No longer possible to select the Anthropic model used by Claude Code

[ashlineldridge]

It appears there may have been changes in claude-code-acp that are preventing the Anthropic model being selected either using either the ANTHROPIC_MODEL environment variable or via ~/.claude/settings.json. I am able to control the default model used by Claude using either of these approaches when invoking claude directly. However, when invoking claude via agent-shell and claude-code-acp these methods of setting the model are no longer respected and the default model (currently Sonnet 4.5) is always selected.

Some points of note:

• Please support model selection. I need sonnet[1m] zed-industries/claude-code-acp#30 suggests that model selection is supported in claude-code-acp and Zed
• I am using Claude models via Vertex (i.e. CLAUDE_CODE_USE_VERTEX=1) though I suspect this regression would still be present when using Anthropic API's directly
• If I override CLAUDE_CODE_EXECUTABLE and dump the environment I can see that ANTHROPIC_MODEL and other vars are correctly set but not being respected by claude when invoked via ACP
• I'm raising this issue here rather than at claude-code-acp as the issue linked above suggests this is working in Zed which makes me think changes chould be required in agent-shell so figured I check here first
• If changes are required to agent-shell I'm happy to raise a PR and if changes are required to agent-shell-acp I can raise an issue there

Versions:

• agent-shell: a16e1e0
• claude-code-acp: v0.10.0
• claude-code: 2.0.36

Checklist

• I've read the README's Issues section.
• For requesting new agent support, I'm including a link to the ACP-capable agent or related ACP package.
• For issues, I'm including ACP traffic (as per README).

[ashlineldridge]

FYI, this workaround to claude-code-acp fixes the issue but I suspect that the intention is to use the ACP protocol to select the model.

[xenodium]

Thanks for filing. From agent-shell's perspective, we can influence agents via either environment variables or ACP requests. If you could find out which of these are needed for claude-code-acp, we can make the appropriate changes in agent-shell.

You mentioned the functionality used to be in claude-code-acp? If so, it may worth either searching their issues or asking for what the alternative mechanism is.

[ashlineldridge]

It appears that claude-code-acp was updated in zed-industries/claude-code-acp#93 to support the currently unstable session/set_model part of ACP. I'm guessing that prior to this change, because the model wasn't being set here, claude was falling back to ANTHROPIC_MODEL.

It looks like we'd need to update both acp.el and agent-shell to support this. Are you open to supporting unstable parts of the ACP protocol? In terms of UX, do you see this working similarly to agent-shell-set-session-mode?

[djgoku]

This was able to unblock me so I could still use agent-shell and claude-code and a different model.

acp.el:

@@ -495,6 +495,23 @@ See https://agentclientprotocol.com/protocol/session-modes"
     (:params . ((sessionId . ,session-id)
                 (modeId . ,mode-id)))))
 
+(cl-defun acp-make-session-set-model-request (&key session-id model-id)
+  "Instantiate a \"session/set_model\" request.
+
+SESSION-ID is the ID of the session to change the model for.
+MODEL-ID is the model to set (e.g., \"claude-sonnet-4-5\", \"claude-opus-4\").
+
+This is a claude-code-acp extension to the ACP protocol.
+
+See https://docs.claude.com/en/api/agent-sdk/typescript"
+  (unless session-id
+    (error ":session-id is required"))
+  (unless model-id
+    (error ":model-id is required"))
+  `((:method . "session/set_model")
+    (:params . ((sessionId . ,session-id)
+                (modelId . ,model-id)))))
+
 (cl-defun acp-make-session-request-permission-response (&key request-id option-id cancelled)
   "Instantiate a \"session/request_permission\" response.

Change model of running session:

(acp-send-request
          :client (map-elt (agent-shell--state) :client)
          :request (acp-make-session-set-model-request
                    :session-id (map-nested-elt (agent-shell--state) '(:session :id))
                    :model-id "claude-opus-4-5-20251101")
          :on-success (lambda (response) (message "Success: %S" response))
          :on-failure (lambda (error raw) (message "Error: %S | Raw: %S" error raw)))

[xenodium]

Thanks for this @djgoku. Need to work out how we want to offer the switching feature. Maybe like we do for modes. For now, I'm starting by listing available models in 37b6355

[xenodium]

Ok, I've added M-x agent-shell-set-session-model. The header also displays current model too. You'll need to update both agent-shell and acp.el. Can you give the latest a try and see if that works for you?

[dangom]

Also affected. This patch works but one needs to first start the agent and then set the model. It would be nice if it were already taken by default from the ANTHROPIC_MODEL env. Would that be possible?

[xenodium]

I need to see if I can squeeze another message during the handshake sequence when the first prompt is sent. May also need to guard this feature for agents that support swapping models via ACP.

ps. Ignore my other comment to file a separate feature request. We can continue here.