feat: add span export hooks

.changeset/pi-tool-current-span.md

@@ -2,4 +2,4 @@
 "braintrust": patch
 ---
 
-Keep Pi Coding Agent tool spans current during execution so nested spans attach to the tool span.
+fix(pi-coding-agent): keep tool spans current during execution so nested spans attach to the tool span

.changeset/span-export-hooks.md

@@ -0,0 +1,9 @@
+---
+"braintrust": minor
+---
+
+feat: add span export hooks
+
+Support synchronous `onSpanExport` customizers for incremental instrumentation
+span records. Customizers can add, modify, delete, or replace fields before export,
+with callbacks applied once per record rather than once per transport retry.

AGENTS.md

@@ -79,6 +79,8 @@ pnpm run test       # Run all workspace tests via turbo
 
 Run from the repo root. **Always run `fix:formatting` before committing** — there is a pre-commit hook that will reject unformatted code.
 
+Agents MUST run Prettier on every file they create or edit before handing work back, even when no commit is requested. Include Markdown, changelogs, config files, and generated files supported by Prettier—not just source code. From the repo root, run `pnpm exec prettier --write <edited-files>` followed by `pnpm exec prettier --check <edited-files>`. If further edits are made, repeat formatting and verification after the final edit. Do not rely on tests, typechecks, CI, or the pre-commit hook to catch formatting issues.
+
 ```bash
 pnpm run formatting      # Check formatting (prettier)
 pnpm run lint            # Run eslint checks

js/CHANGELOG.md

@@ -1,5 +1,11 @@
 # braintrust
 
+## Unreleased
+
+### Minor Changes
+
+- feat: Run configured `onSpanExport` customizers on incremental instrumentation span records, supporting field mutation, deletion, and replacement before export.
+
 ## 3.33.0
 
 ### Minor Changes

js/src/exports.ts

@@ -374,6 +374,10 @@ export {
   braintrustFlueObserver,
   braintrustFlueInstrumentation,
 } from "./instrumentation";
-export type { InstrumentationConfig } from "./instrumentation";
+export type {
+  InstrumentationConfig,
+  SpanCustomizer,
+  SpanExportData,
+} from "./instrumentation";
 
 export { wrapElevenLabs } from "./wrappers/elevenlabs";

js/src/instrumentation/README.md

@@ -180,6 +180,49 @@ termination, and async context.
 - Use narrow vendored provider interfaces shared by wrappers and plugins.
 - Keep enable, disable, subscription, and patching behavior idempotent.
 
+## Export Customizers
+
+Configure `spanCustomizers` through the standalone instrumentation entrypoint
+before importing the main SDK, which enables instrumentation during platform
+initialization. Use a bootstrap module before any auto-instrumentation preload
+that initializes the SDK. Static imports of the main SDK are hoisted; use a
+dynamic import after configuration:
+
+```ts
+import { configureInstrumentation } from "braintrust/instrumentation";
+
+configureInstrumentation({
+  spanCustomizers: [
+    {
+      onSpanExport(data) {
+        data.tags = ["reviewed"];
+        if ("output" in data) data.output = "[redacted]";
+        delete data.error;
+        return data;
+      },
+    },
+  ],
+});
+
+const { initLogger } = await import("braintrust");
+initLogger({ projectName: "my-project" });
+// Import and use instrumented provider SDKs here.
+```
+
+`onSpanExport` receives each incremental record from an instrumentation-created
+span after lazy values resolve, before attachment processing, merging, masking,
+and JSON serialization. It can run before the span ends; fields may be absent.
+Ordinary manually created spans, dataset rows, and feedback are not customized.
+
+Callbacks run synchronously in registration order. Mutate and return the record,
+or return a replacement for the next callback. Preserve identity and routing
+fields and return JSON-serializable data. Exceptions are swallowed; remaining
+customizers and export continue. Export retries reuse the transformed record
+without invoking callbacks again. Configuration is shared across SDK bundles.
+
+Customizers receive only the outgoing record, not a live span or provider
+instrumentation context.
+
 ## Testing
 
 Test at the narrowest useful layers:

js/src/instrumentation/config.ts

@@ -1,3 +1,17 @@
+export type SpanExportData = Record<string, unknown>;
+
+export interface SpanCustomizer {
+  /**
+   * Customize an outgoing span record after lazy values resolve, before JSON
+   * serialization. Records are incremental and may not contain every span field.
+   *
+   * Add, change, or delete fields, then return the record or a replacement.
+   * Preserve identity and routing fields, including id, span_id, root_span_id,
+   * and span_parents.
+   */
+  onSpanExport?(data: SpanExportData): SpanExportData;
+}
+
 export interface InstrumentationIntegrationsConfig {
   openai?: boolean;
   anthropic?: boolean;
@@ -46,6 +60,12 @@ export interface InstrumentationConfig {
    * Set to false to disable instrumentation for that SDK.
    */
   integrations?: InstrumentationIntegrationsConfig;
+
+  /**
+   * Instrumentation-wide customizers, in callback execution order.
+   * Configure before instrumentation is enabled.
+   */
+  spanCustomizers?: readonly SpanCustomizer[];
 }
 
 const envIntegrationAliases: Record<

js/src/instrumentation/index.ts

@@ -45,3 +45,4 @@ export {
 // Configuration API
 export { configureInstrumentation } from "./registry";
 export type { InstrumentationConfig } from "./registry";
+export type { SpanCustomizer, SpanExportData } from "./config";

js/src/instrumentation/registry.ts

@@ -13,6 +13,7 @@ import {
   type InstrumentationConfig,
 } from "./config";
 import { GLOBAL_INSTRUMENTATION_HOOKS_PROTOCOL_VERSION } from "../global-instrumentation-hooks";
+import { setSpanCustomizers } from "../span-customizer";
 
 export type { InstrumentationConfig } from "./config";
 
@@ -62,6 +63,9 @@ class PluginRegistry {
       return;
     }
     this.config = { ...this.config, ...config };
+    if ("spanCustomizers" in config) {
+      setSpanCustomizers(config.spanCustomizers);
+    }
   }
 
   /**

js/src/logger.ts

@@ -207,6 +207,7 @@ import {
   mergeSpanOriginContext,
   type SpanOriginEnvironment,
 } from "./span-origin";
+import { customizeSpanExport } from "./span-customizer";
 
 // Manual type definition for inline attachments (not in generated_types)
 const InlineAttachmentReferenceSchema = z.object({
@@ -8215,6 +8216,7 @@ export class SpanImpl implements Span {
 
   private isMerge: boolean;
   private loggedEndTime: number | undefined;
+  private readonly isInstrumented: boolean;
   private propagatedEvent: StartSpanEventArgs | undefined;
 
   // For internal use only.
@@ -8255,6 +8257,8 @@ export class SpanImpl implements Span {
     const instrumentationName =
       getSpanInstrumentationName(args) ??
       INSTRUMENTATION_NAMES.BRAINTRUST_JS_LOGGER;
+    this.isInstrumented =
+      instrumentationName !== INSTRUMENTATION_NAMES.BRAINTRUST_JS_LOGGER;
 
     const spanAttributes = args.spanAttributes ?? {};
     const rawEvent = args.event ?? {};
@@ -8422,21 +8426,28 @@ export class SpanImpl implements Span {
       );
     }
 
-    const computeRecord = async () => ({
-      ...partialRecord,
-      ...Object.fromEntries(
-        await Promise.all(
-          Object.entries(lazyInternalData).map(async ([key, value]) => [
-            key,
-            await value.get(),
-          ]),
+    const computeRecord = async () => {
+      const record = {
+        ...partialRecord,
+        ...Object.fromEntries(
+          await Promise.all(
+            Object.entries(lazyInternalData).map(async ([key, value]) => [
+              key,
+              await value.get(),
+            ]),
+          ),
         ),
-      ),
-      ...new SpanComponentsV3({
-        object_type: this.parentObjectType,
-        object_id: await this.parentObjectId.get(),
-      }).objectIdFields(),
-    });
+        ...new SpanComponentsV3({
+          object_type: this.parentObjectType,
+          object_id: await this.parentObjectId.get(),
+        }).objectIdFields(),
+      };
+      // Customize inside the memoized lazy value, before attachment processing,
+      // merging, and masking. Retries reuse the already-customized record.
+      return this.isInstrumented
+        ? (customizeSpanExport(record) as BackgroundLogEvent)
+        : record;
+    };
     this._state.bgLogger().log([new LazyValue(computeRecord)]);
   }