diff --git a/crates/server-api/Cargo.toml b/crates/server-api/Cargo.toml
index 75daa75fc..ba51f5424 100644
--- a/crates/server-api/Cargo.toml
+++ b/crates/server-api/Cargo.toml
@@ -43,6 +43,10 @@ name = "generate_worker_launch_api_types"
 [[example]]
 name = "generate_companion_api_types"
 
+[[example]]
+name = "generate_chat_api_types"
+required-features = ["typescript"]
+
 [[example]]
 name = "generate_memory_api_types"
 required-features = ["typescript"]
diff --git a/crates/server-api/examples/generate_chat_api_types.rs b/crates/server-api/examples/generate_chat_api_types.rs
new file mode 100644
index 000000000..3b11df745
--- /dev/null
+++ b/crates/server-api/examples/generate_chat_api_types.rs
@@ -0,0 +1,6 @@
+fn main() {
+    print!(
+        "{}",
+        server_api::normalized_typescript(server_api::chat_api_typescript())
+    );
+}
diff --git a/crates/server-api/src/chat.rs b/crates/server-api/src/chat.rs
index cfff0a836..f2b4bcc7e 100644
--- a/crates/server-api/src/chat.rs
+++ b/crates/server-api/src/chat.rs
@@ -129,11 +129,53 @@ pub struct ChatMessagePage {
     pub has_more: bool,
 }
 
+#[cfg(feature = "typescript")]
+pub fn chat_api_typescript() -> String {
+    use crate::{ChatDelivery, ChatReceiveResponse};
+    use ts_rs::TS;
+
+    let config = ts_rs::Config::default();
+    let declarations = [
+        ChatConversation::decl(&config),
+        ChatMessageSender::decl(&config),
+        ChatMessage::decl(&config),
+        ChatCreateConversationRequest::decl(&config),
+        ChatUserMessageRequest::decl(&config),
+        ChatMessagePageQuery::decl(&config),
+        ChatMessagePage::decl(&config),
+        ChatDelivery::decl(&config),
+        ChatReceiveResponse::decl(&config),
+    ]
+    .into_iter()
+    .map(|declaration| format!("export {declaration}"))
+    .collect::<Vec<_>>()
+    .join("\n\n");
+    format!(
+        "// Generated from server-api. Do not edit by hand.\n// Regenerate: cargo run -q -p server-api --features typescript --example generate_chat_api_types > web/workspace/src/lib/generated/chat-api.ts\n\nexport const CHAT_LIMITS = {{ maxTextBytes: {CHAT_MAX_TEXT_BYTES}, maxPageLimit: {CHAT_MAX_PAGE_LIMIT}, maxCounter: {CHAT_MAX_COUNTER} }} as const;\n\n{declarations}\n"
+    )
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
     use serde_json::json;
 
+    #[cfg(feature = "typescript")]
+    #[test]
+    fn generated_chat_browser_contract_is_current() {
+        let path = std::path::Path::new(env!("CARGO_MANIFEST_DIR"))
+            .join("../../web/workspace/src/lib/generated/chat-api.ts");
+        let actual = std::fs::read_to_string(&path)
+            .unwrap_or_else(|error| panic!("failed to read {}: {error}", path.display()));
+        let normalize = |value: &str| {
+            value
+                .chars()
+                .filter(|c| !c.is_whitespace())
+                .collect::<String>()
+        };
+        assert_eq!(normalize(&actual), normalize(&chat_api_typescript()));
+    }
+
     #[test]
     fn text_only_requests_reject_attachments_and_claimed_sender() {
         for field in ["attachments", "sender", "subject_id", "account_id"] {
diff --git a/web/workspace/deno.json b/web/workspace/deno.json
index 26070d9ba..b56315e6c 100644
--- a/web/workspace/deno.json
+++ b/web/workspace/deno.json
@@ -6,7 +6,7 @@
     "dev": "deno run -A npm:vite@8.3.2 dev",
     "dev:backend": "cd ../.. && cargo run -p yoi-workspace-server --bin yoi-server -- serve --listen 127.0.0.1:8787",
     "check": "deno run -A npm:@sveltejs/kit@3.0.0 sync && deno run -A npm:svelte-check@4.7.6 --tsconfig ./tsconfig.json",
-    "test": "deno test --allow-read=src,test,tests,../../docs,../../resources/config-schema --allow-env=LOG,VSCODE_TEXTMATE_DEBUG,NODE_ENV tests/workspace-model.test.ts tests/repository-api.test.ts tests/workspace-catalog.test.ts tests/profile-api.test.ts tests/skill-api.test.ts tests/tooltip-contract.test.ts tests/bevel-contract.test.ts tests/bevel-line-contract.test.ts tests/typography-contract.test.ts src/lib/workspace/navigation/deferred.test.ts src/lib/workspace/auth/model.test.ts src/lib/workspace/auth/login.test.ts src/lib/workspace/auth/routes.test.ts src/lib/workspace/auth/hosting.test.ts src/lib/workspace/auth/hosting-enrollment.test.ts src/lib/workspace/auth/activation.test.ts src/lib/workspace/auth/hosting-accounts.test.ts src/lib/workspace/protocol-validator.test.ts tests/auth-api.test.ts src/lib/workspace/api/http.test.ts src/lib/workspace/api/workers.test.ts src/lib/workspace/api/ticket-browser.test.ts src/lib/workspace/api/ticket-case.test.ts src/lib/workspace/api/ticket-operations.test.ts src/lib/workspace/header/breadcrumb-model.test.ts src/lib/workspace/console/chat-submit.test.ts src/lib/workspace/console/composer-delivery.test.ts test/composer-history.test.ts tests/composer-paste.test.ts src/lib/workspace/console/composer-command.test.ts src/lib/workspace/console/composer-draft.test.ts src/lib/workspace/console/composer-completion.test.ts src/lib/workspace/console/composer-invocation.test.ts src/lib/workspace/console/file-completions.test.ts test/console/ansi.test.ts src/lib/workspace/console/history.test.ts src/lib/workspace/console/model.test.ts src/lib/workspace/console/alert-order.test.ts src/lib/workspace/console/turn-navigation.test.ts src/lib/workspace/console/run-status.test.ts src/lib/workspace/companion/api.test.ts tests/workdir-api.test.ts src/lib/workspace/settings/workdir-checkout.test.ts tests/workdir-removal.test.ts tests/workdir-permissions.test.ts src/lib/workspace/console/tasks.test.ts test/ticket-detail-route-reuse.test.ts test/spa-shell.test.ts test/objective-pages.test.ts test/repositories/ui.test.ts src/lib/workspace/console/worker-console.ui.test.ts src/lib/workspace/settings/model.test.ts tests/runtime-connection.test.ts tests/runtime-management.test.ts tests/runtime-management-source.test.ts src/lib/workspace/sidebar/override-stack.test.ts src/lib/workspace/sidebar/route-ownership.test.ts src/lib/workspace/sidebar/workers.test.ts src/lib/workspace/sidebar/workspace-switcher.test.ts src/lib/workspace/sidebar/worker-subscription.test.ts src/lib/workspace/sidebar/worker-workdir-meta.test.ts src/lib/workspace/sidebar/worker-launch.test.ts src/lib/workspace/workers/worker-launch.test.ts tests/runtime-workers.test.ts test/sidebar/worker-actions.test.ts src/lib/workspace/tickets/merge-request-resources.test.ts src/lib/workspace/tickets/ticket-panel.test.ts src/lib/workspace/tickets/ticket-search.test.ts test/merge-request-status.test.ts test/config-source/decodal-grammar.test.ts test/config-source/editor-state.test.ts test/config-source/fixed-schema-wrapper.test.ts test/config-source/toolchain.test.ts test/config-source/wasm-parity.test.ts test/config-source/api.test.ts tests/memory-api.test.ts src/lib/workspace/memory/permissions.test.ts tests/subject-delete.test.ts tests/journal-api.test.ts test/repository-access/api.test.ts test/repository-access/loader.test.ts test/repository-access/model.test.ts test/repository-access/structure.test.ts test/repository-ssh-connection-ui.test.ts src/lib/workspace/drive/api.test.ts src/lib/workspace/drive/controller.test.ts src/lib/workspace/drive-ui/policy.test.ts",
+    "test": "deno test --allow-read=src,test,tests,../../docs,../../resources/config-schema --allow-env=LOG,VSCODE_TEXTMATE_DEBUG,NODE_ENV tests/workspace-model.test.ts tests/repository-api.test.ts tests/workspace-catalog.test.ts tests/profile-api.test.ts tests/skill-api.test.ts tests/tooltip-contract.test.ts tests/bevel-contract.test.ts tests/bevel-line-contract.test.ts tests/typography-contract.test.ts src/lib/workspace/navigation/deferred.test.ts src/lib/workspace/auth/model.test.ts src/lib/workspace/auth/login.test.ts src/lib/workspace/auth/routes.test.ts src/lib/workspace/auth/hosting.test.ts src/lib/workspace/auth/hosting-enrollment.test.ts src/lib/workspace/auth/activation.test.ts src/lib/workspace/auth/hosting-accounts.test.ts src/lib/workspace/protocol-validator.test.ts tests/auth-api.test.ts src/lib/workspace/api/http.test.ts src/lib/workspace/api/workers.test.ts src/lib/workspace/api/ticket-browser.test.ts src/lib/workspace/api/ticket-case.test.ts src/lib/workspace/api/ticket-operations.test.ts src/lib/workspace/header/breadcrumb-model.test.ts src/lib/workspace/console/chat-submit.test.ts src/lib/workspace/console/composer-delivery.test.ts test/composer-history.test.ts tests/composer-paste.test.ts src/lib/workspace/console/composer-command.test.ts src/lib/workspace/console/composer-draft.test.ts src/lib/workspace/console/composer-completion.test.ts src/lib/workspace/console/composer-invocation.test.ts src/lib/workspace/console/file-completions.test.ts test/console/ansi.test.ts src/lib/workspace/console/history.test.ts src/lib/workspace/console/model.test.ts src/lib/workspace/console/alert-order.test.ts src/lib/workspace/console/turn-navigation.test.ts src/lib/workspace/console/run-status.test.ts src/lib/workspace/companion/api.test.ts tests/workdir-api.test.ts src/lib/workspace/settings/workdir-checkout.test.ts tests/workdir-removal.test.ts tests/workdir-permissions.test.ts src/lib/workspace/console/tasks.test.ts test/ticket-detail-route-reuse.test.ts test/spa-shell.test.ts test/objective-pages.test.ts test/repositories/ui.test.ts src/lib/workspace/console/worker-console.ui.test.ts src/lib/workspace/settings/model.test.ts tests/runtime-connection.test.ts tests/runtime-management.test.ts tests/runtime-management-source.test.ts src/lib/workspace/sidebar/override-stack.test.ts src/lib/workspace/sidebar/route-ownership.test.ts src/lib/workspace/sidebar/workers.test.ts src/lib/workspace/sidebar/workspace-switcher.test.ts src/lib/workspace/sidebar/worker-subscription.test.ts src/lib/workspace/sidebar/worker-workdir-meta.test.ts src/lib/workspace/sidebar/worker-launch.test.ts src/lib/workspace/workers/worker-launch.test.ts tests/runtime-workers.test.ts test/sidebar/worker-actions.test.ts src/lib/workspace/tickets/merge-request-resources.test.ts src/lib/workspace/tickets/ticket-panel.test.ts src/lib/workspace/tickets/ticket-search.test.ts test/merge-request-status.test.ts test/config-source/decodal-grammar.test.ts test/config-source/editor-state.test.ts test/config-source/fixed-schema-wrapper.test.ts test/config-source/toolchain.test.ts test/config-source/wasm-parity.test.ts test/config-source/api.test.ts tests/memory-api.test.ts src/lib/workspace/memory/permissions.test.ts src/lib/workspace/memory/conversation-attempt.test.ts src/lib/workspace/memory/conversation-api.test.ts tests/subject-delete.test.ts tests/journal-api.test.ts test/repository-access/api.test.ts test/repository-access/loader.test.ts test/repository-access/model.test.ts test/repository-access/structure.test.ts test/repository-ssh-connection-ui.test.ts src/lib/workspace/drive/api.test.ts src/lib/workspace/drive/controller.test.ts src/lib/workspace/drive-ui/policy.test.ts",
     "test:journal:backend-contract": "cd ../.. && mkdir -p target && cargo test -p yoi-workspace-server --lib journal_browser_web_contract -- --nocapture > target/journal-browser-contract.log && cd web/workspace && deno run --allow-read=../../target/journal-browser-contract.log test/journal-backend-contract.ts ../../target/journal-browser-contract.log",
     "test:component": "deno run -A npm:vitest@5.0.3 run --config vitest.config.ts",
     "test:browser:activation": "mkdir -p ../../target/web-ux/tmp && TMPDIR=../../target/web-ux/tmp deno test --config ../../tools/web-ux/deno.json --allow-env --allow-net --allow-read --allow-write --allow-run --allow-sys test/activation-browser/activation-shell.browser.test.ts",
diff --git a/web/workspace/src/lib/workspace/memory/SubjectConversation.svelte b/web/workspace/src/lib/workspace/memory/SubjectConversation.svelte
new file mode 100644
index 000000000..b21acdb7d
--- /dev/null
+++ b/web/workspace/src/lib/workspace/memory/SubjectConversation.svelte
@@ -0,0 +1,235 @@
+<script lang="ts">
+  import { untrack } from 'svelte';
+  import { CHAT_LIMITS, type ChatConversation, type ChatDelivery, type ChatMessage } from '#lib/generated/chat-api.ts';
+  import { browserSession } from '#lib/workspace/auth/session-state.ts';
+  import { ConversationApiError, appendConversationMessages, getConversationPage, openSubjectConversation, sendConversationInput } from './conversation-api.ts';
+  import { clearConversationAttempt, readConversationAttempt, saveConversationAttempt, type ConversationAttempt, type ConversationScope } from './conversation-attempt.ts';
+
+  // 本体の現在状態・起動停止はroute側の共有SubjectBodyが所有する。会話ACKから推測しない。
+  let { workspaceId, subjectId, accountId, subjectName }: {
+    workspaceId: string;
+    subjectId: string;
+    accountId: string;
+    subjectName: string;
+  } = $props();
+  let conversation = $state<ChatConversation | null>(null);
+  let messages = $state<ChatMessage[]>([]);
+  let hasMore = $state(false);
+  let reading = $state(false);
+  let sending = $state(false);
+  let draft = $state('');
+  let attempt = $state<ConversationAttempt | null>(null);
+  let delivery = $state<ChatDelivery | null>(null);
+  let historyError = $state<string | null>(null);
+  let sendError = $state<string | null>(null);
+  let storageBlocked = $state(false);
+  let inaccessible = $state(false);
+  let active: { scope: ConversationScope; controller: AbortController; authGeneration: number; authStamp: string | null } | null = null;
+  const draftBytes = $derived(new TextEncoder().encode(draft).length);
+
+  function current(context: NonNullable<typeof active>): boolean {
+    const snapshot = browserSession.snapshot;
+    return active === context && !context.controller.signal.aborted &&
+      workspaceId === context.scope.workspaceId && subjectId === context.scope.subjectId && accountId === context.scope.accountId &&
+      browserSession.authenticationGeneration === context.authGeneration && browserSession.authenticationStamp === context.authStamp &&
+      snapshot.kind === 'ready' && snapshot.value.session.kind === 'authenticated' && snapshot.value.actor?.account_id === context.scope.accountId;
+  }
+
+  $effect(() => {
+    const scope: ConversationScope = { workspaceId, subjectId, accountId };
+    return untrack(() => {
+      const context = { scope, controller: new AbortController(), authGeneration: browserSession.authenticationGeneration, authStamp: browserSession.authenticationStamp };
+      active = context;
+      conversation = null;
+      messages = [];
+      hasMore = false;
+      reading = false;
+      sending = false;
+      draft = '';
+      attempt = null;
+      delivery = null;
+      historyError = null;
+      sendError = null;
+      storageBlocked = false;
+      inaccessible = false;
+      try { attempt = readConversationAttempt(sessionStorage, scope); }
+      catch { storageBlocked = true; sendError = 'Saved send input cannot be read. No new send will be started.'; }
+      void refresh(context);
+      // 公開履歴だけを再取得する。内部Session購読やモデルのpollingはしない。
+      const timer = setInterval(() => { if (!document.hidden) void refresh(context); }, 4000);
+      const reconnect = () => { if (!document.hidden) void refresh(context); };
+      window.addEventListener('online', reconnect);
+      document.addEventListener('visibilitychange', reconnect);
+      const unsubscribe = browserSession.subscribe(() => {
+        if (!current(context)) {
+          context.controller.abort();
+          messages = [];
+          conversation = null;
+          inaccessible = true;
+        }
+      });
+      return () => {
+        context.controller.abort();
+        if (active === context) active = null;
+        clearInterval(timer);
+        window.removeEventListener('online', reconnect);
+        document.removeEventListener('visibilitychange', reconnect);
+        unsubscribe();
+      };
+    });
+  });
+
+  async function refresh(context = active): Promise<void> {
+    if (!context || !current(context) || reading || inaccessible) return;
+    reading = true;
+    try {
+      const opened = conversation ?? await openSubjectConversation(fetch, context.scope, context.controller.signal);
+      if (!current(context)) return;
+      if (attempt && attempt.conversationId !== opened.id) {
+        storageBlocked = true;
+        sendError = 'The saved send belongs to another conversation. It has not been replaced.';
+      }
+      const after = messages.at(-1)?.sequence ?? 0;
+      const page = await getConversationPage(fetch, context.scope, opened.id, after, context.controller.signal);
+      if (!current(context)) return;
+      // POSTのmessageをcursorにしない。先行する未読の公開返信を飛ばさない。
+      conversation = page.conversation;
+      messages = appendConversationMessages(messages, page.messages);
+      hasMore = page.has_more;
+      historyError = null;
+    } catch (error) {
+      if (!current(context)) return;
+      historyError = error instanceof ConversationApiError ? error.message : 'Conversation history could not be loaded.';
+      if (error instanceof ConversationApiError && [401, 403, 404].includes(error.status ?? 0)) {
+        inaccessible = true;
+        conversation = null;
+        messages = [];
+      }
+    } finally { if (current(context)) reading = false; }
+  }
+
+  async function send(retry = false): Promise<void> {
+    const context = active;
+    if (!context || !current(context) || sending || inaccessible || storageBlocked || !conversation) return;
+    if (!retry && (attempt || !draft.trim() || draftBytes > CHAT_LIMITS.maxTextBytes)) return;
+    const request = retry ? attempt : Object.freeze({ conversationId: conversation.id, operationId: crypto.randomUUID(), text: draft, replyTo: null, workRef: null });
+    if (!request || request.conversationId !== conversation.id) return;
+    // 同期保存が成功するまでPOSTしない。unknownでの編集や再訪でも同じ全入力を使う。
+    try { saveConversationAttempt(sessionStorage, context.scope, request); }
+    catch { storageBlocked = true; sendError = 'Send input could not be retained safely. Nothing new was dispatched.'; return; }
+    attempt = request;
+    if (!retry) draft = '';
+    sending = true;
+    delivery = null;
+    sendError = null;
+    try {
+      const response = await sendConversationInput(fetch, context.scope, request, context.controller.signal);
+      if (!current(context)) return;
+      delivery = response.delivery;
+      if (response.delivery.state === 'accepted') {
+        try {
+          clearConversationAttempt(sessionStorage, context.scope, request);
+          attempt = null;
+        } catch {
+          storageBlocked = true;
+          sendError = 'Runtime accepted this input, but its local retry record could not be cleared. No new send will be started.';
+        }
+      }
+      void refresh(context);
+    } catch (error) {
+      if (!current(context)) return;
+      sendError = error instanceof ConversationApiError ? error.message : 'Send outcome unknown. Keep the saved request.';
+      if (error instanceof ConversationApiError && [401, 403, 404].includes(error.status ?? 0)) {
+        inaccessible = true;
+        messages = [];
+        conversation = null;
+      }
+    } finally { if (current(context)) sending = false; }
+  }
+
+  function recheckStorage(): void {
+    const context = active;
+    if (!context || !current(context) || sending) return;
+    try {
+      const saved = readConversationAttempt(sessionStorage, context.scope);
+      if (attempt) {
+        // clear失敗後の再確認でも新しい配送は行わない。
+        if (delivery?.state === 'accepted') {
+          clearConversationAttempt(sessionStorage, context.scope, attempt);
+          attempt = null;
+        } else {
+          saveConversationAttempt(sessionStorage, context.scope, attempt);
+        }
+      } else attempt = saved;
+      if (attempt && conversation && attempt.conversationId !== conversation.id) throw new Error();
+      storageBlocked = false;
+      sendError = null;
+    } catch { sendError = 'The original send input still cannot be retained or reconciled safely. No new send was started.'; }
+  }
+
+  function deliveryLabel(value: ChatDelivery): string {
+    switch (value.state) {
+      case 'accepted': return 'Runtime accepted this input. Processing and replies are not confirmed.';
+      case 'no_current_body': return 'Message saved; no current body was available. Start the Subject, then retry this same input.';
+      case 'rejected': return 'Message saved; Runtime did not accept this input. Check the body state before retrying.';
+      case 'unknown': return 'Message saved; Runtime acceptance is unknown. Retry only this same input.';
+      case 'unavailable': return 'Message saved; delivery is unavailable. Retry only this same input.';
+    }
+  }
+</script>
+
+<section class="subject-conversation" aria-label="Conversation with {subjectName}">
+  <header>
+    <h2>Conversation</h2>
+    {#if !inaccessible}<button type="button" onclick={() => refresh()} disabled={reading}>{reading ? 'Loading history…' : 'Refresh history'}</button>{/if}
+  </header>
+  {#if historyError}<p class="error" role="alert">{historyError}</p>{/if}
+  {#if !conversation && !historyError}<p role="status">Loading conversation…</p>{/if}
+  <ol aria-label="Saved public messages">
+    {#each messages as message (message.id)}
+      <li>
+        <strong>{message.sender.kind === 'user' ? 'You' : subjectName}</strong>
+        <p class="message-text">{message.text}</p>
+      </li>
+    {/each}
+  </ol>
+  {#if conversation && messages.length === 0}<p>No public messages yet.</p>{/if}
+  {#if hasMore && !inaccessible}<button type="button" onclick={() => refresh()} disabled={reading}>Load newer messages</button>{/if}
+
+  <div class="send-status" aria-live="polite">
+    {#if sending}<p>Sending saved input…</p>
+    {:else if delivery}<p>{deliveryLabel(delivery)}</p>
+    {:else if attempt}<p>Previous send outcome is unknown. Its original input is retained.</p>{/if}
+  </div>
+  {#if sendError}<p class="error" role="alert">{sendError}</p>{/if}
+  {#if storageBlocked}<button type="button" onclick={recheckStorage} disabled={sending}>Recheck retry storage</button>{/if}
+  {#if attempt}
+    <details><summary>Retained send input</summary><p class="message-text">{attempt.text}</p></details>
+    <p>Retry uses the same ID and full input. Runtime duplicate suppression has bounded receipt retention; it is not an indefinite delivery guarantee.</p>
+    {#if !inaccessible && !storageBlocked}<button type="button" onclick={() => send(true)} disabled={sending || !conversation}>Retry same input</button>{/if}
+  {/if}
+  {#if !inaccessible}
+    <form onsubmit={(event) => { event.preventDefault(); void send(); }}>
+      <label for="subject-conversation-input">Message to {subjectName}</label>
+      <textarea id="subject-conversation-input" bind:value={draft} rows="4" aria-describedby="conversation-constraint"></textarea>
+      <p id="conversation-constraint">Text only, up to {CHAT_LIMITS.maxTextBytes} UTF-8 bytes. Attachments are not supported.</p>
+      {#if draftBytes > CHAT_LIMITS.maxTextBytes}<p class="error">The message exceeds the text limit.</p>{/if}
+      {#if attempt}<p>Resolve the retained send before sending another message. Your next draft remains editable.</p>{/if}
+      <button type="submit" disabled={sending || !!attempt || storageBlocked || !conversation || !draft.trim() || draftBytes > CHAT_LIMITS.maxTextBytes}>Send message</button>
+    </form>
+  {/if}
+</section>
+
+<style>
+  .subject-conversation { display: grid; gap: var(--space-4); font-size: var(--font-size-body); line-height: var(--line-height-body); min-width: 0; }
+  header { display: flex; align-items: center; justify-content: space-between; flex-wrap: wrap; gap: var(--space-3); }
+  h2, p { margin: 0; }
+  h2 { font-size: var(--font-size-body); }
+  ol { list-style: none; margin: 0; padding: 0; display: grid; gap: var(--space-5); }
+  li { display: grid; gap: var(--space-1); }
+  .message-text { white-space: pre-wrap; overflow-wrap: anywhere; }
+  form { display: grid; gap: var(--space-2); }
+  textarea { width: 100%; box-sizing: border-box; resize: vertical; font: inherit; color: var(--text); background: var(--bg); border: 1px solid var(--line-strong); padding: var(--space-2); }
+  button { justify-self: start; }
+  .error { color: var(--danger); }
+</style>
diff --git a/web/workspace/src/lib/workspace/memory/conversation-api.test.ts b/web/workspace/src/lib/workspace/memory/conversation-api.test.ts
new file mode 100644
index 000000000..ed8966545
--- /dev/null
+++ b/web/workspace/src/lib/workspace/memory/conversation-api.test.ts
@@ -0,0 +1,207 @@
+// HTTP境界のfixture試験。実Backend/Runtime接続の証拠ではない。
+import {
+  appendConversationMessages,
+  ConversationApiError,
+  getConversationPage,
+  openSubjectConversation,
+  parseConversationMessage,
+  parseConversationPage,
+  sendConversationInput,
+} from "./conversation-api.ts";
+import { CHAT_LIMITS } from "#lib/generated/chat-api.ts";
+import {
+  readConversationAttempt,
+  saveConversationAttempt,
+} from "./conversation-attempt.ts";
+
+declare const Deno: {
+  test(name: string, fn: () => void | Promise<void>): void;
+};
+function assert(value: unknown): asserts value {
+  if (!value) throw new Error("Assertion failed");
+}
+function rejects(fn: () => unknown) {
+  let failed = false;
+  try {
+    fn();
+  } catch {
+    failed = true;
+  }
+  assert(failed);
+}
+const scope = {
+  workspaceId: "w/1",
+  subjectId: "subject",
+  accountId: "account",
+};
+const conversation = {
+  id: "c/1",
+  subject_id: scope.subjectId,
+  participant_account_id: scope.accountId,
+  last_user_message_id: null,
+};
+const attempt = {
+  conversationId: conversation.id,
+  operationId: "send-id",
+  text: "  exact\n本文  ",
+  replyTo: null,
+  workRef: null,
+};
+function message(sequence = 1) {
+  return {
+    id: `m${sequence}`,
+    conversation_id: conversation.id,
+    sequence,
+    sender: { kind: "user", account_id: scope.accountId },
+    text: attempt.text,
+    reply_to: null,
+    work_ref: null,
+    operation_id: attempt.operationId,
+    last_user_message_id: `m${sequence}`,
+  };
+}
+function page(messages: unknown[], has_more = false) {
+  return { conversation, messages, has_more };
+}
+
+Deno.test("public conversation boundary rejects wrong identity internal senders and unsafe cursor/order", () => {
+  for (
+    const invalid of [
+      { ...message(), conversation_id: "other" },
+      { ...message(), sender: { kind: "assistant" } },
+      { ...message(), sender: { kind: "tool" } },
+      { ...message(), sender: { kind: "subject", subject_id: "other" } },
+      { ...message(), sender: { kind: "user", account_id: "other" } },
+      { ...message(), sequence: Number.MAX_SAFE_INTEGER + 1 },
+      { ...message(), sequence: 0 },
+      { ...message(), attachments: [] },
+    ]
+  ) rejects(() => parseConversationMessage(invalid, scope, conversation.id));
+  for (
+    const invalid of [
+      page([message(2), message(1)]),
+      page([message(1), message(1)]),
+      page([], true),
+      { ...page([]), conversation: { ...conversation, subject_id: "other" } },
+      {
+        ...page([]),
+        conversation: { ...conversation, participant_account_id: "other" },
+      },
+    ]
+  ) rejects(() => parseConversationPage(invalid, scope, conversation.id, 0));
+  rejects(() =>
+    parseConversationPage(page([message(1)]), scope, conversation.id, 1)
+  );
+});
+
+Deno.test("exclusive sequence pages append without duplicating an earlier public message", () => {
+  const first = parseConversationPage(
+    page([message(1), message(2)], true),
+    scope,
+    conversation.id,
+    0,
+  );
+  const second = parseConversationPage(
+    page([message(3)]),
+    scope,
+    conversation.id,
+    2,
+  );
+  const merged = appendConversationMessages(first.messages, second.messages);
+  assert(merged.map((m) => m.sequence).join() === "1,2,3");
+  rejects(() => appendConversationMessages(merged, [merged[0]]));
+  const repeat = { ...second.messages[0], sequence: 4 };
+  rejects(() => appendConversationMessages(merged, [repeat]));
+});
+
+Deno.test("revisit uses the same account-scoped create input then reads authoritative pages", async () => {
+  const requests: { path: string; body: unknown }[] = [];
+  const fetchFn: typeof fetch = (input, init) => {
+    requests.push({
+      path: String(input),
+      body: init?.body ? JSON.parse(String(init.body)) : null,
+    });
+    return Promise.resolve(
+      Response.json(
+        init?.method === "POST" ? conversation : page([message(1)]),
+      ),
+    );
+  };
+  const signal = new AbortController().signal;
+  const first = await openSubjectConversation(fetchFn, scope, signal);
+  const revisit = await openSubjectConversation(fetchFn, scope, signal);
+  assert(first.id === revisit.id);
+  assert(JSON.stringify(requests[0]) === JSON.stringify(requests[1]));
+  await getConversationPage(fetchFn, scope, first.id, 1, signal);
+  assert(
+    requests[2].path ===
+      `/api/w/w%2F1/chat/conversations/c%2F1/messages?after_sequence=1&limit=${CHAT_LIMITS.maxPageLimit}`,
+  );
+});
+
+Deno.test("ACK loss retains exact request bytes for explicit replay and accepted is not a reply", async () => {
+  const saved = new Map<string, string>();
+  const storage = {
+    getItem: (k: string) => saved.get(k) ?? null,
+    setItem: (k: string, v: string) => {
+      saved.set(k, v);
+    },
+    removeItem: (k: string) => {
+      saved.delete(k);
+    },
+  };
+  const bodies: string[] = [];
+  const fetchFn: typeof fetch = (_input, init) => {
+    bodies.push(String(init?.body));
+    return bodies.length === 1
+      ? Promise.reject(new TypeError("ACK lost"))
+      : Promise.resolve(
+        Response.json({
+          message: message(),
+          delivery: { state: "accepted", runtime_id: "r", worker_id: "w" },
+        }),
+      );
+  };
+  const signal = new AbortController().signal;
+  saveConversationAttempt(storage, scope, attempt);
+  try {
+    await sendConversationInput(fetchFn, scope, attempt, signal);
+    throw new Error("Expected unknown");
+  } catch (error) {
+    assert(error instanceof ConversationApiError && error.status === null);
+  }
+  const retained = readConversationAttempt(storage, scope);
+  assert(retained !== null);
+  const receipt = await sendConversationInput(fetchFn, scope, retained, signal);
+  assert(bodies[0] === bodies[1]);
+  assert(receipt.message.sender.kind === "user");
+  assert(receipt.delivery.state === "accepted");
+});
+
+Deno.test("HTTP rejection and saved-message delivery failure never become an accepted reply", async () => {
+  const signal = new AbortController().signal;
+  for (const status of [401, 403, 404, 409, 500]) {
+    const fetchFn: typeof fetch = () =>
+      Promise.resolve(new Response(null, { status }));
+    try {
+      await sendConversationInput(fetchFn, scope, attempt, signal);
+      throw new Error("Expected rejection");
+    } catch (error) {
+      assert(error instanceof ConversationApiError && error.status === status);
+    }
+  }
+  for (
+    const delivery of [
+      { state: "no_current_body" },
+      { state: "rejected", runtime_id: "r", worker_id: "w", detail: "stopped" },
+      { state: "unknown", runtime_id: "r", worker_id: "w", detail: "lost" },
+      { state: "unavailable", detail: "unavailable" },
+    ]
+  ) {
+    const fetchFn: typeof fetch = () =>
+      Promise.resolve(Response.json({ message: message(), delivery }));
+    const result = await sendConversationInput(fetchFn, scope, attempt, signal);
+    assert(result.delivery.state === delivery.state);
+    assert(result.message.sender.kind === "user");
+  }
+});
diff --git a/web/workspace/src/lib/workspace/memory/conversation-api.ts b/web/workspace/src/lib/workspace/memory/conversation-api.ts
new file mode 100644
index 000000000..fb6fcdf37
--- /dev/null
+++ b/web/workspace/src/lib/workspace/memory/conversation-api.ts
@@ -0,0 +1,304 @@
+import { CHAT_LIMITS } from "#lib/generated/chat-api.ts";
+import type {
+  ChatConversation,
+  ChatCreateConversationRequest,
+  ChatDelivery,
+  ChatMessage,
+  ChatMessagePage,
+  ChatReceiveResponse,
+  ChatUserMessageRequest,
+} from "#lib/generated/chat-api.ts";
+import { readBoundedJson, workspaceApiPath } from "#lib/workspace/api/http.ts";
+import {
+  type ConversationAttempt,
+  conversationOperationId,
+  type ConversationScope,
+} from "./conversation-attempt.ts";
+
+export class ConversationApiError extends Error {
+  constructor(readonly status: number | null) {
+    super(
+      status === 401 || status === 403
+        ? "You do not have access to this conversation."
+        : status === 404
+        ? "The Subject or conversation is no longer available."
+        : status === 409
+        ? "The saved operation conflicts with another input. Keep the original request; do not send it under a new ID."
+        : "The request outcome could not be confirmed. Keep the saved input and retry the same request.",
+    );
+  }
+}
+
+function invalid(): never {
+  throw new ConversationApiError(null);
+}
+function object(value: unknown, keys: string[]): Record<string, unknown> {
+  if (typeof value !== "object" || value === null || Array.isArray(value)) {
+    invalid();
+  }
+  const record = value as Record<string, unknown>;
+  if (
+    Object.keys(record).length !== keys.length ||
+    keys.some((key) => !(key in record))
+  ) invalid();
+  return record;
+}
+function text(value: unknown, maxBytes = 512): string {
+  if (
+    typeof value !== "string" || !value.trim() ||
+    new TextEncoder().encode(value).length > maxBytes
+  ) invalid();
+  return value;
+}
+function reference(value: unknown): string | null {
+  return value === null ? null : text(value);
+}
+
+export function parseConversation(
+  value: unknown,
+  scope: ConversationScope,
+): ChatConversation {
+  const v = object(value, [
+    "id",
+    "subject_id",
+    "participant_account_id",
+    "last_user_message_id",
+  ]);
+  if (
+    v.subject_id !== scope.subjectId ||
+    v.participant_account_id !== scope.accountId
+  ) invalid();
+  return {
+    id: text(v.id),
+    subject_id: scope.subjectId,
+    participant_account_id: scope.accountId,
+    last_user_message_id: reference(v.last_user_message_id),
+  };
+}
+
+export function parseConversationMessage(
+  value: unknown,
+  scope: ConversationScope,
+  conversationId: string,
+): ChatMessage {
+  const v = object(value, [
+    "id",
+    "conversation_id",
+    "sequence",
+    "sender",
+    "text",
+    "reply_to",
+    "work_ref",
+    "operation_id",
+    "last_user_message_id",
+  ]);
+  if (
+    v.conversation_id !== conversationId || typeof v.sequence !== "number" ||
+    !Number.isSafeInteger(v.sequence) || v.sequence < 1 ||
+    v.sequence > CHAT_LIMITS.maxCounter
+  ) invalid();
+  if (typeof v.sender !== "object" || v.sender === null) invalid();
+  const kind = (v.sender as Record<string, unknown>).kind;
+  let sender: ChatMessage["sender"];
+  if (kind === "user") {
+    const s = object(v.sender, ["kind", "account_id"]);
+    if (s.account_id !== scope.accountId) invalid();
+    sender = { kind, account_id: scope.accountId };
+  } else if (kind === "subject") {
+    const s = object(v.sender, ["kind", "subject_id"]);
+    if (s.subject_id !== scope.subjectId) invalid();
+    sender = { kind, subject_id: scope.subjectId };
+  } else invalid();
+  return {
+    id: text(v.id),
+    conversation_id: conversationId,
+    sequence: v.sequence,
+    sender,
+    text: text(v.text, CHAT_LIMITS.maxTextBytes),
+    reply_to: reference(v.reply_to),
+    work_ref: reference(v.work_ref),
+    operation_id: text(v.operation_id),
+    last_user_message_id: reference(v.last_user_message_id),
+  };
+}
+
+export function parseConversationPage(
+  value: unknown,
+  scope: ConversationScope,
+  conversationId: string,
+  after: number,
+): ChatMessagePage {
+  const v = object(value, ["conversation", "messages", "has_more"]);
+  const conversation = parseConversation(v.conversation, scope);
+  if (
+    conversation.id !== conversationId || !Array.isArray(v.messages) ||
+    v.messages.length > CHAT_LIMITS.maxPageLimit ||
+    typeof v.has_more !== "boolean"
+  ) invalid();
+  const messages = v.messages.map((message) =>
+    parseConversationMessage(message, scope, conversationId)
+  );
+  let previous = after;
+  const ids = new Set<string>();
+  for (const message of messages) {
+    if (message.sequence <= previous || ids.has(message.id)) invalid();
+    previous = message.sequence;
+    ids.add(message.id);
+  }
+  if (v.has_more && messages.length === 0) invalid();
+  return { conversation, messages, has_more: v.has_more };
+}
+
+export function appendConversationMessages(
+  saved: ChatMessage[],
+  incoming: ChatMessage[],
+): ChatMessage[] {
+  let previous = saved.at(-1)?.sequence ?? 0;
+  const ids = new Set(saved.map((message) => message.id));
+  for (const message of incoming) {
+    if (message.sequence <= previous || ids.has(message.id)) invalid();
+    previous = message.sequence;
+    ids.add(message.id);
+  }
+  return [...saved, ...incoming];
+}
+
+function parseDelivery(value: unknown): ChatDelivery {
+  if (typeof value !== "object" || value === null) invalid();
+  const state = (value as Record<string, unknown>).state;
+  if (state === "no_current_body") {
+    object(value, ["state"]);
+    return { state };
+  }
+  if (state === "unavailable") {
+    const v = object(value, ["state", "detail"]);
+    return {
+      state,
+      detail: typeof v.detail === "string" ? v.detail : invalid(),
+    };
+  }
+  if (state !== "accepted" && state !== "rejected" && state !== "unknown") {
+    invalid();
+  }
+  const v = object(
+    value,
+    state === "accepted"
+      ? ["state", "runtime_id", "worker_id"]
+      : ["state", "runtime_id", "worker_id", "detail"],
+  );
+  const target = {
+    runtime_id: text(v.runtime_id),
+    worker_id: text(v.worker_id),
+  };
+  if (state === "accepted") return { state, ...target };
+  return {
+    state,
+    ...target,
+    detail: typeof v.detail === "string" ? v.detail : invalid(),
+  };
+}
+
+async function request(
+  fetchFn: typeof fetch,
+  path: string,
+  signal: AbortSignal,
+  body?: unknown,
+): Promise<unknown> {
+  let response: Response;
+  try {
+    response = await fetchFn(path, {
+      signal,
+      credentials: "same-origin",
+      cache: "no-store",
+      ...(body === undefined ? {} : {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify(body),
+      }),
+    });
+  } catch {
+    throw new ConversationApiError(null);
+  }
+  if (!response.ok) {
+    await response.body?.cancel();
+    throw new ConversationApiError(response.status);
+  }
+  try {
+    return await readBoundedJson(response, 2 * 1024 * 1024);
+  } catch {
+    throw new ConversationApiError(null);
+  }
+}
+
+function base(scope: ConversationScope): string {
+  return workspaceApiPath(scope.workspaceId, "/chat/conversations");
+}
+export async function openSubjectConversation(
+  fetchFn: typeof fetch,
+  scope: ConversationScope,
+  signal: AbortSignal,
+): Promise<ChatConversation> {
+  const body: ChatCreateConversationRequest = {
+    operation_id: conversationOperationId(scope.subjectId),
+    subject_id: scope.subjectId,
+  };
+  return parseConversation(
+    await request(fetchFn, base(scope), signal, body),
+    scope,
+  );
+}
+export async function getConversationPage(
+  fetchFn: typeof fetch,
+  scope: ConversationScope,
+  conversationId: string,
+  after: number,
+  signal: AbortSignal,
+): Promise<ChatMessagePage> {
+  const query = new URLSearchParams({
+    after_sequence: String(after),
+    limit: String(CHAT_LIMITS.maxPageLimit),
+  });
+  return parseConversationPage(
+    await request(
+      fetchFn,
+      `${base(scope)}/${encodeURIComponent(conversationId)}/messages?${query}`,
+      signal,
+    ),
+    scope,
+    conversationId,
+    after,
+  );
+}
+export async function sendConversationInput(
+  fetchFn: typeof fetch,
+  scope: ConversationScope,
+  attempt: ConversationAttempt,
+  signal: AbortSignal,
+): Promise<ChatReceiveResponse> {
+  const body: ChatUserMessageRequest = {
+    operation_id: attempt.operationId,
+    text: attempt.text,
+    reply_to: attempt.replyTo,
+    work_ref: attempt.workRef,
+  };
+  const v = object(
+    await request(
+      fetchFn,
+      `${base(scope)}/${encodeURIComponent(attempt.conversationId)}/messages`,
+      signal,
+      body,
+    ),
+    ["message", "delivery"],
+  );
+  const message = parseConversationMessage(
+    v.message,
+    scope,
+    attempt.conversationId,
+  );
+  if (
+    message.sender.kind !== "user" ||
+    message.operation_id !== body.operation_id || message.text !== body.text ||
+    message.reply_to !== body.reply_to || message.work_ref !== body.work_ref
+  ) invalid();
+  return { message, delivery: parseDelivery(v.delivery) };
+}
diff --git a/web/workspace/src/lib/workspace/memory/conversation-attempt.test.ts b/web/workspace/src/lib/workspace/memory/conversation-attempt.test.ts
new file mode 100644
index 000000000..3f2345e53
--- /dev/null
+++ b/web/workspace/src/lib/workspace/memory/conversation-attempt.test.ts
@@ -0,0 +1,194 @@
+import {
+  clearConversationAttempt,
+  type ConversationAttempt,
+  conversationAttemptKey,
+  conversationOperationId,
+  type ConversationScope,
+  readConversationAttempt,
+  saveConversationAttempt,
+} from "./conversation-attempt.ts";
+
+declare const Deno: { test(name: string, fn: () => void): void };
+function assert(condition: unknown): asserts condition {
+  if (!condition) throw new Error("Assertion failed");
+}
+function rejects(fn: () => void): void {
+  let threw = false;
+  try {
+    fn();
+  } catch {
+    threw = true;
+  }
+  assert(threw);
+}
+function storage() {
+  const values = new Map<string, string>();
+  return {
+    getItem: (key: string) => values.get(key) ?? null,
+    setItem: (key: string, value: string) => {
+      values.set(key, value);
+    },
+    removeItem: (key: string) => {
+      values.delete(key);
+    },
+  };
+}
+const scope: ConversationScope = {
+  workspaceId: "workspace",
+  subjectId: "subject",
+  accountId: "account",
+};
+const attempt: ConversationAttempt = {
+  conversationId: "conversation",
+  operationId: "operation",
+  text: "  original\n本文  ",
+  replyTo: null,
+  workRef: null,
+};
+
+Deno.test("reload and explicit retry preserve the entire original send, not the edited draft", () => {
+  const s = storage();
+  saveConversationAttempt(s, scope, attempt);
+  const restored = readConversationAttempt(s, scope);
+  assert(JSON.stringify(restored) === JSON.stringify(attempt));
+  assert(Object.isFrozen(restored));
+  saveConversationAttempt(s, scope, { ...attempt });
+  assert(readConversationAttempt(s, scope)?.text === "  original\n本文  ");
+  rejects(() =>
+    saveConversationAttempt(s, scope, { ...attempt, text: "edited" })
+  );
+  assert(readConversationAttempt(s, scope)?.text === attempt.text);
+});
+
+Deno.test("unresolved input cannot be overwritten by a new ID or changed reply/work/conversation", () => {
+  const s = storage();
+  saveConversationAttempt(s, scope, attempt);
+  for (
+    const changed of [
+      { ...attempt, operationId: "new" },
+      { ...attempt, conversationId: "other" },
+      { ...attempt, replyTo: "reply" },
+      { ...attempt, workRef: "work" },
+    ]
+  ) {
+    rejects(() => saveConversationAttempt(s, scope, changed));
+    rejects(() => clearConversationAttempt(s, scope, changed));
+    assert(
+      readConversationAttempt(s, scope)?.operationId === attempt.operationId,
+    );
+  }
+  clearConversationAttempt(s, scope, { ...attempt });
+  assert(readConversationAttempt(s, scope) === null);
+});
+
+Deno.test("Workspace Subject and authenticated Account have separate retained input", () => {
+  const s = storage();
+  saveConversationAttempt(s, scope, attempt);
+  for (
+    const other of [
+      { ...scope, workspaceId: "other" },
+      { ...scope, subjectId: "other" },
+      { ...scope, accountId: "other" },
+    ]
+  ) {
+    assert(readConversationAttempt(s, other) === null);
+    // 区切り文字を含む識別子でも別scopeに衝突しない。
+    assert(conversationAttemptKey(other) !== conversationAttemptKey(scope));
+  }
+  assert(
+    conversationAttemptKey({
+      workspaceId: "a:b",
+      subjectId: "c",
+      accountId: "d",
+    }) !==
+      conversationAttemptKey({
+        workspaceId: "a",
+        subjectId: "b:c",
+        accountId: "d",
+      }),
+  );
+  assert(
+    conversationOperationId(scope.subjectId) ===
+      conversationOperationId(scope.subjectId),
+  );
+  assert(
+    conversationOperationId("other") !==
+      conversationOperationId(scope.subjectId),
+  );
+});
+
+Deno.test("corrupt or cross-scope retained input fails closed without replacement or clearing", () => {
+  const s = storage();
+  const key = conversationAttemptKey(scope);
+  for (
+    const bad of [
+      "not json",
+      JSON.stringify({
+        version: 1,
+        scope: { ...scope, accountId: "other" },
+        attempt,
+      }),
+      JSON.stringify({
+        version: 1,
+        scope,
+        attempt: { ...attempt, operationId: "" },
+      }),
+      JSON.stringify({
+        version: 1,
+        scope,
+        attempt: { ...attempt, replyTo: undefined },
+      }),
+      JSON.stringify({
+        version: 1,
+        scope,
+        attempt: { ...attempt, attachments: [] },
+      }),
+    ]
+  ) {
+    s.setItem(key, bad);
+    rejects(() => readConversationAttempt(s, scope));
+    rejects(() => saveConversationAttempt(s, scope, attempt));
+    rejects(() => clearConversationAttempt(s, scope, attempt));
+    assert(s.getItem(key) === bad);
+  }
+});
+
+Deno.test("storage refusal is not a retained send or successful clear", () => {
+  const s = storage();
+  rejects(() =>
+    saveConversationAttempt(
+      {
+        ...s,
+        setItem: () => {
+          throw new Error("quota");
+        },
+      },
+      scope,
+      attempt,
+    )
+  );
+  assert(readConversationAttempt(s, scope) === null);
+  saveConversationAttempt(s, scope, attempt);
+  rejects(() =>
+    clearConversationAttempt(
+      {
+        ...s,
+        removeItem: () => {
+          throw new Error("blocked");
+        },
+      },
+      scope,
+      attempt,
+    )
+  );
+  assert(
+    readConversationAttempt(s, scope)?.operationId === attempt.operationId,
+  );
+  rejects(() =>
+    readConversationAttempt({
+      getItem: () => {
+        throw new Error("blocked");
+      },
+    }, scope)
+  );
+});
diff --git a/web/workspace/src/lib/workspace/memory/conversation-attempt.ts b/web/workspace/src/lib/workspace/memory/conversation-attempt.ts
new file mode 100644
index 000000000..945341b12
--- /dev/null
+++ b/web/workspace/src/lib/workspace/memory/conversation-attempt.ts
@@ -0,0 +1,127 @@
+// 会話の正本や配送queueではなく、同じtabでの明示再送に必要な全入力だけを保持する。
+// 送信結果が不明な間は次の要求で上書きせず、本文の後続編集とは分離する。
+export type ConversationScope = Readonly<{
+  workspaceId: string;
+  subjectId: string;
+  accountId: string;
+}>;
+
+export type ConversationAttempt = Readonly<{
+  conversationId: string;
+  operationId: string;
+  text: string;
+  replyTo: string | null;
+  workRef: string | null;
+}>;
+
+type AttemptStorage = Pick<Storage, "getItem" | "setItem" | "removeItem">;
+
+export function conversationOperationId(subjectId: string): string {
+  // createのoperationはAccount/Workspaceに束縛される。再訪・別tabでも同じ会話を解決する。
+  return `web-subject-conversation/v1/${subjectId}`;
+}
+
+export function conversationAttemptKey(scope: ConversationScope): string {
+  return `yoi.conversation-attempt:${
+    JSON.stringify([scope.workspaceId, scope.subjectId, scope.accountId])
+  }`;
+}
+
+function record(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null && !Array.isArray(value);
+}
+
+function exact(value: Record<string, unknown>, keys: string[]): boolean {
+  return Object.keys(value).length === keys.length &&
+    keys.every((key) => key in value);
+}
+
+function id(value: unknown): value is string {
+  return typeof value === "string" && value.trim().length > 0 &&
+    new TextEncoder().encode(value).length <= 512 &&
+    !/[\u0000-\u001f\u007f-\u009f]/u.test(value);
+}
+
+function nullableId(value: unknown): value is string | null {
+  return value === null || id(value);
+}
+
+export function readConversationAttempt(
+  storage: Pick<AttemptStorage, "getItem">,
+  scope: ConversationScope,
+): ConversationAttempt | null {
+  const raw = storage.getItem(conversationAttemptKey(scope));
+  if (raw === null) return null;
+  const value: unknown = JSON.parse(raw);
+  if (
+    !record(value) || !exact(value, ["version", "scope", "attempt"]) ||
+    value.version !== 1 ||
+    !record(value.scope) ||
+    !exact(value.scope, ["workspaceId", "subjectId", "accountId"]) ||
+    value.scope.workspaceId !== scope.workspaceId ||
+    value.scope.subjectId !== scope.subjectId ||
+    value.scope.accountId !== scope.accountId || !record(value.attempt)
+  ) {
+    throw new Error(
+      "Saved send input cannot be read. Do not replace it with a new send.",
+    );
+  }
+  const attempt = value.attempt;
+  if (
+    !exact(attempt, [
+      "conversationId",
+      "operationId",
+      "text",
+      "replyTo",
+      "workRef",
+    ]) ||
+    !id(attempt.conversationId) || !id(attempt.operationId) ||
+    typeof attempt.text !== "string" ||
+    !attempt.text.trim() || !nullableId(attempt.replyTo) ||
+    !nullableId(attempt.workRef)
+  ) {
+    throw new Error(
+      "Saved send input is invalid. Do not replace it with a new send.",
+    );
+  }
+  return Object.freeze({
+    conversationId: attempt.conversationId,
+    operationId: attempt.operationId,
+    text: attempt.text,
+    replyTo: attempt.replyTo,
+    workRef: attempt.workRef,
+  });
+}
+
+function sameAttempt(a: ConversationAttempt, b: ConversationAttempt): boolean {
+  return a.conversationId === b.conversationId &&
+    a.operationId === b.operationId &&
+    a.text === b.text && a.replyTo === b.replyTo && a.workRef === b.workRef;
+}
+
+export function saveConversationAttempt(
+  storage: AttemptStorage,
+  scope: ConversationScope,
+  attempt: ConversationAttempt,
+): void {
+  const saved = readConversationAttempt(storage, scope);
+  if (saved !== null && !sameAttempt(saved, attempt)) {
+    throw new Error("Resolve the saved send before starting another send.");
+  }
+  storage.setItem(
+    conversationAttemptKey(scope),
+    JSON.stringify({ version: 1, scope, attempt }),
+  );
+}
+
+export function clearConversationAttempt(
+  storage: AttemptStorage,
+  scope: ConversationScope,
+  expected: ConversationAttempt,
+): void {
+  const saved = readConversationAttempt(storage, scope);
+  if (saved !== null && !sameAttempt(saved, expected)) {
+    throw new Error("The saved send changed. It has not been cleared.");
+  }
+  storage.removeItem(conversationAttemptKey(scope));
+}