fix(session): type turn/end error as one structured failure

TurnEndReasonMap.error now carries a single `error: LlmFailure` field:
an LlmError keeps its structured facts, any other error flattens to
errorChain text under the UNKNOWN code. Consumers read message/code
directly instead of defending against an unknown union — this also fixes
errorChain() rendering structured failures as '[object Object]' in the
TUI and ACP error paths. Document the turn-stopping contract: a
concludesTurn result never short-circuits already-submitted next-step
work (same-step additionalContexts or racing steering still runs), data
decides.

packages/core/agent-loop/src/agent.ts

@@ -60,6 +60,20 @@ function requestProposal(header: EpochHeader): LlmCallConfig {
 }
 
 /** Drives one session through turn and step boundaries. */
+/**
+ * Shape a caught turn error into its durable reason: every failure is a
+ * structured LlmFailure — an `LlmError` keeps its facts, anything else
+ * flattens to `errorChain` text under the `UNKNOWN` code.
+ */
+function turnErrorReason(error: unknown): Extract<TurnEndReason, { kind: 'error' }> {
+  return {
+    kind: 'error',
+    error: error instanceof LlmError
+      ? error.failure
+      : { message: errorChain(error), code: 'UNKNOWN' },
+  }
+}
+
 export class ReactLoopAgent implements Agent {
   readonly inbox: Inbox
   private phase: Phase
@@ -279,10 +293,7 @@ export class ReactLoopAgent implements Agent {
         turnEnds = { kind: 'aborted', reason: signal.reason as AgentCancelCause }
         throw error
       }
-      turnEnds = {
-        kind: 'error',
-        error: error instanceof LlmError ? error.failure : errorChain(error),
-      }
+      turnEnds = turnErrorReason(error)
       this.throwError(error)
     } finally {
       try {

packages/core/agent-loop/tests/contract-regressions.spec.ts

@@ -339,7 +339,7 @@ describe('plugin exceptions are contained', () => {
     send(agent, 'first')
     await waitForIdle(ctx, agent)
     expect(agent.session.events.findLast(event => event.type === 'turn/end')).toMatchObject({
-      data: { step: 1, reason: { kind: 'error', error: 'broken continuation plugin' } },
+      data: { step: 1, reason: { kind: 'error', error: { message: 'broken continuation plugin', code: 'UNKNOWN' } } },
     })
 
     // the loop is still alive: a second send works normally
@@ -426,10 +426,10 @@ describe('adapter registration, routing, and accepted-input ownership', () => {
     await waitForIdle(ctx, agent)
     const turnEnd = agent.session.events.findLast(event => event.type === 'turn/end')
     expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind === 'error'
-      ? turnEnd.data.reason.error
+      ? turnEnd.data.reason.error.message
       : undefined).toContain('has no provider/model')
     expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind === 'error'
-      ? turnEnd.data.reason.error
+      ? turnEnd.data.reason.error.message
       : undefined).toContain('agent/request')
   })
 
@@ -762,7 +762,7 @@ describe('turn and step boundary recovery', () => {
       errors: 1,
     })
     expect(agent.session.events.findLast(event => event.type === 'turn/end')).toMatchObject({
-      data: { step: 0, reason: { kind: 'error', error: 'reject step-start before commit' } },
+      data: { step: 0, reason: { kind: 'error', error: { message: 'reject step-start before commit', code: 'UNKNOWN' } } },
     })
   })
 

packages/core/agent-loop/tests/coverage-edges.spec.ts

@@ -155,7 +155,7 @@ describe('thrown-value propagation', () => {
     await waitForIdle(ctx, agent)
     const turnEnd = agent.session.events.find(e => e.type === 'turn/end')
     expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind === 'error'
-      ? turnEnd.data.reason.error
+      ? turnEnd.data.reason.error.message
       : undefined).toBe('[object Object]')
   })
 })
@@ -464,7 +464,7 @@ describe('unrenderable failure settlement', () => {
     if (end?.type === 'turn/end' && end.data.reason.kind === 'error') {
       // The durable failure keeps the adapter facts' message, not the
       // unrenderable chain.
-      expect(errorChain(end.data.reason.error)).not.toBe('<unrenderable value>')
+      expect(errorChain(end.data.reason.error.message)).not.toBe('<unrenderable value>')
     }
   })
 })

packages/core/agent-loop/tests/loop.spec.ts

@@ -235,7 +235,7 @@ describe('agent loop', () => {
     const turnEnd = agent.session.events.find(e => e.type === 'turn/end')
     expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind).toBe('error')
     expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind === 'error'
-      ? turnEnd.data.reason.error
+      ? turnEnd.data.reason.error.message
       : '').toContain('no value for this assembly')
 
     // The loop survived: a waterfall listener rescues {{cwd}} and the SAME

packages/core/agent-loop/tests/request-reconstruction.spec.ts

@@ -361,10 +361,9 @@ describe('request stability across the loop', () => {
       expect(agent.session.events.findLast(event => event.type === 'turn/end')).toMatchObject({
         data: {
           step: 1,
-          reason: {
-            kind: 'error',
-            error: failure instanceof LlmError ? failure.failure : failure.message,
-          },
+          reason: failure instanceof LlmError
+            ? { kind: 'error', error: failure.failure }
+            : { kind: 'error', error: { message: failure.message, code: 'UNKNOWN' } },
         },
       })
       expect(adapter.requests).toHaveLength(0)
@@ -505,7 +504,7 @@ describe('request stability across the loop', () => {
     const turnEnd = agent.session.events.findLast(event => event.type === 'turn/end')
     expect(turnEnd).toMatchObject({ data: { reason: { kind: 'error' } } })
     if (turnEnd?.type !== 'turn/end' || turnEnd.data.reason.kind !== 'error') throw new Error()
-    expect(turnEnd.data.reason.error).toMatch(/not extensible|frozen|read only|readonly/i)
+    expect(turnEnd.data.reason.error.message).toMatch(/not extensible|frozen|read only|readonly/i)
   })
 
   it('a fresh loop instance over a seeded log anchors with a resume snapshot and stays cache-aligned', async () => {

packages/core/agent-loop/tests/tool-calls.spec.ts

@@ -683,7 +683,7 @@ describe('tool-call scheduler: failure quiescence', () => {
     expect(turnEndBeforeDrain).toBeUndefined()
     expect(gated.pending()).toEqual([])
     expect(events(agent).findLast(event => event.type === 'turn/end')).toMatchObject({
-      data: { step: 1, reason: { kind: 'error', error: schedulerError.message } },
+      data: { step: 1, reason: { kind: 'error', error: { message: schedulerError.message, code: 'UNKNOWN' } } },
     })
   })
 })