[BUG] Fable 5 thinking blocks render as empty unclickable stubs in VS Code extension (v2.1.170)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

What's Wrong?

In the Claude Code VS Code extension, thinking blocks for claude-fable-5 render as empty, unclickable stubs. The conversation shows "Thought for Xs" labels above responses, but the blocks cannot be expanded by click and Ctrl+O does nothing. This appears to be the same regression class as #49757 (Opus 4.7), now affecting the newly released Fable 5 model.

What Should Happen?

Expected Behavior

A collapsible "Thought for Xs" block appears above the final answer. Clicking the block (or pressing Ctrl+O) expands it to reveal Claude's reasoning summary, the same way it works on Sonnet 4.6.

Actual Behavior

The "Thought for Xs" label appears but the block is an empty stub:

• Clicking the block does nothing — no expand, no highlight, no cursor change.
• Ctrl+O does not toggle anything.
• Closing and re-opening the panel does not restore expandability.
• Opening a fresh panel reproduces the issue from the first response onward.
• Switching the same panel to Sonnet 4.6 and sending a new prompt produces a clickable, expandable thinking block — confirming the panel/session UI itself is healthy and the issue is tied to the Fable 5 response payload.

Hypothesis

Likely the same root cause as #49757: Fable 5 (as part of the new Mythos-class / Claude 5 model line) defaults thinking.display to "omitted". The Claude Code client does not pass display: "summarized" for claude-fable-5, so the thinking field arrives empty and the UI has nothing to render.

If this is correct, the fix is the same one-line client update: pass display: "summarized" for Fable 5 (and presumably any future Mythos-class models) the same way it presumably should be passing it for Opus 4.7 and Haiku 4.5.

Environment

• Claude Code VS Code extension: anthropic.claude-code v2.1.170 (updated ~3 hours ago)
• VS Code: [your version — Help → About]
• OS: Windows
• Models tested:
• claude-fable-5 — broken (empty unclickable stub)
• claude-sonnet-4-6 — working (clickable, expandable)
• claude-opus-4-7 — broken (matches #49757)
• claude-haiku-4-5 — broken (same symptom)
• Extended Thinking: toggled on via / command menu
• ~/.claude/settings.json: default (no showThinkingSummaries key; confirmed in #49757 that this key is not real)

Related

• #49757 — same symptom on Opus 4.7
• #51131 — extended-thinking dropdown no longer expandable on Opus 4.7
• Fable 5 release announcement: https://www.anthropic.com/claude/fable (2026-06-09)

Workarounds

• Switch the panel to Sonnet 4.6 — loses Fable 5 capability but restores visible reasoning.
• Enable Use Terminal mode in extension settings — CLI renderer reportedly handles thinking blocks for newer models correctly. (Untested for Fable 5 at the time of filing; please verify.)

Error Messages/Logs

Steps to Reproduce

Sonnet 4.6 in the same extension version renders thinking blocks correctly, confirming the issue is model-specific rather than a general UI break.

Steps to Reproduce

1. Update Claude Code VS Code extension to v2.1.170 (latest, released ~2026-06-10).
2. Open a new Claude Code panel.
3. Open the command menu (/) and confirm Extended Thinking is toggled on.
4. Open / → Switch model… → select Fable 5 (or leave on "Default (recommended)", which currently maps to Fable 5 on Pro/Max plans).
5. Send any prompt that triggers reasoning, e.g. "think hard about how to refactor this function."
6. Wait for the response to render.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

sonnet 4.6

Claude Code Version

anthropic.claude-code Version 2.1.170

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_