feat(otel): make attribute truncation configurable

Adds `github.copilot.chat.otel.maxAttributeSizeChars` setting and
`COPILOT_OTEL_MAX_ATTRIBUTE_SIZE_CHARS` env var to control truncation
of free-form OTel content attributes (prompts, responses, tool
arguments/results, hook input/output).

Default is `0` (unlimited), matching the OTel spec's
`AttributeValueLengthLimit` default of `Infinity`. Users on backends
with per-attribute size limits can set a positive value to keep OTLP
batches under the backend cap.

Plumbs the resolved limit through every call site that previously hit a
hardcoded 64KB fallback. Drops the `DEFAULT_MAX_OTEL_ATTRIBUTE_LENGTH`
constant; `truncateForOTel`'s default arg is now `0` (unlimited).

Refs #299952

extensions/copilot/assets/prompts/skills/troubleshoot/SKILL.md

@@ -66,7 +66,7 @@ Key attrs: `args` (JSON string of tool input), `result` (tool output or error te
 ```jsonl
 {"ts":1773200231010,"dur":3001,"sid":"62f52dec","type":"llm_request","name":"chat:gpt-4o","spanId":"000000000000000c","parentSpanId":"0000000000000003","status":"ok","attrs":{"model":"gpt-4o","inputTokens":15025,"outputTokens":126,"ttft":1987,"maxTokens":32000,"systemPromptFile":"system_prompt_0.json","userRequest":"echo hello","inputMessages":"[{...}]"}}
 ```
-Key attrs: `model`, `inputTokens`, `outputTokens`, `ttft` (time to first token in ms), `maxTokens`, `temperature`, `topP`, `systemPromptFile` (references a system prompt file in the session directory), `toolsFile` (references a tools file in the session directory), `userRequest` (the full user message content, untruncated), `inputMessages` (full messages array as JSON, pre-truncated at 64KB), `error` (when failed).
+Key attrs: `model`, `inputTokens`, `outputTokens`, `ttft` (time to first token in ms), `maxTokens`, `temperature`, `topP`, `systemPromptFile` (references a system prompt file in the session directory), `toolsFile` (references a tools file in the session directory), `userRequest` (the full user message content, untruncated), `inputMessages` (full messages array as JSON, truncated to the configured `maxAttributeSizeChars` — unlimited by default), `error` (when failed).
 
 #### agent_response — model output (text + tool calls)
 ```jsonl

extensions/copilot/docs/monitoring/agent_monitoring.md

@@ -70,6 +70,7 @@ Open **Settings** (`Ctrl+,`) and search for `copilot otel`:
 | `github.copilot.chat.otel.exporterType` | string | `"otlp-http"` | `otlp-http`, `otlp-grpc`, `console`, or `file` |
 | `github.copilot.chat.otel.otlpEndpoint` | string | `"http://localhost:4318"` | OTLP collector endpoint |
 | `github.copilot.chat.otel.captureContent` | boolean | `false` | Capture full prompt/response content |
+| `github.copilot.chat.otel.maxAttributeSizeChars` | integer | `0` | Max characters per OTel content attribute (prompts, tool args/results, hook input/output). `0` (the default) disables truncation so backends with no per-attribute limit get full payloads. Set to a positive value to match your backend's per-attribute size limit — consult your backend's documentation. The value counts JavaScript string characters (UTF-16 code units); for non-ASCII content one character can be multiple UTF-8 bytes on the wire. |
 | `github.copilot.chat.otel.outfile` | string | `""` | File path for JSON-lines output |
 | `github.copilot.chat.otel.dbSpanExporter.enabled` | boolean | `false` | Persist OTel spans to a local SQLite database for the **Chat: Export Agent Traces DB** command. Implicitly enables OTel. |
 
@@ -87,6 +88,7 @@ Environment variables **always take precedence** over VS Code settings.
 | `OTEL_SERVICE_NAME` | `copilot-chat` | Service name in resource attributes |
 | `OTEL_RESOURCE_ATTRIBUTES` | — | Extra resource attributes (`key1=val1,key2=val2`) |
 | `COPILOT_OTEL_CAPTURE_CONTENT` | `false` | Capture full prompt/response content |
+| `COPILOT_OTEL_MAX_ATTRIBUTE_SIZE_CHARS` | `0` | Override the max character size for OTel content attributes. `0` (default) disables truncation; set to a positive value when your backend has a per-attribute limit. Takes precedence over the `maxAttributeSizeChars` setting. |
 | `COPILOT_OTEL_LOG_LEVEL` | `info` | Min log level: `trace`, `debug`, `info`, `warn`, `error` |
 | `COPILOT_OTEL_FILE_EXPORTER_PATH` | — | Write all signals to this file (JSON-lines) |
 | `COPILOT_OTEL_HTTP_INSTRUMENTATION` | `false` | Enable HTTP-level OTel instrumentation |

extensions/copilot/docs/monitoring/agent_monitoring_arch.md

@@ -321,6 +321,19 @@ if (this._otelService.config.captureContent) {
 }
 ```
 
+### Attribute Truncation
+
+All free-form content attributes (prompts, tool arguments/results, hook input/output, reasoning text) **must** be passed through `truncateForOTel(value, otel.config.maxAttributeSizeChars)` before being set on a span. The default `maxAttributeSizeChars` is `0` (no truncation), matching the [OTel spec default of `Infinity`](https://opentelemetry.io/docs/specs/otel/common/#attribute-limits) for `AttributeValueLengthLimit`. Users whose OTel backend caps per-attribute size should set `github.copilot.chat.otel.maxAttributeSizeChars` (or the `COPILOT_OTEL_MAX_ATTRIBUTE_SIZE_CHARS` env var) to a positive value so OTLP batches stay under the backend cap — consult the backend's documentation for the appropriate value.
+
+> **Chars vs. bytes.** The [OTel spec](https://opentelemetry.io/docs/specs/otel/common/#attribute-limits) defines string attribute limits as "counting any character in it as 1". `truncateForOTel` approximates this using `value.length`, which counts UTF-16 code units rather than Unicode code points — so astral-plane characters (e.g. some emoji) count as 2 against the limit. Backends typically apply a separate UTF-8 byte limit downstream, so the on-wire size for non-ASCII content can be larger than the configured character count.
+
+```typescript
+const maxLen = this._otelService.config.maxAttributeSizeChars; // 0 = unlimited
+span.setAttribute(GenAiAttr.TOOL_CALL_ARGUMENTS, truncateForOTel(JSON.stringify(args), maxLen));
+```
+
+The two-arg form is preferred at every call site that has access to `IOTelService`. The default-arg form (`truncateForOTel(value)`) is unlimited (no truncation) and should only be used from helpers that do not receive an `IOTelService` (e.g. tests, fixtures).
+
 ---
 
 ## Adding Instrumentation

extensions/copilot/package.json

@@ -4909,6 +4909,15 @@
 							"advanced"
 						]
 					},
+					"github.copilot.chat.otel.maxAttributeSizeChars": {
+						"type": "integer",
+						"default": 0,
+						"minimum": 0,
+						"markdownDescription": "Maximum size **in characters** for free-form OTel content attributes (prompts, responses, tool arguments/results, hook input/output). `0` (the default) disables truncation so backends without per-attribute size limits receive full JSON payloads. Set to a positive value when your OTel backend caps attribute size — consult your backend's documentation for its per-attribute limit. Truncated values are suffixed with `...[truncated, original N chars]`. Env var `COPILOT_OTEL_MAX_ATTRIBUTE_SIZE_CHARS` takes precedence. Requires window reload.",
+						"tags": [
+							"advanced"
+						]
+					},
 					"github.copilot.chat.otel.outfile": {
 						"type": "string",
 						"default": "",

extensions/copilot/src/extension/byok/vscode-node/anthropicProvider.ts

@@ -342,7 +342,7 @@ export class AnthropicLMProvider extends AbstractLanguageModelChatProvider {
 						if (responseText) { parts.push({ type: 'text', content: responseText }); }
 						parts.push(...toolCalls);
 						if (parts.length > 0) {
-							otelSpan.setAttribute(GenAiAttr.OUTPUT_MESSAGES, truncateForOTel(JSON.stringify([{ role: 'assistant', parts }])));
+							otelSpan.setAttribute(GenAiAttr.OUTPUT_MESSAGES, truncateForOTel(JSON.stringify([{ role: 'assistant', parts }]), this._otelService.config.maxAttributeSizeChars));
 						}
 					}
 				}
@@ -485,7 +485,7 @@ export class AnthropicLMProvider extends AbstractLanguageModelChatProvider {
 				// per OTel GenAI semantic conventions (issue #300318).
 				const toolDefs = toToolDefinitions(options.tools);
 				if (toolDefs) {
-					otelSpan.setAttribute(GenAiAttr.TOOL_DEFINITIONS, truncateForOTel(JSON.stringify(toolDefs)));
+					otelSpan.setAttribute(GenAiAttr.TOOL_DEFINITIONS, truncateForOTel(JSON.stringify(toolDefs), this._otelService.config.maxAttributeSizeChars));
 				}
 				try {
 					const roleNames: Record<number, string> = { 1: 'user', 2: 'assistant', 3: 'system' };
@@ -510,7 +510,7 @@ export class AnthropicLMProvider extends AbstractLanguageModelChatProvider {
 						}
 						return { role, parts };
 					});
-					otelSpan.setAttribute(GenAiAttr.INPUT_MESSAGES, truncateForOTel(JSON.stringify(inputMsgs)));
+					otelSpan.setAttribute(GenAiAttr.INPUT_MESSAGES, truncateForOTel(JSON.stringify(inputMsgs), this._otelService.config.maxAttributeSizeChars));
 				} catch { /* swallow */ }
 			}
 			try {

extensions/copilot/src/extension/byok/vscode-node/geminiNativeProvider.ts

@@ -217,7 +217,7 @@ export class GeminiNativeBYOKLMProvider extends AbstractLanguageModelChatProvide
 						if (responseText) { parts.push({ type: 'text', content: responseText }); }
 						parts.push(...toolCalls);
 						if (parts.length > 0) {
-							otelSpan.setAttribute(GenAiAttr.OUTPUT_MESSAGES, truncateForOTel(JSON.stringify([{ role: 'assistant', parts }])));
+							otelSpan.setAttribute(GenAiAttr.OUTPUT_MESSAGES, truncateForOTel(JSON.stringify([{ role: 'assistant', parts }]), this._otelService.config.maxAttributeSizeChars));
 						}
 					}
 				}
@@ -350,7 +350,7 @@ export class GeminiNativeBYOKLMProvider extends AbstractLanguageModelChatProvide
 				// per OTel GenAI semantic conventions (issue #300318).
 				const toolDefs = toToolDefinitions(options.tools);
 				if (toolDefs) {
-					otelSpan.setAttribute(GenAiAttr.TOOL_DEFINITIONS, truncateForOTel(JSON.stringify(toolDefs)));
+					otelSpan.setAttribute(GenAiAttr.TOOL_DEFINITIONS, truncateForOTel(JSON.stringify(toolDefs), this._otelService.config.maxAttributeSizeChars));
 				}
 				try {
 					const roleNames: Record<number, string> = { 1: 'user', 2: 'assistant', 3: 'system' };
@@ -372,7 +372,7 @@ export class GeminiNativeBYOKLMProvider extends AbstractLanguageModelChatProvide
 						}
 						return { role, parts };
 					});
-					otelSpan.setAttribute(GenAiAttr.INPUT_MESSAGES, truncateForOTel(JSON.stringify(inputMsgs)));
+					otelSpan.setAttribute(GenAiAttr.INPUT_MESSAGES, truncateForOTel(JSON.stringify(inputMsgs), this._otelService.config.maxAttributeSizeChars));
 				} catch { /* swallow */ }
 			}
 			try {

extensions/copilot/src/extension/chat/vscode-node/chatHookService.ts

@@ -166,7 +166,7 @@ export class ChatHookService implements IChatHookService {
 					try {
 						// Capture hook input for debug panel resolve
 						try {
-							span.setAttribute(CopilotChatAttr.HOOK_INPUT, truncateForOTel(JSON.stringify(commandInput)));
+							span.setAttribute(CopilotChatAttr.HOOK_INPUT, truncateForOTel(JSON.stringify(commandInput), this._otelService.config.maxAttributeSizeChars));
 						} catch { /* swallow serialization errors */ }
 
 						const sw = StopWatch.create();
@@ -195,7 +195,7 @@ export class ChatHookService implements IChatHookService {
 							try {
 								const output = typeof commandResult.result === 'string' ? commandResult.result : JSON.stringify(commandResult.result);
 								if (output) {
-									span.setAttribute(CopilotChatAttr.HOOK_OUTPUT, truncateForOTel(output));
+									span.setAttribute(CopilotChatAttr.HOOK_OUTPUT, truncateForOTel(output, this._otelService.config.maxAttributeSizeChars));
 								}
 							} catch { /* swallow serialization errors */ }
 						}

extensions/copilot/src/extension/chatSessions/claude/common/claudeMessageDispatch.ts

@@ -243,6 +243,7 @@ function logToolResult(
 	requestLogger: IRequestLogger,
 	otelToolSpans: Map<string, ISpanHandle>,
 	capturingToken: CapturingToken | undefined,
+	maxAttributeSizeChars: number,
 ): void {
 	// OTel span
 	const toolSpan = otelToolSpans.get(toolUseId);
@@ -250,13 +251,13 @@ function logToolResult(
 		if (toolResult.is_error) {
 			const errContent = typeof toolResult.content === 'string' ? toolResult.content : 'tool error';
 			toolSpan.setStatus(SpanStatusCode.ERROR, errContent);
-			toolSpan.setAttribute(GenAiAttr.TOOL_CALL_RESULT, truncateForOTel(`ERROR: ${errContent}`));
+			toolSpan.setAttribute(GenAiAttr.TOOL_CALL_RESULT, truncateForOTel(`ERROR: ${errContent}`, maxAttributeSizeChars));
 		} else {
 			toolSpan.setStatus(SpanStatusCode.OK);
 			if (toolResult.content !== undefined) {
 				try {
 					const result = typeof toolResult.content === 'string' ? toolResult.content : JSON.stringify(toolResult.content);
-					toolSpan.setAttribute(GenAiAttr.TOOL_CALL_RESULT, truncateForOTel(result));
+					toolSpan.setAttribute(GenAiAttr.TOOL_CALL_RESULT, truncateForOTel(result, maxAttributeSizeChars));
 				} catch (e) {
 					logService.warn(`[ClaudeMessageDispatch] Failed to serialize tool result: ${e}`);
 				}
@@ -293,6 +294,7 @@ function processToolResult(
 	const logService = accessor.get(ILogService);
 	const requestLogger = accessor.get(IRequestLogger);
 	const claudeSessionStateService = accessor.get(IClaudeSessionStateService);
+	const otelService = accessor.get(IOTelService);
 
 	const { stream } = request;
 	const { unprocessedToolCalls, otelToolSpans } = state;
@@ -313,7 +315,8 @@ function processToolResult(
 		logService,
 		requestLogger,
 		otelToolSpans,
-		claudeSessionStateService.getCapturingTokenForSession(sessionId)
+		claudeSessionStateService.getCapturingTokenForSession(sessionId),
+		otelService.config.maxAttributeSizeChars,
 	);
 
 	// Tool-specific handling
@@ -511,6 +514,7 @@ export function handleHookResponse(
 	state: MessageHandlerState,
 ): void {
 	const logService = accessor.get(ILogService);
+	const otelService = accessor.get(IOTelService);
 	// TODO: can we map these types better
 	const hookType = message.hook_event as ChatHookType;
 
@@ -528,7 +532,7 @@ export function handleHookResponse(
 			span.setAttribute('copilot_chat.hook_exit_code', message.exit_code);
 		}
 		if (message.output) {
-			span.setAttribute('copilot_chat.hook_output', truncateForOTel(message.output));
+			span.setAttribute('copilot_chat.hook_output', truncateForOTel(message.output, otelService.config.maxAttributeSizeChars));
 		}
 		span.end();
 		state.otelHookSpans.delete(message.hook_id);

extensions/copilot/src/extension/chatSessions/claude/common/test/claudeMessageDispatch.spec.ts

@@ -60,7 +60,7 @@ interface TestServices {
 function createTestServices(): TestServices {
 	return {
 		logService: new TestLogService(),
-		otelService: { startSpan: () => noopSpan } as Pick<IOTelService, 'startSpan'> as IOTelService,
+		otelService: { startSpan: () => noopSpan, config: { maxAttributeSizeChars: 0 } } as unknown as IOTelService,
 		toolsService: { invokeTool: vi.fn() } as Pick<IToolsService, 'invokeTool'> as IToolsService,
 		requestLogger: { logToolCall: vi.fn(), captureInvocation: vi.fn() },
 		sessionStateService: { setPermissionModeForSession: vi.fn(), getCapturingTokenForSession: vi.fn().mockReturnValue(undefined) },

extensions/copilot/src/extension/chatSessions/claude/node/claudeOTelTracker.ts

@@ -86,7 +86,7 @@ export class ClaudeOTelTracker {
 			},
 			parentTraceContext: this._currentTraceContext,
 		});
-		const userContent = truncateForOTel(promptLabel);
+		const userContent = truncateForOTel(promptLabel, this._otelService.config.maxAttributeSizeChars);
 		userMsgSpan.setAttribute(CopilotChatAttr.USER_REQUEST, userContent);
 		userMsgSpan.addEvent('user_message', { content: userContent, [CopilotChatAttr.CHAT_SESSION_ID]: this._sessionId });
 		userMsgSpan.end();

extensions/copilot/src/extension/chatSessions/copilotcli/node/copilotcliSession.ts

@@ -966,13 +966,13 @@ export class CopilotCLISession extends DisposableStore implements ICopilotCLISes
 					[CopilotChatAttr.SESSION_ID]: this.sessionId,
 					[CopilotChatAttr.CHAT_SESSION_ID]: this.sessionId,
 					...(modelId ? { [GenAiAttr.REQUEST_MODEL]: modelId } : {}),
-					[CopilotChatAttr.USER_REQUEST]: truncateForOTel(promptLabel),
+					[CopilotChatAttr.USER_REQUEST]: truncateForOTel(promptLabel, this._otelService.config.maxAttributeSizeChars),
 					...workspaceMetadataToOTelAttributes(resolveWorkspaceOTelMetadata(this._gitService)),
 				},
 			},
 			async span => {
 				// Emit user_message event so chronicle can extract turns and summary
-				span.addEvent('user_message', { content: truncateForOTel(promptLabel) });
+				span.addEvent('user_message', { content: truncateForOTel(promptLabel, this._otelService.config.maxAttributeSizeChars) });
 
 				// Register the trace context so the bridge processor can inject CHAT_SESSION_ID
 				const traceCtx = span.getSpanContext();
@@ -1439,7 +1439,7 @@ export class CopilotCLISession extends DisposableStore implements ICopilotCLISes
 				this.logService.trace(`[CopilotCLISession] Hook ${event.data.hookType} started (${event.data.hookInvocationId})`);
 				let input: string | undefined;
 				try {
-					input = truncateForOTel(JSON.stringify(event.data.input));
+					input = truncateForOTel(JSON.stringify(event.data.input), this._otelService.config.maxAttributeSizeChars);
 				} catch { /* swallow serialization errors */ }
 				this._bridgeProcessor?.stashHookInput(event.data.hookInvocationId, event.data.hookType, input);
 			})));
@@ -1449,7 +1449,7 @@ export class CopilotCLISession extends DisposableStore implements ICopilotCLISes
 				let output: string | undefined;
 				if (event.data.success) {
 					try {
-						output = truncateForOTel(JSON.stringify(event.data.output));
+						output = truncateForOTel(JSON.stringify(event.data.output), this._otelService.config.maxAttributeSizeChars);
 					} catch { /* swallow serialization errors */ }
 				}
 				this._bridgeProcessor?.stashHookEnd(

extensions/copilot/src/extension/extension/vscode-node/services.ts

@@ -292,6 +292,7 @@ export function registerServices(builder: IInstantiationServiceBuilder, extensio
 		settingExporterType: otelSettings.get<'otlp-grpc' | 'otlp-http' | 'console' | 'file'>('exporterType'),
 		settingOtlpEndpoint: otelSettings.get<string>('otlpEndpoint'),
 		settingCaptureContent: otelSettings.get<boolean>('captureContent'),
+		settingMaxAttributeSizeChars: otelSettings.get<number>('maxAttributeSizeChars'),
 		settingOutfile: otelSettings.get<string>('outfile') || undefined,
 		settingDbSpanExporter: otelSettings.get<boolean>('dbSpanExporter.enabled'),
 		extensionVersion: extensionContext.extension.packageJSON.version ?? '0.0.0',

extensions/copilot/src/extension/intents/node/toolCallingLoop.ts

@@ -831,16 +831,17 @@ export abstract class ToolCallingLoop<TOptions extends IToolCallingLoopOptions =
 				// Always capture user input message for the debug panel
 				{
 					const userMessage = this.turn.request.message;
+					const maxLen = this._otelService.config.maxAttributeSizeChars;
 					span.setAttribute(GenAiAttr.INPUT_MESSAGES, truncateForOTel(JSON.stringify([
 						{ role: 'user', parts: [{ type: 'text', content: userMessage }] }
-					])));
+					]), maxLen));
 					// Set USER_REQUEST so event translator can emit user.message
 					if (userMessage) {
-						span.setAttribute(CopilotChatAttr.USER_REQUEST, truncateForOTel(userMessage));
+						span.setAttribute(CopilotChatAttr.USER_REQUEST, truncateForOTel(userMessage, maxLen));
 					}
 					// Emit user_message span event for real-time debug panel streaming
 					if (userMessage) {
-						span.addEvent('user_message', { content: userMessage, ...(chatSessionId ? { [CopilotChatAttr.CHAT_SESSION_ID]: chatSessionId } : {}) });
+						span.addEvent('user_message', { content: truncateForOTel(userMessage, maxLen), ...(chatSessionId ? { [CopilotChatAttr.CHAT_SESSION_ID]: chatSessionId } : {}) });
 					}
 				}
 

extensions/copilot/src/extension/prompt/node/chatMLFetcher.ts

@@ -264,7 +264,7 @@ export class ChatMLFetcherImpl extends AbstractChatMLFetcher {
 						const userContent = typeof lastUserMsg.content === 'string'
 							? lastUserMsg.content
 							: JSON.stringify(lastUserMsg.content);
-						otelInferenceSpan.setAttribute(CopilotChatAttr.USER_REQUEST, truncateForOTel(userContent));
+						otelInferenceSpan.setAttribute(CopilotChatAttr.USER_REQUEST, truncateForOTel(userContent, this._otelService.config.maxAttributeSizeChars));
 					}
 					// System instructions — check messages array, top-level system (Anthropic), or instructions (Responses API)
 					const systemMsg = capiMessages?.find(m => m.role === 'system');
@@ -297,14 +297,14 @@ export class ChatMLFetcherImpl extends AbstractChatMLFetcher {
 					const capiMessages = (requestBody.messages ?? requestBody.input) as ReadonlyArray<Record<string, unknown>> | undefined;
 					if (capiMessages) {
 						// Normalize provider-specific content (Anthropic tool_use/tool_result, OpenAI tool messages) to OTel schema
-						otelInferenceSpan.setAttribute(GenAiAttr.INPUT_MESSAGES, truncateForOTel(JSON.stringify(normalizeProviderMessages(capiMessages))));
+						otelInferenceSpan.setAttribute(GenAiAttr.INPUT_MESSAGES, truncateForOTel(JSON.stringify(normalizeProviderMessages(capiMessages)), this._otelService.config.maxAttributeSizeChars));
 					}
 					// Tool definitions: emit on every chat span so trace viewers can render the
 					// tool catalog per LLM call (issue #299934). Includes `parameters` per
 					// OTel GenAI semantic conventions (issue #300318).
 					const toolDefs = toToolDefinitions(requestBody.tools);
 					if (toolDefs) {
-						otelInferenceSpan.setAttribute(GenAiAttr.TOOL_DEFINITIONS, truncateForOTel(JSON.stringify(toolDefs)));
+						otelInferenceSpan.setAttribute(GenAiAttr.TOOL_DEFINITIONS, truncateForOTel(JSON.stringify(toolDefs), this._otelService.config.maxAttributeSizeChars));
 					}
 					// Cache-relevant request options. Anything in this blob, when changed
 					// between two requests, will invalidate the prompt cache even when
@@ -313,7 +313,7 @@ export class ChatMLFetcherImpl extends AbstractChatMLFetcher {
 					// signature diff.
 					const requestOptions = pickCacheRelevantRequestOptions(requestBody);
 					if (requestOptions) {
-						otelInferenceSpan.setAttribute(CopilotChatAttr.REQUEST_OPTIONS, truncateForOTel(JSON.stringify(requestOptions)));
+						otelInferenceSpan.setAttribute(CopilotChatAttr.REQUEST_OPTIONS, truncateForOTel(JSON.stringify(requestOptions), this._otelService.config.maxAttributeSizeChars));
 					}
 				}
 				tokenCount = await countTokens();
@@ -434,7 +434,7 @@ export class ChatMLFetcherImpl extends AbstractChatMLFetcher {
 						}
 						parts.push(...toolCalls);
 						if (parts.length > 0) {
-							otelInferenceSpan.setAttribute(GenAiAttr.OUTPUT_MESSAGES, truncateForOTel(JSON.stringify([{ role: 'assistant', parts }])));
+							otelInferenceSpan.setAttribute(GenAiAttr.OUTPUT_MESSAGES, truncateForOTel(JSON.stringify([{ role: 'assistant', parts }]), this._otelService.config.maxAttributeSizeChars));
 						}
 						// Capture reasoning/thinking text if present
 						const hasThinking = streamRecorder.deltas.some(d => d.thinking);
@@ -447,7 +447,7 @@ export class ChatMLFetcherImpl extends AbstractChatMLFetcher {
 									return Array.isArray(t.text) ? t.text.join('') : (t.text ?? '');
 								});
 							const reasoningText = thinkingTexts.join('');
-							otelInferenceSpan.setAttribute(CopilotChatAttr.REASONING_CONTENT, truncateForOTel(reasoningText || '[encrypted]'));
+							otelInferenceSpan.setAttribute(CopilotChatAttr.REASONING_CONTENT, truncateForOTel(reasoningText || '[encrypted]', this._otelService.config.maxAttributeSizeChars));
 						}
 					}
 

extensions/copilot/src/extension/tools/vscode-node/toolsService.ts

@@ -143,7 +143,7 @@ export class ToolsService extends BaseToolsService {
 		// Always capture tool call arguments for the debug panel
 		if (options.input !== undefined) {
 			try {
-				span.setAttribute(GenAiAttr.TOOL_CALL_ARGUMENTS, truncateForOTel(JSON.stringify(options.input)));
+				span.setAttribute(GenAiAttr.TOOL_CALL_ARGUMENTS, truncateForOTel(JSON.stringify(options.input), this._otelService.config.maxAttributeSizeChars));
 			} catch { /* swallow serialization errors */ }
 		}
 
@@ -210,7 +210,7 @@ export class ToolsService extends BaseToolsService {
 						}
 					}
 					if (parts.length > 0) {
-						span.setAttribute(GenAiAttr.TOOL_CALL_RESULT, truncateForOTel(parts.join('')));
+						span.setAttribute(GenAiAttr.TOOL_CALL_RESULT, truncateForOTel(parts.join(''), this._otelService.config.maxAttributeSizeChars));
 					}
 				} catch { /* swallow */ }
 				span.end();
@@ -223,7 +223,7 @@ export class ToolsService extends BaseToolsService {
 			err => {
 				span.setStatus(SpanStatusCode.ERROR, err instanceof Error ? err.message : String(err));
 				span.setAttribute(StdAttr.ERROR_TYPE, err instanceof Error ? err.constructor.name : 'Error');
-				span.setAttribute(GenAiAttr.TOOL_CALL_RESULT, truncateForOTel(`ERROR: ${err instanceof Error ? err.message : String(err)}`));
+				span.setAttribute(GenAiAttr.TOOL_CALL_RESULT, truncateForOTel(`ERROR: ${err instanceof Error ? err.message : String(err)}`, this._otelService.config.maxAttributeSizeChars));
 				span.recordException(err);
 				span.end();
 				const durationMs = Date.now() - startTime;

extensions/copilot/src/platform/configuration/common/configurationService.ts

@@ -716,6 +716,7 @@ export namespace ConfigKey {
 		export const OTelExporterType = defineSetting<string>('chat.otel.exporterType', ConfigType.Simple, 'otlp-http');
 		export const OTelOtlpEndpoint = defineSetting<string>('chat.otel.otlpEndpoint', ConfigType.Simple, 'http://localhost:4318');
 		export const OTelCaptureContent = defineSetting<boolean>('chat.otel.captureContent', ConfigType.Simple, false);
+		export const OTelMaxAttributeSizeChars = defineSetting<number>('chat.otel.maxAttributeSizeChars', ConfigType.Simple, 0);
 		export const OTelOutfile = defineSetting<string>('chat.otel.outfile', ConfigType.Simple, '');
 		export const OTelDbSpanExporter = defineSetting<boolean>('chat.otel.dbSpanExporter.enabled', ConfigType.Simple, false);
 

extensions/copilot/src/platform/otel/common/genAiEvents.ts

@@ -51,14 +51,15 @@ export function emitInferenceDetailsEvent(
 		attributes[StdAttr.ERROR_TYPE] = error.type;
 	}
 
-	// Full content capture with truncation to prevent OTLP batch failures
-	// Normalize to OTel GenAI semantic convention format
+	// Full content capture (optionally truncated per OTelConfig.maxAttributeSizeChars).
+	// Normalize to OTel GenAI semantic convention format.
 	if (otel.config.captureContent) {
+		const maxLen = otel.config.maxAttributeSizeChars;
 		if (request.messages !== undefined) {
 			const msgs = Array.isArray(request.messages) ? request.messages as ReadonlyArray<Record<string, unknown>> : undefined;
 			attributes[GenAiAttr.INPUT_MESSAGES] = truncateForOTel(JSON.stringify(
 				msgs ? normalizeProviderMessages(msgs) : request.messages
-			));
+			), maxLen);
 		}
 		if (request.systemMessage !== undefined) {
 			const systemText = typeof request.systemMessage === 'string'
@@ -66,11 +67,11 @@ export function emitInferenceDetailsEvent(
 				: JSON.stringify(request.systemMessage);
 			const systemInstructions = toSystemInstructions(systemText);
 			if (systemInstructions !== undefined) {
-				attributes[GenAiAttr.SYSTEM_INSTRUCTIONS] = truncateForOTel(JSON.stringify(systemInstructions));
+				attributes[GenAiAttr.SYSTEM_INSTRUCTIONS] = truncateForOTel(JSON.stringify(systemInstructions), maxLen);
 			}
 		}
 		if (request.tools !== undefined) {
-			attributes[GenAiAttr.TOOL_DEFINITIONS] = truncateForOTel(JSON.stringify(request.tools));
+			attributes[GenAiAttr.TOOL_DEFINITIONS] = truncateForOTel(JSON.stringify(request.tools), maxLen);
 		}
 	}
 

extensions/copilot/src/platform/otel/common/messageFormatters.ts

@@ -9,22 +9,28 @@
  * @see https://github.com/open-telemetry/semantic-conventions/blob/main/docs/gen-ai/gen-ai-output-messages.json
  */
 
-/**
- * Maximum size (in characters) for a single OTel span/log attribute value.
- * Aligned with common backend limits (Jaeger 64KB, Tempo 100KB).
- * Matches gemini-cli's approach of capping content to prevent OTLP batch failures.
- */
-const MAX_OTEL_ATTRIBUTE_LENGTH = 64_000;
-
 /**
  * Truncate a string to fit within OTel attribute size limits.
  * Returns the original string if within bounds, otherwise truncates with a suffix.
+ *
+ * @param value The string to truncate.
+ * @param maxLength The maximum length in characters. A value of `0` (the
+ * default) or any non-positive number disables truncation entirely, matching
+ * the OTel spec's `AttributeValueLengthLimit` default of `Infinity` for string
+ * attributes (see https://opentelemetry.io/docs/specs/otel/common/#attribute-limits).
+ * Production call sites should pass `OTelConfig.maxAttributeSizeChars` so
+ * users can configure truncation to match their backend's per-attribute limit.
  */
-export function truncateForOTel(value: string, maxLength: number = MAX_OTEL_ATTRIBUTE_LENGTH): string {
-	if (value.length <= maxLength) {
+export function truncateForOTel(value: string, maxLength: number = 0): string {
+	if (maxLength <= 0 || value.length <= maxLength) {
 		return value;
 	}
 	const suffix = `...[truncated, original ${value.length} chars]`;
+	// If maxLength is too small to fit the suffix, fall back to a hard cut so
+	// the result is always <= maxLength.
+	if (maxLength <= suffix.length) {
+		return value.substring(0, maxLength);
+	}
 	return value.substring(0, maxLength - suffix.length) + suffix;
 }
 

extensions/copilot/src/platform/otel/common/otelConfig.ts

@@ -20,6 +20,15 @@ export interface OTelConfig {
 	readonly otlpEndpoint: string;
 	readonly otlpProtocol: 'grpc' | 'http';
 	readonly captureContent: boolean;
+	/**
+	 * Maximum size (in characters) for free-form content attributes (prompts,
+	 * tool args, etc.). A value of `0` disables truncation entirely (the
+	 * default), matching the OTel spec's `AttributeValueLengthLimit` default of
+	 * `Infinity`. Set to a positive value when targeting backends that cap
+	 * per-attribute size to keep OTLP batches under the backend limit; consult
+	 * the backend's documentation for the appropriate value.
+	 */
+	readonly maxAttributeSizeChars: number;
 	readonly fileExporterPath?: string;
 	readonly dbSpanExporter: boolean;
 	readonly logLevel: 'trace' | 'debug' | 'info' | 'warn' | 'error';
@@ -75,6 +84,7 @@ export interface OTelConfigInput {
 	settingExporterType?: OTelExporterType;
 	settingOtlpEndpoint?: string;
 	settingCaptureContent?: boolean;
+	settingMaxAttributeSizeChars?: number;
 	settingOutfile?: string;
 	settingDbSpanExporter?: boolean;
 	extensionVersion: string;
@@ -157,6 +167,11 @@ export function resolveOTelConfig(input: OTelConfigInput): OTelConfig {
 		?? input.settingCaptureContent
 		?? false;
 
+	// Max attribute size in characters: env > setting > default(0 = unlimited).
+	const maxAttributeSizeChars = parseMaxAttributeSizeChars(env['COPILOT_OTEL_MAX_ATTRIBUTE_SIZE_CHARS'])
+		?? input.settingMaxAttributeSizeChars
+		?? 0;
+
 	// Log level
 	const validLogLevels = new Set<OTelConfig['logLevel']>(['trace', 'debug', 'info', 'warn', 'error']);
 	const rawLogLevel = env['COPILOT_OTEL_LOG_LEVEL'];
@@ -181,6 +196,7 @@ export function resolveOTelConfig(input: OTelConfigInput): OTelConfig {
 		otlpEndpoint,
 		otlpProtocol: protocol,
 		captureContent,
+		maxAttributeSizeChars: maxAttributeSizeChars < 0 ? 0 : maxAttributeSizeChars,
 		fileExporterPath,
 		dbSpanExporter,
 		logLevel,
@@ -201,6 +217,7 @@ function createDisabledConfig(input: OTelConfigInput): OTelConfig {
 		otlpEndpoint: '',
 		otlpProtocol: 'http' as const,
 		captureContent: false,
+		maxAttributeSizeChars: 0,
 		dbSpanExporter: false,
 		logLevel: 'info' as const,
 		httpInstrumentation: false,
@@ -217,3 +234,20 @@ function envBool(val: string | undefined): boolean | undefined {
 	}
 	return val === 'true' || val === '1';
 }
+
+/**
+ * Parse a numeric env var representing the max attribute size in characters.
+ * Accepts non-negative safe integers; fractional, unsafe, or non-numeric input
+ * returns `undefined` so the caller can fall back to the next config layer.
+ * Negative values are clamped to `0` (no truncation) by the caller.
+ */
+function parseMaxAttributeSizeChars(val: string | undefined): number | undefined {
+	if (val === undefined || val === '') {
+		return undefined;
+	}
+	const n = Number(val);
+	if (!Number.isSafeInteger(n)) {
+		return undefined;
+	}
+	return n;
+}

extensions/copilot/src/platform/otel/common/test/agentOTelEnv.spec.ts

@@ -16,6 +16,7 @@ function makeConfig(overrides: Partial<OTelConfig> = {}): OTelConfig {
 		otlpEndpoint: 'http://localhost:4318',
 		otlpProtocol: 'http',
 		captureContent: false,
+		maxAttributeSizeChars: 0,
 		dbSpanExporter: false,
 		logLevel: 'info',
 		httpInstrumentation: false,

extensions/copilot/src/platform/otel/common/test/messageFormatters.spec.ts

@@ -322,15 +322,25 @@ describe('truncateForOTel', () => {
 		expect(result).toContain('...[truncated, original 200 chars]');
 	});
 
-	it('uses default 64000 limit', () => {
-		const s = 'x'.repeat(64_001);
-		const result = truncateForOTel(s);
-		expect(result.length).toBeLessThanOrEqual(64_000);
-		expect(result).toContain('...[truncated');
-	});
-
-	it('does not truncate at exactly 64000', () => {
-		const s = 'x'.repeat(64_000);
+	it('default (no maxLength) is unlimited', () => {
+		const s = 'x'.repeat(200_000);
 		expect(truncateForOTel(s)).toBe(s);
 	});
+
+	it('returns string unchanged when maxLength is 0 (unlimited)', () => {
+		const s = 'a'.repeat(200_000);
+		expect(truncateForOTel(s, 0)).toBe(s);
+	});
+
+	it('returns string unchanged when maxLength is negative (unlimited)', () => {
+		const s = 'a'.repeat(200_000);
+		expect(truncateForOTel(s, -1)).toBe(s);
+	});
+
+	it('falls back to a hard cut when maxLength is too small to fit the suffix', () => {
+		const s = 'a'.repeat(200);
+		const result = truncateForOTel(s, 5);
+		expect(result.length).toBeLessThanOrEqual(5);
+		expect(result).toBe('aaaaa');
+	});
 });

extensions/copilot/src/platform/otel/common/test/otelConfig.spec.ts

@@ -230,4 +230,92 @@ describe('resolveOTelConfig', () => {
 			expect(config.enabledVia).toBe('envVar');
 		});
 	});
+
+	describe('maxAttributeSizeChars', () => {
+		it('defaults to 0 (unlimited) when nothing is set', () => {
+			const config = resolveOTelConfig(makeInput({ settingEnabled: true }));
+			expect(config.maxAttributeSizeChars).toBe(0);
+		});
+
+		it('defaults to 0 (unlimited) even when OTel is disabled', () => {
+			const config = resolveOTelConfig(makeInput());
+			expect(config.maxAttributeSizeChars).toBe(0);
+		});
+
+		it('uses VS Code setting when env var is unset', () => {
+			const config = resolveOTelConfig(makeInput({
+				settingEnabled: true,
+				settingMaxAttributeSizeChars: 64_000,
+			}));
+			expect(config.maxAttributeSizeChars).toBe(64_000);
+		});
+
+		it('treats setting value 0 as unlimited (no truncation)', () => {
+			const config = resolveOTelConfig(makeInput({
+				settingEnabled: true,
+				settingMaxAttributeSizeChars: 0,
+			}));
+			expect(config.maxAttributeSizeChars).toBe(0);
+		});
+
+		it('env var overrides VS Code setting', () => {
+			const config = resolveOTelConfig(makeInput({
+				env: {
+					'COPILOT_OTEL_ENABLED': 'true',
+					'COPILOT_OTEL_MAX_ATTRIBUTE_SIZE_CHARS': '128000',
+				},
+				settingMaxAttributeSizeChars: 32_000,
+			}));
+			expect(config.maxAttributeSizeChars).toBe(128_000);
+		});
+
+		it('env var value 0 disables truncation', () => {
+			const config = resolveOTelConfig(makeInput({
+				env: {
+					'COPILOT_OTEL_ENABLED': 'true',
+					'COPILOT_OTEL_MAX_ATTRIBUTE_SIZE_CHARS': '0',
+				},
+				settingMaxAttributeSizeChars: 32_000,
+			}));
+			expect(config.maxAttributeSizeChars).toBe(0);
+		});
+
+		it('clamps negative values to 0 (unlimited)', () => {
+			const config = resolveOTelConfig(makeInput({
+				settingEnabled: true,
+				settingMaxAttributeSizeChars: -1,
+			}));
+			expect(config.maxAttributeSizeChars).toBe(0);
+		});
+
+		it('falls back to default (0) when env var is non-numeric', () => {
+			const config = resolveOTelConfig(makeInput({
+				env: {
+					'COPILOT_OTEL_ENABLED': 'true',
+					'COPILOT_OTEL_MAX_ATTRIBUTE_SIZE_CHARS': 'not-a-number',
+				},
+			}));
+			expect(config.maxAttributeSizeChars).toBe(0);
+		});
+
+		it('falls back to default (0) when env var is fractional', () => {
+			const config = resolveOTelConfig(makeInput({
+				env: {
+					'COPILOT_OTEL_ENABLED': 'true',
+					'COPILOT_OTEL_MAX_ATTRIBUTE_SIZE_CHARS': '1.5',
+				},
+			}));
+			expect(config.maxAttributeSizeChars).toBe(0);
+		});
+
+		it('falls back to default (0) when env var exceeds safe integer range', () => {
+			const config = resolveOTelConfig(makeInput({
+				env: {
+					'COPILOT_OTEL_ENABLED': 'true',
+					'COPILOT_OTEL_MAX_ATTRIBUTE_SIZE_CHARS': '1e308',
+				},
+			}));
+			expect(config.maxAttributeSizeChars).toBe(0);
+		});
+	});
 });