta on the tool's block.
 func handleCCAnthropicToolCall(state *ChatCompletionsToAnthropicStreamState, toolCall *ChatToolCall) []AnthropicStreamEvent {
 	idx := 0
 	if toolCall.Index != nil {
@@ -694,81 +758,39 @@ func handleCCAnthropicToolCall(state *ChatCompletionsToAnthropicStreamState, too
 
 	var events []AnthropicStreamEvent
 
-	if _, ok := state.toolBlockIndex[idx]; !ok {
-		// New tool call. Close any open non-tool block first.
+	if _, seen := state.toolAnnounced[idx]; !seen {
+		// New tool call: it ends whatever block is currently streaming.
 		events = append(events, closeCCAnthropicBlock(state)...)
-		blockIdx := state.ContentBlockIndex
-		state.toolBlockIndex[idx] = blockIdx
 		state.HasToolCall = true
 
-		// Open the tool_use block immediately if we have an ID + name; otherwise
-		// defer the content_block_start until the name arrives.
 		callID := toolCall.ID
 		if callID == "" {
 			callID = generateItemID()
 		}
-		name := toolCall.Function.Name
-		if name != "" {
-			state.toolAnnounced[idx] = true
-			state.toolName[idx] = name
-			state.CurrentToolName = name
-			state.ContentBlockOpen = true
-			state.CurrentBlockType = "tool_use"
-			events = append(events, AnthropicStreamEvent{
-				Type:  "content_block_start",
-				Index: &blockIdx,
-				ContentBlock: &AnthropicContentBlock{
-					Type:  "tool_use",
-					ID:    fromResponsesCallID(callID),
-					Name:  name,
-					Input: json.RawMessage("{}"),
-				},
-			})
+		if name := toolCall.Function.Name; name != "" {
+			events = append(events, announceCCAnthropicToolBlock(state, idx, callID, name)...)
 		} else {
 			state.toolAnnounced[idx] = false
-			// Store the call ID so we can emit content_block_start when the
-			// name arrives. We stash it in toolName prefixed with the ID marker
-			// is unnecessary — keep the pending ID separately is cleaner, but
-			// to avoid another map we re-derive: the next delta for this idx
-			// with a name will announce. We still need the ID though.
-			// Store ID in toolName as "id\x00" sentinel? No — add a field.
 			state.pendingToolCallID[idx] = callID
 		}
-	} else {
-		// Existing tool call: update ID/name if provided.
-		if toolCall.Function.Name != "" && !state.toolAnnounced[idx] {
-			blockIdx := state.toolBlockIndex[idx]
-			name := toolCall.Function.Name
-			state.toolAnnounced[idx] = true
-			state.toolName[idx] = name
-			state.CurrentToolName = name
-			state.ContentBlockOpen = true
-			state.CurrentBlockType = "tool_use"
-			callID := state.pendingToolCallID[idx]
-			if toolCall.ID != "" {
-				callID = toolCall.ID
-			}
-			if callID == "" {
-				callID = generateItemID()
-			}
-			events = append(events, AnthropicStreamEvent{
-				Type:  "content_block_start",
-				Index: &blockIdx,
-				ContentBlock: &AnthropicContentBlock{
-					Type:  "tool_use",
-					ID:    fromResponsesCallID(callID),
-					Name:  name,
-					Input: json.RawMessage("{}"),
-				},
-			})
+	} else if !state.toolAnnounced[idx] && toolCall.Function.Name != "" {
+		// Deferred announcement: the name has arrived.
+		callID := state.pendingToolCallID[idx]
+		if toolCall.ID != "" {
+			callID = toolCall.ID
 		}
+		events = append(events, closeCCAnthropicBlock(state)...)
+		events = append(events, announceCCAnthropicToolBlock(state, idx, callID, toolCall.Function.Name)...)
 	}
 
-	// Argument fragment → input_json_delta on this tool's block.
+	// Argument fragment → input_json_delta on the tool's block once announced,
+	// buffered until the deferred announcement otherwise.
 	if toolCall.Function.Arguments != "" {
-		state.CurrentToolArgs += toolCall.Function.Arguments
-		state.CurrentToolHadDelta = true
-		if blockIdx, ok := state.toolBlockIndex[idx]; ok && state.toolAnnounced[idx] {
+		if state.toolAnnounced[idx] {
+			blockIdx := state.toolBlockIndex[idx]
+			if state.ContentBlockOpen && blockIdx == state.ContentBlockIndex {
+				state.CurrentToolHadDelta = true
+			}
 			events = append(events, AnthropicStreamEvent{
 				Type:  "content_block_delta",
 				Index: &blockIdx,
@@ -777,12 +799,53 @@ func handleCCAnthropicToolCall(state *ChatCompletionsToAnthropicStreamState, too
 					PartialJSON: toolCall.Function.Arguments,
 				},
 			})
+		} else {
+			state.pendingToolArgs[idx] += toolCall.Function.Arguments
 		}
 	}
 
 	return events
 }
 
+// announceCCAnthropicToolBlock assigns the next Anthropic block index to the
+// tool, emits its content_block_start, and flushes any argument fragments
+// buffered while the announcement was deferred.
+func announceCCAnthropicToolBlock(state *ChatCompletionsToAnthropicStreamState, idx int, callID, name string) []AnthropicStreamEvent {
+	blockIdx := state.ContentBlockIndex
+	state.toolBlockIndex[idx] = blockIdx
+	state.toolAnnounced[idx] = true
+	state.toolName[idx] = name
+	state.CurrentToolName = name
+	state.CurrentToolHadDelta = false
+	state.ContentBlockOpen = true
+	state.CurrentBlockType = "tool_use"
+	delete(state.pendingToolCallID, idx)
+
+	events := []AnthropicStreamEvent{{
+		Type:  "content_block_start",
+		Index: &blockIdx,
+		ContentBlock: &AnthropicContentBlock{
+			Type:  "tool_use",
+			ID:    fromResponsesCallID(callID),
+			Name:  name,
+			Input: json.RawMessage("{}"),
+		},
+	}}
+	if pending := state.pendingToolArgs[idx]; pending != "" {
+		delete(state.pendingToolArgs, idx)
+		state.CurrentToolHadDelta = true
+		events = append(events, AnthropicStreamEvent{
+			Type:  "content_block_delta",
+			Index: &blockIdx,
+			Delta: &AnthropicDelta{
+				Type:        "input_json_delta",
+				PartialJSON: pending,
+			},
+		})
+	}
+	return events
+}
+
 // ccAnthropicDelta emits a content_block_delta on the current block.
 func ccAnthropicDelta(state *ChatCompletionsToAnthropicStreamState, delta *AnthropicDelta) []AnthropicStreamEvent {
 	if !state.ContentBlockOpen {
@@ -805,22 +868,35 @@ func closeCCAnthropicBlockIfOpen(state *ChatCompletionsToAnthropicStreamState, b
 	return closeCCAnthropicBlock(state)
 }
 
-// closeCCAnthropicBlock closes the currently open content block.
+// closeCCAnthropicBlock closes the currently open content block. A tool_use
+// block that streamed no argument delta gets a final input_json_delta "{}"
+// first — the double-conversion path normalizes empty tool arguments to "{}",
+// and some clients assemble tool input exclusively from deltas.
 func closeCCAnthropicBlock(state *ChatCompletionsToAnthropicStreamState) []AnthropicStreamEvent {
 	if !state.ContentBlockOpen {
 		return nil
 	}
 	idx := state.ContentBlockIndex
+	var events []AnthropicStreamEvent
+	if state.CurrentBlockType == "tool_use" && !state.CurrentToolHadDelta {
+		events = append(events, AnthropicStreamEvent{
+			Type:  "content_block_delta",
+			Index: &idx,
+			Delta: &AnthropicDelta{
+				Type:        "input_json_delta",
+				PartialJSON: "{}",
+			},
+		})
+	}
 	state.ContentBlockOpen = false
 	state.ContentBlockIndex++
 	state.CurrentBlockType = ""
 	state.CurrentToolName = ""
-	state.CurrentToolArgs = ""
 	state.CurrentToolHadDelta = false
-	return []AnthropicStreamEvent{{
+	return append(events, AnthropicStreamEvent{
 		Type:  "content_block_stop",
 		Index: &idx,
-	}}
+	})
 }
 
 // ccFinishReasonToAnthropicStopReason maps a Chat Completions finish_reason
diff --git a/backend/internal/pkg/apicompat/chatcompletions_anthropic_bridge_test.go b/backend/internal/pkg/apicompat/chatcompletions_anthropic_bridge_test.go
index c569d818a2..bd850aa3ab 100644
--- a/backend/internal/pkg/apicompat/chatcompletions_anthropic_bridge_test.go
+++ b/backend/internal/pkg/apicompat/chatcompletions_anthropic_bridge_test.go
@@ -808,3 +808,264 @@ func TestChatCompletionsToAnthropicStreamState_ToolCallNameArrivesLate(t *testin
 	}
 	require.Equal(t, "late_tool", toolName)
 }
+
+// assembleToolUseBlocks rebuilds tool_use blocks from a stream the way an
+// Anthropic client does: content_block_start announces id/name, input_json_delta
+// fragments concatenate into the input JSON.
+type assembledToolUse struct {
+	ID    string
+	Name  string
+	Input string
+}
+
+func assembleToolUseBlocks(events []AnthropicStreamEvent) []assembledToolUse {
+	blockByIdx := map[int]int{} // anthropic block index → position in out
+	var out []assembledToolUse
+	for _, e := range events {
+		switch e.Type {
+		case "content_block_start":
+			if e.ContentBlock != nil && e.ContentBlock.Type == "tool_use" && e.Index != nil {
+				blockByIdx[*e.Index] = len(out)
+				out = append(out, assembledToolUse{ID: e.ContentBlock.ID, Name: e.ContentBlock.Name})
+			}
+		case "content_block_delta":
+			if e.Delta != nil && e.Delta.Type == "input_json_delta" && e.Index != nil {
+				if pos, ok := blockByIdx[*e.Index]; ok {
+					out[pos].Input += e.Delta.PartialJSON
+				}
+			}
+		}
+	}
+	return out
+}
+
+func TestChatCompletionsToAnthropicStreamState_ToolCallArgsArriveBeforeName(t *testing.T) {
+	// Some upstreams stream argument fragments before the tool name. The
+	// fragments buffered while the announcement is deferred must be flushed
+	// with the content_block_start, so the client rebuilds complete JSON.
+	events := collectAnthropicStreamEvents(t, []string{
+		`{"choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"id":"call_early","function":{"arguments":"{\"city\":"}}]}}]}`,
+		`{"choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"function":{"arguments":"\"SF\""}}]}}]}`,
+		`{"choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"function":{"name":"get_weather","arguments":"}"}}]}}]}`,
+		`{"choices":[{"index":0,"delta":{},"finish_reason":"tool_calls"}],"usage":{"prompt_tokens":1,"completion_tokens":1,"total_tokens":2}}`,
+	})
+
+	tools := assembleToolUseBlocks(events)
+	require.Len(t, tools, 1)
+	require.Equal(t, "call_early", tools[0].ID)
+	require.Equal(t, "get_weather", tools[0].Name)
+	require.JSONEq(t, `{"city":"SF"}`, tools[0].Input)
+
+	// No delta may precede the block's content_block_start.
+	started := map[int]bool{}
+	for _, e := range events {
+		switch e.Type {
+		case "content_block_start":
+			started[*e.Index] = true
+		case "content_block_delta":
+			require.True(t, started[*e.Index], "delta before content_block_start on index %d", *e.Index)
+		}
+	}
+}
+
+func TestChatCompletionsToAnthropicStreamState_ToolCallNameNeverArrives(t *testing.T) {
+	// If the name never arrives, the tool is announced at finalize with an
+	// empty name (like the double-conversion path) so its arguments are not
+	// silently dropped — stop_reason still reports tool_use.
+	events := collectAnthropicStreamEvents(t, []string{
+		`{"choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"id":"call_anon","function":{"arguments":"{\"a\":1}"}}]}}]}`,
+		`{"choices":[{"index":0,"delta":{},"finish_reason":"tool_calls"}],"usage":{"prompt_tokens":1,"completion_tokens":1,"total_tokens":2}}`,
+	})
+
+	tools := assembleToolUseBlocks(events)
+	require.Len(t, tools, 1)
+	require.Equal(t, "call_anon", tools[0].ID)
+	require.Equal(t, "", tools[0].Name)
+	require.JSONEq(t, `{"a":1}`, tools[0].Input)
+
+	// Block lifecycle must stay balanced and terminate before message_stop.
+	types := anthropicEventTypes(events)
+	require.Equal(t, []string{
+		"message_start",
+		"content_block_start",
+		"content_block_delta",
+		"content_block_stop",
+		"message_delta",
+		"message_stop",
+	}, types)
+}
+
+func TestChatCompletionsToAnthropicStreamState_EmptyArgsToolEmitsPlaceholderDelta(t *testing.T) {
+	// A tool call whose arguments never arrive gets a final input_json_delta
+	// "{}" before its stop — the double-conversion path normalizes empty
+	// arguments to "{}", and some clients assemble input only from deltas.
+	events := collectAnthropicStreamEvents(t, []string{
+		`{"choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"id":"call_empty","type":"function","function":{"name":"noop","arguments":""}}]}}]}`,
+		`{"choices":[{"index":0,"delta":{},"finish_reason":"tool_calls"}],"usage":{"prompt_tokens":1,"completion_tokens":1,"total_tokens":2}}`,
+	})
+
+	tools := assembleToolUseBlocks(events)
+	require.Len(t, tools, 1)
+	require.Equal(t, "noop", tools[0].Name)
+	require.JSONEq(t, `{}`, tools[0].Input)
+}
+
+func TestAnthropicToChatCompletionsRequest_UserArrayContentFoldsToString(t *testing.T) {
+	// Text-only array content folds into a single string joined with "\n\n",
+	// like the double-conversion path — strict chat upstreams reject array
+	// content when no image forces the parts form.
+	req := &AnthropicRequest{
+		Model:     "deepseek-v4-pro",
+		MaxTokens: 100,
+		Messages: []AnthropicMessage{
+			{Role: "user", Content: json.RawMessage(`[{"type":"text","text":"first"},{"type":"text","text":"second"}]`)},
+		},
+	}
+
+	out, err := AnthropicToChatCompletionsRequest(req)
+	require.NoError(t, err)
+	require.Len(t, out.Messages, 1)
+	require.Equal(t, `"first\n\nsecond"`, string(out.Messages[0].Content))
+}
+
+func TestDirectBridge_RequestMatchesDoubleConversion_ArrayUserContent(t *testing.T) {
+	// Array-form user content: text-only folds to a string, image-bearing stays
+	// in parts form — both must match the double-conversion chain exactly.
+	req := &AnthropicRequest{
+		Model:     "deepseek-v4-pro",
+		MaxTokens: 100,
+		Messages: []AnthropicMessage{
+			{Role: "user", Content: json.RawMessage(`[{"type":"text","text":"first"},{"type":"text","text":"second"}]`)},
+			{Role: "assistant", Content: json.RawMessage(`"ok"`)},
+			{Role: "user", Content: json.RawMessage(`[{"type":"text","text":"look"},{"type":"image","source":{"type":"base64","media_type":"image/png","data":"aGVsbG8="}}]`)},
+		},
+	}
+
+	direct, err := AnthropicToChatCompletionsRequest(req)
+	require.NoError(t, err)
+
+	responsesReq, err := AnthropicToResponses(req)
+	require.NoError(t, err)
+	double, err := ResponsesToChatCompletionsRequest(responsesReq)
+	require.NoError(t, err)
+
+	require.Len(t, direct.Messages, len(double.Messages), "message count mismatch")
+	for i := range direct.Messages {
+		require.Equal(t, double.Messages[i].Role, direct.Messages[i].Role, "msg %d role mismatch", i)
+		var dContent, dblContent any
+		require.NoError(t, json.Unmarshal(double.Messages[i].Content, &dblContent))
+		require.NoError(t, json.Unmarshal(direct.Messages[i].Content, &dContent))
+		require.Equal(t, dblContent, dContent, "msg %d content mismatch", i)
+	}
+}
+
+func TestDirectBridge_NonStreamingMatchesDoubleConversion_CacheWriteTokens(t *testing.T) {
+	// cache_write_tokens and cache_creation_tokens are alternate spellings, not
+	// additive — when both are set, the double-conversion path prefers write.
+	resp := &ChatCompletionsResponse{
+		ID:    "chatcmpl-cache",
+		Model: "deepseek-v4-pro",
+		Choices: []ChatChoice{{
+			Message:      ChatMessage{Role: "assistant", Content: json.RawMessage(`"hi"`)},
+			FinishReason: "stop",
+		}},
+		Usage: &ChatUsage{
+			PromptTokens:     100,
+			CompletionTokens: 10,
+			TotalTokens:      110,
+			PromptTokensDetails: &ChatTokenDetails{
+				CachedTokens:        20,
+				CacheCreationTokens: 7,
+				CacheWriteTokens:    9,
+			},
+		},
+	}
+
+	direct := ChatCompletionsResponseToAnthropic(resp, "claude-sonnet-4-20250514")
+
+	responsesResp := ChatCompletionsResponseToResponses(resp, "claude-sonnet-4-20250514", nil, false, nil)
+	double := ResponsesToAnthropic(responsesResp, "claude-sonnet-4-20250514")
+
+	require.Equal(t, double.Usage.InputTokens, direct.Usage.InputTokens)
+	require.Equal(t, double.Usage.OutputTokens, direct.Usage.OutputTokens)
+	require.Equal(t, double.Usage.CacheReadInputTokens, direct.Usage.CacheReadInputTokens)
+	require.Equal(t, double.Usage.CacheCreationInputTokens, direct.Usage.CacheCreationInputTokens)
+	require.Equal(t, 9, direct.Usage.CacheCreationInputTokens)
+}
+
+func TestChatCompletionsResponseToAnthropic_GeneratesIDWhenMissing(t *testing.T) {
+	resp := &ChatCompletionsResponse{
+		Model: "deepseek-v4-pro",
+		Choices: []ChatChoice{{
+			Message:      ChatMessage{Role: "assistant", Content: json.RawMessage(`"hi"`)},
+			FinishReason: "stop",
+		}},
+	}
+
+	out := ChatCompletionsResponseToAnthropic(resp, "claude-sonnet-4-20250514")
+	require.NotEmpty(t, out.ID, "response id must be generated when the upstream omits one")
+}
+
+func TestAnthropicToChatCompletionsRequest_ToolChoiceUndeclaredDropped(t *testing.T) {
+	// A named tool_choice pointing at a dropped/unknown tool is not forwarded —
+	// chat upstreams 400 on tool_choice referencing an undeclared tool.
+	base := AnthropicRequest{
+		Model:     "deepseek-v4-pro",
+		MaxTokens: 100,
+		Tools: []AnthropicTool{
+			{Name: "get_weather", InputSchema: json.RawMessage(`{"type":"object"}`)},
+			{Name: "web_search_20250305", Type: "web_search_20250305", InputSchema: json.RawMessage(`{"type":"object"}`)},
+		},
+		Messages: []AnthropicMessage{
+			{Role: "user", Content: json.RawMessage(`"hi"`)},
+		},
+	}
+
+	undeclared := base
+	undeclared.ToolChoice = json.RawMessage(`{"type":"tool","name":"nonexistent"}`)
+	out, err := AnthropicToChatCompletionsRequest(&undeclared)
+	require.NoError(t, err)
+	require.Empty(t, out.ToolChoice, "tool_choice for an undeclared tool must be dropped")
+
+	droppedServerTool := base
+	droppedServerTool.ToolChoice = json.RawMessage(`{"type":"tool","name":"web_search_20250305"}`)
+	out, err = AnthropicToChatCompletionsRequest(&droppedServerTool)
+	require.NoError(t, err)
+	require.Empty(t, out.ToolChoice, "tool_choice for a dropped server tool must be dropped")
+
+	unknownType := base
+	unknownType.ToolChoice = json.RawMessage(`{"type":"mystery"}`)
+	out, err = AnthropicToChatCompletionsRequest(&unknownType)
+	require.NoError(t, err)
+	require.Empty(t, out.ToolChoice, "unknown tool_choice types must be dropped")
+
+	declared := base
+	declared.ToolChoice = json.RawMessage(`{"type":"tool","name":"get_weather"}`)
+	out, err = AnthropicToChatCompletionsRequest(&declared)
+	require.NoError(t, err)
+	require.JSONEq(t, `{"type":"function","function":{"name":"get_weather"}}`, string(out.ToolChoice))
+}
+
+func TestChatCompletionsResponseToAnthropic_ContentFilterWithToolUse(t *testing.T) {
+	// content_filter (and unknown finish reasons) derive stop_reason from the
+	// blocks, like the double-conversion path.
+	resp := &ChatCompletionsResponse{
+		ID:    "chatcmpl-cf",
+		Model: "deepseek-v4-pro",
+		Choices: []ChatChoice{{
+			Message: ChatMessage{
+				Role:    "assistant",
+				Content: json.RawMessage(`"partial"`),
+				ToolCalls: []ChatToolCall{{
+					ID:       "call_cf",
+					Type:     "function",
+					Function: ChatFunctionCall{Name: "search", Arguments: `{"q":"x"}`},
+				}},
+			},
+			FinishReason: "content_filter",
+		}},
+	}
+
+	out := ChatCompletionsResponseToAnthropic(resp, "claude-sonnet-4-20250514")
+	require.Equal(t, "tool_use", out.StopReason)
+}