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fix: align mcp transform pipeline with Anthropic Claude Code 2.1.128 #417

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Merged
claude-code-best merged 1 commit into from
May 6, 2026
+141 −36
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fix: align mcp transform pipeline with Anthropic Claude Code 2.1.128 #417

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118 changes: 104 additions & 14 deletions src/services/mcp/client.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -74,7 +74,10 @@ import {
} from '../../utils/errors.js'
import { getMCPUserAgent } from '../../utils/http.js'
import { maybeNotifyIDEConnected } from '../../utils/ide.js'
import { maybeResizeAndDownsampleImageBuffer } from '../../utils/imageResizer.js'
import {
type ImageLimits,
maybeResizeAndDownsampleImageBuffer,
} from '../../utils/imageResizer.js'
import { logMCPDebug, logMCPError } from '../../utils/log.js'
import {
getBinaryBlobSavedMessage,
Expand Down Expand Up @@ -102,6 +105,7 @@ import {
isPersistError,
persistToolResult,
} from '../../utils/toolResultStorage.js'
import { checkStatsigFeatureGate_CACHED_MAY_BE_STALE } from '../analytics/growthbook.js'
import {
type AnalyticsMetadata_I_VERIFIED_THIS_IS_NOT_CODE_OR_FILEPATHS,
logEvent,
Expand Down Expand Up @@ -2486,15 +2490,23 @@ export function prefetchAllMcpResources(
export async function transformResultContent(
resultContent: PromptMessage['content'],
serverName: string,
limits?: ImageLimits,
includeMeta = false,
): Promise<Array<ContentBlockParam>> {
switch (resultContent.type) {
case 'text':
return [
{
type: 'text',
text: resultContent.text,
},
]
case 'text': {
const block: ContentBlockParam = {
type: 'text',
text: resultContent.text,
}
if (includeMeta) {
const meta = resultContent._meta
if (meta) {
;(block as { _meta?: unknown })._meta = meta
}
}
return [block]
Comment on lines +2497 to +2508
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@coderabbitai coderabbitai Bot May 5, 2026

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⚠️ Potential issue | 🟠 Major | ⚡ Quick win

Preserve annotations before the single-text unwrap path runs.

unwrapSingleTextBlock() explicitly refuses annotated text blocks, but this transformer only carries _meta. A single annotated MCP text result is therefore converted into a bare text block here and later qualifies for plain-text unwrapping, which drops the annotations on the persisted large-output path.

Suggested fix 
     case 'text': {
       const block: ContentBlockParam = {
         type: 'text',
         text: resultContent.text,
       }
+      if ('annotations' in resultContent && resultContent.annotations) {
+        ;(block as { annotations?: unknown }).annotations =
+          resultContent.annotations
+      }
       if (includeMeta) {
         const meta = resultContent._meta
         if (meta) {
           ;(block as { _meta?: unknown })._meta = meta
         }
🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/services/mcp/client.ts` around lines 2497 - 2508, The text-case
transformer currently only copies `_meta` and thus strips `annotations` before
`unwrapSingleTextBlock()` runs; change the `case 'text'` path (where `block:
ContentBlockParam` is constructed from `resultContent.text`) to also preserve
any `resultContent.annotations` by assigning them onto the block (e.g. if
(resultContent.annotations) (block as { annotations?: unknown }).annotations =
resultContent.annotations), so annotated text blocks are not converted into bare
text and lost by the unwrapping logic.

}
case 'audio': {
const audioData = resultContent as {
type: 'audio'
Expand All @@ -2516,6 +2528,7 @@ export async function transformResultContent(
imageBuffer,
imageBuffer.length,
ext,
limits,
)
return [
{
Expand Down Expand Up @@ -2551,6 +2564,7 @@ export async function transformResultContent(
imageBuffer,
imageBuffer.length,
ext,
limits,
)
const content: MessageParam['content'] = []
if (prefix) {
Expand Down Expand Up @@ -2671,6 +2685,7 @@ export async function transformMCPResult(
result: unknown,
tool: string, // Tool name for validation (e.g., "search")
name: string, // Server name for transformation (e.g., "slack")
limits?: ImageLimits, // Image processing limits, plumbed to transformResultContent
): Promise<TransformedMCPResult> {
if (result && typeof result === 'object') {
if ('toolResult' in result) {
Expand All @@ -2694,7 +2709,9 @@ export async function transformMCPResult(
if ('content' in result && Array.isArray(result.content)) {
const transformedContent = (
await Promise.all(
result.content.map(item => transformResultContent(item, name)),
result.content.map(item =>
transformResultContent(item, name, limits, true),
),
)
).flat()
return {
Expand Down Expand Up @@ -2729,15 +2746,28 @@ export async function processMCPResult(
result: unknown,
tool: string, // Tool name for validation (e.g., "search")
name: string, // Server name for IDE check and transformation (e.g., "slack")
limits?: ImageLimits, // Image processing limits, plumbed to transformMCPResult
skipLargeOutput = false, // If true, skip large-output handling for non-image content
): Promise<MCPToolResult> {
const { content, type, schema } = await transformMCPResult(result, tool, name)
const { content, type, schema } = await transformMCPResult(
result,
tool,
name,
limits,
)

// IDE tools are not going to the model directly, so we don't need to
// handle large output.
if (name === 'ide') {
return content
}

// Caller opted out of large-output handling (e.g., result already truncated
// upstream); only continue if the content has images that may need handling.
if (skipLargeOutput && !contentContainsImages(content)) {
return content
}

// Check if content needs truncation (i.e., is too large)
if (!(await mcpContentNeedsTruncation(content))) {
return content
Expand Down Expand Up @@ -2775,9 +2805,15 @@ export async function processMCPResult(
// Generate a unique ID for the persisted file (server__tool-timestamp)
const timestamp = Date.now()
const persistId = `mcp-${normalizeNameForMCP(name)}-${normalizeNameForMCP(tool)}-${timestamp}`
// Convert to string for persistence (persistToolResult expects string or specific block types)
// When the large-string format gate is on, unwrap a single bare text block
// (no annotations, no _meta) into raw text so the model gets plain text in
// the persisted file instead of a JSON-wrapped block. The `_meta` check is
// why transformResultContent preserves _meta on text blocks.
const unwrappedText = unwrapSingleTextBlock(content)
const contentStr =
typeof content === 'string' ? content : jsonStringify(content, null, 2)
typeof content === 'string'
? content
: (unwrappedText ?? jsonStringify(content, null, 2))
const persistResult = await persistToolResult(contentStr, persistId)

if (isPersistError(persistResult)) {
Expand All @@ -2798,14 +2834,50 @@ export async function processMCPResult(
persistedSizeChars: persistResult.originalSize,
} as AnalyticsMetadata_I_VERIFIED_THIS_IS_NOT_CODE_OR_FILEPATHS)

const formatDescription = getFormatDescription(type, schema)
const formatDescription =
unwrappedText !== undefined
? getFormatDescription('toolResult')
: getFormatDescription(type, schema)
return getLargeOutputInstructions(
persistResult.filepath,
persistResult.originalSize,
formatDescription,
)
}

/**
* Returns true when the large-string output format is enabled (matches
* binary's `sf8()`). When enabled, processMCPResult unwraps a single bare
* text block to raw text for persistence instead of JSON-wrapping it.
*
* Gating sources, in order:
* 1. MCP_TRUNCATION_PROMPT_OVERRIDE env var (anything except "legacy" enables)
* 2. Statsig gate `tengu_mcp_subagent_prompt`
*/
function isLargeStringFormatEnabled(): boolean {
const override = process.env.MCP_TRUNCATION_PROMPT_OVERRIDE
if (override) return override !== 'legacy'
return checkStatsigFeatureGate_CACHED_MAY_BE_STALE(
'tengu_mcp_subagent_prompt',
)
}

/**
* Unwraps a single bare text content block to its raw text when the
* large-string format gate is on. Returns undefined when the gate is off,
* the content is not an array, the array doesn't contain exactly one text
* block, or the block carries annotations or _meta. Matches binary mg5's
* `M=...` computation.
*/
function unwrapSingleTextBlock(content: MCPToolResult): string | undefined {
if (!isLargeStringFormatEnabled()) return undefined
if (!Array.isArray(content) || content.length !== 1) return undefined
const block = content[0]
if (!block || block.type !== 'text') return undefined
if ('annotations' in block || '_meta' in block) return undefined
return block.text
}

/**
* Call an MCP tool, handling UrlElicitationRequiredError (-32042) by
* displaying the URL elicitation to the user, waiting for the completion
Expand All @@ -2827,6 +2899,8 @@ export async function callMCPToolWithUrlElicitationRetry({
signal,
setAppState,
onProgress,
imageLimits,
hasResultSizeAnnotation = false,
callToolFn = callMCPTool,
handleElicitation,
}: {
Expand All @@ -2838,6 +2912,8 @@ export async function callMCPToolWithUrlElicitationRetry({
signal: AbortSignal
setAppState: (f: (prev: AppState) => AppState) => void
onProgress?: (data: MCPProgress) => void
imageLimits?: ImageLimits
hasResultSizeAnnotation?: boolean
/** Injectable for testing. Defaults to callMCPTool. */
callToolFn?: (opts: {
client: ConnectedMCPServer
Expand All @@ -2846,6 +2922,8 @@ export async function callMCPToolWithUrlElicitationRetry({
meta?: Record<string, unknown>
signal: AbortSignal
onProgress?: (data: MCPProgress) => void
imageLimits?: ImageLimits
hasResultSizeAnnotation?: boolean
}) => Promise<MCPToolCallResult>
/** Handler for URL elicitations when no hook handles them.
* In print/SDK mode, delegates to structuredIO. In REPL, falls back to queue. */
Expand All @@ -2865,6 +2943,8 @@ export async function callMCPToolWithUrlElicitationRetry({
meta,
signal,
onProgress,
imageLimits,
hasResultSizeAnnotation,
})
} catch (error) {
// The MCP SDK's Protocol creates plain McpError (not UrlElicitationRequiredError)
Expand Down Expand Up @@ -3041,13 +3121,17 @@ async function callMCPTool({
meta,
signal,
onProgress,
imageLimits,
hasResultSizeAnnotation = false,
}: {
client: ConnectedMCPServer
tool: string
args: Record<string, unknown>
meta?: Record<string, unknown>
signal: AbortSignal
onProgress?: (data: MCPProgress) => void
imageLimits?: ImageLimits
hasResultSizeAnnotation?: boolean
}): Promise<{
content: MCPToolResult
_meta?: Record<string, unknown>
Expand Down Expand Up @@ -3176,7 +3260,13 @@ async function callMCPTool({
})
}

const content = await processMCPResult(result, tool, name)
const content = await processMCPResult(
result,
tool,
name,
imageLimits,
hasResultSizeAnnotation,
)
return {
content,
_meta: result._meta as Record<string, unknown> | undefined,
Expand Down
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