Add MAX_THINKING_TOKENS env support and fix file-read-info format (#226)

PR for awareness and discussion.

1) claude-code-acp doesn't currently support configuring
"maxThinkingTokens"
(https://platform.claude.com/docs/en/agent-sdk/typescript) nor is it
configured in the query. Result, at least for me, is that agent thinking
chunks are never flowing through the chain. I added environment support
to my ACP host, added env variable to query init for maxThinkingTokens
following the pyton sdk env "MAX_THINKING_TOKENS". Result is now agent
thinking chunks DO flow. And I can see Claude reason. Before, no chunks
ever. This is perhaps based on configuration that Claude Agent SDK reads
but could be made more flexibel. Such as this suggestions.

2) Fix file-read-info message: was showing nonsensical range format
'Read lines 160-40', now shows 'Read 40 lines starting at line 160'. The
way this was written was confusing for myself and for Claude. If the
file-read-info is supposed to give information, then lines like "Read
lines 160-40" says absolutely nothing.

Reproduce:

Configure your ACP host to parse thinking tokens with no other external
configuration. Claude will not send any.
Enable maxThinkingTokens in ACP and set it to something like 8096. Try
your ACP implementation again, Claude now sends thinking chunks.

Bugs:

Anthropic has a meta issue. Don't talk about the "thinking" tag with
Claude. It will break output regardless of what client you use. (even on
Claude Code CLI)

---------

Co-authored-by: Ben Brandt <[email protected]>

Author: omgpointless

src/acp-agent.ts

@@ -670,10 +670,16 @@ export class ClaudeAcpAgent implements Agent {
       extraArgs["session-id"] = sessionId;
     }
 
+    // Configure thinking tokens from environment variable
+    const maxThinkingTokens = process.env.MAX_THINKING_TOKENS
+      ? parseInt(process.env.MAX_THINKING_TOKENS, 10)
+      : undefined;
+
     const options: Options = {
       systemPrompt,
       settingSources: ["user", "project", "local"],
       stderr: (err) => this.logger.error(err),
+      ...(maxThinkingTokens !== undefined && { maxThinkingTokens }),
       ...userProvidedOptions,
       // Override certain fields that must be controlled by ACP
       cwd: params.cwd,

src/mcp-server.ts

@@ -175,8 +175,8 @@ Usage:
 
             if (result.wasLimited) {
               readInfo += `Read ${result.linesRead} lines (hit 50KB limit). `;
-            } else {
-              readInfo += `Read lines ${input.offset}-${result.linesRead}. `;
+            } else if (input.offset && input.offset > 1) {
+              readInfo += `Read lines ${input.offset}-${input.offset + result.linesRead}.`;
             }
 
             if (result.wasLimited) {