diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index d57df01d..ca10fbde 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -12,6 +12,7 @@
 - Fixed HTML export losing indentation in ANSI-rendered tool output (e.g. JSON code blocks in custom tool results) ([#1269](https://github.com/badlogic/pi-mono/pull/1269) by [@aliou](https://github.com/aliou))
 - Fixed images being silently dropped when `prompt()` is called with both `images` and `streamingBehavior` during streaming. `steer()`, `followUp()`, and the corresponding RPC commands now accept optional images. ([#1271](https://github.com/badlogic/pi-mono/pull/1271) by [@aliou](https://github.com/aliou))
 - CLI `--help`, `--version`, `--list-models`, and `--export` now exit even if extensions keep the event loop alive ([#1285](https://github.com/badlogic/pi-mono/pull/1285) by [@ferologics](https://github.com/ferologics))
+- Fixed crash when models send malformed tool arguments (objects instead of strings) ([#1259](https://github.com/badlogic/pi-mono/issues/1259))
 
 ## [0.51.6] - 2026-02-04
 
diff --git a/packages/coding-agent/src/core/export-html/template.css b/packages/coding-agent/src/core/export-html/template.css
index bbf8e326..fbcec72a 100644
--- a/packages/coding-agent/src/core/export-html/template.css
+++ b/packages/coding-agent/src/core/export-html/template.css
@@ -693,6 +693,9 @@
       color: var(--error);
       padding: 0 var(--line-height);
     }
+    .tool-error {
+      color: var(--error);
+    }
 
     /* Images */
     .message-images {
diff --git a/packages/coding-agent/src/core/export-html/template.js b/packages/coding-agent/src/core/export-html/template.js
index 7d5edae0..551e34f5 100644
--- a/packages/coding-agent/src/core/export-html/template.js
+++ b/packages/coding-agent/src/core/export-html/template.js
@@ -540,6 +540,7 @@
       // ============================================================
 
       function shortenPath(p) {
+        if (typeof p !== 'string') return '';
         if (p.startsWith('/Users/')) {
           const parts = p.split('/');
           if (parts.length > 2) return '~' + p.slice(('/Users/' + parts[2]).length);
@@ -771,6 +772,13 @@
         return text.replace(/\t/g, '   ');
       }
 
+      /** Safely coerce value to string for display. Returns null if invalid type. */
+      function str(value) {
+        if (typeof value === 'string') return value;
+        if (value == null) return '';
+        return null;
+      }
+
       function getLanguageFromPath(filePath) {
         const ext = filePath.split('.').pop()?.toLowerCase();
         const extToLang = {
@@ -880,10 +888,13 @@
         const args = call.arguments || {};
         const name = call.name;
 
+        const invalidArg = '<span class="tool-error">[invalid arg]</span>';
+
         switch (name) {
           case 'bash': {
-            const command = args.command || '';
-            html += `<div class="tool-command">$ ${escapeHtml(command)}</div>`;
+            const command = str(args.command);
+            const cmdDisplay = command === null ? invalidArg : escapeHtml(command || '...');
+            html += `<div class="tool-command">$ ${cmdDisplay}</div>`;
             if (result) {
               const output = getResultText().trim();
               if (output) html += formatExpandableOutput(output, 5);
@@ -891,13 +902,12 @@
             break;
           }
           case 'read': {
-            const filePath = args.file_path || args.path || '';
+            const filePath = str(args.file_path ?? args.path);
             const offset = args.offset;
             const limit = args.limit;
-            const lang = getLanguageFromPath(filePath);
 
-            let pathHtml = escapeHtml(shortenPath(filePath));
-            if (offset !== undefined || limit !== undefined) {
+            let pathHtml = filePath === null ? invalidArg : escapeHtml(shortenPath(filePath || ''));
+            if (filePath !== null && (offset !== undefined || limit !== undefined)) {
               const startLine = offset ?? 1;
               const endLine = limit !== undefined ? startLine + limit - 1 : '';
               pathHtml += `<span class="line-numbers">:${startLine}${endLine ? '-' + endLine : ''}</span>`;
@@ -907,21 +917,28 @@
             if (result) {
               html += renderResultImages();
               const output = getResultText();
+              const lang = filePath ? getLanguageFromPath(filePath) : null;
               if (output) html += formatExpandableOutput(output, 10, lang);
             }
             break;
           }
           case 'write': {
-            const filePath = args.file_path || args.path || '';
-            const content = args.content || '';
-            const lines = content.split('\n');
-            const lang = getLanguageFromPath(filePath);
+            const filePath = str(args.file_path ?? args.path);
+            const content = str(args.content);
 
-            html += `<div class="tool-header"><span class="tool-name">write</span> <span class="tool-path">${escapeHtml(shortenPath(filePath))}</span>`;
-            if (lines.length > 10) html += ` <span class="line-count">(${lines.length} lines)</span>`;
+            html += `<div class="tool-header"><span class="tool-name">write</span> <span class="tool-path">${filePath === null ? invalidArg : escapeHtml(shortenPath(filePath || ''))}</span>`;
+            if (content !== null && content) {
+              const lines = content.split('\n');
+              if (lines.length > 10) html += ` <span class="line-count">(${lines.length} lines)</span>`;
+            }
             html += '</div>';
 
-            if (content) html += formatExpandableOutput(content, 10, lang);
+            if (content === null) {
+              html += `<div class="tool-error">[invalid content arg - expected string]</div>`;
+            } else if (content) {
+              const lang = filePath ? getLanguageFromPath(filePath) : null;
+              html += formatExpandableOutput(content, 10, lang);
+            }
             if (result) {
               const output = getResultText().trim();
               if (output) html += `<div class="tool-output"><div>${escapeHtml(output)}</div></div>`;
@@ -929,8 +946,8 @@
             break;
           }
           case 'edit': {
-            const filePath = args.file_path || args.path || '';
-            html += `<div class="tool-header"><span class="tool-name">edit</span> <span class="tool-path">${escapeHtml(shortenPath(filePath))}</span></div>`;
+            const filePath = str(args.file_path ?? args.path);
+            html += `<div class="tool-header"><span class="tool-name">edit</span> <span class="tool-path">${filePath === null ? invalidArg : escapeHtml(shortenPath(filePath || ''))}</span></div>`;
 
             if (result?.details?.diff) {
               const diffLines = result.details.diff.split('\n');
diff --git a/packages/coding-agent/src/modes/interactive/components/tool-execution.ts b/packages/coding-agent/src/modes/interactive/components/tool-execution.ts
index 35bad0e6..6678e0d1 100644
--- a/packages/coding-agent/src/modes/interactive/components/tool-execution.ts
+++ b/packages/coding-agent/src/modes/interactive/components/tool-execution.ts
@@ -29,7 +29,8 @@ const BASH_PREVIEW_LINES = 5;
 /**
  * Convert absolute path to tilde notation if it's in home directory
  */
-function shortenPath(path: string): string {
+function shortenPath(path: unknown): string {
+	if (typeof path !== "string") return "";
 	const home = os.homedir();
 	if (path.startsWith(home)) {
 		return `~${path.slice(home.length)}`;
@@ -44,6 +45,13 @@ function replaceTabs(text: string): string {
 	return text.replace(/\t/g, "   ");
 }
 
+/** Safely coerce value to string for display. Returns null if invalid type. */
+function str(value: unknown): string | null {
+	if (typeof value === "string") return value;
+	if (value == null) return "";
+	return null; // Invalid type
+}
+
 export interface ToolExecutionOptions {
 	showImages?: boolean; // default: true (only used if terminal supports images)
 }
@@ -341,17 +349,15 @@ export class ToolExecutionComponent extends Container {
 	 * Render bash content using visual line truncation (like bash-execution.ts)
 	 */
 	private renderBashContent(): void {
-		const command = this.args?.command || "";
+		const command = str(this.args?.command);
 		const timeout = this.args?.timeout as number | undefined;
 
 		// Header
 		const timeoutSuffix = timeout ? theme.fg("muted", ` (timeout ${timeout}s)`) : "";
+		const commandDisplay =
+			command === null ? theme.fg("error", "[invalid arg]") : command ? command : theme.fg("toolOutput", "...");
 		this.contentBox.addChild(
-			new Text(
-				theme.fg("toolTitle", theme.bold(`$ ${command || theme.fg("toolOutput", "...")}`)) + timeoutSuffix,
-				0,
-				0,
-			),
+			new Text(theme.fg("toolTitle", theme.bold(`$ ${commandDisplay}`)) + timeoutSuffix, 0, 0),
 		);
 
 		if (this.result) {
@@ -450,13 +456,15 @@ export class ToolExecutionComponent extends Container {
 
 	private formatToolExecution(): string {
 		let text = "";
+		const invalidArg = theme.fg("error", "[invalid arg]");
 
 		if (this.toolName === "read") {
-			const path = shortenPath(this.args?.file_path || this.args?.path || "");
+			const rawPath = str(this.args?.file_path ?? this.args?.path);
+			const path = rawPath !== null ? shortenPath(rawPath) : null;
 			const offset = this.args?.offset;
 			const limit = this.args?.limit;
 
-			let pathDisplay = path ? theme.fg("accent", path) : theme.fg("toolOutput", "...");
+			let pathDisplay = path === null ? invalidArg : path ? theme.fg("accent", path) : theme.fg("toolOutput", "...");
 			if (offset !== undefined || limit !== undefined) {
 				const startLine = offset ?? 1;
 				const endLine = limit !== undefined ? startLine + limit - 1 : "";
@@ -467,8 +475,8 @@ export class ToolExecutionComponent extends Container {
 
 			if (this.result) {
 				const output = this.getTextOutput();
-				const rawPath = this.args?.file_path || this.args?.path || "";
-				const lang = getLanguageFromPath(rawPath);
+				const rawPath = str(this.args?.file_path ?? this.args?.path);
+				const lang = rawPath ? getLanguageFromPath(rawPath) : undefined;
 				const lines = lang ? highlightCode(replaceTabs(output), lang) : output.split("\n");
 
 				const maxLines = this.expanded ? lines.length : 10;
@@ -511,23 +519,21 @@ export class ToolExecutionComponent extends Container {
 				}
 			}
 		} else if (this.toolName === "write") {
-			const rawPath = this.args?.file_path || this.args?.path || "";
-			const path = shortenPath(rawPath);
-			const fileContent = this.args?.content || "";
-			const lang = getLanguageFromPath(rawPath);
-			const lines = fileContent
-				? lang
-					? highlightCode(replaceTabs(fileContent), lang)
-					: fileContent.split("\n")
-				: [];
-			const totalLines = lines.length;
+			const rawPath = str(this.args?.file_path ?? this.args?.path);
+			const fileContent = str(this.args?.content);
+			const path = rawPath !== null ? shortenPath(rawPath) : null;
 
 			text =
 				theme.fg("toolTitle", theme.bold("write")) +
 				" " +
-				(path ? theme.fg("accent", path) : theme.fg("toolOutput", "..."));
+				(path === null ? invalidArg : path ? theme.fg("accent", path) : theme.fg("toolOutput", "..."));
 
-			if (fileContent) {
+			if (fileContent === null) {
+				text += `\n\n${theme.fg("error", "[invalid content arg - expected string]")}`;
+			} else if (fileContent) {
+				const lang = rawPath ? getLanguageFromPath(rawPath) : undefined;
+				const lines = lang ? highlightCode(replaceTabs(fileContent), lang) : fileContent.split("\n");
+				const totalLines = lines.length;
 				const maxLines = this.expanded ? lines.length : 10;
 				const displayLines = lines.slice(0, maxLines);
 				const remaining = lines.length - maxLines;
@@ -552,11 +558,11 @@ export class ToolExecutionComponent extends Container {
 				}
 			}
 		} else if (this.toolName === "edit") {
-			const rawPath = this.args?.file_path || this.args?.path || "";
-			const path = shortenPath(rawPath);
+			const rawPath = str(this.args?.file_path ?? this.args?.path);
+			const path = rawPath !== null ? shortenPath(rawPath) : null;
 
 			// Build path display, appending :line if we have diff info
-			let pathDisplay = path ? theme.fg("accent", path) : theme.fg("toolOutput", "...");
+			let pathDisplay = path === null ? invalidArg : path ? theme.fg("accent", path) : theme.fg("toolOutput", "...");
 			const firstChangedLine =
 				(this.editDiffPreview && "firstChangedLine" in this.editDiffPreview
 					? this.editDiffPreview.firstChangedLine
@@ -578,20 +584,21 @@ export class ToolExecutionComponent extends Container {
 				// Tool executed successfully - use the diff from result
 				// This takes priority over editDiffPreview which may have a stale error
 				// due to race condition (async preview computed after file was modified)
-				text += `\n\n${renderDiff(this.result.details.diff, { filePath: rawPath })}`;
+				text += `\n\n${renderDiff(this.result.details.diff, { filePath: rawPath ?? undefined })}`;
 			} else if (this.editDiffPreview) {
 				// Use cached diff preview (before tool executes)
 				if ("error" in this.editDiffPreview) {
 					text += `\n\n${theme.fg("error", this.editDiffPreview.error)}`;
 				} else if (this.editDiffPreview.diff) {
-					text += `\n\n${renderDiff(this.editDiffPreview.diff, { filePath: rawPath })}`;
+					text += `\n\n${renderDiff(this.editDiffPreview.diff, { filePath: rawPath ?? undefined })}`;
 				}
 			}
 		} else if (this.toolName === "ls") {
-			const path = shortenPath(this.args?.path || ".");
+			const rawPath = str(this.args?.path);
+			const path = rawPath !== null ? shortenPath(rawPath || ".") : null;
 			const limit = this.args?.limit;
 
-			text = `${theme.fg("toolTitle", theme.bold("ls"))} ${theme.fg("accent", path)}`;
+			text = `${theme.fg("toolTitle", theme.bold("ls"))} ${path === null ? invalidArg : theme.fg("accent", path)}`;
 			if (limit !== undefined) {
 				text += theme.fg("toolOutput", ` (limit ${limit})`);
 			}
@@ -624,15 +631,16 @@ export class ToolExecutionComponent extends Container {
 				}
 			}
 		} else if (this.toolName === "find") {
-			const pattern = this.args?.pattern || "";
-			const path = shortenPath(this.args?.path || ".");
+			const pattern = str(this.args?.pattern);
+			const rawPath = str(this.args?.path);
+			const path = rawPath !== null ? shortenPath(rawPath || ".") : null;
 			const limit = this.args?.limit;
 
 			text =
 				theme.fg("toolTitle", theme.bold("find")) +
 				" " +
-				theme.fg("accent", pattern) +
-				theme.fg("toolOutput", ` in ${path}`);
+				(pattern === null ? invalidArg : theme.fg("accent", pattern || "")) +
+				theme.fg("toolOutput", ` in ${path === null ? invalidArg : path}`);
 			if (limit !== undefined) {
 				text += theme.fg("toolOutput", ` (limit ${limit})`);
 			}
@@ -665,16 +673,17 @@ export class ToolExecutionComponent extends Container {
 				}
 			}
 		} else if (this.toolName === "grep") {
-			const pattern = this.args?.pattern || "";
-			const path = shortenPath(this.args?.path || ".");
-			const glob = this.args?.glob;
+			const pattern = str(this.args?.pattern);
+			const rawPath = str(this.args?.path);
+			const path = rawPath !== null ? shortenPath(rawPath || ".") : null;
+			const glob = str(this.args?.glob);
 			const limit = this.args?.limit;
 
 			text =
 				theme.fg("toolTitle", theme.bold("grep")) +
 				" " +
-				theme.fg("accent", `/${pattern}/`) +
-				theme.fg("toolOutput", ` in ${path}`);
+				(pattern === null ? invalidArg : theme.fg("accent", `/${pattern || ""}/`)) +
+				theme.fg("toolOutput", ` in ${path === null ? invalidArg : path}`);
 			if (glob) {
 				text += theme.fg("toolOutput", ` (${glob})`);
 			}