Function withSharedRoot

withSharedRoot<T>(
opts: SharedRootOptions,
body: (root: Branch, sharedPrefixLength: number) => Operation<T>,
): Operation<T>
Scoped shared root branch with guaranteed cleanup

Creates a root branch, prefills the system prompt, and passes it to the body function. The root is pruned via try/finally when the body returns or throws, regardless of whether children still exist.

Use this for the cold-path pattern where multiple agents share a tokenized system prompt prefix. The sharedPrefixLength passed to the body enables KV savings calculation.
Type Parameters
- T
Parameters
- opts: SharedRootOptions
  System prompt, tools, and sampling parameters
- body: (root: Branch, sharedPrefixLength: number) => Operation<T>
  Operation that receives the root branch and prefix length. Typically calls runAgents or useAgentPool inside.
Returns Operation<T>
The body's return value
Example: Cold-path research with shared prefix
```
const { result, prefixLen } = yield* withSharedRoot(
  { systemPrompt: RESEARCH_PROMPT, tools: toolsJson },
  function*(root, prefixLen) {
    const result = yield* runAgents({
      tasks: questions.map(q => ({
        systemPrompt: RESEARCH_PROMPT,
        content: q,
        tools: toolsJson,
        parent: root,
      })),
      tools: toolMap,
    });
    return { result, prefixLen };
  },
);
```
- Defined in agents/src/shared-root.ts:60