release: v0.13.5

[ThomasK33]

Summary

Upgrades the ACP schema to v0.13.5 (make release VERSION=0.13.5) and regenerates the Go bindings.

⚠️ Heads-up: session/set_model is gone — but model selection is not

The 0.13.5 protocol release removed the unstable-only session model API:

• session/set_model method
• SetSessionModelRequest / SetSessionModelResponse
• ModelId, ModelInfo, SessionModelState (availableModels / currentModelId)
• the models fields on session responses

This is not functionality being removed — it has moved. Model selection now
flows through the generic, stable session/set_config_option framework, where
a session config option carries a semantic category of "model" ("Model selector"),
alongside the mode and thought_level categories. The same uniform "session config
options" API now covers model, mode, and reasoning-level selection instead of a bespoke
method per selectable thing.

Concretely, instead of session/set_model, a client now:

1. Reads the agent-advertised configOptions (a SessionConfigSelect with currentValue + options) where category == "model".
2. Calls session/set_config_option { sessionId, configId, value } to switch model.
3. Receives the full updated configOptions back (and may get config_option_update via session/update).

session/set_config_option and the SessionConfigOptionCategory enum already existed
in v0.13.4 and are unchanged by this bump — 0.13.5 simply retired the redundant
experimental path. acp.SessionConfigOptionCategoryModel is present in the regenerated
types_gen.go.

Upstream rationale (maintainer)

From the upstream PR that removed it:

"Session config options have been around long enough, so cleaning up the old iteration."

And the doc line added by that PR:

"The never-stabilized session/set_model API and related session model response fields
have been removed from the protocol artifacts. Agents should continue to expose model
selection through Session Config Options."

A companion PR removed the dedicated model/mode APIs from the draft v2 protocol too —
so there is no separate "v2" model API; v2 also routes through session config options.

• Removal: feat(unstable): Remove unstable session model API agentclientprotocol/agent-client-protocol#1325 ("feat(unstable): Remove unstable session model API")
• v2 cleanup: feat(unstable-v2): Remove dedicated session modes and models apis from v2 agentclientprotocol/agent-client-protocol#1324 ("feat(unstable-v2): Remove dedicated session modes and models apis from v2")
• Release: https://github.com/agentclientprotocol/agent-client-protocol/releases/tag/v0.13.5

Impact

Because the model API was never stabilized (it only ever lived in schema.unstable.json),
no stable surface breaks. The only affected code is anything implementing/calling the
experimental UnstableSetSessionModel* symbols.

Changes

• Regenerated: types_gen.go, agent_gen.go, client_gen.go, constants_gen.go, schema JSON, README.md, version files.
• Hand-edited (non-generated) follow-ups to keep the tree compiling:
  • example/agent/main.go — dropped the now-removed UnstableSetSessionModel implementation.
  • acp_test.go — dropped the UnstableSetSessionModelFunc field and its method.

Verification

• make test (go test ./... + go build ./example/...) passes.
• Ran the example/client ↔ example/agent end-to-end ACP session over stdio: Initialize → NewSession → Prompt → permission round-trip → end-of-turn, exit 0.
• Probed the removed method directly: session/set_model now returns -32601 Method not found, while a still-supported method (initialize) returns a normal result.

🤖 Generated with Claude Code