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@W-21474595 - add MCP tool-level response token usage (responseCharCount) #400

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@W-21474595 - add MCP tool-level response token usage (responseCharCount) #400

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@W-21474595 - add MCP tool-level response token usage (responseCharCo…
smuduganti-sf Mar 4, 2026
c75c061
@W-21474595 - include structuredContent in MCP tool-level response to…
smuduganti-sf Mar 11, 2026
83e34f2
Merge branch 'main' into W-20995739-add-responseCharCount
smuduganti-sf Mar 11, 2026
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36 changes: 36 additions & 0 deletions packages/mcp/src/sf-mcp-server.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -88,6 +88,38 @@ export class SfMcpServer extends McpServer implements ToolMethodSignatures {
};
}

/**
* Calculates the total character count from tool result content and structured output.
* Used for token usage. Accounts for both:
* - content: text (and other) content items
* - structuredContent: structured tool output when the tool defines an outputSchema
* @see https://modelcontextprotocol.io/specification/2025-11-25/server/tools#output-schema
* @param result - The CallToolResult from tool execution
* @returns Total character count across text content and structured content
*/
private calculateResponseCharCount(result: CallToolResult): number {
let total = 0;

// Plain text (and other) content items
if (result.content && Array.isArray(result.content)) {
total += result.content
.filter((item): item is { type: 'text'; text: string } => item.type === 'text')
.reduce((sum, item) => sum + item.text.length, 0);
}

// Structured content (JSON object per outputSchema)
const structured = (result as CallToolResult & { structuredContent?: unknown }).structuredContent;
if (structured !== undefined && structured !== null && typeof structured === 'object') {
try {
total += JSON.stringify(structured).length;
} catch {
// ignore serialization errors
}
}

return total;
}

public registerTool<InputArgs extends ZodRawShape, OutputArgs extends ZodRawShape>(
name: string,
config: {
Expand Down Expand Up @@ -141,6 +173,9 @@ export class SfMcpServer extends McpServer implements ToolMethodSignatures {
this.logger.debug(`Tool ${name} completed in ${runtimeMs}ms`);
if (result.isError) this.logger.debug(`Tool ${name} errored`);

// Calculate response character count for token usage
const responseCharCount = this.calculateResponseCharCount(result);
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@cristiand391 cristiand391 Mar 10, 2026

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This method only considers plain text from tool responses but tools can also have structured output, see:

Image

https://modelcontextprotocol.io/specification/2025-11-25/server/tools#output-schema

mcp/packages/mcp-provider-code-analyzer/src/tools/run_code_analyzer.ts

Line 111 in b7efb41

structuredContent: output

I think the safe way would be to account for both since a tool seems to be able to return plain text + structured output


this.telemetry?.sendEvent('TOOL_CALLED', {
name,
runtimeMs,
Expand All @@ -151,6 +186,7 @@ export class SfMcpServer extends McpServer implements ToolMethodSignatures {
//
// https://modelcontextprotocol.io/specification/2025-06-18/schema#calltoolresult
isError: result.isError ?? false,
responseCharCount: responseCharCount.toString(),
});

this.telemetry?.sendPdpEvent({
Expand Down
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