fix(contracts): Claude-only setups title threads with Haiku 5.5, not legacy Haiku 4.5

[josephv123]

Problem

When Codex isn't available, T3 falls back to Claude for text generation, and the fallback still uses claude-haiku-4-5. The catalog has marked that model legacy since #16903, which moved the bare haiku alias to Haiku 5.5 but missed this default. Through Claude Code, Haiku 4.5 takes about 10 seconds to produce a thread title (p90 about 20 seconds) and gives somewhat worse titles. Fixes #18748.

Change

DEFAULT_TEXT_GENERATION_MODEL_BY_PROVIDER now maps Claude to claude-haiku-5-5. Effort is left unset, so T3 sends the model's catalog default, --effort medium. Saved text generation selections don't change. This affects only installs that use the default.

The existing Codex-disabled fallback test now also asserts the model.

Scope and approval

This is one changed default, with the triage issue #18748. I think it's a stale value left behind when #16903 retired Haiku 4.5, not a new behavior choice. It does change a product default, though, so I'm happy to move it to a discussion if a maintainer prefers.

Verification

Title quality and speed. I ran T3's real title prompt (buildThreadTitlePrompt, imported from this branch) through claude -p with the same flags ClaudeTextGeneration uses: --output-format json --json-schema … --settings {"disableAllHooks":true} --tools "" --disable-slash-commands --strict-mcp-config --permission-mode dontAsk, from an empty temp directory with the prompt on stdin. I confirmed with getProviderOptionDescriptors that T3 sends no thinking setting for Haiku 4.5 and --effort medium for Haiku 5.5, and benchmarked exactly those. The test used 10 realistic first messages, 3 runs each, on Claude Code 2.1.296.

	Haiku 4.5	Haiku 5.5 (medium, default)	Haiku 5.5 (low)
Median time per title	10.1 s	3.9 s	3.5 s
p90 time per title	20.6 s	4.9 s	4.6 s
Median cost per title (Claude Code list price)	$0.0185	$0.0006	$0.0007
Failures	0/30	0/30	0/30

Before (Haiku 4.5) → after (Haiku 5.5), for the clearest differences:

User message	Before	After
"…when an agent finishes a turn in an old thread it jumps to the top but when i just open it it also jumps?? i only want it to move when there's new activity i haven't seen…"	Fix sidebar sorting on thread open (backwards: opening a thread is what should stop moving it)	Sidebar sort only on unseen activity (same meaning in all 3 runs)
Pasted TypeError … at resolveAttachmentPreview, "every time i open a thread with a lot of images"	Fix image attachment preview error (names internals)	Fix crash opening threads with many images
Long report about duplicated messages when several people share a thread over Tailscale	Diagnose T3 message duplication (project name and a process verb, both of which the prompt rules out)	Duplicate Messages in Shared Threads (3/3)
"babysit …/pull/18332 … it's the one that makes the context meter show the configured auto-compact threshold for claude"	Context meter auto-compact threshold	Claude auto-compact threshold in meter

The models tie on simpler messages. Haiku 5.5 sometimes uses Title Case, and both models sometimes include "T3 Code" in a title.

All 10 examples × 3 runs

User message (abridged)	Haiku 4.5 (3 runs)	Haiku 5.5 (3 runs)
ok so the sidebar sorting is driving me crazy. when an agent finishes a turn in an old thread it jumps to the …	Fix sidebar sorting on thread open Sidebar jump only on new activity Sidebar sort by unseen activity only	Sidebar sort: move only on unseen activity Sidebar sort only on unseen activity Sidebar sort on unseen activity only
on iphone the composer is kind of broken: 1) when the keyboard opens the send button gets hidden behind it 2) …	Fix iPhone composer layout bugs Fix iPhone composer layout Fix iPhone composer layout issues	Fix iPhone composer layout issues Fix iPhone composer layout issues iPhone composer layout fixes
Make me an HTML mock (just a single file, inline css) of what a redesigned provider settings page could look l…	Redesign provider settings table Provider settings table redesign Provider settings table redesign	Provider Settings Status Table Mockups Provider Settings Status Table Compact Provider Settings Status Table
how does checkpointing actually work here? like if i revert to an earlier turn does it touch my uncommitted ch…	Checkpointing with manual edits Checkpointing behavior with manual edits Checkpoint revert mechanics	Checkpoint Revert and Manual Edits Checkpoint Revert and Manual Edits Checkpoint Revert vs Uncommitted Edits
getting this every time i open a thread with a lot of images ⏎ ⏎ TypeError: Cannot read properties of undefin…	Fix image attachment preview error Fix image attachment rendering Attachment preview crash with many images	Fix crash opening threads with many images Fix crash in threads with many images Crash opening threads with many images
babysit #18332 — poll CI every few minutes, fix anything real the bots…	Context meter auto-compact threshold Auto-compact threshold in context meter Show context meter auto-compact threshold	Claude auto-compact threshold in meter Context Meter Auto-Compact Threshold Claude auto-compact threshold in meter
help me write an email to my 15-213 professor asking for a 2 day extension on malloc lab. i was sick last week…	Malloc lab extension request Draft malloc extension request email Request malloc lab extension	15-213 Malloc Lab Extension Email 15-213 Malloc Lab Extension Email Malloc Lab Extension Request Email
what llm model/harness doe t3code use to title threads	T3Code thread titling model and harness T3 Code's thread titling system T3 Code thread title model	T3 Code thread title generation model T3 Code Thread Titling Model Model Behind Thread Title Generation
1. pull latest main on both machines 2. rebuild the desktop app on the mac 3. check if the bug where the termi…	Terminal scrollback on thread switch Terminal drawer scrollback bug Terminal drawer loses scrollback	Terminal Drawer Scrollback Loss Terminal Drawer Scrollback Loss Bug Terminal drawer scrollback loss bug
Context: we run T3 on a shared dev box and 4 of us connect over Tailscale. Since the last nightly, when two pe…	Diagnose T3 message duplication Diagnose T3 duplicate messages Shared thread message duplication	Duplicate Messages in Shared Threads Duplicate Messages in Shared Threads Duplicate Messages in Shared Threads

The 4.5 latency comes from thinking and caching. Claude Code thinks by default on Haiku 4.5 (about 500 thinking tokens per title), and each call writes a fresh 6.6k-token prompt cache (cache_read_input_tokens: 0). Haiku 5.5 reads Claude Code's warm cache.

Older Claude Code. The catalog lists 2.1.293 as Haiku 5.5's minVersion, and text generation sends the model ID without a version check. I ran the same title call (--model claude-haiku-5-5 --effort medium) on Claude Code 2.1.111 (the oldest graceful version), 2.1.280 (the oldest supported version), and 2.1.292. All three passed the ID through and returned a valid title.

Tests.

vp test run apps/server/src/serverSettings.test.ts packages/shared/src/serverSettings.test.ts apps/web/src/modelSelection.test.ts
 Test Files  3 passed (3)
      Tests  143 passed (143)

With the old default restored, the new assertion fails as expected: AssertionError: expected 'claude-haiku-4-5' to equal 'claude-haiku-5-5'. vp run --filter @t3tools/contracts typecheck reports no errors.

Not checked: branch name, commit message, and PR text generation. They use the same model selection, but I benchmarked only titles.

Done by Claude Opus 5.5 in Claude Code, running in T3 Code.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This changes the Claude text-generation fallback used by existing installations, replacing Haiku 4.5 with Haiku 5.5 for the product default. The change is small and tested, but its production default change warrants human review.

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[coderabbitai]

📝 Walkthrough

📝 Walkthrough

Walkthrough

Claude’s default text-generation model changes from Haiku 4.5 to Haiku 5.5. The server fallback test now expects Haiku 5.5 when the default Codex instance is disabled and selection falls back to Claude.

Changes

Claude text-generation default

Layer / File(s)	Summary
Update the default and fallback expectation packages/contracts/src/model.ts, apps/server/src/serverSettings.test.ts	The Claude default text-generation model changes to claude-haiku-5-5. The fallback test asserts this model when selection falls back to Claude.

Priority: ⬇️ Low

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

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: juliusmarminge

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Merge Risk: 🟡 Moderate · up to 4a3b4

Claude-only installations using an older supported Claude Code version may lose default text generation. Confirm compatibility or provide a version-aware fallback before merging.

Security Architecture Review

Security architecture risk: ⚪ Minimal · up to 4a3b4

The updated default retains existing access restrictions. No material security exposure introduced or worsened by this change was identified.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The effective change is bounded to consumers using the Claude text-generation default, including fallback selection. The traced execution retains the selected provider instance and its existing credential authority; it does not route requests to another identity or service.

Trust Boundaries and Controls

• observed — The title-generation path retains a temporary working directory, disabled hooks, an empty tool set, strict MCP configuration, and dontAsk permission mode. Investigation tools depend on a separate operation flag that this PR does not change. These controls constrain the model's available actions; they are not a claim of complete credential isolation.
• observed — Server selection still checks provider enablement and rejects ACP Registry instances for application text generation before retaining the current selection. The changed default does not bypass those checks.

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Pre-merge checks · 4

Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check		Issue #18748 requires Claude-only fallback selection to use Haiku 5.5 when no text-generation model is set. The PR changes DEFAULT_TEXT_GENERATION_MODEL_BY_PROVIDER from claude-haiku-4-5 to `claud…
Out of Scope Changes check		The diff contains one default-model change in packages/contracts/src/model.ts and one assertion in the related fallback test. Both changes directly support issue #18748. The diff contains no unrelat…
Description check		The description covers the problem, change, scope and issue reference, verification results, limitations, and the agent used. It provides sufficient technical detail for review.
Title check		The title clearly and concisely identifies the main change: Claude-only thread titles now use Haiku 5.5 instead of legacy Haiku 4.5.

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
Review comments at @packages/contracts/src/model.ts:
- Line 195: Gate the default in the model configuration identified by
CLAUDE_DRIVER_KIND so Claude Code versions below 2.1.293 use claude-haiku-4-5,
while versions 2.1.293 and newer use claude-haiku-5-5. Reuse the existing
version-scoped model selection mechanism.

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📥 Commits

Reviewing files that changed from the base of the PR and between 9e55228 and 4a3b43e.

📒 Files selected for processing (2)

• apps/server/src/serverSettings.test.ts
• packages/contracts/src/model.ts

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[juliusmarminge]

Note

Grok responding on behalf of Julius.

Superseded by #18781, which landed the same change: Claude's default text generation model is now claude-haiku-5-5 on main. Closing this one. Thanks for the benchmarking and the issue!