diff --git a/packages/agent-core/src/prompt.ts b/packages/agent-core/src/prompt.ts
index 28dbe058..65efb126 100644
--- a/packages/agent-core/src/prompt.ts
+++ b/packages/agent-core/src/prompt.ts
@@ -339,6 +339,9 @@ export function buildSystemPrompt(options: {
     parts.push(
       `\n## Workspace\nYour workspace directory is \`${options.workspaceDir}\`. ` +
         `Shell commands, file ops, and the Python REPL default to this location. ` +
+        `Shell and process commands also receive \`WORKSPACE_HOME\` for this ` +
+        `directory and \`AGENT_HOME\` for its parent, so prefer those when ` +
+        `you need stable absolute paths. ` +
         `Your shell maintains state across calls in this session — \`cd\`, ` +
         `exported environment variables, and shell functions all persist. ` +
         `Absolute paths are allowed; what you can actually read and write ` +
diff --git a/packages/agent-core/test/prompt.test.ts b/packages/agent-core/test/prompt.test.ts
index 6380e0eb..bfc75c10 100644
--- a/packages/agent-core/test/prompt.test.ts
+++ b/packages/agent-core/test/prompt.test.ts
@@ -299,6 +299,8 @@ describe("Workspace section (per-agent default cwd)", () => {
     });
     expect(prompt).toContain("## Workspace");
     expect(prompt).toContain("/data/agents/alice/workspace");
+    expect(prompt).toContain("WORKSPACE_HOME");
+    expect(prompt).toContain("AGENT_HOME");
     expect(prompt).toContain("persist");
   });
 
diff --git a/packages/tools/src/builtins/process.ts b/packages/tools/src/builtins/process.ts
index 52f8d52d..d8d54194 100644
--- a/packages/tools/src/builtins/process.ts
+++ b/packages/tools/src/builtins/process.ts
@@ -3,6 +3,8 @@ import { spawn, type ChildProcess } from "node:child_process";
 import { randomUUID } from "node:crypto";
 import { registry } from "../registry.js";
 import { getCurrentWorkspaceDir } from "../session-context.js";
+import { buildToolHomeEnv } from "../tool-env.js";
+import { resolveShellForSpawn } from "../internal/shell-executable.js";
 
 /**
  * Background process management. One tool with an action enum, mirroring
@@ -151,7 +153,8 @@ function startProc(args: {
   const effectiveCwd = args.cwd ?? baseCwd;
   const child = spawn(args.command, {
     cwd: effectiveCwd,
-    shell: process.platform === "win32" ? true : "/bin/bash",
+    env: { ...process.env, ...buildToolHomeEnv() },
+    shell: resolveShellForSpawn(),
     detached: process.platform !== "win32", // for process-group kill
     stdio: ["pipe", "pipe", "pipe"],
   });
@@ -192,6 +195,10 @@ function startProc(args: {
     }
   });
   child.on("error", (err) => {
+    if (e.overallTimer) clearTimeout(e.overallTimer);
+    if (e.silenceTimer) clearTimeout(e.silenceTimer);
+    e.overallTimer = null;
+    e.silenceTimer = null;
     appendOutput(e, `\n[spawn error: ${err.message}]\n`);
     e.endedAt = Date.now();
     if (e.status === "running") e.status = "killed";
@@ -231,7 +238,8 @@ registry.register({
     "`poll` (status + new output since last poll, then clears pending buffer), " +
     "`log` (full transcript), " +
     "`write` (send to stdin; `data` may end with \\n), " +
-    "`kill` (SIGTERM then SIGKILL).",
+    "`kill` (SIGTERM then SIGKILL). Started commands receive `WORKSPACE_HOME` " +
+    "and `AGENT_HOME` environment variables for stable absolute path references.",
   parameters: z.object({
     action: z.enum(["start", "list", "status", "poll", "log", "write", "kill"]),
     id: z.string().optional().describe("Process id (required for all actions except start/list)"),
diff --git a/packages/tools/src/builtins/shell.ts b/packages/tools/src/builtins/shell.ts
index 986e247a..7b9e42bc 100644
--- a/packages/tools/src/builtins/shell.ts
+++ b/packages/tools/src/builtins/shell.ts
@@ -7,6 +7,8 @@ import {
   getCurrentSessionId,
 } from "../session-context.js";
 import { getShellSession } from "../internal/shell-session.js";
+import { buildToolHomeEnv } from "../tool-env.js";
+import { resolveShellForExec } from "../internal/shell-executable.js";
 
 const DESTRUCTIVE_PATTERNS = /(?:^|\s|&&|\|\||;|`)(?:rm\s|rmdir\s|cp\s|mv\s|sed\s+-i|truncate\s|dd\s|shred\s|git\s+(?:reset|clean|checkout)\s)/;
 
@@ -23,7 +25,8 @@ registry.register({
     "installing packages, checking system state, and any terminal operations. " +
     "Runs from your agent's workspace dir by default. State persists across " +
     "calls in this session — `cd`, exported env vars, shell functions, and " +
-    "history are preserved.",
+    "history are preserved. Commands receive `WORKSPACE_HOME` and `AGENT_HOME` " +
+    "environment variables for stable absolute path references.",
   parameters: z.object({
     command: z.string().describe("The shell command to execute"),
     timeout: z
@@ -49,11 +52,12 @@ registry.register({
     const workspaceDir = getCurrentWorkspaceDir();
     const agentId = getCurrentAgentId();
     const sessionId = getCurrentSessionId();
+    const toolHomeEnv = buildToolHomeEnv(workspaceDir);
 
     // Persistent path: a real bash subprocess per (agentId, sessionId)
     // so `cd`, env vars, and shell functions survive across calls.
     if (workspaceDir && agentId && sessionId) {
-      const session = getShellSession(agentId, sessionId, workspaceDir);
+      const session = getShellSession(agentId, sessionId, workspaceDir, toolHomeEnv);
       try {
         const res = await session.exec(command, timeout);
         const trimmed = res.output.trimEnd();
@@ -97,8 +101,9 @@ registry.register({
         timeout,
         encoding: "utf-8",
         maxBuffer: 1024 * 1024 * 10, // 10MB
-        shell: "/bin/bash",
+        shell: resolveShellForExec(),
         cwd: baseCwd,
+        env: { ...process.env, ...toolHomeEnv },
         stdio: ["pipe", "pipe", "pipe"],
       });
 
diff --git a/packages/tools/src/internal/shell-executable.ts b/packages/tools/src/internal/shell-executable.ts
new file mode 100644
index 00000000..f7dbfead
--- /dev/null
+++ b/packages/tools/src/internal/shell-executable.ts
@@ -0,0 +1,67 @@
+import { existsSync } from "node:fs";
+import * as path from "node:path";
+
+export type ResolvedShellKind = "bash" | "sh";
+
+export interface ResolvedShell {
+  command: string;
+  args: string[];
+  kind: ResolvedShellKind;
+}
+
+type Exists = (candidate: string) => boolean;
+
+function isAbsoluteExecutable(candidate: string | undefined, exists: Exists): string | null {
+  if (!candidate || !path.isAbsolute(candidate)) return null;
+  return exists(candidate) ? candidate : null;
+}
+
+function findOnPath(command: string, env: NodeJS.ProcessEnv, exists: Exists): string | null {
+  const pathValue = env.PATH;
+  if (!pathValue) return null;
+  for (const dir of pathValue.split(path.delimiter)) {
+    if (!dir) continue;
+    const candidate = path.join(dir, command);
+    if (exists(candidate)) return candidate;
+  }
+  return null;
+}
+
+export function resolveShell(
+  env: NodeJS.ProcessEnv = process.env,
+  exists: Exists = existsSync
+): ResolvedShell {
+  if (process.platform === "win32") {
+    return { command: "cmd.exe", args: [], kind: "sh" };
+  }
+
+  const bash =
+    isAbsoluteExecutable(env.BASH, exists) ??
+    isAbsoluteExecutable("/bin/bash", exists) ??
+    findOnPath("bash", env, exists);
+  if (bash) {
+    return { command: bash, args: ["--norc", "--noprofile"], kind: "bash" };
+  }
+
+  const sh =
+    isAbsoluteExecutable("/bin/sh", exists) ??
+    findOnPath("sh", env, exists) ??
+    "sh";
+  return { command: sh, args: [], kind: "sh" };
+}
+
+export function resolveShellForSpawn(
+  env: NodeJS.ProcessEnv = process.env,
+  exists: Exists = existsSync
+): string | boolean {
+  if (process.platform === "win32") return true;
+  return resolveShell(env, exists).command;
+}
+
+export function resolveShellForExec(
+  env: NodeJS.ProcessEnv = process.env,
+  exists: Exists = existsSync
+): string | undefined {
+  if (process.platform === "win32") return undefined;
+  return resolveShell(env, exists).command;
+}
diff --git a/packages/tools/src/internal/shell-session.ts b/packages/tools/src/internal/shell-session.ts
index 771ea3c1..cc0f82f5 100644
--- a/packages/tools/src/internal/shell-session.ts
+++ b/packages/tools/src/internal/shell-session.ts
@@ -31,6 +31,7 @@ import { randomUUID } from "node:crypto";
 import * as fs from "node:fs";
 import * as os from "node:os";
 import * as path from "node:path";
+import { resolveShell } from "./shell-executable.js";
 
 export interface ShellExecResult {
   output: string;
@@ -60,7 +61,7 @@ export class ShellSession {
   private currentCwd: string;
   lastUsedAt = Date.now();
 
-  constructor(public readonly initialCwd: string) {
+  constructor(public readonly initialCwd: string, private readonly baseEnv: NodeJS.ProcessEnv = {}) {
     this.currentCwd = initialCwd;
   }
 
@@ -68,17 +69,16 @@ export class ShellSession {
     return this.pending !== null;
   }
 
-  /** True while a bash subprocess is attached. */
+  /** True while a shell subprocess is attached. */
   get live(): boolean {
     return this.proc !== null;
   }
 
-  private spawnBash(): ChildProcessWithoutNullStreams {
-    // --norc / --noprofile keep bash predictable across user environments
-    // — no surprise aliases from a developer's ~/.bashrc bleeding in.
-    const proc = spawn("/bin/bash", ["--norc", "--noprofile"], {
+  private spawnShell(): ChildProcessWithoutNullStreams {
+    const shell = resolveShell();
+    const proc = spawn(shell.command, shell.args, {
       cwd: fs.existsSync(this.currentCwd) ? this.currentCwd : this.initialCwd,
-      env: { ...process.env, PS1: "" },
+      env: { ...process.env, ...this.baseEnv, PS1: "" },
       stdio: ["pipe", "pipe", "pipe"],
     });
     proc.unref();
@@ -151,7 +151,7 @@ export class ShellSession {
 
     if (!this.proc || this.dead) {
       this.dead = false;
-      this.proc = this.spawnBash();
+      this.proc = this.spawnShell();
     }
 
     const uuid = randomUUID().replace(/-/g, "").slice(0, 16);
@@ -281,13 +281,14 @@ function pruneTrackedIfFull(): void {
 export function getShellSession(
   agentId: string,
   sessionId: string,
-  initialCwd: string
+  initialCwd: string,
+  baseEnv: NodeJS.ProcessEnv = {}
 ): ShellSession {
   const key = `${agentId}:${sessionId}`;
   let s = sessions.get(key);
   if (!s) {
     pruneTrackedIfFull();
-    s = new ShellSession(initialCwd);
+    s = new ShellSession(initialCwd, baseEnv);
     sessions.set(key, s);
   }
   // The caller is about to exec — make room under the live cap and make
diff --git a/packages/tools/src/tool-env.ts b/packages/tools/src/tool-env.ts
new file mode 100644
index 00000000..1a66b01a
--- /dev/null
+++ b/packages/tools/src/tool-env.ts
@@ -0,0 +1,12 @@
+import * as path from "node:path";
+import { getCurrentWorkspaceDir } from "./session-context.js";
+
+export function buildToolHomeEnv(
+  workspaceDir = getCurrentWorkspaceDir(),
+): NodeJS.ProcessEnv {
+  if (!workspaceDir) return {};
+  return {
+    WORKSPACE_HOME: workspaceDir,
+    AGENT_HOME: path.dirname(workspaceDir),
+  };
+}
diff --git a/packages/tools/test/process.test.ts b/packages/tools/test/process.test.ts
new file mode 100644
index 00000000..deaf6e1c
--- /dev/null
+++ b/packages/tools/test/process.test.ts
@@ -0,0 +1,49 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { registry } from "../src/registry.js";
+import { _resetProcessRegistry } from "../src/builtins/process.js";
+import "../src/builtins/process.js";
+
+afterEach(() => {
+  _resetProcessRegistry();
+});
+
+async function runProcessTool<R = Record<string, unknown>>(
+  args: Record<string, unknown>
+): Promise<R> {
+  const tool = registry.get("process");
+  if (!tool) throw new Error("process tool not registered");
+  return JSON.parse(await tool.handler(args)) as R;
+}
+
+async function pollUntilExited(id: string): Promise<{ output: string; status: string }> {
+  let output = "";
+  let status = "running";
+  for (let i = 0; i < 20; i++) {
+    const result = await runProcessTool<{ output: string; status: string }>({
+      action: "poll",
+      id,
+    });
+    output += result.output ?? "";
+    status = result.status;
+    if (status !== "running") return { output, status };
+    await new Promise((resolve) => setTimeout(resolve, 25));
+  }
+  return { output, status };
+}
+
+describe("process tool", () => {
+  it("starts commands through the resolved shell", async () => {
+    const started = await runProcessTool<{ success: boolean; id: string; status: string }>({
+      action: "start",
+      command: "printf process-ok",
+      timeoutMs: 5000,
+      silenceTimeoutMs: 5000,
+    });
+    expect(started.success).toBe(true);
+    expect(started.status).toBe("running");
+
+    const result = await pollUntilExited(started.id);
+    expect(result.status).toBe("exited");
+    expect(result.output).toContain("process-ok");
+  });
+});
diff --git a/packages/tools/test/shell-executable.test.ts b/packages/tools/test/shell-executable.test.ts
new file mode 100644
index 00000000..56b3fcba
--- /dev/null
+++ b/packages/tools/test/shell-executable.test.ts
@@ -0,0 +1,48 @@
+import { describe, expect, it } from "vitest";
+import {
+  resolveShell,
+  resolveShellForExec,
+  resolveShellForSpawn,
+} from "../src/internal/shell-executable.js";
+
+function exists(paths: string[]) {
+  const set = new Set(paths);
+  return (candidate: string) => set.has(candidate);
+}
+
+describe("shell executable resolution", () => {
+  it("prefers a valid BASH env path", () => {
+    const shell = resolveShell(
+      { BASH: "/opt/homebrew/bin/bash", PATH: "/bin" },
+      exists(["/opt/homebrew/bin/bash", "/bin/bash", "/bin/sh"])
+    );
+    expect(shell).toEqual({
+      command: "/opt/homebrew/bin/bash",
+      args: ["--norc", "--noprofile"],
+      kind: "bash",
+    });
+  });
+
+  it("ignores a missing /bin/bash and falls back to bash on PATH", () => {
+    const shell = resolveShell(
+      { PATH: "/nix/store/bin:/bin" },
+      exists(["/nix/store/bin/bash", "/bin/sh"])
+    );
+    expect(shell.command).toBe("/nix/store/bin/bash");
+    expect(shell.kind).toBe("bash");
+  });
+
+  it("falls back to sh when bash is unavailable", () => {
+    const shell = resolveShell(
+      { BASH: "/bin/bash", PATH: "/usr/bin" },
+      exists(["/bin/sh"])
+    );
+    expect(shell).toEqual({ command: "/bin/sh", args: [], kind: "sh" });
+    expect(resolveShellForSpawn({ PATH: "/usr/bin" }, exists(["/bin/sh"]))).toBe(
+      "/bin/sh"
+    );
+    expect(resolveShellForExec({ PATH: "/usr/bin" }, exists(["/bin/sh"]))).toBe(
+      "/bin/sh"
+    );
+  });
+});
diff --git a/packages/tools/test/workspace-cwd.test.ts b/packages/tools/test/workspace-cwd.test.ts
index b09f4602..f67065c0 100644
--- a/packages/tools/test/workspace-cwd.test.ts
+++ b/packages/tools/test/workspace-cwd.test.ts
@@ -5,6 +5,7 @@ import * as os from "node:os";
 import { registry } from "../src/registry.js";
 import { toolCallContext } from "../src/session-context.js";
 import { closeAllShellSessions } from "../src/internal/shell-session.js";
+import { _resetProcessRegistry } from "../src/builtins/process.js";
 // Side-effect imports — each tool self-registers at module load.
 import "../src/builtins/shell.js";
 import "../src/builtins/file.js";
@@ -14,6 +15,7 @@ import "../src/builtins/apply-patch.js";
 afterEach(() => {
   // Reap any per-(agent, session) bash subprocesses spawned during the test.
   closeAllShellSessions();
+  _resetProcessRegistry();
 });
 
 // `<dataDir>/agents/<id>/workspace/` is the default cwd for the agent's
@@ -120,6 +122,76 @@ describe("shell — cwd defaults to workspace", () => {
       res.output === cwd || res.output === path.join("/private", cwd)
     ).toBe(true);
   });
+
+  it("exposes WORKSPACE_HOME and AGENT_HOME in persistent shell sessions", async () => {
+    const ctx = {
+      sessionId: "sess-home-env",
+      agentId: "agent-home-env",
+      workspaceDir,
+    };
+    const res = await toolCallContext.run(ctx, () =>
+      runTool<{ success: boolean; output: string }>("shell", {
+        command:
+          'printf \'workspace=%s\\nagent=%s\\n\' "$WORKSPACE_HOME" "$AGENT_HOME"',
+        timeout: 5000,
+      }),
+    );
+    expect(res.success).toBe(true);
+    expect(res.output).toContain(`workspace=${workspaceDir}`);
+    expect(res.output).toContain(`agent=${path.dirname(workspaceDir)}`);
+  });
+
+  it("exposes WORKSPACE_HOME and AGENT_HOME in non-persistent shell executions", async () => {
+    const res = await toolCallContext.run(
+      { sessionId: "", agentId: "", workspaceDir },
+      () =>
+        runTool<{ success: boolean; output: string }>("shell", {
+          command:
+            'printf \'workspace=%s\\nagent=%s\\n\' "$WORKSPACE_HOME" "$AGENT_HOME"',
+          timeout: 5000,
+        }),
+    );
+    expect(res.success).toBe(true);
+    expect(res.output).toContain(`workspace=${workspaceDir}`);
+    expect(res.output).toContain(`agent=${path.dirname(workspaceDir)}`);
+  });
+});
+
+describe("process — home environment", () => {
+  it("exposes WORKSPACE_HOME and AGENT_HOME to started processes", async () => {
+    const started = await withWorkspace(workspaceDir, () =>
+      runTool<{ success: boolean; id: string }>("process", {
+        action: "start",
+        command:
+          'printf \'workspace=%s\\nagent=%s\\n\' "$WORKSPACE_HOME" "$AGENT_HOME"',
+        timeoutMs: 5000,
+        silenceTimeoutMs: 5000,
+      }),
+    );
+    expect(started.success).toBe(true);
+
+    let output = "";
+    let status = "running";
+    for (let i = 0; i < 20 && status === "running"; i++) {
+      await new Promise((resolve) => setTimeout(resolve, 25));
+      const polled = await withWorkspace(workspaceDir, () =>
+        runTool<{ success: boolean; status: string; output: string }>(
+          "process",
+          {
+            action: "poll",
+            id: started.id,
+          },
+        ),
+      );
+      expect(polled.success).toBe(true);
+      output += polled.output;
+      status = polled.status;
+    }
+
+    expect(status).not.toBe("running");
+    expect(output).toContain(`workspace=${workspaceDir}`);
+    expect(output).toContain(`agent=${path.dirname(workspaceDir)}`);
+  });
 });
 
 describe("write_file + read_file — relative paths resolve to workspaceDir", () => {