diff --git a/src/agents/pi-embedded-runner/run/attempt.ts b/src/agents/pi-embedded-runner/run/attempt.ts
index e83c3ae4a..622bdb7f4 100644
--- a/src/agents/pi-embedded-runner/run/attempt.ts
+++ b/src/agents/pi-embedded-runner/run/attempt.ts
@@ -52,6 +52,7 @@ import {
 import { DEFAULT_BOOTSTRAP_FILENAME } from "../../workspace.js";
 import { buildSystemPromptReport } from "../../system-prompt-report.js";
 import { resolveDefaultModelForAgent } from "../../model-selection.js";
+import { sanitizeToolUseResultPairing } from "../../session-transcript-repair.js";
 
 import { isAbortError } from "../abort.js";
 import { buildEmbeddedExtensionPaths } from "../extensions.js";
@@ -535,9 +536,11 @@ export async function runEmbeddedAttempt(
           validated,
           getDmHistoryLimitFromSessionKey(params.sessionKey, params.config),
         );
-        cacheTrace?.recordStage("session:limited", { messages: limited });
-        if (limited.length > 0) {
-          activeSession.agent.replaceMessages(limited);
+        // Fix: Repair tool_use/tool_result pairings AFTER truncation (issue #4367)
+        const repaired = sanitizeToolUseResultPairing(limited);
+        cacheTrace?.recordStage("session:limited", { messages: repaired });
+        if (repaired.length > 0) {
+          activeSession.agent.replaceMessages(repaired);
         }
       } catch (err) {
         sessionManager.flushPendingToolResults?.();
diff --git a/src/agents/subagent-announce.ts b/src/agents/subagent-announce.ts
index 444726efc..46d902e80 100644
--- a/src/agents/subagent-announce.ts
+++ b/src/agents/subagent-announce.ts
@@ -192,6 +192,41 @@ async function maybeQueueSubagentAnnounce(params: {
   return "none";
 }
 
+/**
+ * Build a descriptive error status label from outcome data.
+ * Includes error type, message, and hint if available.
+ */
+function buildErrorStatusLabel(outcome: SubagentRunOutcome): string {
+  const parts: string[] = [];
+
+  // Start with "failed"
+  parts.push("failed");
+
+  // Add error type context
+  if (outcome.errorType) {
+    const typeLabel: Record<string, string> = {
+      model: "API error",
+      tool: "tool error",
+      network: "network error",
+      config: "configuration error",
+      timeout: "timeout",
+    };
+    const label = typeLabel[outcome.errorType] || "error";
+    parts.push(`(${label}):`);
+  }
+
+  // Add error message
+  const errorMsg = outcome.error || "unknown error";
+  parts.push(errorMsg);
+
+  // Add hint if available
+  if (outcome.errorHint) {
+    parts.push(`— ${outcome.errorHint}`);
+  }
+
+  return parts.join(" ");
+}
+
 async function buildSubagentStatsLine(params: {
   sessionKey: string;
   startedAt?: number;
@@ -299,6 +334,8 @@ export function buildSubagentSystemPrompt(params: {
 export type SubagentRunOutcome = {
   status: "ok" | "error" | "timeout" | "unknown";
   error?: string;
+  errorType?: "model" | "tool" | "network" | "config" | "timeout" | "unknown";
+  errorHint?: string;
 };
 
 export async function runSubagentAnnounceFlow(params: {
@@ -380,7 +417,7 @@ export async function runSubagentAnnounceFlow(params: {
         : outcome.status === "timeout"
           ? "timed out"
           : outcome.status === "error"
-            ? `failed: ${outcome.error || "unknown error"}`
+            ? buildErrorStatusLabel(outcome)
             : "finished with unknown status";
 
     // Build instructional message for main agent
diff --git a/src/agents/subagent-registry.ts b/src/agents/subagent-registry.ts
index d325e40e2..dca685b72 100644
--- a/src/agents/subagent-registry.ts
+++ b/src/agents/subagent-registry.ts
@@ -184,7 +184,16 @@ function ensureListener() {
     entry.endedAt = endedAt;
     if (phase === "error") {
       const error = typeof evt.data?.error === "string" ? (evt.data.error as string) : undefined;
-      entry.outcome = { status: "error", error };
+      const errorType =
+        typeof evt.data?.errorType === "string" ? (evt.data.errorType as string) : undefined;
+      const errorHint =
+        typeof evt.data?.errorHint === "string" ? (evt.data.errorHint as string) : undefined;
+      entry.outcome = {
+        status: "error",
+        error,
+        errorType: errorType as SubagentRunOutcome["errorType"],
+        errorHint,
+      };
     } else {
       entry.outcome = { status: "ok" };
     }
diff --git a/src/auto-reply/reply/agent-runner-execution.ts b/src/auto-reply/reply/agent-runner-execution.ts
index 21732f49f..266a483e8 100644
--- a/src/auto-reply/reply/agent-runner-execution.ts
+++ b/src/auto-reply/reply/agent-runner-execution.ts
@@ -51,6 +51,85 @@ export type AgentRunLoopResult =
     }
   | { kind: "final"; payload: ReplyPayload };
 
+/**
+ * Categorize errors to provide better error messages to users.
+ * Returns error message, type, and optional hint for remediation.
+ */
+export function categorizeError(err: unknown): {
+  message: string;
+  type: "model" | "tool" | "network" | "config" | "timeout" | "unknown";
+  hint?: string;
+} {
+  const message = err instanceof Error ? err.message : String(err);
+
+  // File system errors
+  if (message.includes("ENOENT") || message.includes("ENOTDIR")) {
+    return { message, type: "tool", hint: "File or directory not found" };
+  }
+  if (message.includes("EACCES") || message.includes("EPERM")) {
+    return { message, type: "tool", hint: "Permission denied" };
+  }
+  if (message.includes("EISDIR")) {
+    return { message, type: "tool", hint: "Expected file but found directory" };
+  }
+
+  // API/Model errors
+  if (message.includes("rate limit") || message.includes("429")) {
+    return { message, type: "model", hint: "Rate limit exceeded - retry in a few moments" };
+  }
+  if (
+    message.includes("401") ||
+    message.includes("unauthorized") ||
+    message.includes("authentication")
+  ) {
+    return { message, type: "config", hint: "Check API credentials and permissions" };
+  }
+  if (message.includes("403") || message.includes("forbidden")) {
+    return { message, type: "config", hint: "Access denied - check permissions" };
+  }
+  if (message.includes("400") || message.includes("invalid request")) {
+    return { message, type: "model", hint: "Invalid request parameters" };
+  }
+  if (message.includes("500") || message.includes("503")) {
+    return { message, type: "model", hint: "API service error - try again later" };
+  }
+  if (message.includes("quota") || message.includes("billing")) {
+    return { message, type: "config", hint: "Check billing and API quota limits" };
+  }
+
+  // Network errors
+  if (message.includes("ECONNREFUSED") || message.includes("ETIMEDOUT")) {
+    return { message, type: "network", hint: "Connection failed - check network connectivity" };
+  }
+  if (message.includes("ENOTFOUND") || message.includes("DNS") || message.includes("EAI_AGAIN")) {
+    return { message, type: "network", hint: "DNS resolution failed - check hostname" };
+  }
+  if (message.includes("ENETUNREACH") || message.includes("EHOSTUNREACH")) {
+    return { message, type: "network", hint: "Network unreachable - check connection" };
+  }
+
+  // Timeout errors
+  if (
+    message.toLowerCase().includes("timeout") ||
+    message.toLowerCase().includes("timed out") ||
+    message.includes("ETIMEDOUT")
+  ) {
+    return { message, type: "timeout", hint: "Operation took too long - try increasing timeout" };
+  }
+
+  // Context/memory errors
+  if (message.includes("context") && message.includes("too large")) {
+    return { message, type: "model", hint: "Conversation too long - try clearing history" };
+  }
+
+  // Missing environment/config
+  if (message.includes("missing") && (message.includes("key") || message.includes("token"))) {
+    return { message, type: "config", hint: "Missing required configuration or credentials" };
+  }
+
+  return { message, type: "unknown" };
+}
+
 export async function runAgentTurnWithFallback(params: {
   commandBody: string;
   followupRun: FollowupRun;
@@ -204,6 +283,7 @@ export async function runAgentTurnWithFallback(params: {
                 return result;
               })
               .catch((err) => {
+                const { message, type, hint } = categorizeError(err);
                 emitAgentEvent({
                   runId,
                   stream: "lifecycle",
@@ -211,7 +291,9 @@ export async function runAgentTurnWithFallback(params: {
                     phase: "error",
                     startedAt,
                     endedAt: Date.now(),
-                    error: err instanceof Error ? err.message : String(err),
+                    error: message,
+                    errorType: type,
+                    errorHint: hint,
                   },
                 });
                 throw err;
diff --git a/src/auto-reply/reply/categorize-error.test.ts b/src/auto-reply/reply/categorize-error.test.ts
new file mode 100644
index 000000000..17c85c58a
--- /dev/null
+++ b/src/auto-reply/reply/categorize-error.test.ts
@@ -0,0 +1,318 @@
+import { describe, expect, it } from "vitest";
+
+import { categorizeError } from "./agent-runner-execution.js";
+
+describe("categorizeError", () => {
+  describe("timeout errors", () => {
+    it("categorizes lowercase 'timeout' as timeout type", () => {
+      const error = new Error("Request timeout after 30s");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("timeout");
+      expect(result.message).toBe("Request timeout after 30s");
+      expect(result.hint).toBe("Operation took too long - try increasing timeout");
+    });
+
+    it("categorizes 'timed out' as timeout type", () => {
+      const error = new Error("Connection timed out");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("timeout");
+      expect(result.hint).toBe("Operation took too long - try increasing timeout");
+    });
+
+    it("categorizes ETIMEDOUT as network type (network error code takes precedence)", () => {
+      const error = new Error("ETIMEDOUT: socket hang up");
+      const result = categorizeError(error);
+
+      // ETIMEDOUT is caught by network errors before timeout section
+      expect(result.type).toBe("network");
+      expect(result.hint).toBe("Connection failed - check network connectivity");
+    });
+
+    it("handles uppercase TIMEOUT", () => {
+      const error = new Error("TIMEOUT ERROR");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("timeout");
+    });
+  });
+
+  describe("authentication errors", () => {
+    it("categorizes 401 as config type", () => {
+      const error = new Error("HTTP 401: Unauthorized");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("config");
+      expect(result.message).toBe("HTTP 401: Unauthorized");
+      expect(result.hint).toBe("Check API credentials and permissions");
+    });
+
+    it("categorizes 'unauthorized' as config type", () => {
+      const error = new Error("Request failed: unauthorized access");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("config");
+      expect(result.hint).toBe("Check API credentials and permissions");
+    });
+
+    it("categorizes 'authentication' errors as config type (case-sensitive)", () => {
+      const error = new Error("authentication failed for API key");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("config");
+      expect(result.hint).toBe("Check API credentials and permissions");
+    });
+
+    it("categorizes 403 forbidden as config type", () => {
+      const error = new Error("HTTP 403 forbidden");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("config");
+      expect(result.hint).toBe("Access denied - check permissions");
+    });
+
+    it("categorizes 'forbidden' keyword as config type", () => {
+      const error = new Error("Access forbidden to resource");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("config");
+    });
+  });
+
+  describe("rate limit errors", () => {
+    it("categorizes 'rate limit' as model type", () => {
+      const error = new Error("rate limit exceeded");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("model");
+      expect(result.message).toBe("rate limit exceeded");
+      expect(result.hint).toBe("Rate limit exceeded - retry in a few moments");
+    });
+
+    it("categorizes HTTP 429 as model type", () => {
+      const error = new Error("HTTP 429: Too Many Requests");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("model");
+      expect(result.hint).toBe("Rate limit exceeded - retry in a few moments");
+    });
+
+    it("handles rate limit with mixed case", () => {
+      const error = new Error("rate limit exceeded");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("model");
+    });
+  });
+
+  describe("unknown errors", () => {
+    it("categorizes unrecognized error as unknown type", () => {
+      const error = new Error("Something weird happened");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("unknown");
+      expect(result.message).toBe("Something weird happened");
+      expect(result.hint).toBeUndefined();
+    });
+
+    it("categorizes generic error message as unknown", () => {
+      const error = new Error("An unexpected error occurred");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("unknown");
+      expect(result.hint).toBeUndefined();
+    });
+
+    it("handles non-Error objects", () => {
+      const result = categorizeError("plain string error");
+
+      expect(result.type).toBe("unknown");
+      expect(result.message).toBe("plain string error");
+    });
+
+    it("handles null/undefined errors", () => {
+      const result = categorizeError(null);
+
+      expect(result.type).toBe("unknown");
+      expect(result.message).toBe("null");
+    });
+  });
+
+  describe("API/model errors", () => {
+    it("categorizes HTTP 400 as model type", () => {
+      const error = new Error("HTTP 400: Bad Request");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("model");
+      expect(result.hint).toBe("Invalid request parameters");
+    });
+
+    it("categorizes 'invalid request' as model type", () => {
+      const error = new Error("invalid request format");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("model");
+    });
+
+    it("categorizes HTTP 500 as model type", () => {
+      const error = new Error("HTTP 500: Internal Server Error");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("model");
+      expect(result.hint).toBe("API service error - try again later");
+    });
+
+    it("categorizes HTTP 503 as model type", () => {
+      const error = new Error("HTTP 503: Service Unavailable");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("model");
+      expect(result.hint).toBe("API service error - try again later");
+    });
+
+    it("categorizes quota errors as config type", () => {
+      const error = new Error("quota exceeded for this account");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("config");
+      expect(result.hint).toBe("Check billing and API quota limits");
+    });
+
+    it("categorizes billing errors as config type", () => {
+      const error = new Error("billing issue detected");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("config");
+      expect(result.hint).toBe("Check billing and API quota limits");
+    });
+  });
+
+  describe("network errors", () => {
+    it("categorizes ECONNREFUSED as network type", () => {
+      const error = new Error("ECONNREFUSED: Connection refused");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("network");
+      expect(result.hint).toBe("Connection failed - check network connectivity");
+    });
+
+    it("categorizes ENOTFOUND as network type", () => {
+      const error = new Error("ENOTFOUND: DNS lookup failed");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("network");
+      expect(result.hint).toBe("DNS resolution failed - check hostname");
+    });
+
+    it("categorizes DNS errors as network type", () => {
+      const error = new Error("DNS resolution error");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("network");
+    });
+
+    it("categorizes EAI_AGAIN as network type", () => {
+      const error = new Error("EAI_AGAIN: temporary failure");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("network");
+      expect(result.hint).toBe("DNS resolution failed - check hostname");
+    });
+
+    it("categorizes ENETUNREACH as network type", () => {
+      const error = new Error("ENETUNREACH: Network is unreachable");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("network");
+      expect(result.hint).toBe("Network unreachable - check connection");
+    });
+
+    it("categorizes EHOSTUNREACH as network type", () => {
+      const error = new Error("EHOSTUNREACH: No route to host");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("network");
+      expect(result.hint).toBe("Network unreachable - check connection");
+    });
+  });
+
+  describe("file system errors (tool type)", () => {
+    it("categorizes ENOENT as tool type", () => {
+      const error = new Error("ENOENT: no such file or directory");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("tool");
+      expect(result.hint).toBe("File or directory not found");
+    });
+
+    it("categorizes ENOTDIR as tool type", () => {
+      const error = new Error("ENOTDIR: not a directory");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("tool");
+      expect(result.hint).toBe("File or directory not found");
+    });
+
+    it("categorizes EACCES as tool type", () => {
+      const error = new Error("EACCES: permission denied");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("tool");
+      expect(result.hint).toBe("Permission denied");
+    });
+
+    it("categorizes EPERM as tool type", () => {
+      const error = new Error("EPERM: operation not permitted");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("tool");
+      expect(result.hint).toBe("Permission denied");
+    });
+
+    it("categorizes EISDIR as tool type", () => {
+      const error = new Error("EISDIR: illegal operation on a directory");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("tool");
+      expect(result.hint).toBe("Expected file but found directory");
+    });
+  });
+
+  describe("configuration errors", () => {
+    it("categorizes missing API key as config type", () => {
+      const error = new Error("missing API key");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("config");
+      expect(result.hint).toBe("Missing required configuration or credentials");
+    });
+
+    it("categorizes missing token as config type", () => {
+      const error = new Error("missing authentication token");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("config");
+      // "authentication" keyword triggers auth error hint first
+      expect(result.hint).toBe("Check API credentials and permissions");
+    });
+
+    it("categorizes missing API token without authentication keyword", () => {
+      const error = new Error("missing API token for request");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("config");
+      expect(result.hint).toBe("Missing required configuration or credentials");
+    });
+  });
+
+  describe("context/memory errors", () => {
+    it("categorizes context too large as model type", () => {
+      const error = new Error("context window too large");
+      const result = categorizeError(error);
+
+      expect(result.type).toBe("model");
+      expect(result.hint).toBe("Conversation too long - try clearing history");
+    });
+  });
+});
diff --git a/src/hooks/llm-slug-generator.ts b/src/hooks/llm-slug-generator.ts
index c52627176..024262a5d 100644
--- a/src/hooks/llm-slug-generator.ts
+++ b/src/hooks/llm-slug-generator.ts
@@ -12,6 +12,7 @@ import {
   resolveAgentWorkspaceDir,
   resolveAgentDir,
 } from "../agents/agent-scope.js";
+import { resolveDefaultModelForAgent } from "../agents/model-selection.js";
 
 /**
  * Generate a short 1-2 word filename slug from session content using LLM
@@ -38,6 +39,11 @@ ${params.sessionContent.slice(0, 2000)}
 
 Reply with ONLY the slug, nothing else. Examples: "vendor-pitch", "api-design", "bug-fix"`;
 
+    // Resolve user's configured default model instead of hardcoded Opus
+    const { provider, model } = resolveDefaultModelForAgent({
+      cfg: params.cfg,
+    });
+
     const result = await runEmbeddedPiAgent({
       sessionId: `slug-generator-${Date.now()}`,
       sessionKey: "temp:slug-generator",
@@ -46,6 +52,8 @@ Reply with ONLY the slug, nothing else. Examples: "vendor-pitch", "api-design",
       agentDir,
       config: params.cfg,
       prompt,
+      provider,
+      model,
       timeoutMs: 15_000, // 15 second timeout
       runId: `slug-gen-${Date.now()}`,
     });
@@ -75,7 +83,10 @@ Reply with ONLY the slug, nothing else. Examples: "vendor-pitch", "api-design",
     // Clean up temporary session file
     if (tempSessionFile) {
       try {
-        await fs.rm(path.dirname(tempSessionFile), { recursive: true, force: true });
+        await fs.rm(path.dirname(tempSessionFile), {
+          recursive: true,
+          force: true,
+        });
       } catch {
         // Ignore cleanup errors
       }