fix: normalize message trimming behavior across all model providers

e2e/testdata/cassettes/TestDebug_Title/Anthropic.yaml

@@ -8,7 +8,7 @@ interactions:
         proto_minor: 1
         content_length: 0
         host: api.anthropic.com
-        body: '{"max_tokens":20,"messages":[{"content":[{"text":"Based on the following recent user messages from a conversation with an AI assistant, generate a short, descriptive title (maximum 50 characters) that captures the main topic or purpose of the conversation. Return ONLY the title text on a single line, nothing else. Do not include any newlines, explanations, or formatting.\n\nRecent user messages:\n1. What can you do?","cache_control":{"type":"ephemeral"},"type":"text"}],"role":"user"}],"model":"claude-haiku-4-5","system":[{"text":"You are a helpful AI assistant that generates concise, descriptive titles for conversations. You will be given up to 2 recent user messages and asked to create a single-line title that captures the main topic. Never use newlines or line breaks in your response.","type":"text"}],"tools":[],"stream":true}'
+        body: '{"max_tokens":20,"messages":[{"content":[{"text":"Based on the following recent user messages from a conversation with an AI assistant, generate a short, descriptive title (maximum 50 characters) that captures the main topic or purpose of the conversation. Return ONLY the title text on a single line, nothing else. Do not include any newlines, explanations, or formatting.\n\nRecent user messages:\n1. What can you do?\n\n\n","cache_control":{"type":"ephemeral"},"type":"text"}],"role":"user"}],"model":"claude-haiku-4-5","system":[{"text":"You are a helpful AI assistant that generates concise, descriptive titles for conversations. You will be given up to 2 recent user messages and asked to create a single-line title that captures the main topic. Never use newlines or line breaks in your response.","type":"text"}],"tools":[],"stream":true}'
         url: https://api.anthropic.com/v1/messages
         method: POST
       response:

e2e/testdata/cassettes/TestDebug_Title/Anthropic_Opus46.yaml

@@ -8,7 +8,7 @@ interactions:
         proto_minor: 1
         content_length: 0
         host: api.anthropic.com
-        body: '{"max_tokens":20,"messages":[{"content":[{"text":"Based on the following recent user messages from a conversation with an AI assistant, generate a short, descriptive title (maximum 50 characters) that captures the main topic or purpose of the conversation. Return ONLY the title text on a single line, nothing else. Do not include any newlines, explanations, or formatting.\n\nRecent user messages:\n1. What can you do?","cache_control":{"type":"ephemeral"},"type":"text"}],"role":"user"}],"model":"claude-opus-4-6","system":[{"text":"You are a helpful AI assistant that generates concise, descriptive titles for conversations. You will be given up to 2 recent user messages and asked to create a single-line title that captures the main topic. Never use newlines or line breaks in your response.","type":"text"}],"tools":[],"stream":true}'
+        body: '{"max_tokens":20,"messages":[{"content":[{"text":"Based on the following recent user messages from a conversation with an AI assistant, generate a short, descriptive title (maximum 50 characters) that captures the main topic or purpose of the conversation. Return ONLY the title text on a single line, nothing else. Do not include any newlines, explanations, or formatting.\n\nRecent user messages:\n1. What can you do?\n\n\n","cache_control":{"type":"ephemeral"},"type":"text"}],"role":"user"}],"model":"claude-opus-4-6","system":[{"text":"You are a helpful AI assistant that generates concise, descriptive titles for conversations. You will be given up to 2 recent user messages and asked to create a single-line title that captures the main topic. Never use newlines or line breaks in your response.","type":"text"}],"tools":[],"stream":true}'
         url: https://api.anthropic.com/v1/messages
         method: POST
       response:

e2e/testdata/cassettes/TestDebug_Title/Anthropic_Sonnet45.yaml

@@ -8,7 +8,7 @@ interactions:
         proto_minor: 1
         content_length: 0
         host: api.anthropic.com
-        body: '{"max_tokens":20,"messages":[{"content":[{"text":"Based on the following recent user messages from a conversation with an AI assistant, generate a short, descriptive title (maximum 50 characters) that captures the main topic or purpose of the conversation. Return ONLY the title text on a single line, nothing else. Do not include any newlines, explanations, or formatting.\n\nRecent user messages:\n1. What can you do?","cache_control":{"type":"ephemeral"},"type":"text"}],"role":"user"}],"model":"claude-sonnet-4-5","system":[{"text":"You are a helpful AI assistant that generates concise, descriptive titles for conversations. You will be given up to 2 recent user messages and asked to create a single-line title that captures the main topic. Never use newlines or line breaks in your response.","type":"text"}],"tools":[],"stream":true}'
+        body: '{"max_tokens":20,"messages":[{"content":[{"text":"Based on the following recent user messages from a conversation with an AI assistant, generate a short, descriptive title (maximum 50 characters) that captures the main topic or purpose of the conversation. Return ONLY the title text on a single line, nothing else. Do not include any newlines, explanations, or formatting.\n\nRecent user messages:\n1. What can you do?\n\n\n","cache_control":{"type":"ephemeral"},"type":"text"}],"role":"user"}],"model":"claude-sonnet-4-5","system":[{"text":"You are a helpful AI assistant that generates concise, descriptive titles for conversations. You will be given up to 2 recent user messages and asked to create a single-line title that captures the main topic. Never use newlines or line breaks in your response.","type":"text"}],"tools":[],"stream":true}'
         url: https://api.anthropic.com/v1/messages
         method: POST
       response:

e2e/testdata/cassettes/TestExec_Anthropic_ToolCall.yaml

@@ -55,7 +55,7 @@ interactions:
         proto_minor: 1
         content_length: 0
         host: api.anthropic.com
-        body: '{"max_tokens":64000,"messages":[{"content":[{"text":"How many files in testdata/working_dir? Only output the number.","type":"text"}],"role":"user"},{"content":[{"id":"toolu_012gmfqnoTX8c5aV3vMWUnas","input":{"path":"testdata/working_dir"},"name":"list_directory","cache_control":{"type":"ephemeral"},"type":"tool_use"}],"role":"assistant"},{"content":[{"tool_use_id":"toolu_012gmfqnoTX8c5aV3vMWUnas","is_error":false,"cache_control":{"type":"ephemeral"},"content":[{"text":"FILE README.me","type":"text"}],"type":"tool_result"}],"role":"user"}],"model":"claude-sonnet-4-0","system":[{"text":"You are a knowledgeable assistant that helps users with various tasks.\nBe helpful, accurate, and concise in your responses.","type":"text"},{"text":"## Filesystem Tools\n\n- Relative paths resolve from the working directory; absolute paths and \"..\" work as expected\n- Prefer read_multiple_files over sequential read_file calls\n- Use search_files_content to locate code or text across files\n- Use exclude patterns in searches and max_depth in directory_tree to limit output","cache_control":{"type":"ephemeral"},"type":"text"}],"tools":[{"input_schema":{"properties":{"path":{"description":"The directory path to traverse (relative to working directory)","type":"string"}},"required":["path"],"type":"object"},"name":"directory_tree","description":"Get a recursive tree view of files and directories as a JSON structure."},{"input_schema":{"properties":{"edits":{"description":"Array of edit operations","items":{"additionalProperties":false,"properties":{"newText":{"description":"The replacement text","type":"string"},"oldText":{"description":"The exact text to replace","type":"string"}},"required":["oldText","newText"],"type":"object"},"type":["null","array"]},"path":{"description":"The file path to edit","type":"string"}},"required":["path","edits"],"type":"object"},"name":"edit_file","description":"Make line-based edits to a text file. Each edit replaces exact line sequences with new content."},{"input_schema":{"properties":{"path":{"description":"The directory path to list","type":"string"}},"required":["path"],"type":"object"},"name":"list_directory","description":"Get a detailed listing of all files and directories in a specified path."},{"input_schema":{"properties":{"path":{"description":"The file path to read","type":"string"}},"required":["path"],"type":"object"},"name":"read_file","description":"Read the complete contents of a file from the file system. Supports text files and images (jpg, png, gif, webp). Images are returned as image content that you can view directly."},{"input_schema":{"properties":{"json":{"description":"Whether to return the result as JSON","type":"boolean"},"paths":{"description":"Array of file paths to read","items":{"type":"string"},"type":["null","array"]}},"required":["paths"],"type":"object"},"name":"read_multiple_files","description":"Read the contents of multiple files simultaneously."},{"input_schema":{"properties":{"excludePatterns":{"description":"Patterns to exclude from search","items":{"type":"string"},"type":["null","array"]},"is_regex":{"description":"If true, treat query as regex; otherwise literal text","type":"boolean"},"path":{"description":"The starting directory path","type":"string"},"query":{"description":"The text or regex pattern to search for","type":"string"}},"required":["path","query"],"type":"object"},"name":"search_files_content","description":"Searches for text or regex patterns in the content of files matching a GLOB pattern."},{"input_schema":{"properties":{"content":{"description":"The content to write to the file","type":"string"},"path":{"description":"The file path to write","type":"string"}},"required":["path","content"],"type":"object"},"name":"write_file","description":"Create a new file or completely overwrite an existing file with new content."},{"input_schema":{"properties":{"paths":{"description":"Array of directory paths to create","items":{"type":"string"},"type":["null","array"]}},"required":["paths"],"type":"object"},"name":"create_directory","description":"Create one or more new directories or nested directory structures."},{"input_schema":{"properties":{"paths":{"description":"Array of directory paths to remove","items":{"type":"string"},"type":["null","array"]}},"required":["paths"],"type":"object"},"name":"remove_directory","description":"Remove one or more empty directories."}],"stream":true}'
+        body: '{"max_tokens":64000,"messages":[{"content":[{"text":"How many files in testdata/working_dir? Only output the number.","type":"text"}],"role":"user"},{"content":[{"id":"toolu_012gmfqnoTX8c5aV3vMWUnas","input":{"path":"testdata/working_dir"},"name":"list_directory","cache_control":{"type":"ephemeral"},"type":"tool_use"}],"role":"assistant"},{"content":[{"tool_use_id":"toolu_012gmfqnoTX8c5aV3vMWUnas","is_error":false,"cache_control":{"type":"ephemeral"},"content":[{"text":"FILE README.me\n","type":"text"}],"type":"tool_result"}],"role":"user"}],"model":"claude-sonnet-4-0","system":[{"text":"You are a knowledgeable assistant that helps users with various tasks.\nBe helpful, accurate, and concise in your responses.","type":"text"},{"text":"## Filesystem Tools\n\n- Relative paths resolve from the working directory; absolute paths and \"..\" work as expected\n- Prefer read_multiple_files over sequential read_file calls\n- Use search_files_content to locate code or text across files\n- Use exclude patterns in searches and max_depth in directory_tree to limit output","cache_control":{"type":"ephemeral"},"type":"text"}],"tools":[{"input_schema":{"properties":{"path":{"description":"The directory path to traverse (relative to working directory)","type":"string"}},"required":["path"],"type":"object"},"name":"directory_tree","description":"Get a recursive tree view of files and directories as a JSON structure."},{"input_schema":{"properties":{"edits":{"description":"Array of edit operations","items":{"additionalProperties":false,"properties":{"newText":{"description":"The replacement text","type":"string"},"oldText":{"description":"The exact text to replace","type":"string"}},"required":["oldText","newText"],"type":"object"},"type":["null","array"]},"path":{"description":"The file path to edit","type":"string"}},"required":["path","edits"],"type":"object"},"name":"edit_file","description":"Make line-based edits to a text file. Each edit replaces exact line sequences with new content."},{"input_schema":{"properties":{"path":{"description":"The directory path to list","type":"string"}},"required":["path"],"type":"object"},"name":"list_directory","description":"Get a detailed listing of all files and directories in a specified path."},{"input_schema":{"properties":{"path":{"description":"The file path to read","type":"string"}},"required":["path"],"type":"object"},"name":"read_file","description":"Read the complete contents of a file from the file system. Supports text files and images (jpg, png, gif, webp). Images are returned as image content that you can view directly."},{"input_schema":{"properties":{"json":{"description":"Whether to return the result as JSON","type":"boolean"},"paths":{"description":"Array of file paths to read","items":{"type":"string"},"type":["null","array"]}},"required":["paths"],"type":"object"},"name":"read_multiple_files","description":"Read the contents of multiple files simultaneously."},{"input_schema":{"properties":{"excludePatterns":{"description":"Patterns to exclude from search","items":{"type":"string"},"type":["null","array"]},"is_regex":{"description":"If true, treat query as regex; otherwise literal text","type":"boolean"},"path":{"description":"The starting directory path","type":"string"},"query":{"description":"The text or regex pattern to search for","type":"string"}},"required":["path","query"],"type":"object"},"name":"search_files_content","description":"Searches for text or regex patterns in the content of files matching a GLOB pattern."},{"input_schema":{"properties":{"content":{"description":"The content to write to the file","type":"string"},"path":{"description":"The file path to write","type":"string"}},"required":["path","content"],"type":"object"},"name":"write_file","description":"Create a new file or completely overwrite an existing file with new content."},{"input_schema":{"properties":{"paths":{"description":"Array of directory paths to create","items":{"type":"string"},"type":["null","array"]}},"required":["paths"],"type":"object"},"name":"create_directory","description":"Create one or more new directories or nested directory structures."},{"input_schema":{"properties":{"paths":{"description":"Array of directory paths to remove","items":{"type":"string"},"type":["null","array"]}},"required":["paths"],"type":"object"},"name":"remove_directory","description":"Remove one or more empty directories."}],"stream":true}'
         url: https://api.anthropic.com/v1/messages
         method: POST
       response:

pkg/model/provider/anthropic/beta_client_test.go

@@ -215,7 +215,8 @@ func TestExtractBetaSystemBlocks_MultipleSystemMessages(t *testing.T) {
 	assert.Equal(t, "Be concise", blocks[1].Text)
 }
 
-// TestExtractBetaSystemBlocks_SkipsEmptyText tests that empty system text is skipped
+// TestExtractBetaSystemBlocks_SkipsEmptyText tests that empty system text is skipped.
+// System blocks are trimmed because YAML literal-block instructions always append a trailing newline.
 func TestExtractBetaSystemBlocks_SkipsEmptyText(t *testing.T) {
 	msgs := []chat.Message{
 		{

pkg/model/provider/anthropic/beta_converter.go

@@ -43,11 +43,11 @@ func (c *Client) convertBetaMessages(ctx context.Context, messages []chat.Messag
 						Content: contentBlocks,
 					})
 				}
-			} else if txt := strings.TrimSpace(msg.Content); txt != "" {
+			} else {
 				betaMessages = append(betaMessages, anthropic.BetaMessageParam{
 					Role: anthropic.BetaMessageParamRoleUser,
 					Content: []anthropic.BetaContentBlockParamUnion{
-						{OfText: &anthropic.BetaTextBlockParam{Text: txt}},
+						{OfText: &anthropic.BetaTextBlockParam{Text: msg.Content}},
 					},
 				})
 			}
@@ -68,9 +68,9 @@ func (c *Client) convertBetaMessages(ctx context.Context, messages []chat.Messag
 			}
 
 			// Add text content if present
-			if txt := strings.TrimSpace(msg.Content); txt != "" {
+			if msg.Content != "" {
 				contentBlocks = append(contentBlocks, anthropic.BetaContentBlockParamUnion{
-					OfText: &anthropic.BetaTextBlockParam{Text: txt},
+					OfText: &anthropic.BetaTextBlockParam{Text: msg.Content},
 				})
 			}
 
@@ -138,11 +138,17 @@ func (c *Client) convertBetaMessages(ctx context.Context, messages []chat.Messag
 // including any image content from MultiContent.
 func convertBetaToolResultBlock(msg *chat.Message) anthropic.BetaContentBlockParamUnion {
 	if !hasImageMultiContent(msg.MultiContent) {
+		// tool_result must be present for every preceding tool_use; we cannot skip
+		// it. Normalize whitespace-only content to empty string rather than skipping.
+		content := msg.Content
+		if strings.TrimSpace(content) == "" {
+			content = ""
+		}
 		return anthropic.BetaContentBlockParamUnion{
 			OfToolResult: &anthropic.BetaToolResultBlockParam{
 				ToolUseID: msg.ToolCallID,
 				Content: []anthropic.BetaToolResultBlockParamContentUnion{
-					{OfText: &anthropic.BetaTextBlockParam{Text: strings.TrimSpace(msg.Content)}},
+					{OfText: &anthropic.BetaTextBlockParam{Text: content}},
 				},
 			},
 		}
@@ -152,11 +158,9 @@ func convertBetaToolResultBlock(msg *chat.Message) anthropic.BetaContentBlockPar
 	for _, part := range msg.MultiContent {
 		switch part.Type {
 		case chat.MessagePartTypeText:
-			if txt := strings.TrimSpace(part.Text); txt != "" {
-				content = append(content, anthropic.BetaToolResultBlockParamContentUnion{
-					OfText: &anthropic.BetaTextBlockParam{Text: txt},
-				})
-			}
+			content = append(content, anthropic.BetaToolResultBlockParamContentUnion{
+				OfText: &anthropic.BetaTextBlockParam{Text: part.Text},
+			})
 		case chat.MessagePartTypeImageURL:
 			if part.ImageURL == nil {
 				continue
@@ -194,11 +198,9 @@ func (c *Client) convertBetaUserMultiContent(ctx context.Context, parts []chat.M
 	for _, part := range parts {
 		switch part.Type {
 		case chat.MessagePartTypeText:
-			if txt := strings.TrimSpace(part.Text); txt != "" {
-				contentBlocks = append(contentBlocks, anthropic.BetaContentBlockParamUnion{
-					OfText: &anthropic.BetaTextBlockParam{Text: txt},
-				})
-			}
+			contentBlocks = append(contentBlocks, anthropic.BetaContentBlockParamUnion{
+				OfText: &anthropic.BetaTextBlockParam{Text: part.Text},
+			})
 
 		case chat.MessagePartTypeImageURL:
 			if part.ImageURL == nil {