[arthi-arumugam-git]

Problem

_process_response_item treats message_delta.usage.output_tokens as an increment and adds it to the count already stored from message_start:

item_output_tokens = dict(item.usage).get("output_tokens", 0)
existing_output_tokens = complete_response["usage"].get("output_tokens", 0)
complete_response["usage"]["output_tokens"] = (
    item_output_tokens + existing_output_tokens
)

Anthropic sends message_delta.usage.output_tokens as the running total for the whole message. The value in message_start is a partial count that is already contained in it. Adding them inflates output tokens, and therefore total tokens and any cost computed downstream, on every streamed Anthropic span. Nothing raises, so the wrong number just propagates.

The vendor SDK assigns rather than adds, in anthropic/lib/streaming/_messages.py:

current_snapshot.usage.output_tokens = event.usage.output_tokens

This repo's own Bedrock streaming wrapper also overwrites rather than accumulates (bedrock/streaming_wrapper.py, usage.update(...)).

Reproduction

Replaying the SSE events from this repo's existing cassette, tests/cassettes/test_messages/test_anthropic_message_streaming_legacy.yaml, through both accumulators:

message_start.usage : {'input_tokens': 17, 'output_tokens': 3}
message_delta.usage : {'output_tokens': 171}

openllmetry  output_tokens : 174
anthropic SDK output_tokens: 171   <- ground truth

A 1.75% overcount on this recording. The gap is whatever message_start reported, so it grows with prompt-cache and tool-use shapes where that initial count is larger.

Fix

Overwrite with the delta's value instead of adding, and leave the stored count alone when the delta omits output_tokens.

Test

test_anthropic_message_streaming_legacy already asserted input_tokens == 17 and the input + output == total identity, but never asserted the output value itself, so the bug passed CI. This PR adds that one assertion. It fails with assert 174 == 171 on main and passes with the fix.

Summary by CodeRabbit

• Bug Fixes
  • Corrected output token counts in streamed responses to reflect the provider’s reported totals rather than adding them repeatedly.
  • Preserved the existing output token count when a streaming update does not include a new total.
  • Updated streaming usage checks to match the corrected token counts across standard and prompt-caching scenarios.

[CLAassistant]

All committers have signed the CLA.

[coderabbitai]

Walkthrough

Anthropic streaming usage handling now treats output_tokens as an upstream running total. Tests verify the resulting span value and updated prompt-caching token counts.

Changes: Anthropic streaming usage

Layer / File(s)	Summary
Record output token totals packages/opentelemetry-instrumentation-anthropic/.../streaming.py, packages/opentelemetry-instrumentation-anthropic/tests/test_messages.py, packages/opentelemetry-instrumentation-anthropic/tests/test_prompt_caching.py	message_delta handling overwrites explicitly provided output token totals instead of accumulating them. Streaming tests assert the resulting output-token counts.

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: dinmukhamedm

Merge Risk: Minimal · up to 36a04

No actionable merge-blocking risk remains in the reviewed changes.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title names the main change: streamed Anthropic output-token usage is no longer double-counted. The raw summary confirms that streaming.py now overwrites output_tokens from message_delta instead o…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Title check

Explanation

The title names the main change: streamed Anthropic output-token usage is no longer double-counted. The raw summary confirms that streaming.py now overwrites output_tokens from message_delta instead of adding to the message_start value, and the objectives describe the same fix. The title is specific, concise, and matches the changeset.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 6 functions across 1 files.

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

Finishing Touches

Generate unit tests (beta)

• Create a new PR

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[arthi-arumugam-git]

Gentle ping: CodeRabbit, GitGuardian, and the CLA are all green, and the branch is current as of yesterday. The test workflows are still waiting on the fork workflow approval click; would a maintainer be able to approve the run so CI can report on this change?

[roy-tong]

Adding a piece of authoritative evidence to this thread: the official Anthropic SDK documents message_delta.usage as cumulative — anthropic-sdk-python _messages.py:520-524 implements accumulate as "cumulative totals... overwrite rather than add". So summing output_tokens across message_delta events multiplies the final value by the event count (a recorded stream with deltas 2 then 6 reports 8 instead of 6 — reproducible with the cassette in this repo). The overwrite-style accumulation the SDK ships is the shape the fix here needs. Same class we've confirmed in the wild in a few downstream tools (e.g. one-api-pro#13, +50.7% billed input with no cache).

[chrikrah]

@arthi-arumugam-git the streaming.py change looks right to me, and it can be merged once six stream tests catch up. blocking: in test_prompt_caching.py they still expect the one-token overcount, so each drops by one in both output counts.

# packages/opentelemetry-instrumentation-anthropic, base 62e24c2, main 8d9c93f (only a version bump to this package since).
# CPython 3.10.21, anthropic 0.86.0 from uv.lock, pytest 8.4.2, uv 0.12.19, ANTHROPIC_* unset
$ uv run pytest -q tests                                    # 8d9c93f
270 passed
$ uv run pytest -q tests                                    # 7c33857
6 failed, 264 passed
E       assert 201 == 202    # 3 sync tests, test_prompt_caching.py:39 from :762, :823, :989
E       assert 289 == 290    # 3 async tests, from :1092, :1154, :1331
$ uv run pytest -q tests                                    # 7c33857, streaming.py from 8d9c93f: your new assertion catches the old code
1 failed, 269 passed         # test_messages.py::test_anthropic_message_streaming_legacy, assert 174 == 171
$ uv run pytest -q tests                                    # 7c33857 plus the six lines below
270 passed
$ for pr in 7f99923 620bb1c; do git merge-tree --write-tree 7c33857 $pr >/dev/null && echo clean; done    # #4539, #4644 also edit streaming.py
clean
clean

--- a/packages/opentelemetry-instrumentation-anthropic/tests/test_prompt_caching.py
+++ b/packages/opentelemetry-instrumentation-anthropic/tests/test_prompt_caching.py
@@ -762 +762 @@
-    _verify_caching_attributes(cache_creation_span, cache_read_span, 1169, 202, 222, 1165)
+    _verify_caching_attributes(cache_creation_span, cache_read_span, 1169, 201, 221, 1165)
@@ -823 +823 @@
-    _verify_caching_attributes(cache_creation_span, cache_read_span, 1169, 202, 222, 1165)
+    _verify_caching_attributes(cache_creation_span, cache_read_span, 1169, 201, 221, 1165)
@@ -989 +989 @@
-    _verify_caching_attributes(cache_creation_span, cache_read_span, 1169, 202, 222, 1165)
+    _verify_caching_attributes(cache_creation_span, cache_read_span, 1169, 201, 221, 1165)
@@ -1092 +1092 @@
-    _verify_caching_attributes(cache_creation_span, cache_read_span, 1171, 290, 257, 1167)
+    _verify_caching_attributes(cache_creation_span, cache_read_span, 1171, 289, 256, 1167)
@@ -1154 +1154 @@
-    _verify_caching_attributes(cache_creation_span, cache_read_span, 1171, 290, 257, 1167)
+    _verify_caching_attributes(cache_creation_span, cache_read_span, 1171, 289, 256, 1167)
@@ -1331 +1331 @@
-    _verify_caching_attributes(cache_creation_span, cache_read_span, 1171, 290, 257, 1167)
+    _verify_caching_attributes(cache_creation_span, cache_read_span, 1171, 289, 256, 1167)

No test job has run in CI on this head yet, so these failures show up only locally. Can you push the six lines?

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