feat(agent): implement reasoning mode in engine and CLI

Wire reasoningMode through Agent runner, CLI output, and all CLI
entry points (run, interactive, non-interactive). In stream mode,
thinking content is printed to stderr in real-time. In on mode,
thinking is shown after message completion. Default is stream.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agent/cli/commands/run.ts

@@ -18,6 +18,7 @@ type RunOptions = {
   baseUrl?: string;
   system?: string;
   thinking?: string;
+  reasoning?: string;
   cwd?: string;
   session?: string;
   debug?: boolean;
@@ -40,6 +41,7 @@ ${cyan("Options:")}
   ${yellow("--base-url")} URL      Custom base URL for provider
   ${yellow("--system")} TEXT       System prompt (ignored if --profile set)
   ${yellow("--thinking")} LEVEL    Thinking level
+  ${yellow("--reasoning")} MODE   Reasoning display mode (off, on, stream)
   ${yellow("--cwd")} DIR           Working directory
   ${yellow("--session")} ID        Session ID for persistence
   ${yellow("--debug")}             Enable debug logging
@@ -106,6 +108,10 @@ function parseArgs(argv: string[]): { opts: RunOptions; prompt: string } {
       opts.thinking = args.shift();
       continue;
     }
+    if (arg === "--reasoning") {
+      opts.reasoning = args.shift();
+      continue;
+    }
     if (arg === "--cwd") {
       opts.cwd = args.shift();
       continue;
@@ -192,6 +198,7 @@ export async function runCommand(args: string[]): Promise<void> {
     baseUrl: opts.baseUrl,
     systemPrompt: opts.system,
     thinkingLevel: opts.thinking as any,
+    reasoningMode: (opts.reasoning as any) ?? undefined,
     cwd: opts.cwd,
     sessionId: opts.session,
     debug: opts.debug,

src/agent/cli/interactive.ts

@@ -12,6 +12,7 @@ type CliOptions = {
   model?: string | undefined;
   system?: string | undefined;
   thinking?: string | undefined;
+  reasoning?: string | undefined;
   cwd?: string | undefined;
   session?: string | undefined;
   help?: boolean | undefined;
@@ -35,6 +36,7 @@ function printUsage() {
   console.log(`  ${yellow("--model")} NAME     Model name`);
   console.log(`  ${yellow("--system")} TEXT    System prompt (ignored if --profile is set)`);
   console.log(`  ${yellow("--thinking")} LEVEL Thinking level`);
+  console.log(`  ${yellow("--reasoning")} MODE Reasoning display mode (off, on, stream)`);
   console.log(`  ${yellow("--cwd")} DIR        Working directory for commands`);
   console.log(`  ${yellow("--session")} ID     Session ID to resume`);
   console.log(`  ${yellow("--help")}, -h       Show this help`);
@@ -76,6 +78,10 @@ function parseArgs(argv: string[]) {
       opts.thinking = args.shift();
       continue;
     }
+    if (arg === "--reasoning") {
+      opts.reasoning = args.shift();
+      continue;
+    }
     if (arg === "--cwd") {
       opts.cwd = args.shift();
       continue;
@@ -343,6 +349,7 @@ class InteractiveCLI {
       model: this.opts.model,
       systemPrompt: this.opts.system,
       thinkingLevel: this.opts.thinking as AgentOptions["thinkingLevel"],
+      reasoningMode: (this.opts.reasoning as AgentOptions["reasoningMode"]) ?? undefined,
       cwd: this.opts.cwd,
       sessionId,
     });

src/agent/cli/non-interactive.ts

@@ -9,6 +9,7 @@ type CliOptions = {
   baseUrl?: string | undefined;
   system?: string | undefined;
   thinking?: string | undefined;
+  reasoning?: string | undefined;
   cwd?: string | undefined;
   session?: string | undefined;
   debug?: boolean | undefined;
@@ -31,6 +32,7 @@ function printUsage() {
   console.log("  --base-url URL   Custom base URL for the provider");
   console.log("  --system TEXT    System prompt (ignored if --profile is set)");
   console.log("  --thinking LEVEL Thinking level");
+  console.log("  --reasoning MODE Reasoning display mode (off, on, stream)");
   console.log("  --cwd DIR        Working directory for commands");
   console.log("  --session ID     Session ID for conversation persistence");
   console.log("  --debug          Enable debug logging");
@@ -87,6 +89,10 @@ function parseArgs(argv: string[]) {
       opts.thinking = args.shift();
       continue;
     }
+    if (arg === "--reasoning") {
+      opts.reasoning = args.shift();
+      continue;
+    }
     if (arg === "--cwd") {
       opts.cwd = args.shift();
       continue;
@@ -171,6 +177,7 @@ async function main() {
     baseUrl: opts.baseUrl,
     systemPrompt: opts.system,
     thinkingLevel: opts.thinking as any,
+    reasoningMode: (opts.reasoning as any) ?? undefined,
     cwd: opts.cwd,
     sessionId: opts.session,
     debug: opts.debug,

src/agent/cli/output.ts

@@ -1,10 +1,13 @@
 import type { AgentEvent, AgentMessage } from "@mariozechner/pi-agent-core";
-import { colors, createSpinner } from "./colors.js";
-import { extractText } from "../extract-text.js";
+import { colors, createSpinner, dim } from "./colors.js";
+import { extractText, extractThinking } from "../extract-text.js";
+import type { ReasoningMode } from "../types.js";
 
 export type AgentOutputState = {
   lastAssistantText: string;
+  lastAssistantThinking: string;
   printedLen: number;
+  printedThinkingLen: number;
   streaming: boolean;
 };
 
@@ -171,10 +174,14 @@ export function formatResultSummary(name: string, result: unknown): string {
 export function createAgentOutput(params: {
   stdout: NodeJS.WritableStream;
   stderr: NodeJS.WritableStream;
+  reasoningMode?: ReasoningMode;
 }): AgentOutput {
+  const reasoningMode = params.reasoningMode ?? "stream";
   const state: AgentOutputState = {
     lastAssistantText: "",
+    lastAssistantThinking: "",
     printedLen: 0,
+    printedThinkingLen: 0,
     streaming: false,
   };
 
@@ -194,11 +201,20 @@ export function createAgentOutput(params: {
           }
           state.streaming = true;
           state.printedLen = 0;
+          state.printedThinkingLen = 0;
           const text = extractText(msg);
           if (text.length > 0) {
             params.stdout.write(text);
             state.printedLen = text.length;
           }
+          // Stream thinking content in real-time
+          if (reasoningMode === "stream") {
+            const thinking = extractThinking(msg);
+            if (thinking.length > 0) {
+              params.stderr.write(dim(thinking));
+              state.printedThinkingLen = thinking.length;
+            }
+          }
         }
         break;
       }
@@ -210,6 +226,14 @@ export function createAgentOutput(params: {
             params.stdout.write(text.slice(state.printedLen));
             state.printedLen = text.length;
           }
+          // Stream thinking content in real-time
+          if (reasoningMode === "stream") {
+            const thinking = extractThinking(msg);
+            if (thinking.length > state.printedThinkingLen) {
+              params.stderr.write(dim(thinking.slice(state.printedThinkingLen)));
+              state.printedThinkingLen = thinking.length;
+            }
+          }
         }
         break;
       }
@@ -224,6 +248,21 @@ export function createAgentOutput(params: {
           if (state.streaming) params.stdout.write("\n");
           state.streaming = false;
           state.lastAssistantText = text;
+
+          // Extract and store thinking content
+          const thinking = extractThinking(msg);
+          state.lastAssistantThinking = thinking;
+
+          // Show thinking at end for "on" mode
+          if (reasoningMode === "on" && thinking) {
+            params.stderr.write(`\n${dim("--- Thinking ---")}\n`);
+            params.stderr.write(dim(thinking));
+            params.stderr.write(`\n${dim("--- End Thinking ---")}\n`);
+          }
+          // Finish streaming thinking with a newline
+          if (reasoningMode === "stream" && state.printedThinkingLen > 0) {
+            params.stderr.write("\n");
+          }
         }
         break;
       }