diff --git a/examples/echo-agent/src/mock-llm.ts b/examples/echo-agent/src/mock-llm.ts
index 441e5639e3..93132711de 100644
--- a/examples/echo-agent/src/mock-llm.ts
+++ b/examples/echo-agent/src/mock-llm.ts
@@ -1,6 +1,6 @@
 import type { Context } from 'cordis'
 import type { GenerateOptions, StreamChunk } from '@deepseek-ai/dsh-llm'
-import { LlmAdapter } from '@deepseek-ai/dsh-llm'
+import { CallId, LlmAdapter } from '@deepseek-ai/dsh-llm'
 
 /**
  * Demo adapter for the `mock-echo` model.
@@ -30,8 +30,8 @@ class MockEchoAdapter extends LlmAdapter {
       }
       yield { type: 'block-end', index: 0, block: { type: 'text', text: 'Let me echo that for you.' } }
       yield { type: 'block-start', index: 1, blockType: 'tool-call' }
-      yield { type: 'tool-call-delta', index: 1, id: 'call-echo', name: 'echo', argumentsDelta: args }
-      yield { type: 'block-end', index: 1, block: { type: 'tool-call', id: 'call-echo', name: 'echo', arguments: args } }
+      yield { type: 'tool-call-delta', index: 1, id: CallId('call-echo'), name: 'echo', argumentsDelta: args }
+      yield { type: 'block-end', index: 1, block: { type: 'tool-call', id: CallId('call-echo'), name: 'echo', arguments: args } }
       yield { type: 'usage', usage: { inputTokens: 20, outputTokens: 10 } }
       yield { type: 'finish', reason: { kind: 'tool-calls' } }
       return
diff --git a/packages/agent-loop/src/agent.ts b/packages/agent-loop/src/agent.ts
index 3c9a2ad914..bc8c72650f 100644
--- a/packages/agent-loop/src/agent.ts
+++ b/packages/agent-loop/src/agent.ts
@@ -7,7 +7,7 @@
  */
 
 import type { Context } from 'cordis'
-import type { AgentOptions, AgentStatus, SendOptions } from '@deepseek-ai/dsh-agent'
+import type { AgentId, AgentOptions, AgentStatus, SendOptions } from '@deepseek-ai/dsh-agent'
 import type { Agent } from '@deepseek-ai/dsh-agent'
 import type { ContentBlock, MessageSource } from '@deepseek-ai/dsh-llm'
 import type { Session } from '@deepseek-ai/dsh-session'
@@ -33,7 +33,7 @@ export class LoopAgent implements Agent {
 
   constructor(
     private ctx: Context,
-    public readonly id: string,
+    public readonly id: AgentId,
     public readonly options: AgentOptions,
     public readonly session: Session,
   ) {
diff --git a/packages/agent-loop/src/index.ts b/packages/agent-loop/src/index.ts
index 6484ec9ace..460bf52a88 100644
--- a/packages/agent-loop/src/index.ts
+++ b/packages/agent-loop/src/index.ts
@@ -9,6 +9,7 @@
 
 import { Context, Service } from 'cordis'
 import z from 'schemastery'
+import { AgentId } from '@deepseek-ai/dsh-agent'
 import type { AgentOptions } from '@deepseek-ai/dsh-agent'
 import type {} from '@deepseek-ai/dsh-llm'
 import type {} from '@deepseek-ai/dsh-session'
@@ -67,7 +68,7 @@ export class AgentLoop extends Service {
    */
   create(id: string, options: AgentOptions = {}): LoopAgent {
     const session = this.ctx.sessions.create(`${id}-session`)
-    const agent = new LoopAgent(this.ctx, id, options, session)
+    const agent = new LoopAgent(this.ctx, AgentId(id), options, session)
     // Generator effect: stop and unregister are independent disposables
     // (LIFO), so a throwing stop() cannot leak the registry entry.
     this.ctx.effect(function* (this: AgentLoop) {
diff --git a/packages/agent-loop/tests/agent.spec.ts b/packages/agent-loop/tests/agent.spec.ts
index 8cd591a6f2..26933188fc 100644
--- a/packages/agent-loop/tests/agent.spec.ts
+++ b/packages/agent-loop/tests/agent.spec.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
+import { AgentId } from '@deepseek-ai/dsh-agent'
 import LlmService from '@deepseek-ai/dsh-llm'
 import SessionStore from '@deepseek-ai/dsh-session'
 import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
@@ -101,7 +102,7 @@ describe('LoopAgent', () => {
     const ctx = new Context()
     await ctx.plugin(SessionStore)
     const session = ctx.sessions.create('test')
-    const agent = new LoopAgent(ctx, 'bare', { model: 'mock' }, session)
+    const agent = new LoopAgent(ctx, AgentId('bare'), { model: 'mock' }, session)
 
     // Start the loop to get the disposer; the agent waits for messages
     // (idle, never-resolving cancel), so it will stay idle.
diff --git a/packages/agent-loop/tests/coverage-edges.spec.ts b/packages/agent-loop/tests/coverage-edges.spec.ts
index 77d39d15d8..4d1f03c01e 100644
--- a/packages/agent-loop/tests/coverage-edges.spec.ts
+++ b/packages/agent-loop/tests/coverage-edges.spec.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
-import LlmService, { LlmError, StreamChunk } from '@deepseek-ai/dsh-llm'
+import LlmService, { CallId, LlmError, StreamChunk } from '@deepseek-ai/dsh-llm'
 import SessionStore, { TurnEndReason } from '@deepseek-ai/dsh-session'
 import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
 import ToolRegistry, { defineTool } from '@deepseek-ai/dsh-tools'
@@ -100,7 +100,7 @@ describe('tool JSON parse', () => {
       // model emits tool-call with malformed arguments (not valid JSON)
       [
         { type: 'block-start' as const, index: 0, blockType: 'tool-call' as const },
-        { type: 'block-end' as const, index: 0, block: { type: 'tool-call' as const, id: 'c1', name: 'echo', arguments: 'not json' } },
+        { type: 'block-end' as const, index: 0, block: { type: 'tool-call' as const, id: CallId('c1'), name: 'echo', arguments: 'not json' } },
         { type: 'finish' as const, reason: { kind: 'tool-calls' as const } },
       ] satisfies StreamChunk[],
       textResponse('done'),
@@ -133,7 +133,7 @@ describe('tool JSON parse', () => {
     const adapter = new MockAdapter([
       [
         { type: 'block-start' as const, index: 0, blockType: 'tool-call' as const },
-        { type: 'block-end' as const, index: 0, block: { type: 'tool-call' as const, id: 'c1', name: 'noarg', arguments: '' } },
+        { type: 'block-end' as const, index: 0, block: { type: 'tool-call' as const, id: CallId('c1'), name: 'noarg', arguments: '' } },
         { type: 'finish' as const, reason: { kind: 'tool-calls' as const } },
       ] satisfies StreamChunk[],
       textResponse('done'),
diff --git a/packages/agent-loop/tests/mock-adapter.ts b/packages/agent-loop/tests/mock-adapter.ts
index 416f3b444d..f6cb47cda3 100644
--- a/packages/agent-loop/tests/mock-adapter.ts
+++ b/packages/agent-loop/tests/mock-adapter.ts
@@ -1,5 +1,5 @@
 import type { GenerateOptions, StreamChunk } from '@deepseek-ai/dsh-llm'
-import { LlmAdapter } from '@deepseek-ai/dsh-llm'
+import { CallId, LlmAdapter } from '@deepseek-ai/dsh-llm'
 
 /** Helpers to write scripted responses tersely. */
 export function textResponse(text: string): StreamChunk[] {
@@ -12,7 +12,8 @@ export function textResponse(text: string): StreamChunk[] {
   ]
 }
 
-export function toolCallResponse(callId: string, name: string, args: object, text?: string): StreamChunk[] {
+export function toolCallResponse(rawCallId: string, name: string, args: object, text?: string): StreamChunk[] {
+  const callId = CallId(rawCallId)
   const argumentsJson = JSON.stringify(args)
   const chunks: StreamChunk[] = []
   let index = 0
diff --git a/packages/agent-loop/tests/review-fixes.spec.ts b/packages/agent-loop/tests/review-fixes.spec.ts
index f5cea25ef2..4ac6152308 100644
--- a/packages/agent-loop/tests/review-fixes.spec.ts
+++ b/packages/agent-loop/tests/review-fixes.spec.ts
@@ -1,10 +1,10 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
-import LlmService, { ContentBlock, MessageSource, StreamChunk } from '@deepseek-ai/dsh-llm'
-import SessionStore, { Session, SessionEvent, TurnEndReason } from '@deepseek-ai/dsh-session'
+import LlmService, { CallId, ContentBlock, MessageSource, StreamChunk } from '@deepseek-ai/dsh-llm'
+import SessionStore, { Session, SessionEvent, SessionId, TurnEndReason } from '@deepseek-ai/dsh-session'
 import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
 import ToolRegistry, { defineTool } from '@deepseek-ai/dsh-tools'
-import AgentRegistry from '@deepseek-ai/dsh-agent'
+import AgentRegistry, { AgentId } from '@deepseek-ai/dsh-agent'
 import AgentLoop, { LoopAgent } from '@deepseek-ai/dsh-agent-loop'
 import { MockAdapter, textResponse, toolCallResponse } from './mock-adapter.ts'
 
@@ -65,7 +65,7 @@ describe('HIGH: session log records what agent/step-result actually produced', (
         role: 'assistant' as const,
         content: [
           { type: 'text' as const, text: 'rewritten' },
-          { type: 'tool-call' as const, id: 'c-injected', name: 'injected-tool', arguments: '{}' },
+          { type: 'tool-call' as const, id: CallId('c-injected'), name: 'injected-tool', arguments: '{}' },
         ],
       }
     })
@@ -96,9 +96,9 @@ describe('HIGH: abort during tool execution ends the turn', () => {
       // model asks for two tool calls in one step
       [
         { type: 'block-start', index: 0, blockType: 'tool-call' },
-        { type: 'block-end', index: 0, block: { type: 'tool-call', id: 'c1', name: 'aborter', arguments: '{}' } },
+        { type: 'block-end', index: 0, block: { type: 'tool-call', id: CallId('c1'), name: 'aborter', arguments: '{}' } },
         { type: 'block-start', index: 1, blockType: 'tool-call' },
-        { type: 'block-end', index: 1, block: { type: 'tool-call', id: 'c2', name: 'second', arguments: '{}' } },
+        { type: 'block-end', index: 1, block: { type: 'tool-call', id: CallId('c2'), name: 'second', arguments: '{}' } },
         { type: 'finish', reason: { kind: 'tool-calls' } },
       ] satisfies StreamChunk[],
       textResponse('should never be requested'),
@@ -429,7 +429,7 @@ describe('MEDIUM: turn numbering continues across seeded (forked) sessions', ()
     ctx2.llm.registerAdapter(['mock'], second)
 
     const seeded = ctx2.sessions.create('forked', [...agent.session.events])
-    const forked = new LoopAgent(ctx2, 'forked-agent', { model: 'mock' }, seeded)
+    const forked = new LoopAgent(ctx2, AgentId('forked-agent'), { model: 'mock' }, seeded)
     ctx2.effect(() => forked.start())
 
     const turns: number[] = []
@@ -459,10 +459,10 @@ describe('LOW: BlockAssembler and streamBlocks edge cases', () => {
   it('assembles tool-call blocks from deltas without block-end', async () => {
     const { BlockAssembler } = await import('@deepseek-ai/dsh-llm')
     const assembler = new BlockAssembler()
-    assembler.push({ type: 'tool-call-delta', index: 0, id: 'c9', name: 'echo', argumentsDelta: '{"a"' })
-    assembler.push({ type: 'tool-call-delta', index: 0, id: 'c9', argumentsDelta: ':1}' })
+    assembler.push({ type: 'tool-call-delta', index: 0, id: CallId('c9'), name: 'echo', argumentsDelta: '{"a"' })
+    assembler.push({ type: 'tool-call-delta', index: 0, id: CallId('c9'), argumentsDelta: ':1}' })
     expect(assembler.blocks()).toEqual([
-      { type: 'tool-call', id: 'c9', name: 'echo', arguments: '{"a":1}' },
+      { type: 'tool-call', id: CallId('c9'), name: 'echo', arguments: '{"a":1}' },
     ])
   })
 
@@ -531,9 +531,9 @@ describe('LOW: BlockAssembler and streamBlocks edge cases', () => {
 
 describe('LOW: discriminated SessionEvent narrows without casts', () => {
   it('narrows event.data from event.type', () => {
-    const session = new Session('s')
+    const session = new Session(SessionId('s'))
     const appended: SessionEvent = session.append('tool/call', {
-      turn: 1, step: 1, callId: 'c1', name: 'echo', arguments: '{}',
+      turn: 1, step: 1, callId: CallId('c1'), name: 'echo', arguments: '{}',
     })
     // compile-time: this switch narrows; runtime: values flow through
     switch (appended.type) {
diff --git a/packages/agent/src/types.ts b/packages/agent/src/types.ts
index f54f8d5d8e..0441938b6d 100644
--- a/packages/agent/src/types.ts
+++ b/packages/agent/src/types.ts
@@ -9,7 +9,15 @@
  * @module @deepseek-ai/dsh-agent/types
  */
 
-import type { ContentBlock, GenerateOptions, Message, MessageSource, StreamChunk } from '@deepseek-ai/dsh-llm'
+import type { Branded, ContentBlock, GenerateOptions, Message, MessageSource, StreamChunk } from '@deepseek-ai/dsh-llm'
+
+/** Identifies one live agent in the registry. */
+export type AgentId = Branded<'AgentId'>
+
+/** Brand a string as an {@link AgentId}. */
+export function AgentId(id: string): AgentId {
+  return id as AgentId
+}
 import type { Session, TurnEndReason } from '@deepseek-ai/dsh-session'
 
 /**
@@ -36,7 +44,7 @@ export type AgentStatus = 'idle' | 'running' | 'disposed'
  * package should depend on the implementation.
  */
 export interface Agent {
-  readonly id: string
+  readonly id: AgentId
   readonly options: AgentOptions
   readonly session: Session
   readonly status: AgentStatus
diff --git a/packages/agent/tests/agent.spec.ts b/packages/agent/tests/agent.spec.ts
index a90918044a..5d66035421 100644
--- a/packages/agent/tests/agent.spec.ts
+++ b/packages/agent/tests/agent.spec.ts
@@ -1,13 +1,14 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
-import { Session } from '@deepseek-ai/dsh-session'
-import AgentRegistry, { Agent } from '@deepseek-ai/dsh-agent'
+import { Session, SessionId } from '@deepseek-ai/dsh-session'
+import AgentRegistry, { Agent, AgentId } from '@deepseek-ai/dsh-agent'
 
-function stubAgent(id: string): Agent {
+function stubAgent(rawId: string): Agent {
+  const id = AgentId(rawId)
   return {
     id,
     options: {},
-    session: new Session(`${id}-session`),
+    session: new Session(SessionId(`${id}-session`)),
     status: 'idle',
     send() {},
     steer() {},
diff --git a/packages/llm/src/assembler.ts b/packages/llm/src/assembler.ts
index 65cd98dcda..a75b9a7418 100644
--- a/packages/llm/src/assembler.ts
+++ b/packages/llm/src/assembler.ts
@@ -5,12 +5,13 @@
  * @module @deepseek-ai/dsh-llm/assembler
  */
 
+import { CallId } from './brand.ts'
 import type { ContentBlock, FinishReason, GenerateResult, Message, StreamChunk, TokenUsage } from './types.ts'
 
 interface PartialBlock {
   blockType: string
   text: string
-  toolCallId?: string
+  toolCallId?: CallId
   toolCallName?: string
   toolCallArguments: string
   /** Set by `block-end` — authoritative, and freezes the partial. */
@@ -101,7 +102,7 @@ export class BlockAssembler {
       case 'reasoning': return { type: 'reasoning', text: partial.text }
       case 'tool-call': return {
         type: 'tool-call',
-        id: partial.toolCallId ?? `call-${index}`,
+        id: partial.toolCallId ?? CallId(`call-${index}`),
         name: partial.toolCallName ?? '',
         arguments: partial.toolCallArguments,
       }
diff --git a/packages/llm/src/brand.ts b/packages/llm/src/brand.ts
new file mode 100644
index 0000000000..38d69fe37b
--- /dev/null
+++ b/packages/llm/src/brand.ts
@@ -0,0 +1,32 @@
+/**
+ * Branded (nominal) ID types.
+ *
+ * A brand makes structurally-identical strings non-interchangeable at the
+ * type level: an `AgentId` cannot be passed where a `CallId` is expected,
+ * even though both are strings at runtime. Construction goes through the
+ * per-type factory (a plain cast inside — zero runtime cost); comparison,
+ * logging, and serialization all behave as ordinary strings.
+ *
+ * Policy: core packages brand the IDs they own — `CallId` here (tool-call
+ * correlation), `SessionId` in dsh-session, `AgentId` in dsh-agent. Branding
+ * is for IDs that cross package boundaries and could plausibly be confused;
+ * not every string needs a brand.
+ *
+ * @module @deepseek-ai/dsh-llm/brand
+ */
+
+declare const BRAND: unique symbol
+
+/** A string carrying a compile-time-only brand `B`. */
+export type Branded<B extends string> = string & { readonly [BRAND]: B }
+
+/**
+ * Correlates a model-issued tool call with its result. Provider-issued for
+ * real adapters; synthesized by mocks/assembler fallbacks.
+ */
+export type CallId = Branded<'CallId'>
+
+/** Brand a string as a {@link CallId}. */
+export function CallId(id: string): CallId {
+  return id as CallId
+}
diff --git a/packages/llm/src/index.ts b/packages/llm/src/index.ts
index 9a1bad3f29..4158853d84 100644
--- a/packages/llm/src/index.ts
+++ b/packages/llm/src/index.ts
@@ -10,6 +10,7 @@ import { Context, Service } from 'cordis'
 import type { ContentBlock, GenerateOptions, GenerateResult, StreamChunk } from './types.ts'
 import { BlockAssembler } from './assembler.ts'
 
+export * from './brand.ts'
 export * from './types.ts'
 export { BlockAssembler } from './assembler.ts'
 
diff --git a/packages/llm/src/types.ts b/packages/llm/src/types.ts
index 2b16095e74..484a172b38 100644
--- a/packages/llm/src/types.ts
+++ b/packages/llm/src/types.ts
@@ -19,6 +19,8 @@
  * ```
  */
 
+import type { CallId } from './brand.ts'
+
 /** Cache hint attached to a content block (provider-interpreted). */
 export type CacheHint = 'ephemeral'
 
@@ -39,7 +41,7 @@ export interface ReasoningBlock {
 export interface ToolCallBlock {
   type: 'tool-call'
   /** Provider-issued call id; correlates with the matching tool result. */
-  id: string
+  id: CallId
   name: string
   /** Raw JSON string as produced by the model. */
   arguments: string
@@ -48,7 +50,7 @@ export interface ToolCallBlock {
 /** The result of a tool invocation, sent back to the model. */
 export interface ToolResultBlock {
   type: 'tool-result'
-  toolCallId: string
+  toolCallId: CallId
   content: ContentBlock[]
   isError?: boolean
   cache?: CacheHint
@@ -134,7 +136,7 @@ export type StreamChunk =
   | { type: 'block-start'; index: number; blockType: ContentBlockType }
   | { type: 'text-delta'; index: number; text: string }
   | { type: 'reasoning-delta'; index: number; text: string }
-  | { type: 'tool-call-delta'; index: number; id: string; name?: string; argumentsDelta: string }
+  | { type: 'tool-call-delta'; index: number; id: CallId; name?: string; argumentsDelta: string }
   | { type: 'block-end'; index: number; block: ContentBlock }
   | { type: 'usage'; usage: TokenUsage }
   | { type: 'finish'; reason: FinishReason }
diff --git a/packages/llm/tests/assembler.spec.ts b/packages/llm/tests/assembler.spec.ts
index 9fdf3d541f..db8cb32820 100644
--- a/packages/llm/tests/assembler.spec.ts
+++ b/packages/llm/tests/assembler.spec.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest'
-import { BlockAssembler, type StreamChunk } from '@deepseek-ai/dsh-llm'
+import { BlockAssembler, CallId, type StreamChunk } from '@deepseek-ai/dsh-llm'
 
 describe('BlockAssembler', () => {
   it('assembles interleaved text, reasoning, and tool-call deltas', () => {
@@ -11,8 +11,8 @@ describe('BlockAssembler', () => {
       { type: 'text-delta', index: 1, text: 'Hello' },
       { type: 'text-delta', index: 1, text: ' world' },
       { type: 'block-start', index: 2, blockType: 'tool-call' },
-      { type: 'tool-call-delta', index: 2, id: 'call-1', name: 'echo', argumentsDelta: '{"text":' },
-      { type: 'tool-call-delta', index: 2, id: 'call-1', argumentsDelta: '"hi"}' },
+      { type: 'tool-call-delta', index: 2, id: CallId('call-1'), name: 'echo', argumentsDelta: '{"text":' },
+      { type: 'tool-call-delta', index: 2, id: CallId('call-1'), argumentsDelta: '"hi"}' },
       { type: 'usage', usage: { inputTokens: 10, outputTokens: 5 } },
       { type: 'finish', reason: { kind: 'tool-calls' } },
     ]
@@ -22,7 +22,7 @@ describe('BlockAssembler', () => {
     expect(assembler.blocks()).toEqual([
       { type: 'reasoning', text: 'thinking…' },
       { type: 'text', text: 'Hello world' },
-      { type: 'tool-call', id: 'call-1', name: 'echo', arguments: '{"text":"hi"}' },
+      { type: 'tool-call', id: CallId('call-1'), name: 'echo', arguments: '{"text":"hi"}' },
     ])
     expect(assembler.usage).toEqual({ inputTokens: 10, outputTokens: 5 })
     expect(assembler.finish).toEqual({ kind: 'tool-calls' })
@@ -125,11 +125,11 @@ describe('BlockAssembler', () => {
   it('ignores tool-call-delta stragglers after block-end', () => {
     const assembler = new BlockAssembler()
     assembler.push({ type: 'block-start', index: 0, blockType: 'tool-call' })
-    assembler.push({ type: 'tool-call-delta', index: 0, id: 'c1', name: 'echo', argumentsDelta: '{}' })
-    assembler.push({ type: 'block-end', index: 0, block: { type: 'tool-call', id: 'c1', name: 'echo', arguments: '{}' } })
+    assembler.push({ type: 'tool-call-delta', index: 0, id: CallId('c1'), name: 'echo', argumentsDelta: '{}' })
+    assembler.push({ type: 'block-end', index: 0, block: { type: 'tool-call', id: CallId('c1'), name: 'echo', arguments: '{}' } })
     // straggler after block-end — partial.block is set, so early return
-    assembler.push({ type: 'tool-call-delta', index: 0, id: 'c1', name: 'evil', argumentsDelta: 'oops' })
-    expect(assembler.blocks()).toEqual([{ type: 'tool-call', id: 'c1', name: 'echo', arguments: '{}' }])
+    assembler.push({ type: 'tool-call-delta', index: 0, id: CallId('c1'), name: 'evil', argumentsDelta: 'oops' })
+    expect(assembler.blocks()).toEqual([{ type: 'tool-call', id: CallId('c1'), name: 'echo', arguments: '{}' }])
   })
 
   it('assembles tool-call with generated id fallback when no id provided', () => {
@@ -138,7 +138,7 @@ describe('BlockAssembler', () => {
     // No id and no name provided — uses fallback id `call-{index}` and empty name
     const blocks = assembler.blocks()
     expect(blocks).toEqual([
-      { type: 'tool-call', id: 'call-0', name: '', arguments: '{}' },
+      { type: 'tool-call', id: CallId('call-0'), name: '', arguments: '{}' },
     ])
   })
 
diff --git a/packages/session/src/index.ts b/packages/session/src/index.ts
index b19fc47336..7b3b04812c 100644
--- a/packages/session/src/index.ts
+++ b/packages/session/src/index.ts
@@ -8,6 +8,7 @@
 
 import { Context, Service } from 'cordis'
 import type { ContentBlock, Message, MessageSource } from '@deepseek-ai/dsh-llm'
+import { SessionId } from './types.ts'
 import type { SessionEvent, SessionEventMap, SessionEventType } from './types.ts'
 
 export * from './types.ts'
@@ -60,7 +61,7 @@ export class Session {
   /** Set by the store so appends are observable; undefined when detached. */
   onAppend: ((event: SessionEvent) => void) | undefined
 
-  constructor(public readonly id: string, seed?: SessionEvent[]) {
+  constructor(public readonly id: SessionId, seed?: SessionEvent[]) {
     if (seed) this.log = [...seed]
   }
 
@@ -155,16 +156,16 @@ export class SessionStore extends Service {
    * session from the store.
    */
   create(id?: string, seed?: SessionEvent[]): Session {
-    id ??= `session-${++this.counter}`
-    if (this.store.has(id)) throw new Error(`session "${id}" already exists`)
-    const session = new Session(id, seed)
+    const sessionId = SessionId(id ?? `session-${++this.counter}`)
+    if (this.store.has(sessionId)) throw new Error(`session "${sessionId}" already exists`)
+    const session = new Session(sessionId, seed)
     this.ctx.effect(() => {
       session.onAppend = (event) => { this.ctx.emit('session/event', session, event) }
-      this.store.set(id, session)
+      this.store.set(sessionId, session)
       this.ctx.emit('session/created', session)
       return () => {
         session.onAppend = undefined
-        this.store.delete(id)
+        this.store.delete(sessionId)
       }
     }, 'sessions.create()')
     return session
diff --git a/packages/session/src/types.ts b/packages/session/src/types.ts
index 44e904a55c..25628bc3a6 100644
--- a/packages/session/src/types.ts
+++ b/packages/session/src/types.ts
@@ -1,4 +1,12 @@
-import type { ContentBlock, MessageSource, StreamChunk, TokenUsage } from '@deepseek-ai/dsh-llm'
+import type { Branded, CallId, ContentBlock, MessageSource, StreamChunk, TokenUsage } from '@deepseek-ai/dsh-llm'
+
+/** Identifies one session in the store (and its persistence artifacts). */
+export type SessionId = Branded<'SessionId'>
+
+/** Brand a string as a {@link SessionId}. */
+export function SessionId(id: string): SessionId {
+  return id as SessionId
+}
 
 /**
  * What started a turn.
@@ -53,8 +61,8 @@ export interface SessionEventMap {
   'assistant/chunk': { turn: number; step: number; chunk: StreamChunk }
   /** Assembled assistant message for one step (derived history uses this). */
   'assistant/message': { turn: number; step: number; content: ContentBlock[] }
-  'tool/call': { turn: number; step: number; callId: string; name: string; arguments: string }
-  'tool/result': { turn: number; step: number; callId: string; content: ContentBlock[]; isError: boolean }
+  'tool/call': { turn: number; step: number; callId: CallId; name: string; arguments: string }
+  'tool/result': { turn: number; step: number; callId: CallId; content: ContentBlock[]; isError: boolean }
   /** Steering content injected between steps of a running turn. */
   'steering/message': { turn: number; content: ContentBlock[]; source: MessageSource }
   'usage': { turn: number; step: number; usage: TokenUsage }
diff --git a/packages/session/tests/session.spec.ts b/packages/session/tests/session.spec.ts
index 68e5d69458..44f2bae980 100644
--- a/packages/session/tests/session.spec.ts
+++ b/packages/session/tests/session.spec.ts
@@ -1,10 +1,11 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
-import SessionStore, { Session, SessionEvent } from '@deepseek-ai/dsh-session'
+import { CallId } from '@deepseek-ai/dsh-llm'
+import SessionStore, { Session, SessionEvent, SessionId } from '@deepseek-ai/dsh-session'
 
 describe('Session', () => {
   it('derives message history from the event log', () => {
-    const session = new Session('s1')
+    const session = new Session(SessionId('s1'))
     session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
     session.append('user/message', { content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } })
     session.append('assistant/chunk', { turn: 1, step: 1, chunk: { type: 'text-delta', index: 0, text: 'hi' } })
@@ -12,21 +13,21 @@ describe('Session', () => {
       turn: 1, step: 1,
       content: [
         { type: 'text', text: 'let me check' },
-        { type: 'tool-call', id: 'c1', name: 'echo', arguments: '{}' },
+        { type: 'tool-call', id: CallId('c1'), name: 'echo', arguments: '{}' },
       ],
     })
-    session.append('tool/result', { turn: 1, step: 1, callId: 'c1', content: [{ type: 'text', text: 'ok' }], isError: false })
+    session.append('tool/result', { turn: 1, step: 1, callId: CallId('c1'), content: [{ type: 'text', text: 'ok' }], isError: false })
     session.append('turn/end', { turn: 1, reason: { kind: 'completed' } })
 
     const messages = session.deriveMessages()
     expect(messages.map(m => m.role)).toEqual(['user', 'assistant', 'user'])
     // raw chunks must NOT appear in derived history
     expect(messages[1]!.content).toHaveLength(2)
-    expect(messages[2]!.content[0]).toMatchObject({ type: 'tool-result', toolCallId: 'c1' })
+    expect(messages[2]!.content[0]).toMatchObject({ type: 'tool-result', toolCallId: CallId('c1') })
   })
 
   it('renders context and steering messages as tagged synthetic user content', () => {
-    const session = new Session('s2')
+    const session = new Session(SessionId('s2'))
     session.append('context/message', {
       content: [{ type: 'text', text: 'file changed: a.ts' }],
       source: { kind: 'plugin', plugin: 'watcher' },
@@ -45,11 +46,11 @@ describe('Session', () => {
   })
 
   it('replays identically from a seeded event log', () => {
-    const original = new Session('s3')
+    const original = new Session(SessionId('s3'))
     original.append('user/message', { content: [{ type: 'text', text: 'q' }], source: { kind: 'user' } })
     original.append('assistant/message', { turn: 1, step: 1, content: [{ type: 'text', text: 'a' }] })
 
-    const replayed = new Session('s3-replay', [...original.events])
+    const replayed = new Session(SessionId('s3-replay'), [...original.events])
     expect(replayed.deriveMessages()).toEqual(original.deriveMessages())
     expect(replayed.seq).toBe(original.seq)
   })
diff --git a/packages/tools/src/index.ts b/packages/tools/src/index.ts
index 05b84dda3d..abb6085982 100644
--- a/packages/tools/src/index.ts
+++ b/packages/tools/src/index.ts
@@ -8,7 +8,7 @@
  */
 
 import { Context, Service } from 'cordis'
-import type { ContentBlock, ToolSchema } from '@deepseek-ai/dsh-llm'
+import type { CallId, ContentBlock, ToolSchema } from '@deepseek-ai/dsh-llm'
 import type { Agent } from '@deepseek-ai/dsh-agent'
 import type {} from '@deepseek-ai/dsh-system-prompt'
 
@@ -54,7 +54,7 @@ export interface ToolDefinition extends ToolSchema {
 
 /** One pending tool call, as it flows through the execution waterfall. */
 export interface ToolExecution {
-  callId: string
+  callId: CallId
   name: string
   /** Parsed JSON arguments (unknown — tools validate their own input). */
   arguments: unknown
@@ -65,7 +65,7 @@ export interface ToolExecution {
 
 /** The outcome of one tool call. */
 export interface ToolExecutionResult {
-  callId: string
+  callId: CallId
   content: ContentBlock[]
   isError: boolean
 }
diff --git a/packages/tools/tests/tools.spec.ts b/packages/tools/tests/tools.spec.ts
index 40bed2c936..b4500bcfe3 100644
--- a/packages/tools/tests/tools.spec.ts
+++ b/packages/tools/tests/tools.spec.ts
@@ -1,5 +1,6 @@
 import { describe, expect, expectTypeOf, it } from 'vitest'
 import { Context } from 'cordis'
+import { CallId } from '@deepseek-ai/dsh-llm'
 import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
 import ToolRegistry, {
   defineTool, schemaSpecToJsonSchema,
@@ -43,8 +44,8 @@ describe('ToolRegistry', () => {
   it('executes a tool and returns its content', async () => {
     const ctx = await setup()
     ctx.tools.register(echoTool)
-    const result = await ctx.tools.execute({ callId: 'c1', name: 'echo', arguments: { text: 'hi' } })
-    expect(result).toEqual({ callId: 'c1', content: [{ type: 'text', text: 'hi' }], isError: false })
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'hi' } })
+    expect(result).toEqual({ callId: CallId('c1'), content: [{ type: 'text', text: 'hi' }], isError: false })
   })
 
   it('returns isError results for unknown tools and throwing tools', async () => {
@@ -57,10 +58,10 @@ describe('ToolRegistry', () => {
       },
     })
 
-    const unknown = await ctx.tools.execute({ callId: 'c1', name: 'nope', arguments: {} })
+    const unknown = await ctx.tools.execute({ callId: CallId('c1'), name: 'nope', arguments: {} })
     expect(unknown.isError).toBe(true)
 
-    const thrown = await ctx.tools.execute({ callId: 'c2', name: 'boom', arguments: {} })
+    const thrown = await ctx.tools.execute({ callId: CallId('c2'), name: 'boom', arguments: {} })
     expect(thrown.isError).toBe(true)
     expect(thrown.content[0]).toMatchObject({ text: 'Error: exploded' })
   })
@@ -80,7 +81,7 @@ describe('ToolRegistry', () => {
       return next()
     })
 
-    const result = await ctx.tools.execute({ callId: 'c1', name: 'echo', arguments: { text: 'hi' } })
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'hi' } })
     expect(result.isError).toBe(true)
     expect(result.content[0]).toMatchObject({ text: 'denied by policy' })
   })
@@ -103,7 +104,7 @@ describe('ToolRegistry', () => {
       return result
     })
 
-    const result = await ctx.tools.execute({ callId: 'c1', name: 'echo', arguments: { text: 'x' } })
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'x' } })
     expect(result.isError).toBe(false)
     expect(order).toEqual(['first:before', 'second:before', 'second:after', 'first:after'])
   })
@@ -220,7 +221,7 @@ describe('defineTool / schema DSL', () => {
     }])
 
     const result = await ctx.tools.execute({
-      callId: 'c1',
+      callId: CallId('c1'),
       name: 'typed-echo',
       arguments: { text: 'hello', uppercase: true },
     })
@@ -274,7 +275,7 @@ describe('defineTool / schema DSL', () => {
 
     // Execution round-trip
     const result = await ctx.tools.execute({
-      callId: 'c1',
+      callId: CallId('c1'),
       name: 'roundtrip',
       arguments: { req: 'hello' },
     })
@@ -306,7 +307,7 @@ describe('defineTool / schema DSL', () => {
     })
 
     const result = await ctx.tools.execute({
-      callId: 'c1',
+      callId: CallId('c1'),
       name: 'raw-tool',
       arguments: { path: '/tmp' },
     })
@@ -519,7 +520,7 @@ describe('schema DSL regressions (Codex review round 2)', () => {
         throw { message: 'denied by object' }
       },
     })
-    const result = await ctx.tools.execute({ callId: 'c1', name: 'object-thrower', arguments: {} })
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'object-thrower', arguments: {} })
     expect(result.isError).toBe(true)
     expect(result.content[0]).toMatchObject({ text: 'Error: denied by object' })
   })
@@ -534,7 +535,7 @@ describe('schema DSL regressions (Codex review round 2)', () => {
         throw 'kaboom'
       },
     })
-    const result = await ctx.tools.execute({ callId: 'c1', name: 'string-thrower', arguments: {} })
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'string-thrower', arguments: {} })
     expect(result.isError).toBe(true)
     expect(result.content[0]).toMatchObject({ text: 'Error: kaboom' })
   })
@@ -549,7 +550,7 @@ describe('schema DSL regressions (Codex review round 2)', () => {
         throw { code: 500 }
       },
     })
-    const result = await ctx.tools.execute({ callId: 'c1', name: 'object-no-message', arguments: {} })
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'object-no-message', arguments: {} })
     expect(result.isError).toBe(true)
     const firstContent = result.content[0]!
     expect(firstContent.type).toBe('text')