diff --git a/packages/opencode/src/tool/workflow.ts b/packages/opencode/src/tool/workflow.ts
index 470f72fb83..4cae53c6e6 100644
--- a/packages/opencode/src/tool/workflow.ts
+++ b/packages/opencode/src/tool/workflow.ts
@@ -107,7 +107,7 @@ export const Parameters = Schema.Struct({
   action: Schema.Literals(["start", "extend", "control", "status", "list"]).annotate({ description: "start: create workflow; extend: add nodes; control: pause/resume/cancel/replan/step/complete; status: inspect durable workflow and node state; list: show saved workflow specs in the library (not running workflows)" }),
   spec: Schema.optional(Schema.Record(Schema.String, Schema.Unknown)).annotate({ description: "(start/extend/control replan) Inline structured spec for a one-off graph. Use this or spec_path, never both" }),
   spec_path: Schema.optional(Schema.String).annotate({ description: '(start/extend/control replan) A saved workflow name from the library (e.g. "code-review"), or a path to a YAML workflow spec. Relative paths resolve from the session directory' }),
-  session_id: Schema.optional(Schema.String).annotate({ description: "(start) Parent session ID" }),
+  session_id: Schema.optional(Schema.String).annotate({ description: "(start) Parent session ID; when provided, it must match the calling session" }),
   project_id: Schema.optional(Schema.String).annotate({ description: "(start) Optional Project ID; must match the parent session project" }),
   workflow_id: Schema.optional(Schema.String).annotate({ description: "(extend/control/status) Target workflow ID" }),
   operation: Schema.optional(Schema.Literals(["pause", "resume", "cancel", "replan", "step", "complete"])).annotate({ description: "(control) Operation to perform" }),
@@ -131,6 +131,17 @@ export const WorkflowTool = Tool.define<
     const agents = yield* Agent.Service
     const question = yield* Question.Service
 
+    const requireOwnedWorkflow = Effect.fn("WorkflowTool.requireOwnedWorkflow")(function* (
+      workflowID: string,
+      sessionID: string,
+    ) {
+      const workflow = yield* dag.store.getWorkflow(workflowID).pipe(Effect.orDie)
+      if (!workflow || workflow.sessionId !== sessionID) {
+        return yield* Effect.die(new Error(`Workflow not found: ${workflowID}`))
+      }
+      return workflow
+    })
+
     return {
       description: CommandPlugin.WorkflowContent,
       parameters: Parameters,
@@ -164,8 +175,7 @@ export const WorkflowTool = Tool.define<
             }
             case "status": {
               if (!params.workflow_id) return yield* Effect.die(new Error("status requires 'workflow_id'"))
-              const workflow = yield* dag.store.getWorkflow(params.workflow_id).pipe(Effect.orDie)
-              if (!workflow) return yield* Effect.die(new Error(`Workflow not found: ${params.workflow_id}`))
+              const workflow = yield* requireOwnedWorkflow(params.workflow_id, ctx.sessionID)
               const nodes = yield* dag.store.getNodes(params.workflow_id).pipe(Effect.orDie)
               const config = Dag.parseWorkflowConfig(workflow.config)
               return {
@@ -212,7 +222,10 @@ export const WorkflowTool = Tool.define<
               }
             }
             case "start": {
-              const sessionID = SessionID.make(params.session_id ?? ctx.sessionID)
+              if (params.session_id && params.session_id !== ctx.sessionID) {
+                return yield* Effect.die(new Error("session_id must match the calling session"))
+              }
+              const sessionID = SessionID.make(ctx.sessionID)
               const session = yield* sessions.get(sessionID).pipe(Effect.orDie)
               if (params.project_id && params.project_id !== session.projectID) {
                 return yield* Effect.die(new Error("project_id must match the parent session project"))
@@ -274,6 +287,7 @@ export const WorkflowTool = Tool.define<
             }
             case "extend": {
               if (!params.workflow_id) return yield* Effect.die(new Error("extend requires 'workflow_id'"))
+              yield* requireOwnedWorkflow(params.workflow_id, ctx.sessionID)
               const session = yield* sessions.get(SessionID.make(ctx.sessionID)).pipe(Effect.orDie)
               const specFile = yield* readWorkflowSpec(params.spec, params.spec_path, session.directory, ctx).pipe(Effect.orDie)
               const spec = yield* decodeExtendSpec(specFile.value).pipe(
@@ -297,6 +311,7 @@ export const WorkflowTool = Tool.define<
                 ))
               }
               const wfId = params.workflow_id
+              yield* requireOwnedWorkflow(wfId, ctx.sessionID)
               switch (params.operation) {
                 case "pause":
                   yield* dag.pause(wfId).pipe(Effect.orDie)
diff --git a/packages/opencode/test/dag/workflow-tool.test.ts b/packages/opencode/test/dag/workflow-tool.test.ts
index 4826306568..402136775e 100644
--- a/packages/opencode/test/dag/workflow-tool.test.ts
+++ b/packages/opencode/test/dag/workflow-tool.test.ts
@@ -486,6 +486,45 @@ describe("workflow tool execution", () => {
     }),
   )
 
+  runtime.effect("rejects reads and mutations from a session that does not own the workflow", () =>
+    Effect.gen(function* () {
+      published.length = 0
+      const info = yield* WorkflowTool
+      const workflow = yield* info.init()
+      const foreignContext = {
+        ...toolContext(),
+        sessionID: SessionID.make("ses_foreign"),
+      } satisfies Tool.Context
+
+      const statusExit = yield* Effect.exit(workflow.execute(
+        { action: "status", workflow_id: "dag_status" },
+        foreignContext,
+      ))
+      const extendExit = yield* Effect.exit(workflow.execute(
+        { action: "extend", workflow_id: "dag_defaults", spec: { nodes: [] } },
+        foreignContext,
+      ))
+      const controlExit = yield* Effect.exit(workflow.execute(
+        { action: "control", workflow_id: "dag_status", operation: "pause" },
+        foreignContext,
+      ))
+
+      expect({
+        statusSucceeded: Exit.isSuccess(statusExit),
+        statusLeakedChildSession: Exit.isSuccess(statusExit) && statusExit.value.output.includes("ses_child"),
+        extendSucceeded: Exit.isSuccess(extendExit),
+        controlSucceeded: Exit.isSuccess(controlExit),
+        publishedPause: published.some((event) => event.type === DagEvent.WorkflowPaused.type),
+      }).toEqual({
+        statusSucceeded: false,
+        statusLeakedChildSession: false,
+        extendSucceeded: false,
+        controlSucceeded: false,
+        publishedPause: false,
+      })
+    }),
+  )
+
   runtime.effect("dispatches every public control operation to its durable workflow event", () =>
     Effect.gen(function* () {
       const info = yield* WorkflowTool
@@ -1286,6 +1325,32 @@ config:
       expect(published).toHaveLength(0)
     }),
   )
+
+  runtime.effect("start rejects a parent session other than the calling session", () =>
+    Effect.gen(function* () {
+      published.length = 0
+      const info = yield* WorkflowTool
+      const workflow = yield* info.init()
+      const exit = yield* workflow
+        .execute(
+          {
+            action: "start",
+            session_id: "ses_other_parent",
+            spec: {
+              config: {
+                name: "foreign-parent",
+                nodes: [],
+              },
+            },
+          },
+          toolContext(),
+        )
+        .pipe(Effect.exit)
+
+      expect(Exit.isFailure(exit)).toBe(true)
+      expect(published).toHaveLength(0)
+    }),
+  )
 })
 
 describe("workflow tool saved workflows", () => {