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fix(ai, ai-anthropic): thinking blocks missing on turn 2+ in tool loops #391

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ea96c3d
fix(ai, ai-anthropic): thinking blocks missing on turn 2+ in tool loops
imsherrill Mar 20, 2026
c042c55
ci: apply automated fixes
autofix-ci[bot] Mar 20, 2026
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37 changes: 36 additions & 1 deletion packages/typescript/ai-anthropic/src/adapters/text.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -289,6 +289,9 @@ export class AnthropicTextAdapter<
system: options.systemPrompts?.join('\n'),
tools: tools,
...validProviderOptions,
...(thinkingBudget && {
betas: ['interleaved-thinking-2025-05-14'] as any,
}),
}
validateTextProviderOptions(requestParams)
return requestParams
Expand Down Expand Up @@ -389,6 +392,18 @@ export class AnthropicTextAdapter<
if (role === 'assistant' && message.toolCalls?.length) {
const contentBlocks: AnthropicContentBlocks = []

if (message.thinking?.length) {
for (const thinking of message.thinking) {
if (thinking.signature) {
contentBlocks.push({
type: 'thinking',
thinking: thinking.content,
signature: thinking.signature,
} as unknown as AnthropicContentBlock)
}
}
}

if (message.content) {
const content =
typeof message.content === 'string' ? message.content : ''
Expand Down Expand Up @@ -528,6 +543,7 @@ export class AnthropicTextAdapter<
): AsyncIterable<StreamChunk> {
let accumulatedContent = ''
let accumulatedThinking = ''
let accumulatedSignature = ''
const timestamp = Date.now()
const toolCallsMap = new Map<
number,
Expand Down Expand Up @@ -570,6 +586,7 @@ export class AnthropicTextAdapter<
})
} else if (event.content_block.type === 'thinking') {
accumulatedThinking = ''
accumulatedSignature = ''
// Emit STEP_STARTED for thinking
stepId = genId()
yield {
Expand Down Expand Up @@ -615,6 +632,11 @@ export class AnthropicTextAdapter<
delta,
content: accumulatedThinking,
}
} else if (
(event.delta as { type: string }).type === 'signature_delta'
) {
accumulatedSignature +=
(event.delta as { signature: string }).signature || ''
} else if (event.delta.type === 'input_json_delta') {
const existing = toolCallsMap.get(currentToolIndex)
if (existing) {
Expand Down Expand Up @@ -644,7 +666,20 @@ export class AnthropicTextAdapter<
}
}
} else if (event.type === 'content_block_stop') {
if (currentBlockType === 'tool_use') {
if (currentBlockType === 'thinking') {
// Emit signature so it can be replayed in multi-turn context
if (accumulatedSignature && stepId) {
yield {
type: 'STEP_FINISHED',
stepId,
model,
timestamp,
delta: '',
content: accumulatedThinking,
signature: accumulatedSignature,
}
}
} else if (currentBlockType === 'tool_use') {
const existing = toolCallsMap.get(currentToolIndex)
if (existing) {
// If tool call wasn't started yet (no args), start it now
Expand Down
6 changes: 5 additions & 1 deletion packages/typescript/ai-client/src/chat-client.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -153,7 +153,11 @@ export class ChatClient {
this.events.textUpdated(this.currentStreamId, messageId, content)
}
},
onThinkingUpdate: (messageId: string, content: string) => {
onThinkingUpdate: (
messageId: string,
_stepId: string,
content: string,
) => {
// Emit thinking update to devtools
if (this.currentStreamId) {
this.events.thinkingUpdated(
Expand Down
43 changes: 40 additions & 3 deletions packages/typescript/ai/src/activities/chat/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -253,6 +253,10 @@ class TextEngine<
private totalChunkCount = 0
private currentMessageId: string | null = null
private accumulatedContent = ''
private accumulatedThinking: Array<{ content: string; signature?: string }> =
[]
private currentThinkingContent = ''
private currentThinkingSignature = ''
private eventOptions?: Record<string, unknown>
private eventToolNames?: Array<string>
private finishedEvent: RunFinishedEvent | null = null
Expand Down Expand Up @@ -494,6 +498,9 @@ class TextEngine<
private async beginIteration(): Promise<void> {
this.currentMessageId = this.createId('msg')
this.accumulatedContent = ''
this.accumulatedThinking = []
this.currentThinkingContent = ''
this.currentThinkingSignature = ''
this.finishedEvent = null

// Update mutable context fields
Expand Down Expand Up @@ -585,12 +592,15 @@ class TextEngine<
case 'RUN_ERROR':
this.handleRunErrorEvent(chunk)
break
case 'STEP_STARTED':
this.handleStepStartedEvent()
break
case 'STEP_FINISHED':
this.handleStepFinishedEvent(chunk)
break

default:
// RUN_STARTED, TEXT_MESSAGE_START, TEXT_MESSAGE_END, STEP_STARTED,
// RUN_STARTED, TEXT_MESSAGE_START, TEXT_MESSAGE_END,
// STATE_SNAPSHOT, STATE_DELTA, CUSTOM
// - no special handling needed in chat activity
break
Expand Down Expand Up @@ -633,10 +643,32 @@ class TextEngine<
this.earlyTermination = true
}

private finalizeCurrentThinkingStep(): void {
if (this.currentThinkingContent) {
this.accumulatedThinking.push({
content: this.currentThinkingContent,
...(this.currentThinkingSignature && {
signature: this.currentThinkingSignature,
}),
})
this.currentThinkingContent = ''
this.currentThinkingSignature = ''
}
}

private handleStepStartedEvent(): void {
this.finalizeCurrentThinkingStep()
}

private handleStepFinishedEvent(
_chunk: Extract<StreamChunk, { type: 'STEP_FINISHED' }>,
chunk: Extract<StreamChunk, { type: 'STEP_FINISHED' }>,
): void {
// State tracking for STEP_FINISHED is handled by middleware
if (chunk.delta) {
this.currentThinkingContent += chunk.delta
}
if (chunk.signature) {
this.currentThinkingSignature = chunk.signature
}
}

private async *checkForPendingToolCalls(): AsyncGenerator<
Expand Down Expand Up @@ -939,12 +971,17 @@ class TextEngine<
}

private addAssistantToolCallMessage(toolCalls: Array<ToolCall>): void {
this.finalizeCurrentThinkingStep()

this.messages = [
...this.messages,
{
role: 'assistant',
content: this.accumulatedContent || null,
toolCalls,
...(this.accumulatedThinking.length > 0 && {
thinking: this.accumulatedThinking,
}),
},
]
}
Expand Down
13 changes: 12 additions & 1 deletion packages/typescript/ai/src/activities/chat/messages.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -165,6 +165,7 @@ function isToolCallIncluded(part: ToolCallPart): boolean {
function buildAssistantMessages(uiMessage: UIMessage): Array<ModelMessage> {
const messageList: Array<ModelMessage> = []
let current = createSegment()
let pendingThinking: Array<{ content: string; signature?: string }> = []

Comment on lines +168 to 169
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@coderabbitai coderabbitai bot Mar 20, 2026

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⚠️ Potential issue | 🟠 Major

This still loses thinking/tool-call order within a single turn.

pendingThinking is buffered independently from current.toolCalls, so a sequence like [thinking(step-1), tool-call(a), thinking(step-2), tool-call(b)] gets flattened into one assistant ModelMessage with thinking: [step-1, step-2] and toolCalls: [a, b]. If a provider emits multiple thinking blocks around multiple tool calls in one turn, the original interleaving is gone and the next request cannot faithfully replay that history.

Also applies to: 233-239

🤖 Prompt for AI Agents 
Verify each finding against the current code and only fix it if needed.

In `@packages/typescript/ai/src/activities/chat/messages.ts` around lines 168 -
169, The issue is that pendingThinking is buffered separately from
current.toolCalls, which loses the original interleaving (e.g., thinking,
tool-call, thinking) when constructing ModelMessage; change the buffering to a
single ordered stream of typed entries (e.g., an array of {type:
'thinking'|'toolCall', payload: ...}) instead of separate pendingThinking and
current.toolCalls so you can preserve insert order when emitting/serializing;
update all places that push to pendingThinking and to current.toolCalls (and the
duplicate buffering logic referenced around the 233-239 region) to push a typed
entry into the unified buffer and update the code that builds the assistant
ModelMessage to iterate this unified buffer and emit thinking/toolCall entries
in original sequence.

// Track emitted tool result IDs to avoid duplicates.
// A tool call can have BOTH an explicit tool-result part AND an output
Expand All @@ -181,7 +182,9 @@ function buildAssistantMessages(uiMessage: UIMessage): Array<ModelMessage> {
role: 'assistant',
content,
...(hasToolCalls && { toolCalls: current.toolCalls }),
...(pendingThinking.length > 0 && { thinking: pendingThinking }),
})
pendingThinking = []
}
current = createSegment()
}
Expand Down Expand Up @@ -227,7 +230,15 @@ function buildAssistantMessages(uiMessage: UIMessage): Array<ModelMessage> {
}
break

// thinking parts are skipped - they're UI-only
case 'thinking':
if (part.content) {
pendingThinking.push({
content: part.content,
...(part.signature && { signature: part.signature }),
})
}
break

default:
break
}
Expand Down
13 changes: 10 additions & 3 deletions packages/typescript/ai/src/activities/chat/stream/message-updaters.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -244,28 +244,35 @@ export function updateToolCallApprovalResponse(
}

/**
* Update or add a thinking part to a message.
* Update or add a thinking part to a message, keyed by stepId.
* Each distinct stepId produces its own ThinkingPart.
*/
export function updateThinkingPart(
messages: Array<UIMessage>,
messageId: string,
stepId: string,
content: string,
signature?: string,
): Array<UIMessage> {
return messages.map((msg) => {
if (msg.id !== messageId) {
return msg
}

const parts = [...msg.parts]
const thinkingPartIndex = parts.findIndex((p) => p.type === 'thinking')
const thinkingPartIndex = parts.findIndex(
(p) => p.type === 'thinking' && p.stepId === stepId,
)

const thinkingPart: ThinkingPart = {
type: 'thinking',
content,
stepId,
...(signature && { signature }),
}

if (thinkingPartIndex >= 0) {
// Update existing thinking part
// Update existing thinking part for this step
parts[thinkingPartIndex] = thinkingPart
} else {
// Add new thinking part at the end (preserve natural streaming order)
Expand Down
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