Vertex/Bedrock: empty ThinkingBlock.thinking on Opus 4.8/4.7 — bundled CLI drops thinking.display (post-#831, root cause anthropics/claude-code#71599)

[maxim092001]

Summary

SDK users running on Vertex (or Bedrock) with claude-opus-4-8 / 4-7 get ThinkingBlock.thinking == "" (empty) and no thinking_delta events, even with thinking={"type":"adaptive","display":"summarized"}.

This is not the same as the now-fixed #831 (where the SDK didn't forward display). The SDK does now correctly pass --thinking-display (_internal/transport/subprocess_cli.py resolves thinking["display"] → --thinking-display). The remaining failure is downstream in the bundled CLI, which strips thinking.display from the request body on non-first-party providers.

Full CLI root-cause analysis, request-body captures, and a live Vertex reproduction are in the CLI issue: anthropics/claude-code#71599. Filing here for SDK-user visibility and to track an SDK-side mitigation, since claude-agent-sdk bundles the affected CLI.

Why it still affects the SDK after #831

#831 made the SDK forward the flag. But the bundled CLI (2.1.177 → 2.1.193, latest) gates thinking.display behind a first-party-only provider allow-list (firstParty | anthropicAws | foundry) that omits vertex/bedrock. So with CLAUDE_CODE_USE_VERTEX=1, the CLI builds thinking: {"type":"adaptive"} (no display), the server applies the Opus 4.8 default display:"omitted", and the SDK parses back a ThinkingBlock with empty thinking + a signature. Direct Anthropic (first-party) is unaffected.

Version boundary: bundled CLI 2.1.150 works; 2.1.177+ (and 2.1.193, latest) broken on Vertex/Bedrock.

Reproduction (SDK)

import os
from claude_agent_sdk import ClaudeAgentOptions, query, AssistantMessage, ThinkingBlock

# Vertex env: CLAUDE_CODE_USE_VERTEX=1, CLOUD_ML_REGION=global, ANTHROPIC_VERTEX_PROJECT_ID=<your-project>
options = ClaudeAgentOptions(
    model="claude-opus-4-8[1m]",
    thinking={"type": "adaptive", "display": "summarized"},  # forwarded correctly since #831
)

async for msg in query(prompt="Reason step by step, then answer: a bat and ball cost $1.10; the bat is $1.00 more than the ball. How much is the ball?", options=options):
    if isinstance(msg, AssistantMessage):
        for b in msg.content:
            if isinstance(b, ThinkingBlock):
                print("thinking len:", len(b.thinking))  # 0 on Vertex; non-empty on direct Anthropic

Observed on Vertex: thinking len: 0 (signature present). On the direct Anthropic API: non-empty.

Verified workaround (no downgrade)

Set the CLI extra-body env var on the subprocess; it re-injects display on the wire (verified against live Vertex — empty → 224-char summarized thinking):

os.environ["CLAUDE_CODE_EXTRA_BODY"] = '{"thinking":{"type":"adaptive","display":"summarized"}}'

Must be type:"adaptive" — Opus 4.8 rejects type:"enabled" with HTTP 400.

Requests for the SDK

1. Document this Vertex/Bedrock limitation (and the CLAUDE_CODE_EXTRA_BODY workaround) near the thinking docs, linking Vertex/Bedrock: thinking.display:"summarized" stripped from request body → empty thinking on Opus 4.8/4.7 (regression 2.1.150→2.1.177, still in 2.1.193) claude-code#71599.
2. Optional mitigation: when thinking["display"] is set and a 3P provider is detected (CLAUDE_CODE_USE_VERTEX / CLAUDE_CODE_USE_BEDROCK in env), have the transport set/merge CLAUDE_CODE_EXTRA_BODY with the thinking override so display survives the bundled CLI — until the CLI fix lands.
3. Bump the bundled CLI to whatever version fixes Vertex/Bedrock: thinking.display:"summarized" stripped from request body → empty thinking on Opus 4.8/4.7 (regression 2.1.150→2.1.177, still in 2.1.193) claude-code#71599.

Related

• Opus 4.7 returns empty thinking text: ThinkingConfig.display not forwarded to CLI #831 (closed) — SDK now forwards display; this issue is the remaining CLI-side drop on Vertex/Bedrock.
• tmp-preserve-thinking-2025-10-01 beta header causes failures with AWS Bedrock and Vertex AI #335 (closed) — earlier Vertex/Bedrock thinking-beta issue.
• Vertex/Bedrock: thinking.display:"summarized" stripped from request body → empty thinking on Opus 4.8/4.7 (regression 2.1.150→2.1.177, still in 2.1.193) claude-code#71599 — upstream CLI root cause (request-body thinking.display stripped on Vertex/Bedrock).

[maxim092001]

Hey @ashwin-ant!

Faced this bug recently - TL;DR vertex does not return thinking blocks. However, calls directly with anthropic api key with same configuration do work. Verified both with raw CLI and with SDK, bug is presumably in CLI, so I cannot open PR to SDK (I do not feel that proposed workaround is viable for SDK)