diff --git a/packages/core/src/tool/plugin/shell.ts b/packages/core/src/tool/plugin/shell.ts
index 87be4c9d165..7f7154a671c 100644
--- a/packages/core/src/tool/plugin/shell.ts
+++ b/packages/core/src/tool/plugin/shell.ts
@@ -207,6 +207,10 @@ export const Plugin = {
                 },
                 (invocation) =>
                   Effect.gen(function* () {
+                    invocation.env.AGENT = "1"
+                    invocation.env.OPENCODE = "1"
+                    invocation.env.AI_AGENT ||= "opencode"
+                    invocation.env.OPENCODE_SESSION_ID = context.sessionID
                     finalTimeout = yield* prepare(invocation, context)
                   }),
               )
diff --git a/packages/core/test/tool-shell.test.ts b/packages/core/test/tool-shell.test.ts
index 226c716b190..e706d9b0efd 100644
--- a/packages/core/test/tool-shell.test.ts
+++ b/packages/core/test/tool-shell.test.ts
@@ -956,6 +956,59 @@ describe("ShellTool", () => {
     { timeout: 15_000 },
   )
 
+  productionIt.live(
+    "identifies agent shell commands without inheriting a stale session ID",
+    () =>
+      Effect.acquireUseRelease(
+        Effect.promise(() => tmpdir()),
+        (tmp) => {
+          reset()
+          return withSession(tmp.path, (registry) =>
+            Effect.gen(function* () {
+              const sessions = yield* Session.Service
+              yield* sessions.environment({
+                sessionID,
+                variables: { AGENT: "0", OPENCODE: "0", AI_AGENT: "", OPENCODE_SESSION_ID: "stale" },
+              })
+              const command = isWindows
+                ? '[Console]::Out.Write("$env:AGENT|$env:OPENCODE|$env:AI_AGENT|$env:OPENCODE_SESSION_ID")'
+                : 'printf %s "$AGENT|$OPENCODE|$AI_AGENT|$OPENCODE_SESSION_ID"'
+              const settled = yield* executeTool(registry, call({ command }))
+
+              expect(settled.status).toBe("completed")
+              expect(settled.content?.[0]).toEqual({ type: "text", text: `1|1|opencode|${sessionID}` })
+            }),
+          )
+        },
+        (tmp) => Effect.promise(() => tmp[Symbol.asyncDispose]().then(() => undefined)),
+      ),
+    { timeout: 15_000 },
+  )
+
+  productionIt.live(
+    "preserves an outer AI_AGENT marker in agent shell commands",
+    () =>
+      Effect.acquireUseRelease(
+        Effect.promise(() => tmpdir()),
+        (tmp) => {
+          reset()
+          return withSession(tmp.path, (registry) =>
+            Effect.gen(function* () {
+              const sessions = yield* Session.Service
+              yield* sessions.environment({ sessionID, variables: { AI_AGENT: "outer-agent" } })
+              const command = isWindows ? '[Console]::Out.Write($env:AI_AGENT)' : 'printf %s "$AI_AGENT"'
+              const settled = yield* executeTool(registry, call({ command }))
+
+              expect(settled.status).toBe("completed")
+              expect(settled.content?.[0]).toEqual({ type: "text", text: "outer-agent" })
+            }),
+          )
+        },
+        (tmp) => Effect.promise(() => tmp[Symbol.asyncDispose]().then(() => undefined)),
+      ),
+    { timeout: 15_000 },
+  )
+
   it.live("resolves a relative workdir from the active Location", () =>
     Effect.acquireUseRelease(
       Effect.promise(() => tmpdir()),