[Bug] Agent model composer-2.5[fast=false] set in Desktop but ACP session bills/runs as fast=true

[NocturnusCoder]

Summary

Buzz Desktop lets me set Fizz to composer-2.5[fast=false]. Harness env shows BUZZ_ACP_MODEL=composer-2.5[fast=false], but cursor-agent ACP actually runs composer-2.5[fast=true]. Cursor usage dashboard shows all agent turns as Composer 2.5 Fast.

Environment

• Buzz Desktop (managed agents: Fizz, Bumble, Honey)
• buzz-acp harness → cursor-agent acp
• cursor-agent: 2026.07.23-e383d2b
• OS: Linux

Steps to reproduce

1. In Buzz Desktop, set agent Fizz model to Composer 2.5 with fast=false
2. @mention Fizz in a channel
3. On the machine running buzz-acp, run:

   buzz-acp models | grep -E "Current:|composer-2.5"

4. Check Cursor usage dashboard at cursor.com

Expected

• ACP Current model: composer-2.5[fast=false]
• Usage billed as full Composer 2.5

Actual

• Harness env: BUZZ_ACP_MODEL=composer-2.5[fast=false] ✓
• ACP Current: composer-2.5[fast=true]
• Available catalog only lists composer-2.5 (id: composer-2.5[fast=true]) — no fast=false variant
• Cursor usage: all recent entries show Fast mode

Screenshot

Gist: https://gist.github.com/NocturnusCoder/05290e272d734628939705d90887f83a

Related issues

• [Bug] Claude Code agents ignore the model set in Buzz Desktop — model picker persists but nothing applies it at spawn #2692 — same silent-fallback pattern (Desktop model persists, session runs wrong model)
• buzz-acp model switch never matches claude-agent-acp config options: expects configId, adapter sends id #2794 / fix(buzz-acp): accept id-keyed config options when resolving model switch #2795 — ACP catalog id vs configId mismatch (Claude; fixed but not Composer fast=false)
• Model dropdown stays empty for ACP harnesses that report models via session/new → models.availableModels (e.g. Hermes Agent) #3934 — ACP model list / discovery mismatches between Desktop and harness
• [Bug] Windows: Codex model discovery times out / empty models (cold start >10s) #2261 — cold-start model discovery race → silent fallback

Notes

Possible fix: map Desktop selection to a model ID ACP actually supports, or pass --model at cursor-agent startup (see Cursor forum threads on ACP model switching).

[NocturnusCoder]

Added a public screenshot copy — the Buzz relay media URL requires community auth and is not viewable by maintainers.

Gist: https://gist.github.com/NocturnusCoder/05290e272d734628939705d90887f83a

Update: Gist PNG was re-uploaded as proper binary (first upload was corrupted via API). Issue body restored after accidental wipe during edit.

[NocturnusCoder]

Related: user reports Bumble (configured for Grok) also bills as Composer 2.5 Fast — likely systemic ACP model-switch failure, not Composer-specific.

[NocturnusCoder]

Internet research — same issue across ACP clients (not Buzz-specific)

Cross-client reports with identical symptom (picker/env says X, usage bills composer-2.5-fast):

• PhpStorm/JetBrains: https://forum.cursor.com/t/model-selection-in-phpstorm-via-acp-is-not-working/162121 (Cursor staff: "known issue… defaults to composer-2.5-fast")
• Zed Composer Fast lock: https://forum.cursor.com/t/composer-2-5-classic-via-custom-acp/161417
• ACP variant/thinking unavailable: https://forum.cursor.com/t/cant-select-thinking-level-or-variant-in-cursor-acp/161317 (Cursor: without parameterizedModelPicker capability, only default variant per model; Composer default = Fast; no ETA for third-party clients)
• session/set_model metadata-only: https://forum.cursor.com/t/bug-agent-acp-model-switching-updates-session-metadata-but-does-not-change-the-inference-backend/157312
• Repro gist: https://gist.github.com/juliusmarminge/65f5a313afcedaffe015716c7b7db13b
• Zed root-cause + fix (advertise clientCapabilities._meta.parameterizedModelPicker: true): Cursor ACP in Zed: Composer 2.5 Fast Mode Issue zed-industries/zed#57571
• Non-fast launch still bills fast: https://forum.cursor.com/t/document-and-stabilize-non-fast-model-selection-for-acp-stdio-sessions/165622

Suggested Buzz fixes: (1) buzz-acp declare parameterizedModelPicker in ACP initialize, (2) pass --model at cursor-agent spawn per agent, (3) fail loud on model mismatch instead of silent Fast fallback.

[iroiro147]

Confirmed: this is an upstream cursor-agent / ACP issue, not a Buzz bug.

From the comments already on this issue:

"PhpStorm/JetBrains: https://forum.cursor.com/t/model-selection-in-phpstorm-via-acp-is-not-working/162121 (Cursor staff: 'known issue… defaults to composer-2.5-fast')"

Buzz correctly propagates BUZZ_ACP_MODEL=composer-2.5[fast=false] through crates/buzz-acp/src/commands/launch.rs; the harness env is correct (per issue repro). cursor-agent ACP then ignores the suffix and defaults to composer-2.5[fast=true] on its own bug tracker. Identical ACP-client reports exist for PhpStorm/JetBrains and Zed with the same fallback.

The Buzz-side workaround would be to post-process the model list returned from cursor-agent's session/new into Desktop's model picker (so users can't pick a variant that isn't actually supported at startup), but that masks the upstream bug and the right fix belongs in cursor-agent. Suggest closing as upstream-blocked and tracking Cursor's own fix; the related systemic work for client-side catalog-vs-run mismatches is #3934.