diff --git a/src/main/native-chat/agent-session-wire/structured-agent-session-adapter-errors.ts b/src/main/native-chat/agent-session-wire/structured-agent-session-adapter-errors.ts
new file mode 100644
index 000000000000..75dc165cdafb
--- /dev/null
+++ b/src/main/native-chat/agent-session-wire/structured-agent-session-adapter-errors.ts
@@ -0,0 +1,69 @@
+import type { AgentSessionAccountKind } from '../../../shared/agent-session-availability'
+import type { AgentSessionRefusalReason } from '../../../shared/agent-session-wire-refusals'
+
+// Errors an adapter throws to the host; kept apart so the adapter contract file stays the contract.
+
+export class AgentSessionAcquisitionRefusal extends Error {
+  readonly code = 'agent_session_operation_invalid'
+
+  constructor(
+    message: string,
+    /** UI refusal reason; message stays in logs. Defaults to providerStartFailed. */
+    readonly reason: AgentSessionRefusalReason<'agent_session_operation_invalid'> = 'providerStartFailed',
+    readonly account?: AgentSessionAccountKind
+  ) {
+    super(message)
+    this.name = 'AgentSessionAcquisitionRefusal'
+  }
+
+  static historyTooLarge(message: string): AgentSessionAcquisitionRefusal {
+    return new AgentSessionAcquisitionRefusal(message, 'historyTooLarge')
+  }
+}
+
+export class AgentSessionPromptUnavailableError extends Error {
+  constructor(itemId: string) {
+    super(`The provider is no longer waiting on ${itemId}.`)
+    this.name = 'AgentSessionPromptUnavailableError'
+  }
+}
+
+/** Reject before journal commit when a provider reply cannot be constructed. */
+export class AgentSessionPromptAnswerRejectedError extends Error {
+  constructor(message: string) {
+    super(message)
+    this.name = 'AgentSessionPromptAnswerRejectedError'
+  }
+}
+
+export type AgentSessionPreSpawnReason = Extract<
+  AgentSessionRefusalReason<'agent_session_operation_invalid'>,
+  | 'managedAccountEnvOverride'
+  | 'accountSwitchInProgress'
+  | 'managedAccountUnsupported'
+  | 'launchFolderMissing'
+  | 'historyInOtherAccount'
+  | 'claudeAccountFolderMissing'
+  | 'claudeAccountSetupFailed'
+  | 'agentCommandNotRunnable'
+  | 'runtimeNotInstalled'
+>
+
+/** First-hand proof that acquisition spawned no process. */
+export class AgentSessionPreSpawnError extends Error {
+  /** Absent reasons are log-only; wrapped pre-spawn errors retain theirs. */
+  readonly reason: AgentSessionPreSpawnReason | undefined
+
+  constructor(
+    cause: unknown,
+    options: { reason?: AgentSessionPreSpawnReason; message?: string } = {}
+  ) {
+    super(options.message ?? (cause instanceof Error ? cause.message : String(cause)), { cause })
+    this.name = 'AgentSessionPreSpawnError'
+    this.reason = options.reason ?? (isAgentSessionPreSpawnError(cause) ? cause.reason : undefined)
+  }
+}
+
+export function isAgentSessionPreSpawnError(error: unknown): error is AgentSessionPreSpawnError {
+  return error instanceof Error && error.name === 'AgentSessionPreSpawnError'
+}
diff --git a/src/main/native-chat/agent-session-wire/structured-agent-session-adapter.ts b/src/main/native-chat/agent-session-wire/structured-agent-session-adapter.ts
index 155ce13ccff8..e93a2cab1533 100644
--- a/src/main/native-chat/agent-session-wire/structured-agent-session-adapter.ts
+++ b/src/main/native-chat/agent-session-wire/structured-agent-session-adapter.ts
@@ -1,5 +1,4 @@
 import type { AiVaultConversationCopy } from '../../../shared/ai-vault-resume-preparation'
-import type { AgentSessionAccountKind } from '../../../shared/agent-session-availability'
 import type {
   AgentSessionRewindFork,
   AgentSessionRewindReason,
@@ -29,7 +28,6 @@ import type {
 import type { StructuredAgentSessionAtRestCommands } from './structured-agent-session-at-rest-commands'
 import type { AgentSessionSlashCommand } from '../../../shared/agent-session-wire'
 import type { AgentSessionThreadGoalChange } from '../../../shared/agent-session-thread-goal-wire'
-import type { AgentSessionRefusalReason } from '../../../shared/agent-session-wire-refusals'
 import type { SubmissionRejectionFact } from '../../../shared/agent-session-failure'
 import type {
   AgentSessionCancelOutcome,
@@ -60,38 +58,14 @@ import type {
 } from './structured-agent-session-option-models'
 export type { StructuredAgentSessionLiveOptions } from './structured-agent-session-option-models'
 
-export class AgentSessionAcquisitionRefusal extends Error {
-  readonly code = 'agent_session_operation_invalid'
-
-  constructor(
-    message: string,
-    /** UI refusal reason; message stays in logs. Defaults to providerStartFailed. */
-    readonly reason: AgentSessionRefusalReason<'agent_session_operation_invalid'> = 'providerStartFailed',
-    readonly account?: AgentSessionAccountKind
-  ) {
-    super(message)
-    this.name = 'AgentSessionAcquisitionRefusal'
-  }
-
-  static historyTooLarge(message: string): AgentSessionAcquisitionRefusal {
-    return new AgentSessionAcquisitionRefusal(message, 'historyTooLarge')
-  }
-}
-
-export class AgentSessionPromptUnavailableError extends Error {
-  constructor(itemId: string) {
-    super(`The provider is no longer waiting on ${itemId}.`)
-    this.name = 'AgentSessionPromptUnavailableError'
-  }
-}
-
-/** Reject before journal commit when a provider reply cannot be constructed. */
-export class AgentSessionPromptAnswerRejectedError extends Error {
-  constructor(message: string) {
-    super(message)
-    this.name = 'AgentSessionPromptAnswerRejectedError'
-  }
-}
+export {
+  AgentSessionAcquisitionRefusal,
+  AgentSessionPreSpawnError,
+  AgentSessionPromptAnswerRejectedError,
+  AgentSessionPromptUnavailableError,
+  isAgentSessionPreSpawnError
+} from './structured-agent-session-adapter-errors'
+export type { AgentSessionPreSpawnReason } from './structured-agent-session-adapter-errors'
 
 export {
   AgentSessionAcquisitionExitProvenError,
@@ -107,38 +81,6 @@ export type AgentSessionAcquisition = {
   acquisitionGeneration: string
 }
 
-export type AgentSessionPreSpawnReason = Extract<
-  AgentSessionRefusalReason<'agent_session_operation_invalid'>,
-  | 'managedAccountEnvOverride'
-  | 'accountSwitchInProgress'
-  | 'managedAccountUnsupported'
-  | 'launchFolderMissing'
-  | 'historyInOtherAccount'
-  | 'claudeAccountFolderMissing'
-  | 'claudeAccountSetupFailed'
-  | 'agentCommandNotRunnable'
-  | 'runtimeNotInstalled'
->
-
-/** First-hand proof that acquisition spawned no process. */
-export class AgentSessionPreSpawnError extends Error {
-  /** Absent reasons are log-only; wrapped pre-spawn errors retain theirs. */
-  readonly reason: AgentSessionPreSpawnReason | undefined
-
-  constructor(
-    cause: unknown,
-    options: { reason?: AgentSessionPreSpawnReason; message?: string } = {}
-  ) {
-    super(options.message ?? (cause instanceof Error ? cause.message : String(cause)), { cause })
-    this.name = 'AgentSessionPreSpawnError'
-    this.reason = options.reason ?? (isAgentSessionPreSpawnError(cause) ? cause.reason : undefined)
-  }
-}
-
-export function isAgentSessionPreSpawnError(error: unknown): error is AgentSessionPreSpawnError {
-  return error instanceof Error && error.name === 'AgentSessionPreSpawnError'
-}
-
 /** The host creates the command’s running turn; the child translator ends it and writes its result. */
 export type StructuredAgentSessionCommandRun = {
   clientMessageId: string