diff --git a/CLAUDE.md b/CLAUDE.md
index 47dc3e3d86..c317064255 120000
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -1 +1 @@
-AGENTS.md
\ No newline at end of file
+AGENTS.md
diff --git a/src/agents/cli-runner/helpers.ts b/src/agents/cli-runner/helpers.ts
index 7abed5673d..d19885d26e 100644
--- a/src/agents/cli-runner/helpers.ts
+++ b/src/agents/cli-runner/helpers.ts
@@ -11,6 +11,7 @@ import type { EmbeddedContextFile } from "../pi-embedded-helpers.js";
 import { runExec } from "../../process/exec.js";
 import { buildTtsSystemPromptHint } from "../../tts/tts.js";
 import { resolveDefaultModelForAgent } from "../model-selection.js";
+import { detectRuntimeShell } from "../shell-utils.js";
 import { buildSystemPromptParams } from "../system-prompt-params.js";
 import { buildAgentSystemPrompt } from "../system-prompt.js";
 
@@ -226,6 +227,7 @@ export function buildSystemPrompt(params: {
       node: process.version,
       model: params.modelDisplay,
       defaultModel: defaultModelLabel,
+      shell: detectRuntimeShell(),
     },
   });
   const ttsHint = params.config ? buildTtsSystemPromptHint(params.config) : undefined;
diff --git a/src/agents/pi-embedded-runner/compact.ts b/src/agents/pi-embedded-runner/compact.ts
index 8c5a63d32d..280e7d0aba 100644
--- a/src/agents/pi-embedded-runner/compact.ts
+++ b/src/agents/pi-embedded-runner/compact.ts
@@ -45,6 +45,7 @@ import { resolveSandboxContext } from "../sandbox.js";
 import { repairSessionFileIfNeeded } from "../session-file-repair.js";
 import { guardSessionManager } from "../session-tool-result-guard-wrapper.js";
 import { acquireSessionWriteLock } from "../session-write-lock.js";
+import { detectRuntimeShell } from "../shell-utils.js";
 import {
   applySkillEnvOverrides,
   applySkillEnvOverridesFromSnapshot,
@@ -308,6 +309,7 @@ export async function compactEmbeddedPiSessionDirect(
       arch: os.arch(),
       node: process.version,
       model: `${provider}/${modelId}`,
+      shell: detectRuntimeShell(),
       channel: runtimeChannel,
       capabilities: runtimeCapabilities,
       channelActions,
diff --git a/src/agents/pi-embedded-runner/run/attempt.ts b/src/agents/pi-embedded-runner/run/attempt.ts
index 2e6c702929..cc95a77cf8 100644
--- a/src/agents/pi-embedded-runner/run/attempt.ts
+++ b/src/agents/pi-embedded-runner/run/attempt.ts
@@ -49,6 +49,7 @@ import { resolveSandboxRuntimeStatus } from "../../sandbox/runtime-status.js";
 import { repairSessionFileIfNeeded } from "../../session-file-repair.js";
 import { guardSessionManager } from "../../session-tool-result-guard-wrapper.js";
 import { acquireSessionWriteLock } from "../../session-write-lock.js";
+import { detectRuntimeShell } from "../../shell-utils.js";
 import {
   applySkillEnvOverrides,
   applySkillEnvOverridesFromSnapshot,
@@ -331,6 +332,7 @@ export async function runEmbeddedAttempt(
         node: process.version,
         model: `${params.provider}/${params.modelId}`,
         defaultModel: defaultModelLabel,
+        shell: detectRuntimeShell(),
         channel: runtimeChannel,
         capabilities: runtimeCapabilities,
         channelActions,
diff --git a/src/agents/shell-utils.ts b/src/agents/shell-utils.ts
index 1a392ba09e..386fd65e69 100644
--- a/src/agents/shell-utils.ts
+++ b/src/agents/shell-utils.ts
@@ -67,6 +67,63 @@ function resolveShellFromPath(name: string): string | undefined {
   return undefined;
 }
 
+function normalizeShellName(value: string): string {
+  const trimmed = value.trim();
+  if (!trimmed) {
+    return "";
+  }
+  return path
+    .basename(trimmed)
+    .replace(/\.(exe|cmd|bat)$/i, "")
+    .replace(/[^a-zA-Z0-9_-]/g, "");
+}
+
+export function detectRuntimeShell(): string | undefined {
+  const overrideShell = process.env.CLAWDBOT_SHELL?.trim();
+  if (overrideShell) {
+    const name = normalizeShellName(overrideShell);
+    if (name) {
+      return name;
+    }
+  }
+
+  if (process.platform === "win32") {
+    if (process.env.POWERSHELL_DISTRIBUTION_CHANNEL) {
+      return "pwsh";
+    }
+    return "powershell";
+  }
+
+  const envShell = process.env.SHELL?.trim();
+  if (envShell) {
+    const name = normalizeShellName(envShell);
+    if (name) {
+      return name;
+    }
+  }
+
+  if (process.env.POWERSHELL_DISTRIBUTION_CHANNEL) {
+    return "pwsh";
+  }
+  if (process.env.BASH_VERSION) {
+    return "bash";
+  }
+  if (process.env.ZSH_VERSION) {
+    return "zsh";
+  }
+  if (process.env.FISH_VERSION) {
+    return "fish";
+  }
+  if (process.env.KSH_VERSION) {
+    return "ksh";
+  }
+  if (process.env.NU_VERSION || process.env.NUSHELL_VERSION) {
+    return "nu";
+  }
+
+  return undefined;
+}
+
 export function sanitizeBinaryOutput(text: string): string {
   const scrubbed = text.replace(/[\p{Format}\p{Surrogate}]/gu, "");
   if (!scrubbed) {
diff --git a/src/agents/system-prompt-params.ts b/src/agents/system-prompt-params.ts
index d269253223..e35709009c 100644
--- a/src/agents/system-prompt-params.ts
+++ b/src/agents/system-prompt-params.ts
@@ -16,6 +16,7 @@ export type RuntimeInfoInput = {
   node: string;
   model: string;
   defaultModel?: string;
+  shell?: string;
   channel?: string;
   capabilities?: string[];
   /** Supported message actions for the current channel (e.g., react, edit, unsend) */
diff --git a/src/agents/system-prompt.ts b/src/agents/system-prompt.ts
index 8c8682dff6..17ae800c62 100644
--- a/src/agents/system-prompt.ts
+++ b/src/agents/system-prompt.ts
@@ -190,6 +190,7 @@ export function buildAgentSystemPrompt(params: {
     node?: string;
     model?: string;
     defaultModel?: string;
+    shell?: string;
     channel?: string;
     capabilities?: string[];
     repoRoot?: string;
@@ -616,6 +617,7 @@ export function buildRuntimeLine(
     node?: string;
     model?: string;
     defaultModel?: string;
+    shell?: string;
     repoRoot?: string;
   },
   runtimeChannel?: string,
@@ -634,6 +636,7 @@ export function buildRuntimeLine(
     runtimeInfo?.node ? `node=${runtimeInfo.node}` : "",
     runtimeInfo?.model ? `model=${runtimeInfo.model}` : "",
     runtimeInfo?.defaultModel ? `default_model=${runtimeInfo.defaultModel}` : "",
+    runtimeInfo?.shell ? `shell=${runtimeInfo.shell}` : "",
     runtimeChannel ? `channel=${runtimeChannel}` : "",
     runtimeChannel
       ? `capabilities=${runtimeCapabilities.length > 0 ? runtimeCapabilities.join(",") : "none"}`