diff --git a/dev-packages/e2e-tests/test-applications/effect-3-node/tests/spans.test.ts b/dev-packages/e2e-tests/test-applications/effect-3-node/tests/spans.test.ts
index 3b04189aea2f..9f689d35018b 100644
--- a/dev-packages/e2e-tests/test-applications/effect-3-node/tests/spans.test.ts
+++ b/dev-packages/e2e-tests/test-applications/effect-3-node/tests/spans.test.ts
@@ -58,8 +58,10 @@ test('Sends Effect spans with correct parent-child structure', async ({ baseURL
   expect(parent.parent_span_id).toBe(segment.span_id);
   expect(nested.parent_span_id).toBe(parent.span_id);
   for (const child of children) {
-    expect(getSpanOp(child)).toBe('function');
-    expect(child.attributes['sentry.origin']?.value).toBe('auto.function.effect');
+    // These spans come from user code calling `Effect.withSpan`, so the SDK does not claim an op or
+    // an `auto.*` origin for them.
+    expect(getSpanOp(child)).toBeUndefined();
+    expect(child.attributes['sentry.origin']?.value).toBe('manual');
     expect(child.trace_id).toBe(segment.trace_id);
   }
 });
diff --git a/dev-packages/e2e-tests/test-applications/effect-4-node/tests/spans.test.ts b/dev-packages/e2e-tests/test-applications/effect-4-node/tests/spans.test.ts
index 7477d91a04be..2a468928c798 100644
--- a/dev-packages/e2e-tests/test-applications/effect-4-node/tests/spans.test.ts
+++ b/dev-packages/e2e-tests/test-applications/effect-4-node/tests/spans.test.ts
@@ -58,8 +58,10 @@ test('Sends Effect spans with correct parent-child structure', async ({ baseURL
   expect(parent.parent_span_id).toBe(segment.span_id);
   expect(nested.parent_span_id).toBe(parent.span_id);
   for (const child of children) {
-    expect(getSpanOp(child)).toBe('function');
-    expect(child.attributes['sentry.origin']?.value).toBe('auto.function.effect');
+    // These spans come from user code calling `Effect.withSpan`, so the SDK does not claim an op or
+    // an `auto.*` origin for them.
+    expect(getSpanOp(child)).toBeUndefined();
+    expect(child.attributes['sentry.origin']?.value).toBe('manual');
     expect(child.trace_id).toBe(segment.trace_id);
   }
 });
diff --git a/packages/effect/src/tracer.ts b/packages/effect/src/tracer.ts
index 31ac0be35c29..08157a6e3a89 100644
--- a/packages/effect/src/tracer.ts
+++ b/packages/effect/src/tracer.ts
@@ -1,5 +1,5 @@
 import { SENTRY_OP } from '@sentry/conventions/attributes';
-import { FUNCTION, HTTP_CLIENT, HTTP_SERVER } from '@sentry/conventions/op';
+import { HTTP_CLIENT, HTTP_SERVER } from '@sentry/conventions/op';
 import type { Span, StartSpanOptions } from '@sentry/core';
 import { isObjectLike, getActiveSpan, SEMANTIC_ATTRIBUTE_SENTRY_ORIGIN, withActiveSpan } from '@sentry/core';
 import type * as Context from 'effect/Context';
@@ -7,20 +7,22 @@ import * as Exit from 'effect/Exit';
 import * as Option from 'effect/Option';
 import * as EffectTracer from 'effect/Tracer';
 
-function deriveOrigin(name: string): string {
+function deriveOrigin(name: string): string | undefined {
   if (name.startsWith('http.server') || name.startsWith('http.client')) {
     return 'auto.http.effect';
   }
 
-  return 'auto.function.effect';
+  return undefined;
 }
 
 /**
- * Effect span names are chosen by user code, so the name is the only signal available. `@effect/platform`
- * names its HTTP spans `http.server`/`http.client`, which map onto the matching Sentry ops; everything
- * else is arbitrary user work and falls back to `function`.
+ * Effect span names are chosen by whoever calls `Effect.withSpan`, so the name is the only signal
+ * available. `@effect/platform` names its HTTP spans `http.server`/`http.client`, which map onto the
+ * matching Sentry ops. Every other name comes from user code or a third-party library, whose semantics
+ * we cannot infer, so op and origin stay unset and the span keeps the core defaults: no op, and a
+ * `manual` origin.
  */
-function deriveOp(name: string): string {
+function deriveOp(name: string): string | undefined {
   if (name.startsWith('http.server')) {
     return HTTP_SERVER;
   }
@@ -29,7 +31,7 @@ function deriveOp(name: string): string {
     return HTTP_CLIENT;
   }
 
-  return FUNCTION;
+  return undefined;
 }
 
 type HrTime = [number, number];
@@ -190,12 +192,16 @@ function createSentrySpan(
   const parentSentrySpan =
     Option.isSome(parent) && isSentrySpan(parent.value) ? parent.value.sentrySpan : (getActiveSpan() ?? null);
 
+  const op = deriveOp(name);
+  const origin = deriveOrigin(name);
+
   const newSpan = startInactiveSpan({
     name,
     startTime: nanosToHrTime(startTime),
+    // Setting these to `undefined` would strip the core defaults instead of leaving them in place.
     attributes: {
-      [SENTRY_OP]: deriveOp(name),
-      [SEMANTIC_ATTRIBUTE_SENTRY_ORIGIN]: deriveOrigin(name),
+      ...(op && { [SENTRY_OP]: op }),
+      ...(origin && { [SEMANTIC_ATTRIBUTE_SENTRY_ORIGIN]: origin }),
     },
     ...(parentSentrySpan ? { parentSpan: parentSentrySpan } : {}),
   });
diff --git a/packages/effect/test/tracer.test.ts b/packages/effect/test/tracer.test.ts
index a056227576e1..90429b3a8bcd 100644
--- a/packages/effect/test/tracer.test.ts
+++ b/packages/effect/test/tracer.test.ts
@@ -192,12 +192,16 @@ describe.each(VARIANTS)('SentryEffectTracer ($variant)', ({ tracer, spanApi }) =
       return capturedAttributes;
     }).pipe(withSentryTracer);
 
-  it.effect('sets origin and op for regular spans', () =>
+  // A name we cannot map belongs to user code or a third-party library. Leaving op and origin unset
+  // keeps the core defaults (no op, `manual` origin) rather than claiming we instrumented the span.
+  it.effect('leaves origin and op unset for spans it cannot map', () =>
     Effect.gen(function* () {
       const attributes = yield* attributesFor('my-operation');
 
-      expect(attributes?.[SEMANTIC_ATTRIBUTE_SENTRY_ORIGIN]).toBe('auto.function.effect');
-      expect(attributes?.[SEMANTIC_ATTRIBUTE_SENTRY_OP]).toBe('function');
+      expect(attributes?.[SEMANTIC_ATTRIBUTE_SENTRY_ORIGIN]).toBeUndefined();
+      expect(attributes?.[SEMANTIC_ATTRIBUTE_SENTRY_OP]).toBeUndefined();
+      expect(attributes).not.toHaveProperty(SEMANTIC_ATTRIBUTE_SENTRY_ORIGIN);
+      expect(attributes).not.toHaveProperty(SEMANTIC_ATTRIBUTE_SENTRY_OP);
     }),
   );