feat: display model information in session UI

frontend/src/lib/components/content/MessageContent.svelte

@@ -6,6 +6,7 @@
   } from "../../utils/content-parser.js";
   import { formatTimestamp } from "../../utils/format.js";
   import { copyToClipboard } from "../../utils/clipboard.js";
+  import { messages as messagesStore } from "../../stores/messages.svelte.js";
   import ThinkingBlock from "./ThinkingBlock.svelte";
   import ToolBlock from "./ToolBlock.svelte";
   import CodeBlock from "./CodeBlock.svelte";
@@ -37,6 +38,18 @@
 
   let isUser = $derived(message.role === "user");
 
+  let mainModel = $derived(
+    !isSubagentContext &&
+    messagesStore.sessionId === message.session_id
+      ? messagesStore.mainModel
+      : "",
+  );
+
+  let offMainModel = $derived.by((): string => {
+    if (isUser || !message.model || !mainModel) return "";
+    return message.model !== mainModel ? message.model : "";
+  });
+
   /** Resolve the session that owns this message, falling back to activeSession. */
   let owningSession = $derived(
     sessions.sessions.find((s) => s.id === message.session_id) ??
@@ -202,6 +215,11 @@
     <span class="timestamp">
       {formatTimestamp(message.timestamp)}
     </span>
+    {#if offMainModel}
+      <span class="message-model" title={offMainModel}>
+        {offMainModel}
+      </span>
+    {/if}
   </div>
 
   <div class="message-body">
@@ -295,6 +313,17 @@
     margin-left: auto;
   }
 
+  .message-model {
+    font-size: 10px;
+    color: var(--text-muted);
+    padding: 1px 4px;
+    border-radius: 3px;
+    background: var(--bg-tertiary);
+    white-space: nowrap;
+    flex-shrink: 0;
+    opacity: 0.8;
+  }
+
   .copy-btn {
     display: flex;
     align-items: center;

frontend/src/lib/components/content/SubagentInline.svelte

@@ -4,6 +4,7 @@
   import type { Message, Session } from "../../api/types.js";
   import { getMessages, getSession } from "../../api/client.js";
   import { formatTokenCount } from "../../utils/format.js";
+  import { computeMainModel } from "../../utils/model.js";
   import { sessions } from "../../stores/sessions.svelte.js";
   import { router } from "../../stores/router.svelte.js";
   import MessageContent from "./MessageContent.svelte";
@@ -56,6 +57,12 @@
   let messageCountLabel = $derived(
     sessionMeta ? `${sessionMeta.message_count} messages` : null,
   );
+  let subagentModel = $derived(
+    messages && sessionMeta &&
+    messages.length >= sessionMeta.message_count
+      ? computeMainModel(messages)
+      : "",
+  );
 </script>
 
 <div class="subagent-inline">
@@ -75,6 +82,9 @@
         {#if ctxTokens + subagentSession.total_output_tokens > 0}
           <span class="toggle-tokens">({formatTokenCount(ctxTokens)} ctx / {formatTokenCount(subagentSession.total_output_tokens)} out)</span>
         {/if}
+        {#if subagentModel}
+          <span class="toggle-model" title={subagentModel}>{subagentModel}</span>
+        {/if}
       {/if}
     </button>
     <a
@@ -192,6 +202,12 @@
     flex-shrink: 0;
   }
 
+  .toggle-model {
+    font-size: 10px;
+    color: var(--text-muted);
+    white-space: nowrap;
+    flex-shrink: 0;
+  }
 
   .subagent-messages {
     border-left: 3px solid var(--accent-green);

frontend/src/lib/components/layout/SessionBreadcrumb.svelte

@@ -18,6 +18,7 @@
   } from "../../utils/resume.js";
 
   import { inSessionSearch } from "../../stores/inSessionSearch.svelte.js";
+  import { messages as messagesStore } from "../../stores/messages.svelte.js";
 
   interface Props {
     session: Session | undefined;
@@ -57,6 +58,12 @@
 
   let sessionContextTokens = $derived(session?.peak_context_tokens ?? 0);
 
+  let mainModel = $derived(
+    messagesStore.sessionId === session?.id
+      ? messagesStore.mainModel
+      : "",
+  );
+
   function sessionDisplayId(id: string): string {
     const idx = id.indexOf(":");
     return idx >= 0 ? id.slice(idx + 1) : id;
@@ -447,6 +454,9 @@
           {formatTokenCount(sessionContextTokens)} ctx / {formatTokenCount(session?.total_output_tokens ?? 0)} out
         </span>
       {/if}
+      {#if mainModel}
+        <span class="model-badge" title={mainModel}>{mainModel}</span>
+      {/if}
       <div class="actions-wrapper">
         <button
           class="find-btn"
@@ -703,6 +713,15 @@
     flex-shrink: 0;
   }
 
+  .model-badge {
+    font-size: 10px;
+    color: var(--text-muted);
+    padding: 1px 5px;
+    border-radius: 4px;
+    background: var(--bg-tertiary);
+    white-space: nowrap;
+    flex-shrink: 0;
+  }
 
   .actions-wrapper {
     position: relative;

frontend/src/lib/stores/messages.svelte.ts

@@ -1,5 +1,6 @@
 import * as api from "../api/client.js";
 import type { Message } from "../api/types.js";
+import { computeMainModel } from "../utils/model.js";
 
 const MESSAGE_PAGE_SIZE = 1000;
 const FULL_SESSION_MESSAGE_THRESHOLD = 20_000;
@@ -18,6 +19,9 @@ class MessagesStore {
   messageCount: number = $state(0);
   hasOlder: boolean = $state(false);
   loadingOlder: boolean = $state(false);
+  mainModel: string = $derived(
+    !this.loading ? computeMainModel(this.messages) : "",
+  );
   private abortController: AbortController | null = null;
   private reloadPromise: Promise<void> | null = null;
   private reloadSessionId: string | null = null;

frontend/src/lib/utils/model.test.ts

@@ -0,0 +1,80 @@
+import { describe, it, expect } from "vitest";
+import { computeMainModel } from "./model.js";
+import type { Message } from "../api/types.js";
+
+function msg(role: string, model: string): Message {
+  return {
+    id: 0,
+    session_id: "",
+    ordinal: 0,
+    role,
+    content: "",
+    timestamp: "",
+    has_thinking: false,
+    has_tool_use: false,
+    content_length: 0,
+    model,
+    context_tokens: 0,
+    output_tokens: 0,
+  };
+}
+
+describe("computeMainModel", () => {
+  it("returns empty string for empty array", () => {
+    expect(computeMainModel([])).toBe("");
+  });
+
+  it("returns the single model", () => {
+    expect(
+      computeMainModel([
+        msg("assistant", "claude-sonnet-4.6"),
+      ]),
+    ).toBe("claude-sonnet-4.6");
+  });
+
+  it("returns most frequent model", () => {
+    expect(
+      computeMainModel([
+        msg("assistant", "claude-sonnet-4.6"),
+        msg("assistant", "claude-sonnet-4.6"),
+        msg("assistant", "claude-haiku-4.5"),
+      ]),
+    ).toBe("claude-sonnet-4.6");
+  });
+
+  it("breaks ties alphabetically", () => {
+    expect(
+      computeMainModel([
+        msg("assistant", "b-model"),
+        msg("assistant", "a-model"),
+      ]),
+    ).toBe("a-model");
+  });
+
+  it("ignores user messages", () => {
+    expect(
+      computeMainModel([
+        msg("user", "some-model"),
+        msg("assistant", "claude-sonnet-4.6"),
+      ]),
+    ).toBe("claude-sonnet-4.6");
+  });
+
+  it("ignores empty model strings", () => {
+    expect(
+      computeMainModel([
+        msg("assistant", ""),
+        msg("assistant", "claude-sonnet-4.6"),
+      ]),
+    ).toBe("claude-sonnet-4.6");
+  });
+
+  it("returns empty when no model data", () => {
+    expect(
+      computeMainModel([
+        msg("assistant", ""),
+        msg("user", ""),
+      ]),
+    ).toBe("");
+  });
+});

frontend/src/lib/utils/model.ts

@@ -0,0 +1,24 @@
+import type { Message } from "../api/types.js";
+
+/**
+ * Compute the most frequently used model across assistant messages.
+ * Returns empty string if no model data is present.
+ * Tie-break: alphabetically first model wins.
+ */
+export function computeMainModel(messages: Message[]): string {
+  const counts = new Map<string, number>();
+  for (const m of messages) {
+    if (m.role === "assistant" && m.model) {
+      counts.set(m.model, (counts.get(m.model) ?? 0) + 1);
+    }
+  }
+  let best = "";
+  let bestN = 0;
+  for (const [model, n] of counts) {
+    if (n > bestN || (n === bestN && model < best)) {
+      best = model;
+      bestN = n;
+    }
+  }
+  return best;
+}

internal/parser/copilot.go

@@ -17,6 +17,7 @@ const (
 	copilotEventAssistantMsg    = "assistant.message"
 	copilotEventToolComplete    = "tool.execution_complete"
 	copilotEventAssistantReason = "assistant.reasoning"
+	copilotEventModelChange     = "session.model_change"
 )
 
 // copilotSessionBuilder accumulates state while scanning a
@@ -29,6 +30,7 @@ type copilotSessionBuilder struct {
 	sessionID    string
 	project      string
 	ordinal      int
+	currentModel string
 }
 
 func newCopilotSessionBuilder() *copilotSessionBuilder {
@@ -60,6 +62,10 @@ func (b *copilotSessionBuilder) processLine(line string) {
 		b.handleToolComplete(data, ts)
 	case copilotEventAssistantReason:
 		b.handleAssistantReasoning()
+	case copilotEventModelChange:
+		if v := data.Get("newModel"); v.Exists() {
+			b.currentModel = v.Str
+		}
 	}
 }
 
@@ -165,6 +171,7 @@ func (b *copilotSessionBuilder) handleAssistantMessage(
 		HasToolUse:    hasToolUse,
 		ContentLength: len(displayContent),
 		ToolCalls:     toolCalls,
+		Model:         b.currentModel,
 	})
 	b.ordinal++
 }

internal/parser/copilot_test.go

@@ -313,6 +313,67 @@ func TestCopilotUserMessageCount(t *testing.T) {
 	assertEqual(t, 2, sess.UserMessageCount, "UserMessageCount")
 }
 
+func TestParseCopilotSession_ModelChange(t *testing.T) {
+	path := writeCopilotJSONL(t,
+		`{"type":"session.start","data":{"sessionId":"model-test"},"timestamp":"2025-01-15T10:00:00Z"}`,
+		`{"type":"session.model_change","data":{"newModel":"claude-sonnet-4.6"},"timestamp":"2025-01-15T10:00:01Z"}`,
+		`{"type":"user.message","data":{"content":"Hello"},"timestamp":"2025-01-15T10:00:02Z"}`,
+		`{"type":"assistant.message","data":{"content":"Hi there"},"timestamp":"2025-01-15T10:00:03Z"}`,
+	)
+
+	_, msgs := parseAndValidateHelper(t, path, "m", 2)
+
+	assertEqual(t, "claude-sonnet-4.6", msgs[1].Model, "msgs[1].Model")
+	assertEqual(t, "", msgs[0].Model, "msgs[0].Model")
+}
+
+func TestParseCopilotSession_NoModel(t *testing.T) {
+	path := writeCopilotJSONL(t,
+		`{"type":"session.start","data":{"sessionId":"no-model"},"timestamp":"2025-01-15T10:00:00Z"}`,
+		`{"type":"user.message","data":{"content":"Hello"},"timestamp":"2025-01-15T10:00:01Z"}`,
+		`{"type":"assistant.message","data":{"content":"Hi"},"timestamp":"2025-01-15T10:00:02Z"}`,
+	)
+
+	_, msgs := parseAndValidateHelper(t, path, "m", 2)
+	assertEqual(t, "", msgs[1].Model, "msgs[1].Model")
+}
+
+func TestParseCopilotSession_ModelMidSessionChange(t *testing.T) {
+	path := writeCopilotJSONL(t,
+		`{"type":"session.start","data":{"sessionId":"switch-test"},"timestamp":"2025-01-15T10:00:00Z"}`,
+		`{"type":"session.model_change","data":{"newModel":"claude-sonnet-4.6"},"timestamp":"2025-01-15T10:00:01Z"}`,
+		`{"type":"user.message","data":{"content":"First"},"timestamp":"2025-01-15T10:00:02Z"}`,
+		`{"type":"assistant.message","data":{"content":"Reply one"},"timestamp":"2025-01-15T10:00:03Z"}`,
+		`{"type":"session.model_change","data":{"newModel":"claude-haiku-4.5"},"timestamp":"2025-01-15T10:00:04Z"}`,
+		`{"type":"user.message","data":{"content":"Second"},"timestamp":"2025-01-15T10:00:05Z"}`,
+		`{"type":"assistant.message","data":{"content":"Reply two"},"timestamp":"2025-01-15T10:00:06Z"}`,
+	)
+
+	_, msgs := parseAndValidateHelper(t, path, "m", 4)
+
+	assertEqual(t, "claude-sonnet-4.6", msgs[1].Model, "msgs[1].Model")
+	assertEqual(t, "claude-haiku-4.5", msgs[3].Model, "msgs[3].Model")
+}
+
+func TestParseCopilotSession_ModelReset(t *testing.T) {
+	// An empty newModel clears the active model so
+	// subsequent assistant messages have no model.
+	path := writeCopilotJSONL(t,
+		`{"type":"session.start","data":{"sessionId":"reset-test"},"timestamp":"2025-01-15T10:00:00Z"}`,
+		`{"type":"session.model_change","data":{"newModel":"claude-sonnet-4.6"},"timestamp":"2025-01-15T10:00:01Z"}`,
+		`{"type":"user.message","data":{"content":"First"},"timestamp":"2025-01-15T10:00:02Z"}`,
+		`{"type":"assistant.message","data":{"content":"Reply one"},"timestamp":"2025-01-15T10:00:03Z"}`,
+		`{"type":"session.model_change","data":{"newModel":""},"timestamp":"2025-01-15T10:00:04Z"}`,
+		`{"type":"user.message","data":{"content":"Second"},"timestamp":"2025-01-15T10:00:05Z"}`,
+		`{"type":"assistant.message","data":{"content":"Reply two"},"timestamp":"2025-01-15T10:00:06Z"}`,
+	)
+
+	_, msgs := parseAndValidateHelper(t, path, "m", 4)
+
+	assertEqual(t, "claude-sonnet-4.6", msgs[1].Model, "msgs[1].Model")
+	assertEqual(t, "", msgs[3].Model, "msgs[3].Model (reset)")
+}
+
 func TestSessionIDFromPath(t *testing.T) {
 	tests := []struct {
 		path string