diff --git a/cli/src/agent/backends/acp/AcpMessageHandler.test.ts b/cli/src/agent/backends/acp/AcpMessageHandler.test.ts
index 095dca85..af28ace0 100644
--- a/cli/src/agent/backends/acp/AcpMessageHandler.test.ts
+++ b/cli/src/agent/backends/acp/AcpMessageHandler.test.ts
@@ -779,4 +779,247 @@ describe('AcpMessageHandler', () => {
 
         expect(messages).toHaveLength(0);
     });
+
+    describe('tool_call_update content normalization (Gemini/OpenCode path)', () => {
+        it('unwraps text content block to string output', () => {
+            // Gemini sends content: [{type:'content', content:{type:'text', text:'...'}}]
+            // when the tool has stdout. HAPI must normalize this to a plain string.
+            const messages: AgentMessage[] = [];
+            const handler = new AcpMessageHandler((message) => messages.push(message));
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCall,
+                toolCallId: 'gem-1',
+                title: 'shell',
+                rawInput: { cmd: 'echo hello' },
+                status: 'in_progress'
+            });
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCallUpdate,
+                toolCallId: 'gem-1',
+                status: 'completed',
+                content: [{ type: 'content', content: { type: 'text', text: 'hello\n' } }]
+            });
+
+            const result = getToolResult(messages, 'gem-1');
+            expect(result.status).toBe('completed');
+            expect(result.output).toBe('hello\n');
+        });
+
+        it('normalizes empty content array to empty string output', () => {
+            // Gemini sends content: [] when returnDisplay is falsy (no visible output).
+            // Raw [] must not be forwarded to the web renderer.
+            const messages: AgentMessage[] = [];
+            const handler = new AcpMessageHandler((message) => messages.push(message));
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCall,
+                toolCallId: 'gem-2',
+                title: 'shell',
+                rawInput: { cmd: 'touch file' },
+                status: 'in_progress'
+            });
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCallUpdate,
+                toolCallId: 'gem-2',
+                status: 'completed',
+                content: []
+            });
+
+            const result = getToolResult(messages, 'gem-2');
+            expect(result.status).toBe('completed');
+            expect(result.output).toBe('');
+        });
+
+        it('preserves diff content block fields in output', () => {
+            // Gemini sends content: [{type:'diff', path, oldText, newText, _meta:{kind}}]
+            // for file-edit tools. HAPI must surface these fields intact.
+            const messages: AgentMessage[] = [];
+            const handler = new AcpMessageHandler((message) => messages.push(message));
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCall,
+                toolCallId: 'gem-3',
+                title: 'write_file',
+                rawInput: { path: 'src/foo.ts' },
+                status: 'in_progress'
+            });
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCallUpdate,
+                toolCallId: 'gem-3',
+                status: 'completed',
+                content: [{
+                    type: 'diff',
+                    path: 'src/foo.ts',
+                    oldText: 'old content',
+                    newText: 'new content',
+                    _meta: { kind: 'modify' }
+                }]
+            });
+
+            const result = getToolResult(messages, 'gem-3');
+            expect(result.status).toBe('completed');
+            expect(result.output).toEqual({
+                path: 'src/foo.ts',
+                oldText: 'old content',
+                newText: 'new content',
+                kind: 'modify'
+            });
+        });
+
+        it('prefers rawOutput over content when both are present (regression guard)', () => {
+            // Claude/Codex always send rawOutput. If both fields arrive, rawOutput wins
+            // and the ACP content array is ignored to preserve existing behavior.
+            const messages: AgentMessage[] = [];
+            const handler = new AcpMessageHandler((message) => messages.push(message));
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCall,
+                toolCallId: 'reg-1',
+                title: 'Bash',
+                rawInput: { cmd: 'ls' },
+                status: 'in_progress'
+            });
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCallUpdate,
+                toolCallId: 'reg-1',
+                status: 'completed',
+                rawOutput: { stdout: 'file.txt\n' },
+                content: [{ type: 'content', content: { type: 'text', text: 'should be ignored' } }]
+            });
+
+            const result = getToolResult(messages, 'reg-1');
+            expect(result.status).toBe('completed');
+            expect(result.output).toEqual({ stdout: 'file.txt\n' });
+        });
+
+        it('passes through non-array content value unchanged when rawOutput is absent', () => {
+            // If an ACP agent sends content as a non-array value (e.g. a plain string or
+            // object), normalizeAcpToolContent returns null and we fall back to the
+            // original content to avoid silent data loss.
+            const messages: AgentMessage[] = [];
+            const handler = new AcpMessageHandler((message) => messages.push(message));
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCall,
+                toolCallId: 'reg-2',
+                title: 'Bash',
+                rawInput: { cmd: 'ls' },
+                status: 'in_progress'
+            });
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCallUpdate,
+                toolCallId: 'reg-2',
+                status: 'completed',
+                content: { stdout: 'file.txt\n' }
+            });
+
+            const result = getToolResult(messages, 'reg-2');
+            expect(result.status).toBe('completed');
+            expect(result.output).toEqual({ stdout: 'file.txt\n' });
+        });
+
+        it('falls back to raw content for mixed text+diff array (null from normalizer)', () => {
+            // A mixed array [{type:'content',...}, {type:'diff',...}] cannot be safely
+            // collapsed into either a string or a single diff object without losing data.
+            // normalizeAcpToolContent must return null so the caller falls back to the
+            // original content array, preserving all information.
+            const messages: AgentMessage[] = [];
+            const handler = new AcpMessageHandler((message) => messages.push(message));
+
+            const mixedContent = [
+                { type: 'content', content: { type: 'text', text: 'some stdout' } },
+                { type: 'diff', path: 'src/foo.ts', oldText: 'old', newText: 'new', _meta: { kind: 'modify' } }
+            ];
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCall,
+                toolCallId: 'mixed-1',
+                title: 'run_and_edit',
+                rawInput: { cmd: 'patch' },
+                status: 'in_progress'
+            });
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCallUpdate,
+                toolCallId: 'mixed-1',
+                status: 'completed',
+                content: mixedContent
+            });
+
+            const result = getToolResult(messages, 'mixed-1');
+            expect(result.status).toBe('completed');
+            // Must fall back to original content array — no information loss
+            expect(result.output).toEqual(mixedContent);
+        });
+
+        it('falls back to raw content for multi-diff array (null from normalizer)', () => {
+            // Multiple diff blocks cannot be collapsed into a single diff object.
+            // normalizeAcpToolContent must return null so we keep the full array.
+            const messages: AgentMessage[] = [];
+            const handler = new AcpMessageHandler((message) => messages.push(message));
+
+            const multiDiffContent = [
+                { type: 'diff', path: 'a.ts', oldText: 'a-old', newText: 'a-new', _meta: { kind: 'modify' } },
+                { type: 'diff', path: 'b.ts', oldText: 'b-old', newText: 'b-new', _meta: { kind: 'modify' } }
+            ];
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCall,
+                toolCallId: 'multidiff-1',
+                title: 'edit_files',
+                rawInput: { files: ['a.ts', 'b.ts'] },
+                status: 'in_progress'
+            });
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCallUpdate,
+                toolCallId: 'multidiff-1',
+                status: 'completed',
+                content: multiDiffContent
+            });
+
+            const result = getToolResult(messages, 'multidiff-1');
+            expect(result.status).toBe('completed');
+            // Must fall back to original content array — no information loss
+            expect(result.output).toEqual(multiDiffContent);
+        });
+
+        it('falls back to raw content for unknown block type (null from normalizer)', () => {
+            // An unrecognized block type (e.g. {type:'image',...}) cannot be safely
+            // normalized. We must return null and let the caller fall back to the original
+            // content to avoid silent data loss.
+            const messages: AgentMessage[] = [];
+            const handler = new AcpMessageHandler((message) => messages.push(message));
+
+            const unknownContent = [
+                { type: 'image', url: 'https://example.com/screenshot.png' }
+            ];
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCall,
+                toolCallId: 'unknown-1',
+                title: 'screenshot',
+                rawInput: { url: 'https://example.com' },
+                status: 'in_progress'
+            });
+
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.toolCallUpdate,
+                toolCallId: 'unknown-1',
+                status: 'completed',
+                content: unknownContent
+            });
+
+            const result = getToolResult(messages, 'unknown-1');
+            expect(result.status).toBe('completed');
+            // Must fall back to original content array — no information loss
+            expect(result.output).toEqual(unknownContent);
+        });
+    });
 });
diff --git a/cli/src/agent/backends/acp/AcpMessageHandler.ts b/cli/src/agent/backends/acp/AcpMessageHandler.ts
index c3f934c3..4feec31b 100644
--- a/cli/src/agent/backends/acp/AcpMessageHandler.ts
+++ b/cli/src/agent/backends/acp/AcpMessageHandler.ts
@@ -70,6 +70,77 @@ function extractAudienceField(value: unknown): string[] {
     return value.filter((entry): entry is string => typeof entry === 'string');
 }
 
+/**
+ * Normalizes the ACP `tool_call_update` content array sent by agents (e.g.
+ * Gemini, OpenCode) that do not populate `rawOutput`.
+ *
+ * ACP ToolCallContent union (as emitted by Gemini CLI):
+ *   - `{type:'content', content:{type:'text', text:…}}` — tool stdout/stderr
+ *   - `{type:'diff', path, oldText, newText, _meta:{kind}}` — file edits
+ *
+ * Only normalizes unambiguous cases. Returns null for anything that cannot be
+ * safely collapsed without losing information, so the caller can fall back to
+ * the original content value.
+ *
+ * Returns:
+ *   - string   — concatenated text from a pure-text-block array
+ *   - object   — structured diff from a single-diff-block array
+ *   - ""       — empty string when content array is empty (no visible output)
+ *   - null     — non-array, mixed types, multiple diffs, or unknown block type;
+ *                caller should pass through the original value unchanged
+ */
+function normalizeAcpToolContent(content: unknown): string | object | null {
+    if (!Array.isArray(content)) {
+        return null;
+    }
+    // Empty array: no display output from the agent (e.g. touch, silent command)
+    if (content.length === 0) {
+        return '';
+    }
+    // Classify every block. If any block has an unrecognized type or the array
+    // contains a mix of text and diff blocks we cannot collapse losslessly, so
+    // return null and let the caller fall back to the original content array.
+    let diffCount = 0;
+    let textCount = 0;
+    const parts: string[] = [];
+    let diffBlock: object | null = null;
+
+    for (const block of content) {
+        if (!isObject(block)) {
+            return null; // Non-object element — unrecognized
+        }
+        if (block.type === 'diff') {
+            diffCount++;
+            if (diffCount > 1) {
+                return null; // Multiple diffs cannot be merged into one object
+            }
+            diffBlock = {
+                path: typeof block.path === 'string' ? block.path : undefined,
+                oldText: typeof block.oldText === 'string' ? block.oldText : undefined,
+                newText: typeof block.newText === 'string' ? block.newText : undefined,
+                kind: isObject(block._meta) && typeof block._meta.kind === 'string' ? block._meta.kind : undefined
+            };
+        } else if (block.type === 'content' && isObject(block.content)) {
+            const inner = block.content;
+            if (inner.type === 'text' && typeof inner.text === 'string') {
+                textCount++;
+                parts.push(inner.text);
+            } else {
+                return null; // Unknown inner content type (e.g. image, resource)
+            }
+        } else {
+            return null; // Unknown top-level block type
+        }
+    }
+
+    // Mixed text + diff: cannot represent as a single value without losing data
+    if (diffCount > 0 && textCount > 0) {
+        return null;
+    }
+
+    return diffBlock ?? parts.join('');
+}
+
 function normalizePlanEntries(entries: unknown): PlanItem[] {
     if (!Array.isArray(entries)) return [];
 
@@ -297,11 +368,21 @@ export class AcpMessageHandler {
         }
 
         if (status === 'completed' || status === 'failed') {
-            const result = update.rawOutput ?? update.content;
+            // Prefer rawOutput (Claude/Codex path). When absent, normalize the
+            // ACP content array sent by agents such as Gemini and OpenCode.
+            // If content is not an array (normalizeAcpToolContent returns null),
+            // fall back to the original content value to avoid silent data loss.
+            let output: unknown;
+            if (update.rawOutput !== undefined) {
+                output = update.rawOutput;
+            } else {
+                const normalized = normalizeAcpToolContent(update.content);
+                output = normalized !== null ? normalized : update.content;
+            }
             this.onMessage({
                 type: 'tool_result',
                 id: toolCallId,
-                output: result,
+                output,
                 status: status === 'failed' ? 'failed' : 'completed'
             });
         }